Pneumatic Grippers

328 mm

8 mm

120 mm

42 mm

Pneumatics Electronics Mechanics Sensorics Software

47,3 mm

▼

Handling

Handling Machining Assembly Organisation

▼

160 mm

88,5 mm

Pneumatics Hesse Grippers and their applications

Hesse Grippers and their applications

English

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Blue Digest

195 mm

Blue Digest on Automation 053 435

225 mm

▼

158,5 mm

including vacuum devices

Hesse Grippers and their applications

Handling Pneumatics

Stefan Hesse

Grippers and their applications including vacuum devices

Blue Digest on Automation

Blue Digest on Automation © 2004 by Festo AG & Co. KG Ruiter Straße 82 D-73734 Esslingen Federal Republic of Germany Tel. 0711 347-0 Fax 0711 347-2155 All texts, representations, illustrations and drawings included in this book are the intellectual property of Festo AG & Co. KG, and are protected by copyright law. All rights reserved, including translation rights. No part of this publication may be reproduced or transmitted in any form or by any means, electronic, mechanical, photocopying or otherwise, without the prior written permission of Festo AG & Co. KG.

Foreword

It has been a long held dream by man to one day be free of the drudgery of manual labour through the use of automatic devices. Needless to say, this vision always depends on the technical components available at the time. The automatic production lines of the twenties used by the English company MORRIS MOTORS had to be mechanically controlled to a large extent, which was not very successful. Not until the sixties did a new basic technology become established: The NC machine and the industrial robot. Both are computer-aided and therefore freely programmable as far as movement is concerned. The robot is an important handling machine which roughly reproduces the human arm. In order to be effective, it also requires mechanical hands, which are generally referred to as grippers. These are also required on pick-and-place devices and a wide range of other automatic systems. In principle, there are two basic designs of grippers: Those, designed in the form of fingers and those which do not ressemble fingers in any way. Thousands of individual patents can be found, each of which claiming to be able to solve a gripping problem more successfully than previously known. This demonstrates that the gripper has a key role in automatic handling. For the user, it is becoming increasingly difficult to take in the now wide range of gripper technology. This is the reason why this brief introduction has been published. Above all, it is intended to provide advice and ideas to practical users, since the selection of the right gripper is by no means a trivial task. As with other technologies, there is a risk of making the wrong decision. Nowadays, however, most gripper tasks can be accomplished using standard grippers. Therefore, special grippers are only developed for exceptional cases. A sound basic knowledge of grippers and their use is always a good investment for the future. Stefan Hesse

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Table of contents Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1 Analysis of gripper functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 Gripper applications for component production and assembly . . . . . . . . 16 3 Grippers and hand axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4 Construction of gripping effectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5 Forces acting on grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 6 Technical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 7 Application areas and gripper types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 8 Checklist for grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 9 Suction grippers – abhorred by nature . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 10 Suction cups for every application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 11 Suction cups in handling technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 12 List of illustrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 13 List of special terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

1 Analysis of gripper functions

Grippers are the solution, but to which problem?

Determining the requirement profile

Grippers further enhance the performance capability of automated handling devices. These devices include not only industrial robots but also insertion devices, manipulators and special feed devices, for example for automatic machines, testing machines and batch-assembly systems. Grippers form the link between all kinds of workpieces and the manipulating machine concerned. Whilst man can pick up even complex workpieces easily and without hesitation, gripping in the world of technology requires careful planning to obtain a desired sequence – and a situation which has been defined must then be maintained extremely accurately. This series of articles will deal with a number of aspects which ensure that grippers produce the desired effect.

A general definition of grippers is given in the VDI Guideline 2860. According to this, the distinguishing feature of a gripper is the temporary grasping, retaining and subsequent releasing of objects of a particular geometrical shape. Grippers act like hands in automated machinery. The word “gripper” essentially describes the ever-growing family of accessories used in handling systems. It is not always easy to select the right type of gripper. However, it is not always appreciated that a correctly-formulated problem description can, in itself, be halfway towards solving the problem.

A gripper application can be defined in terms of technological requirements, workpiece parameters, the machine to be used for the handling operation and environmental parameters. The technological requirements may include the time available for gripping, the gripping path, the gripper force curve and the number of workpieces to be gripped simultaneously. The most important workpiece parameters are mass, shape, dimensions with tolerances, the position of the centre of gravity, stability, surface properties, material, strength and temperature. The data required for the handling device comprise its positioning error, axial acceleration and connection conditions. Environmental parameters consist of process forces, the space available for gripping, set-down conditions and the dirt, humidity and vibration present. It is difficult if a gripper is required to handle several different workpieces in succession, since it is then necessary to satisfy several requirement profiles. It must also be borne in mind that it is not always possible to grip a workpiece on all sides; for example, it may be necessary to avoid precisionmachined functional surfaces. A feed sequence may also impose limitations. This can be seen clearly in this example: (Fig. 1-1).


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Fig. 1-1: Division of a workpiece into gripping zone (G), clamping zone (C) and set-down zone (S) 1 2 3 4 5

1 2 4 5

Workpiece Magazine Clamping device Gripper Magnetic gripper

A

S

G G

3

A workpiece is to be picked up and brought into a clamping position. The workpiece cannot be gripped in the area of the set-down zone S, which fits into the magazine, since this is covered. The available gripping zone thus gives rise to the fact that the workpiece might slip away from the desired position. We must therefore clarify for each workpiece the question of which zones can be used for gripping. The situation will of course change if the user selects a different axial position in the magazine or another workpiece magazine. In our example, the set-down and clamping zones are identical. The gripping zone must be defined positively. The example involves external gripping. Workpieces which are hollow or have holes or recesses can also be gripped internally, as shown in Fig. 1-2. Fig. 1-2: How can a workpiece be picked up? a) External gripping b) Internal gripping c) Combination of internal and external gripping

a)

b)

c)

This is an important factor in the selection of a gripper, since in the case of internal gripping the holding force acts from the inside to the outside, requiring a double-acting gripper. External gripping, on the other hand, usually requires more space around the gripper.

10


Positioning accuracy is more important in assembly than in conveying. Positioning errors may result from : • the repetition accuracy of the handling device • gripper positioning errors • workpiece tolerances or • errors in the basic workpiece to which the axis motions are matched. An example is shown in Fig. 1-3. Fig. 1-3: The right choice of gripping point can affect the positioning error during assembly a) Gripping a component by its body b) Gripping by connecting wires

a b

a)

b)

The connecting wires of an electronic component are to be inserted into a printed circuit board. If the component is gripped by its body, distortions such as slightly bent connecting wires will seriously impair the success of the assembly operation; the effect being greater as the distance “a” increases. If, on the other hand, the workpiece is gripped by its wires at a distance “b” from the end of these, the situation is considerably better. Consideration must also be given to the intervals at which the components are to be fitted. Any gripper requires a certain minimum space to operate.

Is a controlled gripper necessary at all?

Workpiece handling does not in itself create added value. Only in assembly operations or when used to guide tools can an industrial robot add value to a workpiece. It is therefore a welcome advantage if handling devices can be simplified or even eliminated altogether. Example – a user wishes to feed workpieces to a clamping device (Fig. 1-4).


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Fig. 1-4: Feeding a clamping device 1 Clamping point 2 Simplest type of gripper (mandrel or plug gripper) 3 Supply magazine

2 1

3

He envisages a controlled mechanical jaw-type gripper with internal gripping. But why? A uncontrolled spring-loaded gripper is fully adequate. This can pick the workpiece from the magazine and take it to the clamping point. The clamping jaws then close and the plug gripper is retracted. Efforts should always be made to find the simplest solution. There are, however, often parameters which prevent this. The principle nonetheless remains – first simplify, then automate!

High point loadings can damage workpieces

12

There are many workpieces which can withstand the necessary gripping force without sustaining damage. But there are other workpieces which are for example polished, thin-walled, soft, brittle or super-finished and which can be damaged during gripping, especially by clamp-type grippers which impose a point loading (See Fig. 1-5).


Fig. 1-5: Types of point loading resulting from gripping a) b) c) d)

Area/area Line/area Point/area Double line/area

a)

b)

c)

d)

Point loading is the contact force per unit gripping area which results from clamp gripping. Deformation occurs at the point of contact. Contact force should not, however, be assumed to be the same as the closing force of the gripper. V-shaped grippers, for example, spread forces. Gravity may also be a factor, depending on the orientation of the gripper, as may the coefficient of friction µ. Excessive point loadings may produce clamping marks on workpieces and dents in hollow workpieces. In the case of grippers with plain-bearing guides, oscillations of the overall system may produces transient effects on the coefficients of static and sliding friction. This means that during a motion (subject to vibration), contact force may increase due to the fact that sliding friction becomes effective for a short time, reducing the friction in the guides which diminishes the gripper force. This phenomenon occurs hardly at all with roller-bearing jaw guides.

Problems with accuracy

When it comes to the fine positioning of objects, the average person relies on eye/hand coordination and manages without difficulty, for example, to thread a needle. Mechanical gripping must be just as precise and trouble-free. Problems can occur in 3 situations: • When picking up a workpiece • When aligning this in the gripper device • When setting the workpiece down in the desired position.


13

Gripping devices have only a limited working range. Workpieces outside or at the limit of this range will not be picked up reliably. The answer is either to use a wide-range gripper (which requires a longer gripping time) or to reduce the workpiece placing error. This can often be achieved by simple means. Fig. 1-6 shows an example. In the old arrangement, the workpiece is positioned very inaccurately at the pick-up point. An improvement is produced by using a template with a centring effect. Fig. 1-6: Unambiguous pick-up points ensure reliable gripping a) Inaccurate workpiece position b) V-shaped template centres workpiece

a)

b)

Workpieces are not aligned to the gripping centre, since the gripper closes in an arc and operates with workpieces of different diameters. The gripping centre, also known as the tool centre point (TCP), is however the value entered in the programming of the handling machine. Deviations of this kind may causes problems with close-tolerance assembly operations. This type of problem is shown in Fig. 1-7 [1]. This effect is not encountered with parallel-jaw grippers. Fig. 1-7: Gripper devices which close in an arc may cause a shift of the gripping centre

1 1 2

a) Scissors-type gripper with 2 different workpieces 1 and 2 b) Parallel-jaw gripper

2

δx a)

b)

Problems may also be encountered in obtaining set-down at precise points. This is subject to a total error comprising the handling machine positioning error, the gripper error and the workpiece error [2]. Shape errors in particular may be problematic with long workpieces and narrow gripper jaws, as shown in Fig. 1-8. The remedy is to use wider gripper devices with a compliant covering on their gripping surface.

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The accuracy situation must therefore be studied thoroughly. A gripper with V-shaped jaws will compensate for preceding placing errors and will align the workpiece to the TCP. A magnetic gripper or suction cup cannot do this. These types of grippers will retain the placing error and add errors of their own. The process technology also has an effect – a clamping collet opens only a short distance and is more difficult to feed than a wide-opening jaw-type chuck. Fig. 1-8: Centre deviation resulting from workpiece form errors

δx

δy

δx

Summary

Publications

A gripping operation can be influenced by a number of parameters which may (but need not necessarily) cause the gripper to “miss” the workpiece in the worst-case scenario. We have discussed a number of reasons for this. A thorough study of the gripper application will help to recognise problems in good time [3].

[1] Volmer, J. (Hrsg.): Industrieroboter – Funktion und Gestaltung (“Industrial Robots: Function and Design”), published by Verlag Technik, Berlin and Munich 1992 [2] Hesse, S.: Montagemaschinen (“Mounting machines”), published by Vogel-Buchverlag, Würzburg 1993 [3] Hesse, S.: Greifer-Praxis (“Praxis of grippers”), published by Vogel-Buchverlag, Würzburg 1991


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2 Gripper applications for component production and assembly

The choice of gripper type is always determined by the properties of the object to be gripped and the purpose of the handling operation concerned. Classic applications include production of components (feed) and assembly (handling of components for assembly). New applications have, however, also emerged, with different parameters from those in production. These applications include packaging (requiring high speeds) and commissioning (with an undefined initial object position). It is thus necessary to adapt grippers constantly for new and important applications. Standard grippers have reached a high level of development and can be used for many applications, not just the classic applications. There are also gripper systems for special applications. Rails and adapter plates can be used, for example, to create double grippers, multi-position grippers and multi-workpiece grippers. We will illustrate this with a few selected examples.

Workpiece handling with standard grippers

3-point grippers are the preferred type for handling cylindrical workpieces. These grippers give both good centring and a high degree of reliability [1]. Careful design of the gripper fingers can provide a certain measure of adaptability to different workpiece dimensions. Fig. 2-1 shows an example, using hardened gripper pins for the internal gripping of small workpieces. These pins can if desired be repositioned in other bores provided in the gripper fingers. The result is an enhanced gripper range, albeit with the need for manual resetting. The gripper pins need not be used concentrically – it may be better to arrange them at gripping points in accordance with the internal contours of the workpiece, for example in order to grip a housing with a rectangular aperture (Fig. 2-1c). It is also possible to grip into hole patterns extremely effectively using gripper pins.

Fig. 2-1: Using a 3-point gripper a) Adjustable gripper pins b) Concentric internal gripping of a flange ring c) Non-concentric internal gripping of a housing

a)

b)

c)

The minimum effective length of the pins should be 5 mm. This also applies to mechanical gripper fingers.

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For workpieces with a length of 200 mm and more, it is better to use multi-point grippers. The best way to produce grippers of this kind is by combining two standard grippers. This is shown in Fig. 2-2, taking the example of the gripping of a sheet-metal profile. For this purpose, the grippers are mounted on an adapter rail. The required gripping force per gripper is of course halved and the problematic torque forces which may result from high-speed manipulation can be absorbed more easily. Fig. 2-2: Multi-point gripper for long workpieces

The centre of gravity of the workpiece should be placed at the exact midpoint between the grippers. When single grippers are used, the centre of gravity should be as close as possible to the gripping point. Frequent use is made of modified gripper systems based on standard grippers. One example of this is in-line arrays of grippers used to pick up workpieces from pallets row by row or to set down rows of workpieces on pallets, in crates, etc. In this case, too, the grippers are mounted on a rail and then act as multiworkpiece grippers. This is illustrated in Fig. 2-3. Since the grippers act at a large number of points simultaneously, the workpiece positions must be maintained to close tolerances. If workpieces need to be picked up, for example from a conveyor belt, it may be necessary to align these on the belt beforehand.

2 Gripper applications for component production and assembly2

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Fig. 2-3: Multi-workpiece gripper for transfer of complete rows of workpieces

The following problem may be encountered when feeding workpieces to clamping points: at the moment of clamping, the flow of forces is a closed kinematic chain, due to the fact that the gripper must also hold the workpiece. This means that the clamping point forces the gripper and thus the handling machine to adopt its position. In the long term, this may cause damage to robots and grippers. Ways must therefore be found of compensating for this effect. There are robots which in these circumstances switch to a “soft” action and do not therefore attempt at all costs to maintain their position but rather yield. Grippers can also be provided with compliant mountings (“floating” grippers), and there are also grippers with an integrated or attached pressure device. In this case, the gripper opens in the clamping device, at which time the workpiece must be held against the clamping device by the pressure device, for example a pneumatic ram. The clamping device then closes and the gripper can withdraw.

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Gripping in assembly operations

There are many details which require consideration in the planning of an automated assembly line. The most important of these, however, are time and accuracy. Time can be gained, for example, by using turret grippers. It is perfectly possible to use standard grippers for this purpose. Fig. 2-4 shows a mounting plate equipped with the necessary number of grippers. Assembly tools can also be fitted in certain cases. The advantage for the user is that a number of idle robot motions within the assembly sequence can be avoided, thus shortening the sequence. The large diameter of the interference circle, on the other hand, is a disadvantage. It must be determined as a first step whether sufficient working space is available.

Fig. 2-4: Multi-workpiece gripper for assembly operations

New technology, such as video recognition systems, has led to new demands being placed on gripper systems. One example of this is the insertion of chocolates into a blister pack. This also falls under the broad heading of “assembly” work. Fig. 2-5 shows a solution in which several suction cups are used to pick-up the rectangular chocolates from a conveyor belt. The suction cups are fitted to single-acting standard cylinders which are protected against torsion. Each suction cup is able to move forward independently at high speed. Once the recognition system system has detected a workpiece and determined the coordinates and the orientation of the workpiece (longitudinal axis), the gripper adjusts its angle accordingly. The suction cup now advances, picks up a workpiece from the moving belt and returns to its initial position. Once all the suction cups have picked up workpieces, the robot swivels to the packing conveyor belt and sets down all the workpieces simultaneously in the nests of the blister packs. Since all workpieces are already correctly aligned relative to the grippers, their alignment with the packaging is also correct.


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Fig. 2-5: Multiple suction-cup grippers for assembly operations

1

1 NC rotary axis (robot hand axis) 2 Angle mounting plate 3 Standard cylinder with non-rotating piston rod 4 Suction cup 5 Conveyor belt 6 Workpiece (confectionery)

2

3

4 6

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5

Positive-locking or force-locking grippers?

Workpieces can be held within gripper fingers by (frictional) force. They can, however, also be held simply by physical systems such as the shape of the gripper or even by adhesion, e.g. adhesives. The theoretical possibilities are shown in Fig. 2-6.

Fig. 2-6: Methods of holding a workpiece (example: ball bearing) 1 Enclosure only without clamping 2 Partial enclosure combined with clamping force 3 Clamping force only (force-locking connection) 4 Holding with suction (force field) 5 Holding with magnetic field 6 Holding with adhesive layer, such as grease

1

2

3

4

5

6

Clamping force is very frequently used to hold workpieces. We must, however, consider the following: in order to hold the workpiece, the fingers must act on the workpiece with a force FG of at least FG = m · g/µ (disregarding safety margins and the effects of other forces for the moment). In the above, m is the coefficient of friction and m the mass of the workpiece. In an assembly operation, however, this force is not sufficient, since a joining force FS is also required. The required gripping force is thus FG = (m · g + FS)/m. If we assume a coefficient of friction of 0.1, the gripping force FG would be 10 times the total weight of the workpiece to be assembled and the joining force. This may lead to deformation of or damage to the workpiece, particularly if this is delicate. It is therefore desirable to use a positive-locking connection. Fig. 2-7b shows how, with this type of gripping, the workpiece is now able to rest on the gripper finger, allowing the clamping force to be kept relatively low.


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Fig. 2-7: Gripping principle: A positive-locking connecion places much less stress on workpieces during gripping and holding

Positive locking

Force locking

a) Gripping an egg with the human hand [2] b) Gripping a workpiece and holding it during assembly c) Attitude of gripper hand during feed motion

a)

m.g

m.g

b)

c)

A positive-locking connection may also be desirable for feed motions. When a workpiece is lifted rapidly, it is subject not only to a weight force m · g but also to an inertia force FT which is a function of vertical accelera-tion. If, however, the gripper hand is turned through 90° before the workpiece is lifted, the previous force-locking connection temporarily becomes a positive-locking connection for this motion sequence. These examples show that the gripper is a component where different influences come to bear and should therefore never be used without taking into considera-tion all the various factors involved.

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Grippers as separators

Grippers are well-proven components and can also be used as workpiece feeders to release a certain number of workpieces from a magazine. Generally, workpieces are released one at a time – we therefore refer to this process as “separation”. Fig. 2-8 shows an example of an application using a gate-type feeder.

Fig. 2-8: Gate feeder using a parallel-jaw gripper 1 Parallel-jaw gripper 2 Sliding gate

1

2

Sliding gates have been fitted in place of the gripper fingers. These should be as short as possible, as is usually the case in gripping applications, to avoid overload of the linear guides of the gripper jaws and reducing their service life. This solution is worth considering only for the feed of small workpieces, since other solutions are available for large heavy workpieces [3]. In order to reduce the load acting on the feed slide, a stepped track has also been developed which enables a proportion of the load due to workpiece build-up to be supported by the step.

Publications

[1] Seegräber, L.: Greifsysteme für Montage, Handhabung und Industrieroboter (“Gripper Systems for Assembly, Handling and Industrial Robots”), published by expert Verlag, Renningen, 1993 [2] Bohmann, J.; Nönnig, R.: Ein Greifer für empfindliche Teile (“Grippers for Sensitive Workpieces”), article in magazine “Konstruktion” No. 45 (1993) pp. 95-97 [3] Hesse, S.: Atlas der modernen Handhabungstechnik (“Handbook of Modern Handling Technology”), published by Vieweg Verlag, Wiesbaden, 1995


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3 Grippers and hand axes

Grippers are holding devices; this is their main function. In order to obtain a given effect, we must provide grippers with the ability to move in three dimensions. This is achieved by using motion axes. A robot application for example with 6 axes is not particularly complicated, since the robot provides the necessary motion capability. In applications where costs are critical but ultra-high speed is not required, it is worth considering hand axes, which are often available as flexible modules and can take the place of a robot. This produces solutions which can be installed quickly at reasonable cost. Depending on the application in question, a hand axis may be of interest for the following motions. Rotation > 360°, swivelling < 360°, thrust motions (generally with short strokes) and screwdriving motions, particularly for the insertion of screws. The most typical motion is, however, swivelling, which is why gripper manufacturers almost always offer compatible swivelling units. Fig. 3-1 shows a two-axis module which can swivel between 0 and 270° and provide a linear thrust stroke of up to 100 mm. The positions are finely adjustable, with a cushioned approach. But what exactly can we do with this motion capability?

Fig. 3-1: Three-point gripper combined with a swivel/linear unit

Degrees of freedom of the hand

Let us first consider the term “degrees of freedom”. A workpiece can have a maximum of 6 degrees of freedom, expressed as 3 linear motions on the 3-dimensional axes x, y and z and 3 rotary motions α1, α2 and α3 about the axes x, y and z. Handling machines, by the way, can have more than 6 degrees of freedom. We then speak of degrees of mechanical freedom or travel freedom. Thrust motions (Fig. 3-2) are described as follows: 1 Vertical up-down 2 Forward/backward 3 Lateral left/right

24


while rotary motions, following aviation practice [1], are designated as follows: α1 Pitch, tilt α2 Roll, twist α3 Yaw, turn. As you will know, the closing motion of the gripper jaws is not considered as a degree of freedom, since this motion has no influence on the motion path of the gripper. Fig. 3-2: The human hand can execute motions with 6 degrees of freedom (according to Bejczy)

1 a) Biological b) Technical

2 α1 α2 3

G α3

a)

Double grippers save process time

b)

Particularly in applications involving the feeding of machine tools, it is desirable for the machine to resume work as quickly as possible after the workpiece has been changed. After this, the handling machine will generally have ample time to set down workpieces and pick up new workpieces from a magazine. The double gripper was developed for this type of application. Fig. 3-3 shows a common design of double gripper.

Fig. 3-3: Double gripper designed as a crown turret a) Radial gripper b) Axial gripper

a)


b)

25

The crown turret is driven by a rotary cylinder with adjustable endposition cushioning at both ends and fine adjustment of its end positions. An important feature is the backlash compensation in the rack-and-pinion unit, since otherwise considerable positioning errors could result at the gripping point. Double grippers of this kind are often used with gantry robots. Whether gripping is carried out radially or axially depends on the workpiece axis position in the magazine. Long and thin workpieces are generally fed horizontally, with radial gripping, while axial gripping is the more common method for short and thick workpieces and also castings with or without flanges. The crown turret is designed to accept standard grippers. Gripper systems are also sometimes modified for a given application. The shaft gripper in Fig. 3-4 is an example of this. In this case, standard grippers are mounted on a swivel plate, and the motion is executed by a suspensionmounted pneumatic cylinder. A fast but cushioned approach is provided to the cylinder end positions. Set-screws are used to adjust the angle. This solution is used, for example, in conjunction with line gantry robots to feed material to machine tools and test machines. Fig. 3-4: Shaft gripper

1 1 2 3 4 5

Connecting flange Cushioned stop Set screw Parallel-jaw gripper Pneumatic cylinder

2 3 5

4

26


Hand axes used to assemble small workpieces

In series production applications, pneumatic components have proved ideal as providers of motion. For example, a swivel/linear drive can be used to create a complete handling module, as shown in Fig. 3-5. In this case, the gripping point is not placed coaxially with the piston rod of the swivel/linear unit but deviates from this. This creates an arc-shaped working area within which the positions to be approached must lie. This is an extremely simple solution for normal insertion tasks. The workpieces are fed stepwise on a cross-table to a fixed pick-up point. If ultra-high speed was required, the answer would be to provide a second gripper opposite the first one. This would allow pick-up from the magazine and insertion of workpieces to be carried out in parallel.

Fig. 3-5: Handling module for assembly of small workpieces

1 1 2 3 4 5 6 7 8 9

Swivel cylinder Lifting cylinder Adapter plate Standard gripper Transfer system Workpiece for insertion Receiver workpiece Gripper finger Magazine

2

3 4 5 6

7

8 9

Simple handling modules can also be quickly and easily assembled using standard suction cups and semi-rotary actuators (Fig. 3-6). An additional shortstroke axis would turn this combination into a pick-and-place device. A hollow flange shaft can be used as a throughfeed for the vacuum line. The actuator can operate at switching frequencies of up to 3 Hz.


27

Fig. 3-6: Handling unit with suction cup and semi-rotary actuator

It is often necessary to rotate or turn over workpieces between workstations, for example, inverting parts such as receiver workpieces in an assembly process. A simple in-line solution for inverting a workpiece is shown in Fig. 3-7. A standard gripper executes a swivel motion of 180°. The gripper fingers in this case are arranged like a mouth. The workpiece runs against the stops in the open “mouth”; the gripper then closes and transfers the workpiece overhead to the next conveyor belt. Fig. 3-7: Inverting workpieces 1 2 3 4 5

Gripper jaw Standard gripper Semi-rotary actuator Workpiece Conveyor belt

28


Feeding double stations

In the interests of fast processing time, machines with rotary indexing tables are often equipped with double clamping devices. Machines of this kind are also referred to as duplex machines, producing two finished workpieces during each working cycle. It would be an attractive idea to construct a quadruple gripper, able to remove two finished workpieces and at the same time bring two fresh blanks. A gripper of this kind would, however, be bulky and difficult to use, particularly with large irregularly-shaped workpieces. This problem can be solved by using a triple turret gripper. This is shown in Fig. 3-8. The free gripper G1 first picks up a finished workpiece from the clamping point S1. A new blank is then inserted into the vacant clamping point, while the vacated gripper G2 can pickup the second finished workpiece from the clamping point S2. The second blank is then positioned by the gripper G3 [2]. If a single gripper were used, it would be necessary for the robot to execute a number of idle strokes, making the operating cycle longer.

Fig. 3-8: Triple gripper installed on a special machine with double stations G Grippers S Clamping points


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Feeding sheet metal

Fig. 3-9 shows a particularly simple solution for feeding ferromagnetic sheet metal. A suction cup is mounted directly on the hollow piston rod of a standard cylinder. The suction cups reach through the gaps between the top rollers of a roller conveyor and contact and pick up a sheet metal workpiece. The workpieces are secured by the permanently-magnetised upper rollers and are then moved on to a normal roller conveyor. The stack of workpieces is progressively raised by a lifting device. If the roller conveyor is inclined by a few degrees, the sheet metal workpieces will move along the conveyor by gravity alone. Further suggestions for the handling of sheet metal can be found in [3,4].

Fig. 3-9: Picking up ferromagnetic sheets from a stack using a suction-cup/lifting module 1 Suction cup 2 Standard cylinder with hollow piston rod 3 Frame 4 Magnetic rollers 5 Roller conveyor 6 Sheet metal stack 7 Lifting table

2

1

3 4

6 7

30


5

Specimen shaker

The excellent quality of pneumatic components has made them popular far beyond the field of mechanical engineering alone. Pneumatic modules are being used for manipulative movements in all kinds of new areas. The specimen shaker shown in Fig. 3-10 is an example from the field of laboratory automation.

Fig. 3-10: Simple specimen shaker made from standard components

This shaker has been created using a standard cylinder, a swivel unit with a hollow flange shaft and a standard gripper and adapter. It would also be possible to create an array of shakers or provide other motion combinations. The critical factor is to create a quick, inexpensive assembly, without the need for a great deal of preparatory work.

Publications

[1] Siegert, H.-J.; Bocionek, S.: Robotik: Programmierung intelligenter Roboter (“Robotics: Programming Intelligent Robots”), published by Springer Verlag, Berlin, Heidelberg et alia. 1996 [2] Breuer, H.J.: Bestehende Fertigungsanlage für Schwenklager mit zehn Industrierobotern automatisiert (“Automating An Existing Production Line For Swivel Bearings Using Ten Industrial Robots”), in the magazine “Werkstatt und Betrieb” 123(1990)12, pp. 929-932 [3] Hesse, S.: Blechteile automatisch handhaben (“Automatic Handling Of Sheet-Metal Workpieces”), in the magazine Bänder, Bleche, Rohre 37(1996)4, pp. 21-23 [4] Hesse, S.: Umformmaschinen (“Shaping Machines”), published by Vogel Buchverlag, Würzburg 1995


31

4 Construction of gripping effectors

It requires many technical components and procedures, such as an industrial robot, a controller, a program, a workpiece magazine, sensors and grippers to enable a handling device actually to pick up an object automatically. At the end of the chain, it is the gripper jaws or similar which provide the contact with the workpiece. This contact is often only a minute point, and it is here that major differences are encountered – a lump of iron is easy to pick up, a pickled herring is not. We can see from this that gripper jaws must be matched to the workpiece. So, how many different designs of gripper jaws are there? “Gripper jaws” are separate components, generally shaped and interchangeable, which provide a positive- or force-locking contact with a workpiece and hold this in place. “Gripper fingers” are elastic or articulated force transmission components which are positioned around a workpiece. Non-articulated components are also in practice referred to as gripper fingers. The gripper jaws are fitted to these, using either a fixed or movable connection. Gripper jaws are generally produced by the user or machine manufacturer. Certain companies help in this process by providing “neutral” universal jaws which require machining to the negative contour of the workpiece but have ready-made connection surfaces. There are also moulded jaws, in which the workpiece contour is produced by being pressed into synthetic resin, vulcanised rubber or molten metal.

Main purpose of grippers is to provide a holding system

32

A “securing” function comprises the temporary holding of an object in a defined position and orientation, followed by release as an inversion of holding or a cancellation of the securing function. In relation to mechanical grippers, we speak of “clamping” and “unclamping”. The holding function is normally provided by gripper jaws, which are thus described as a holding system, as shown in Fig. 4-1. The function of a holding system is an essential feature of all grippers. Holding can be achieved through mechanical clamping and also through fluidic or magnetic force fields or the enmeshment of surfaces [1].


Fig. 4-1: Some of the sub-systems of a mechanical gripper 1 2 3 4

Adapter Force generator Force conversion Force transmission, with finger as transmission components 5 Gripper jaw as gripping component

1

3

2

4

5

Holding system

Interface

Actuation

Kinematics

It is essential when designing gripper jaws to know the points at which the workpiece is to be gripped. Technical parameters naturally also have an certain influence. Fig. 4-2 illustrates this with the example of a two-finger gripper. Area contacts are generally preferable to line or point contacts. Fig. 4-2: The contour at the gripping point of the workpiece determines the jaw shape used, 1, 2 or 3.

1

2

3

Conditions are not always ideal. If we consider the case of “parallel flat surfaces”, we see that some workpieces are not in fact parallel at all; for example, plastic mouldings may have slight moulding bevels. If the deviations from parallel are small, it may be sufficient to fit the gripper jaws with a compliant rubber covering. It is, however, sometimes better to provide pendulum jaws, of which three types are shown in Fig. 4-3.


33

Ball-jointed pressure plates can compensate for angular errors on two planes. Rubber coverings or specially-produced gripper cushions are often sufficiently compliant for this purpose and have the further advantage that they increase the coefficient of friction (µ approx. 0.5), meaning that a lower gripping force can be used. Fig. 4-3: Gripper jaws with compliant surfaces 1 Rubber or plastic covering 2 Pendulum jaw 3 Ball-jointed pressure plate

1

34


2

3

Dealing with differences in workpiece dimensions

With some grippers, the tool centre point changes when workpieces with different dimensions are gripped. Designs include vice-type grippers (one fixed and one mobile finger) and scissor-tong grippers. In the former case, the position changes must be allowed for in programming, while in the case of scissor-tong grippers, compensation can be provided by specially-shaped grippers. The gripping surfaces are arched, as shown in Fig. 4-4, and do not have the usual simple V shape.

Fig. 4-4: Jaw shape with centring effect for scissor-tong grippers TCP = Tool centre point D = Workpiece diameter

α

TCP

D

R2

α R1

R

B A

A C1

Dimensioning should be carried out in accordance with the following: • The ratio between diameter D1 (largest workpiece) and D2 (smallest workpiece) should not be more than 2.5. • The contact point angle α should be roughly 20 to 25°. The following equations are used: D=

D1 + D2 2

A=

0.5 · R tan(α · 3.14/180)

B = 0.5 · R

R1 =

0.5 · R – 0.5 · D sin(α · 3.14/180)

R2 =

0.5 · R + 0.5 · D sin(α · 3.14/180)


35

This calculation also applies to angle grippers (gripper fingers with separate pivot points C1 and C2). Radius R1 then becomes larger, while R2 becomes smaller. The angle β between the lines TCP-C1 and TCP-C2 should lie in the range 0 < β < (2α – 40°). The case is different when several workpieces need to be gripped simultaneously. Here, too, it is essential to achieve a certain degree of equilibrium. The best solution, of course, is to divide up the degree of mobility using the multiple clamp principle, i.e. with individual pressure points as shown in Fig. 4-5. Fig. 4-5: Gripping several workpieces simultaneously, using a pressure distributor to compensate for tolerances

F

F

To enable a larger range of dimensions to be handled, it is also possible to use a stepped V-shaped jaw. Thus, using a jaw stroke of 50 mm in each case, it is possible to grip workpieces with diameters ranging from 1 to 110 mm. The grippers jaws used for this are shown in Fig. 4-6 [2]. We must, however, accept displacements dx of the tool centre point. A typical application of this kind of gripper would be gearbox assembly in an assembly cell, in which shafts, bearings and gearwheels, all with different diameters, must be gripped in succession.

36


Fig. 4-6: Jaws of a parallel gripper for 3 diameter ranges

y

δx

φ1 φ3

0.

φ7

.. 7 0

0.

.. 1 0

...

30

mm

0m

m

50 mm

Jaws for sequence grippers

x

mm

50 mm

Sequence grippers are used to grip a defined number of different objects in an unvarying sequence. The gripper jaws must accordingly have a number of gripping points which match the gripping points of the workpieces in each case. The easiest way to explain this is by using an example, such as the one in Fig. 4-7, with 4 workpieces. The orientations and gripping points required for the process concerned are defined. The gripper jaws must be provided with a negative contour for each gripping point, which makes the jaws extremely complex in shape. This is of course, not always possible, but gripper jaws have been designed which allow 9 different workpieces for a gearbox assembly line to be gripped without changing the gripper jaws. This case, by the way, would be encountered only in assembly cells, in which one robot is required to assemble as many different components as possible. On assembly lines for mass production, robots and thus grippers are generally set up for one specific task.


37

Fig. 4-7: Gripper jaws with specisallyshaped multiple gripping surfaces

D

A B

B C

C D

A

Jaws for special applications

Grippers can be designed for many special applications [3]. There are, for example, grippers with rotary jaws, which can turn a workpiece through 90°. One jaw is passive, while the other is equipped with a rotary actuator. Attempts are, however, always made to use a normal basic gripper before developing a special gripper. Fig. 4-8 shows some examples of special grippers.

Fig. 4-8: Variants of jaws for parallel grippers 1 Moulding jaw with lamellar assembly 2 Width-adjustable jaw 3 Jaw of combination gripper

1

2

3

The moulding jaw incorporates thin movable metal plates. When the gripper closes, the plates are pressed against the workpiece, forming an impression of it’s contour. The entire lamellar assembly is then clamped into place. The impression is reversible, i.e the lamellae can be reset. Gripper jaws can also be provided with serrated adjusters, allowing the gripping width, but not the stroke, to be matched to the workpiece dimensions. The jaws can also be used turned through 180°. In order to grip several workpieces simultaniously 38


(combination grippers), jaws must be provided with an appropriate number of identical indentations. Here, too, we need to consider the tolerance problem [4]. It is even possible to equip gripper fingers with mobile jaws. Fig. 4-9 shows an example. As the gripper fingers close, they contact the workpiece and lift this out of the V-shaped recess. There is no need to provide the handling device with a lifting motion. This does, however, require jaws and workpieces with a suitably smooth surface. The contact point must be below the centre of the workpiece. The purpose of this design is thus to save the need for a motion axis. Fig. 4-9: Mobile gripper jaws lift the workpiece out of the V-shaped recess in the magazine 1 2 3 4 5 6 7

Gripper housing Finger Mobile jaw Rotary axis Support surface Torsion spring Workpiece


39

Taking the jaw stroke into account

Fig. 4-10: The type of approach affects the required opening

In selecting the shape of the gripper jaws, allowance must be made for the type of approach of the gripper to the workpiece, since this may influence the required stroke. The approach may be axial or radial – technical conditions will generally dictate which. Fig. 4-10 shows 2 cases, taking the example of V-shaped jaws for which different gripping strokes “c” are required for the same workpiece. An opening safety margin “a” and clamping safety margin “b” are always required; these compensate for tolerances and provide the necessary latitude.

Radial approach

Axial approach

a Opening safety margin b Clamping safety margin c Required jaw stroke

b

b a

c

Publications

40

a

c

[1] Cardaun, U.: Systematische Auswahl von Greifkonzepten (“Systematic Selection of Gripper Concepts”). Doctoral thesis, University of Hanover 1981 [2] Volmer, J. (Hrsg.): Industrieroboter – Funktion und Gestaltung (“Industrial Robots: Function and Design”), published by Verlag Technik, Berlin and Munich 1992 [3] Hesse, S. (Hrsg.): Industrieroboterperipherie (“Industrial Robotics Peripherals”), published by Hüthig Verlag, Heidelberg 1990 [4] Hesse, S.: Lexikon Handhabungseinrichtungen und Industrierobotik (“Lexicon of Handling Devices and Industrial Robotics”), published by expert Verlag, Renningen 1995


5 Forces acting on grippers

The main purpose of a gripper is to hold objects securely for a certain period. Grippers using the force-locking principle, on which we will concentrate in this article, are required to generate holding forces to balance out all the steadystate and dynamic forces and torque values which occur during a motion sequence. The required gripping force is thus a major criterion for the selection of the right type and size of gripper. The required gripping force can be calculated approximately. This calculation should not be neglected, but may not provide a final answer. In doubtful cases, you should also carry out tests or recommend users to do so, since some of the calculation variables are subjected to fluctuations or are only estimates. If you go too far over to the “safe side”, this may be disadvantageous for the user – a heavy gripper may necessitate a handling device which is one size larger in terms of handling capacity or may reduce the working load of such a device.

This law, formulated by Isaac Newton in 1687, states the following: Newton's Third Law of Motion

The actions of two bodies upon each other are always equal and directly opposite, i.e. reaction is always equal and opposite to action. This means that force and counter-force are in equilibrium. A simple experiment to demonstrate this is shown in Fig. 5-1a. A rod is subjected to a tensile load. In the first case, one end of the rod is clamped, while in the second case a person pulls on each end. The tensile force in the rod in each case is not 400 N as you might think but only 200 N. If we apply this to the parallel gripper shown in Fig. 5-1b, this is subject to the same prin-ciple. It makes no difference whether only one finger moves and applies a gripping force of 200 N or whether two opposed fingers each generate 200 N. The two grippers shown are equivalent in terms of force.


41

Fig. 5-1: The law of interacting forces a) The tensile force in the rod is 200 N in both cases b) Due to the principle of action = reaction, it makes no difference with parallel grippers whether the gripping force FG is applied by one finger or two

200 N

200 N

200 N

a)

b)

FG

FG

FG

200 N

200 N

200 N

Newton’s Third Law of Motion also applies to three-point grippers in somewhat modified form, as we shall see. In this case, forces act in three directions.

Friction forces create a holding effect

If we confine our study of holding principles to the balance of forces with mechanical grippers, we see that the gripping force is only a means to an end. From the point of view of friction, the gripping force acts like a normal force. The actual holding function is produced by friction forces FR which are created in accordance with Coulomb's Law of Friction in the direction opposite to the direction of motion and oppose the weight force G of the gripped object. The simplest case is shown in Fig. 5-2.

Fig. 5-2: Forces acting on gripped object (state of rest)

FR

FR

1 1 Finger 2 Gripper jaw 3 Workpiece

FG

FG

µ Coefficient of friction

2 µ

µ 3

G

This situation applies when the handling device is in a state of rest (or moving very slowly). The following equation is obtained: G = FG · µ · n

42


or expressed another way: FG =

m·G µ·n

Variables in formula: FG = Minimum required gripping force in N G = Weight force of gripped object in N g = Gravitational acceleration in m/s2 m = Workpiece mass in kg n = Number of fingers or gripper jaws µ = Coefficient of friction between gripper jaw and object. As we can see, allowance is made in the formula for the number of fingers, since of course a friction force FR is created at each gripper jaw. With 3 contact points, n = 3. There are several possible variants for the 3-point solution. Fig. 5-3 shows a “true” three-finger gripper and a solution based on a two-finger gripper. In this latter case, the gripping force is split at the V-jaw into the contact forces FKi. Fig. 5-3: Plan view of 2 gripper situations

FG

a) Two-finger gripper with V-jaw b) Three-finger gripper

FK2

FG

β

α

FG = FK1

FG

α

FG

FK3 G = Σ FKi . µ

G = FG . µ . 3

If, in the case of the two-finger gripper, a V-jaw angle of 120° is selected, the same as the finger positioning of a three-finger gripper, the two grip-pers will be the same from the point of view of holding forces. There will be differences with other V-jaw angles. In these cases, we must refer to the contact angles, which can be determined by the following formula if random V-jaw angles are permissible: FKi =

G · sinα1 µ · (sinα1 + sinα2 + sinα3)

in which i = 1, 2, 3 and α1 = 180° – α23 α2 = 180° – α13 α3 = 180° – α12 5 Forces acting on grippers

43

The total of the 3 friction forces FR1 to FR3 (Fig. 5-4) must be at least large enough to compensate for the weight force G produced by gravity. Knowledge of contact forces is also required if we wish to check the gripping pressure per unit area in the case of sensitive workpieces.

Fig. 5-4: Calculation of contact forces for a gripper with a V-jaw on one side

α13

FR1

FG = FK1

FR3 FR2 FK3 α12

α23 FK2

G

Fig. 5-5 shows the mathematical relationships governing gripping force during an upward motion in the case of the commonly-used V-jaw grippers with 3- or 4-point object contact. A distinction can be made between 3 variants of gripping: • Pure positive-locking gripping • Positive locking in combination with friction locking • Pure friction-locking gripping.

44


Fig. 5-5: Forces at the parallel-jaw gripper with V-jaw for workpieces a g m S µ

Sketch

Contact forces

Gripper force upward

Pure positivelocking gripping

Linear acceleration Gravitational acceleration Mass Safety factor Coefficient of friction

FG

a

FK1 =

m(g + a)sinα2 sin(α1 + α2)

m .g

FG = m(g + a) · S FK2 =

FK1 α1

m(g + a)sinα1 sin(α1 + α2)

FK2

α2 FG

Positive locking with friction locking FR1

FR2

a

m(g + a) FK1 =

FG

2cosα1

FG

FK1

FK2 α1

m .g

m(g + a) FG =

FK2 =

α2

2

tanα · S

m(g + a) 2cosα2

FR a

FR

FK1 = m(g + a)tanα2 FG= FK1

FG = FK1 · S

FG FK2

FK2 =

m(g + a) 2cosα2

α

90° m .g

Pure friction locking a FG

m(g + a)

FK

FR

FK =

4µ

m(g + a) FG =

2µ

sinα · S

2α m .g


FG

45

Effective gripping depends strongly on the coefficient of friction. The following can be taken as guide values: • • • •

Workpieces with smooth surfaces, lightly oiled µ = 0.1 Metal-to-metal contact µ = 0.5 to 0.2 Gripper jaws with pointed-tooth surface µ = 0.3 to 0.4 Jaws with non-slip covering or metal/rubber contact µ = 0.5 to 0.7.

The equations given in Fig. 5-5 incorporate allowances for certain factors which we have not yet mentioned: • The coefficient of friction m fluctuates quite widely, like a person’s blood pressure. A safety factor S must therefore be included. In practice, the factor used is between 1.5 and 4. • The weight force G represents only part of the load. Allowance must also be made for other forces, particularly the inertia forces resulting from the acceleration “a” of the robot arm. Process forces may also be involved, for example during assembly, by inserting components.

Check the motion cycle

It may be, in the case of a multi-axis handling machine, that the force situation changes from time to time during a motion cycle. It may be possible to counteract certain forces which occur by using positive-locking gripping (lateral motions), while other forces may call for a higher coefficient of friction. It is therefore a question of identifying the motion phase requiring the highest holding force and selecting a gripper on this basis. Fig. 5-6 shows a number of typical motions and the forces operative during these.

Fig. 5-6: Force situations during gripper motion

v

FR a) b) c) d) e)

Rest state Upward motion Downward motion Lateral motion Inclined upward motion

FR

v

FR

FR

FR

v=0

FB FG a)

FG

FB

FG

G

FG

b)

FG

d)


FG

G

FG

c)

v

FR

46

FR

G

v

FR

FB

FR

FG G

FG

e)

FR

FB

FG G

In the case of an upward motion, the weight force G and the inertia force FB can be compensated by the friction forces FR. During the downward motion, the inertia force acts in opposition to the weight force and makes the object effectively “lighter”. During the braking phase, however, a deceleration force then becomes operative. In the case of lateral motions, inertia force is absorbed by positivelocking gripping, requiring no additional friction forces. The inertia force, attempts, however, to force open the gripper jaw. With inclined motions, consideration must be given to the horizontally- and vertically-acting components of the inertia force. The position is of course somewhat different with V-jaws and other types of handling devices. It is necessary to study the complete handling operation. The inertia forces can be calculated as a general principle as follows: FB = m · a or in the case of a rotary motion: FB = m · r · ε or in the case of a rotary motion: a m r ε

= = = =

Linear acceleration in m/s2 Mass of gripped object in kg Distance from pivot point in m Angular acceleration in rad/s–1.

It would be possible to use the acceleration values specified by brochures as being achieved by robots under normal operating conditions. This would not, however, be quite correct. The critical values are not those of normal conditions but of exceptional situations, particularly those following an EMERGENCY STOP of the robot. We can, for example, take the following standard values for acceleration: Acceleration in normal operation, linear: EMERGENCY STOP acceleration, linear: Acceleration in normal operation, rotary: EMERGENCY STOP acceleration, rotary:

a = 5 m/s2, aN = 10 m/s2, ε = 10 rad/s–2 and εN = 17 rad/s–2.

If more precise data is available from technical documentation, this should be used.


47

Example: An industrial robot is to pick up an object at P1, transport this via P2 to P3 and set it down at P4 (Fig. 5-7). What gripping forces must the two-jaw gripper provide? (Let us assume: µ = 0.2; Safety factor S = 2). Fig. 5-7: Example of handling task

P2

P3

P1 P4

Mass = 1 kg

Gripping force during lifting: m(g + a) · S FG =

µ·n

=

1 · (9.81 + 5) · 2 0.2 · 2

= 74N

Gripping force during lateral stroke: m·g·S FG =

µ·n

+m·a=

1 · 9.81 · 2 0.2 · 2

+ 1 · 5 = 54N

Gripping force during EMERGENCY STOP downwards (after acceleration phase): FG =

m · (g + aN) · S µ·n

=

1 · (9.81 + 10) · 2 0.2 · 2

= 99N

Grippers should in general be selected on the basis of EMERGENCY STOP situations, since the requirement in these situations is that the workpiece should continue to be held by the gripper and not ejected. The highest acceleration values (in fact deceleration values) occur during an EMERGENCY STOP.

48


Problems with torque

In the cases discussed up to now, it has been assumed that the centre of gravity of the workpiece is positioned precisely between the gripper jaws. It can of course also lie elsewhere, but this should be avoided, since this may result in workpieces twisting under heavy acceleration.

Fig. 5-8: Examples of torque created as a result of gripped workpieces

1

a

2

b

3

l

l c

c FK1

FK2 G

FK2 FK1

FK2

FK1 G

G

The following applies in case 1: G·b FK1 =

a+b

and

FK2 =

G·a a+b

For cases 2 and 3 the following applies: G(l + c) FK1 =

l

and

FK2 =

–G · c l

An associated problem is the ability of grippers to handle off-centre loads. Gripper manufacturers generally state in graphs of load capacity, the forces due to finger length resulting in a torque which tends to open the gripper jaws, or to rotate the workpiece in cases where the gripper does not have V-jaws. The wider the gripper jaws, the better the distribution of forces and the lower the gripping forces required when the workpiece centre of gravity lies outside the gripper jaws.


49

Why is it necessary to observe these limits? Fig. 5-9 shows the forces which occur. In accordance with the law of levers (force times force arm equals load times load arm), the gripping force FG produces tilting forces in the finger guide. These in turn produce increased friction. Part of the generated gripping force is thus counteracted by friction forces in the guide. Moreover, wherever friction occurs, wear will soon follow. The higher the tilting moments in the guide, the higher the load. If the permissible limits are exceeded, the gripper will not achieve either its specified service life or the desired gripping force. Fig. 5-9: Eccentric forces acting on a gripper finger

v

FN Normal forces acting on guide x Finger length up to point of action y Distance from point of actIon to centre of gripper

FNy

FNx

x

FR

FG

FNy y FNx

FG

50


Characteristic curve for gripping force

Users need to know whether or not their grippers develop a constant gripping force throughout the stroke. This information is provided by a characteristic curve of gripping force. Fig. 5-10 shows 2 transmission mechanisms as exaples. As the angle lever (angle gripper) swivels, the gripping force varies throughout the gripper stroke as a function of the cosine of the angle of rotation a. In the second example, on the other hand, FG = constant. If the angle of rotation α of an angle gripper is small, this effect can be ignored. Some lever mechanisms, however, have a characteristic gripping force curve with very pronounced variations. A typical case is a lever operated gripper, whose curve has a very steep gradient, with high gripping force available only within a short range of travel. These two examples demonstrate that not every gripper offers a constant force throughout its travel.

Fig. 5-10: Gripping force FG as a function of gripper stroke h a) Angle gripper b) Parallel gripper Q Tensile force h Gripper stroke


51

6 Technical properties

The technical properties of grippers and their price form the basis for an assessment of their suitability for a given application and a comparison with other makers’ products. The more suitably a gripper range is matched to the users’ average requirements, the easier it will be for users to select the “right” gripper. There will of course always be special applications requiring special grippers. But what in fact are the characteristics of a gripper? Any use of grippers must start with a study of the planned application. This will reveal what grippers are required to do, and what loads they must withstand. If studies of this kind are carried out inadequately, over-hastily and incompletely, the result is likely to be annoyance – if grippers are the wrong choice, they may fail quickly and thus not provide the expected performance. Never try to talk someone into using a particular gripper – the choice must be taken on the basis of the degree to which the requirement profile and the gripper performance coincide.

A few words about characteristic data

The properties of a gripper can be illustrated by some characteristic data. The table of Fig. 6-1 gives a list of this data. The data can be sub-divided into primary and secondary characteristic data. In examining the suitability of a gripper, we will proceed step by step, first checking the primary characteristics data and then using the secondary data to make a final selection. Of course, not everything is always important in all cases. It is advisable to weight the characteristic data as appropriate to a given application. For example, the opening and closing times of a gripper are not important if the process concerned is not time-critical, which is the case if the handling time of the gripper is not in series with the main process time but runs parallel to this. As regards the gripping force, only a low value will be required in order to fit components with up to 3 connecting wires to printed circuit boards. Reliable handling can be obtained even with grippers which close only by spring force. If, however, the application also involves gripping heavier components (such as elays), the gripping force will be important and must be checked. In specific cases, it can also be advisable to consider further characteristic values, for example the jaw changing time in applications where the gripper jaws need to be changed frequently, perhaps even several times a day, due to wear or the need to handle different products. A secondary characteristic value may become a primary value, for example if a gripper is required to be suitable for use in clean rooms. Even a consideration of primary data will eliminate a number of possible candidates.

52


Fig. 6-1: Characteristic data for grippers

Characteristic data for grippers • Type designation • Design • Size Primary characteristic data

Secondary characteristic data

• Operating principle - mechanical - fluidic - magnetic - adhesive • Gripping force in N • Gripping force pattern (Gripping force diagram) • Gripper stroke per jaw in mm or opening angle in degrees • Gripping width adjustment • Load capacity max. in N • Closing (gripping time) in s • Opening (release time) in s • Load limit values - Forces - Torques - Finger length • Number of gripper components • Main dimensions in mm • Dead weight in kg

• Performance/mass ratio in N/grams • Mass moment of inertia in kgcm2 • Operating pressure range in bar • Maintenance cycles • Design of bearings and guides • Range of sizes • Repetition accuracy in mm • Operating temperature range in degrees • Mode of operation - Single acting - Double acting • Working frequency max. in Hz • Mounting position • Energy type and consumption • Retention of gripping force in case of power supply failure • Monitoring of gripping stroke • Material specifications • Service life • Interface data - mechanical - fluidic - electrical • Environmental characteristics - Clean-room class - Exhaust air - Abraided particles


53

Accuracy of gripping

Only idealised handling operations run perfectly smoothly. If we take a closer look, we will see that it is always necessary to accommodate tolerances on all axes. Theoretically this is true, the only question is the order of magnitude of the resulting errors. Fig. 6-2 shows an exaggerated form of the general situation. The workpiece feed is subject to errors, the robot motions are imprecise and the target position, for example a basic workpiece to which others are to be fitted, is also subject to tolerances. Only an analysis of the tolerances will show whether the situation is critical or not. Roughly one-third of all assembly applications are a question of inserting pins into holes. Some of these operations can be regarded as precision assembly, with clearances of only a few hundredths of a millimetre. In cases of this kind, it may occur that the repetition accuracy of the gripper and industrial robot used exceeds the permissible limit. The repetition accuracy of a gripper is defined as the variation in the jaw end position during 100 successive gripping operations (closing motions). This figure may, for example, be ± 0.02 mm in the case of a parallel gripper.

Fig. 6-2: General model of a handling operation

1

2

1 Workpiece store on suspended conveyor 2 Industrial robot on a mobile unit 3 Assembly carriers on a transfer line

3

In order to carry out close-tolerance assembly, all the following measures must be adopted: • Improvement of repetition accuracy, particularly that of the handling device • Design of components to be assembled in such a way as to facilitate assembly, particularly by providing guide chamfers • Combination of grippers using joining mechanisms. Joining mechanisms are devices placed ahead of the gripper which are designed to compensate for angular and positioning errors between the connecting part held by the gripper and the connecting axis defined by the basic assembly part. A distinction is made between active (IRCC = instrumented remote centre compliance) and passive joining mechanisms (RCC = remote centre compliance). In the interests of simplicity, most are are of the RCC type. These can easily

54


compensate for position deviations of 2 mm with an orientation error of 2°, and a permissible clearance between the two joining parts of as little as 0.01 mm. In order for these devices to operate, guide chamfers are required on the basic assembly part (bore) and/or the joining part to be added (pin), a factor which should not be forgotten. Fig. 6-3 shows the principle of an RCC unit. Compliance can be provided by using a special configuration of elastomer components or leaf springs. The inner pair of joints correct angular errors, while the outer pair of joints compensate for positioning errors. The apparent (remote) pivot point of the joining part to be inserted lies at the tip of this. It is not necessary for users to make their own joining mechanisms; these are commercially available in a range of sizes. Fig. 6-3: Joining mechanism with combined lateral and angular compensation 1 Lateral (position) compensation 2 Angular compensation 3 Apparent pivot point for angular compensation 4 Gripper support plate 5 Parallel gripper 6 Gripper jaws 7 Joining part 8 Basic assembly part

1

2

4 5

6 3

7 8

The problem of fitting a pin into a hole is similar to the problem encountered in feed motions of inserting a workpiece into a clamping device. There is little fundamental difference between these two operations. We shall consider this type of operation next.

Compensating for axial alignment error

A gripper and clamped workpiece represent a rigid structure. If this is used to feed a clamping device, this may cause overload of the gripper if the clamping device is required to close before the gripper is allowed to open. This is shown in Fig. 6-4, which is based on the assumption that the axes are not perfectly aligned. This is theoretically always the case. The gripper and handling device are pressed in the direction of the clamping axis, which is in effect an overload. There is a brief force-locking connection between the handling device and the machine to which the workpiece is being fed. A flexible flange plate on the gripper can help prevent damage


55

Fig. 6-4: Situation in which an overload of the gripper and handling device can occur if no compensation is provided.

a) a) Feed to a clamping device b) Axial correction during closing of clamping device

b)

Force closure

This problem theoretically also occurs when a gripper picks-up a workpiece from a pallet under conditions of axial misalignment. As the gripper tightens on the workpiece, the robot arm is once again pushed out of position and consequently uses the power of its actuators to attempt to regain its programmed position. This may even lead to damage to the robot drive system. In this case, too, a slightly flexible flange connection provides a remedy (Fig. 6-5). Moreover, the value chosen for the clearance of the workpieces in the workpiece carriers should not be too small in order to allow the workpieces to adapt to the gripper position. Fig. 6-5: Picking-up a workpiece from a magazine pallet a) Axial misalignment x during approach to position b) Compensation during gripping operation

x

a)

56


b)

The situations here are more than the grippers can cope with. Attention must be paid to axial misalignment. What is the answer? There are various possibilities: • Depending on the type of clamping device (clamping collet, vertical insertion), it may be possible to work in the sequence “Set down/open jaws/clamp in machine/retract gripper” or it may be essential to work in the sequence “Insert workpiece/clamp in machine/open jaws/retract gripper”. The former case does not present difficulties, since there is no force-locking connection between the clamping device and gripper. • There are industrial robots which allow a “soft” switching action within certain limits. The arm behaves compliantly and the robot does not attempt to reestablish its old position. This is achieved by means of a larger area of coincidence for the evaluation of the signals from the positional transducers of the respective robot axes. • It is possible to use hand-joint sensors to detect misalignment. The sensor data is used to derive corrective motions for the robot arm. • The simplest method is to use compliant intermediate plates (rubber, springs), which provide adequate compensation at least for minor errors. Even gripper jaws with compliant faces are often enough. • It is also possible to arrange for the gripper to open in stages. In the transitional phase, the gripping force is slightly reduced. Commerciallyavailable grippers with this type of function operate with spring fingers (leaf springs) driven by a three-position cylinder. • Grippers have also been produced with a definite floating mounting for use in cases where large deviations can be expected between the actual position and programmed setpoint position, for example when picking cartons from shelves. Once the workpiece has been gripped and raised, the gripper travels to the centre of the axis and is locked in this position. This requires a locking device, which is integrated into the gripper. • The sequence “Insert workpiece/open jaws/clamp in machine/retract gripper” can, at least with small workpieces, be achieved by using a pressing element as shown in Fig. 6-6. This is an additional facility complementing the gripper function whereby the pressing element is clamped when the workpiece is picked up. Once the clamping position is reached, the gripper opens. The pressing element now acts on the workpiece and presses it against the contact area of the clamping device, into which it is then clamped. The gripper, having completed its task, is retracted without being subjected to overload.


57

It can be seen from the number of remedies available that this is a problem which needs to be taken seriously. Fig. 6-6: Gripper combined with a pressing element 1 2 3 4 5

Pressure spring Gripper Pressure plate Workpiece Gripper jaws

1 2

3

4

5

Fig. 6-7 shows a very simple device which can be used to assist assembly operations. The gripper is mounted on a cone able to tilt by about 15°. This makes the gripper flexible in the x, y and z directions in cases where the joining part misses its destination and rests on the basic assembly part. In these cases, the cone lifts slightly, creating some “breathing space” in the x-y plane. The gripper is now able to deflect in the appropri-ate direction. It is, however, necessary for the mating parts to have guide chamfers. Fig. 6-7: Simple joining mechanism for vertical assembly 1 2 3 4 5 6 7

1

Centring cone Connector plate for gripper Gripper Gripper jaw joining part Basic assembly part Arm of handling device

2

7 3 4 z 5 6

58


y x

Protection against collision

Grippers are final effectors, which is to say that they are positioned at the end of a kinematic chain (free-arm robots) and thus have the greatest radius of action of all the robot components. This in turn means that grippers are subject to the greatest risk of collision. The more complex and delicate a gripper is, the greater the chance of damage in the case of a collision. Collision-protection devices (or shut-off devices) have thus been developed to prevent this. These devices are fitted between the gripper and robot arm and complement the gripper control system. The protective devices are triggered when an adjustable load threshold is exceeded and generate a shut-off signal. In the case of the device shown in Fig. 6-8, a pneumatically-pressurised chamber is used to keep the device stiff. In the case of a collision, the cushion of compressed air is depressurised and the mechanism becomes “soft”, i.e. slightly flexible.

Fig. 6-8: Collision protection with adjustable parameters for a gripper, showing reaction capability

α z

a) Rotational b) Vertical c) Horizontal F Triggering force z Vertical impact path α Angle of deflection resulting from collision β Angle of tilt

β F F F

a)

b)

c)

There are also spring-loaded mechanisms in which the gripper disengages under overload and recoils from an obstacle. These, however, offer little convenience in the form of adjustment but are very simple in design. The deciding factor is of course the application in question and the probability that unexpected obstacles will be encountered. This will indicate whether it is necessary to protect a gripper against collisions. If there are no obstacles anywhere near the gripper, collision protection will certainly not be required.


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Gripper working area

Fig. 6-9: When working with threedimensional object configurations, consideration must be given to the clearance contour of the gripper

Every gripper requires space to operate. The route to the gripping point requires a gripper working area or feed channel which must be free of obstructions. The minimum size of this area is governed by the contour of the gripper with open jaws or with a workpiece if this projects beyond the edges of the gripper. As the result of this, it may prove better to use a parallel-jaw gripper instead of an angle gripper. This is illustrated in Fig. 6-9. It has been assumed that the workpiece to be press-fitted into a basic assembly part needs to be picked up by a gripper making a positive-locking connection in the insertion direction. This can, however, also be achieved by using a parallel gripper, with the advantage that this will permit the storage locations on the magazine pallet to be positioned more closely together. This increases storage capacity, which is generally desirable.

1

a) Radial gripper b) Parallel gripper 1 2 3 4 5 x

Clearance edge Pitch circle of gripper jaws Workpiece Flat pallet Open gripper jaws Distance between storage locations

2 3

5

4 x

x a)

Load-bearing capacity and dead weight

b)

If the gripper is kept as light as possible, this means a higher payload for the handling device and minimum impairment of dynamic machine characteristics. Fig. 6-10 shows the relationship between the nominal load of the handling device and the tool load, with grippers often being regarded as tools. The nominal load specification refers to the interface between the robot arm and the connector flange of the gripper. If a handling device is operated with the maximum possible load, speed and acceleration must be reduced. This may affect one motion axis or several. The handling cycle will thus become slower. This is not a problem if the process times are significantly longer than the cycle time for a handling operation. It is therefore worth considering in appropriate cases whether an increased handling-device load can be used.

60


Fig. 6-10: Specifications of load capacity for industrial robots

Nominal load Tool load

Working load

Additional load Maximum working load Maximum load The performance of a gripper can be expressed by the ratio of gripping force in newtons to dead weight in grams. The performance index for a gripper with a mass of 420 g and a gripping force of 300 N would thus be 0.71 newtons per gram of dead weight. Values of over 1 indicate very good grippers. Most commercially-available grippers, however, have in-dexes well below 1.

Service life

The service life of a gripper is an important selection criterion. Modern grippers are expected to last for at least 10 million gripping cycles. This is achieved by using high-quality materials and providing appropriate treatment of the contact surfaces of active components and precise wear-resistant guides. It must be ensured that the type and level of load specified for the gripper in question are not exceeded. Grippers with additional seals must be used if coolant, casting dust or grinding dust are present.


61

7 Application areas and gripper types

The excellent flexibility of an industrial robot from both the mechanical and control technology points of view and the speed of a pick-and-place device can provide a practical benefit only if the selected gripper meets the requirements of the application in question. The applications of a given gripper are not, however, subject to any rigid definition – with a little imagination, modifications can always be found to provide the optimum solution to a gripper application. The aim of this article is to provide some suggestions for this. One of the main uses of industrial robots and insertion devices is without doubt machine-feed and assembly applications. Both these areas may involve requirements and customer wishes which go well beyond the “aver-age case”. It may also be the case that a single gripper module is required to deal with objects of widely-varying geometry. Each individual gripping task must be thought through thoroughly before a recommendation is made.

Application areas of grippers

62

It is almost impossible to specify particular types of grippers for particular applications, since virtually every type of gripper can be made suitable for a given application by selecting an appropriate size, jaws, peripheral devices, magazining technique and gripping strategy. Fig. 7-1 nonetheless shows a rough correlation between object features and gripper types. This correlation relates to average situations and covers parallel grippers, radial grippers (jaws opening 90°), angle grippers (opening angle per jaw 18°), 3-point grippers and suction grippers. There are always wide variations within each gripper type and special cases, such as combination suction grippers which can lift sheet-metal workpieces weighing several tonnes. The angle-gripper principle is used for large forging manipulators with load-bearing capacities of as much as 250 tonnes.


Fig. 7-1: Approximate correlation between gripped objects and gripper types

Gripper types Gripped objects Mass

Ideal Suitable Suitable in certain cases — Not applicable

0,2 ... 1 kg 1 ... 10 kg

Dimension

10 ... 50 kg

—

> 50 kg

—

20 ... 50 mm 50 ... 300 mm 300 ... 1000 mm

—

> 1000 mm

—

Internal gripping Surface

—

—

—

Smooth Rough

—

Porous Sensitive Round parts

—

Disc

— —

Short cylinder Shaft/Rod Prisms

—

—

Block Flat/short Flat/long

—

— — —

—

—

Textiles

—

—

—

—

Foil

—

—

—

—

Plastics

—

Glass Pottery

The process of selecting grippers is often dominated by operating parameters and special properties. Fig. 7-2, for example, shows a radial gripper whose large swivel angle allows it to grip flanged sheet-metal workpieces when equipped with gripper jaws shaped like spot-welding tongs. This is almost impossible to achieve with other types of grippers.


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Fig. 7-2: Radial gripper holding a sheet-metal workpiece

The size of the workpiece to be handled need not necessarily dictate the size of the gripper. If, for example, a gripper is required for large integrated circuits (ICs) with 40 pins, this gripper will almost always need to be capable of executing a powerful closing motion, since with large IC’s considerable force is required to bend the numerous “legs”. As Fig. 7-3 shows, the reason for this is that the gripper must adjust the legs to a precise spacing during its closing motion. The IC legs are prebent to an angle of approximately 15°, allowing them to conform to the correct spacing in the gripper. Fig. 7-3: ICs have splayed pins which are aligned to the desired spacing during the gripper motion a) IC with straight legs b) Pins splayed out at an angle c) Gripper jaws

Applications such as the feed of automatic machine tools and the removal of processed workpieces may require 2 workpieces to be picked up simultaneously. Special multiple grippers can be designed for this purpose, but it is sometimes possible to use a slightly modified simple parallel gripper for this purpose. This is shown in Fig. 7-4. It is, however, then necessary to use suitable gripper jaws to suit the distance between the workpieces.

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Fig. 7-4: Example of twin-workpiece gripper as a special use of a parallel jaw gripper

These examples demonstrate again that it is not possible to achieve a strict correlation between gripper types and workpiece properties.

Feed gripper as special solution

There are cases where the clamping point of a machine is not freely accessible to the gripper due to the fact that passage is obstructed by tools, safety or test equipment, etc., leaving only a certain “feed channel”. In cases of this kind, the clearance contour of the gripper when holding a workpiece is a critical factor in the selection process. Fig. 7-5 shows a solution in which a workpiece is gripped parallel to the main axis of a 3-point gripper. The gripper has been fitted for this purpose with specially adapted gripper jaws. The workpiece should be gripped close to its centre of gravity to prevent unnecessary moments which would have the effect of rotating the workpiece out of the jaws.

Fig. 7-5: Handling lengths of bar material with a 3-point gripper

1

open

1 Arm of a handling device 2 Three-finger gripper 3 Gripper jaw

clamped 2 3


65

A further gripper application is shown in Fig. 7-6. A rectangular workpiece is to be placed precisely in a clamping device. This requires the workpiece to be aligned on two axes. The 2-point gripper is, however, able only to align the workpiece on the x-axis. The accuracy on the y-axis depends on how accurately the workpiece is positioned at the pick-up point. If the workpiece has suitable geometrical properties, these can be exploited (Fig. 7-6b) to produce an alignment effect on the y-axis as well. It is also possible to provide the workpiece with a suitable geometrical feature just for this purpose. This is a form of automationcompatible design. Fig. 7-6: Gripping a rectangular workpiece a) 2-point gripper b) Exploitation of geometrical features c) Corner-to-corner clamping d) 4-point gripper

a)

b)

c)

d)

y x

An alignment effect can also be obtained by clamping the workpiece on the diagonal (Fig. 7-6c). If, however, the design of the clamping device means that the corners must be left free, a 4-point gripper can be used. A suction gripper would not be suitable due to the interrupted surface of the workpiece and the gripper’s less accurate positioning (displacement of soft sealing lips, floating during pick-up). 4-point grippers are commercially available. It is, however, also possible to combine 2 parallel-jaw grippers to form a gripper system as shown in Fig. 7-7. This type of arrangement is also known as a combination gripper.

66


Fig. 7-7: A 4-point gripper created by combining 2-point grippers

Grippers for assembly applications

Great progress has been achieved in recent years in the automation of assembly work at all technological levels. For example, automatic machines have been developed for the assembly of electronic components which allow cycle times well below 1 second. This cannot be achieved by industrial robots, but these play a valuable role in flexible assembly systems for short-run assembly work. Assembly robots are without doubt an essential element of the “factory of the future”. One of the ways of achieving flexibility is to use automatic gripper changing systems. The idea is to constantly interchange individual specialised grippers as appropriate to technical requirements. In cases of this kind, an effector is more than just a gripper and may also include further function groups (Fig. 7-8).


67

Fig. 7-8: Gripper module for flexible assembly

1

1 Industrial robot connector flange 2 Upper part of changer system 3 Joining part

Collision protection Uncontrolled joining mechanism 2

Gripper with lower changer system

3

The changer system provides a mechanical coupling and connections for signal and power-supply lines, for example for compressed air. Each individual gripper must be equipped with a lower changer system. Automatic changing (setting one gripper down and picking up another) takes approx. 5 seconds. The purpose of the joining mechanism is to provide automatic compensation for axial offset and small anglar deviations. A collision protecting device can also be valuable, particularly in cases where it is necessary to protect a complicated and costly gripper from damage. If the gripper is overloaded, the collision protection device disengages, triggering an emergency stop of the handling device. For long-run assembly operations, other gripper systems can be considered, such as assembly grippers. This term is commonly used to refer to all grippers in assembly operations but should really only be applied to grippers inside which an assembly operation can be carried out. These will of course be special grippers built for a specific purpose or combination grippers. Fig. 7-9 shows an example of these.

68


Fig. 7-9: Assembly gripper 1 2 3 4 5 6

1

Hollow piston rod Connecting piece Parallel gripper Short-stroke cyl. Suction cup Gripper jaws

2 3 4

6

5

The gripper system consists in this case of a suction gripper and parallel-jaw gripper. These are independent and are activated individually. It is perfectly possible to produce this combination gripper from standard components. In accordance with the assembly sequence shown in Fig. 7-10, the joining part is first picked up by the suction gripper. The basic assembly part is then gripped by the parallel gripper. The joining process is carried out while the effector travels to the set-down position. This operation can also be carried out during set-down, by inserting the joining part into the basic assembly part. This method can be used, for example, to place lenses in mounts. The advantage is that no external assembly device is required. Fig. 7-10: Sequence for assembly within a gripper a) Approach to pick-up position b) Gripping the joining part c) Lifting the joining part d) Approach to 2nd pick-up position e) Gripping the basic assembly part f ) Joining process g) Module assembled h) Set-down of completed module

a)

b)

c)

d)

e)

f)

g)

h)


69

8 Checklist for grippers

Grippers are the direct interface between automation devices and the objects to be gripped. The geometry of these objects can vary greatly. Operating conditions, too, are certainly not constant and may be far from ideal. This makes it difficult to select grippers. In individual cases, it may be that no standard gripper is acceptable, making it necessary to develop a special gripper. The rule therefore is – check all the requirements of a given application and consider their feasibility. The selection of grippers is a matter which needs to be taken very seriously! Up to the present time, no uniform guidelines have been developed for the design and sizing of grippers. If a variety of workpieces need to be handled, the selection of grippers will be determined chiefly by object and process parameters and other parameters under the user's control. There is at the moment no universal algorithm to determine the structure of gripper systems and the design of grippers. Programs are, however, avail-able for the calculation of technical/physical parameters.

Many interrelationships between factors

The relationships between the major technical/physical factors governing gripper applications are shown in Fig. 8-1. The critical aspect is not steady-state conditions but the dynamic effects in a moving system. Furthermore, it is not enough to consider random moments which occur at some point during a handling operation. Rather, it is important to determine the maximum values which are encountered at various points in time within a motion sequence. There are then two possibilities: • Relaxation of requirements by changing motion and time parameters and/or • Selection of a gripper on the basis of the maximum parameter values within the motion sequence.

70


Fig. 8-1: Mutually influential factors and basic variables relating to the selection of grippers, from the technical point of view

Configuration Anordnung von of robotund and Roboter machine Maschine Position and

Lage der Teile am components Bereitstellplatz at pick-up point

Bewegungsrichtung Direction of motion ofdes Greifers Greifobjekt gripper zum towards object

Greifzone und Oberfläche Gripping zone and an der Griffstelle surface at gripping point

cLage Position and in der Teile

Lage der Objekte Position of objects iminGreifer gripper

components der Übernahmein transfer device einrichtung Shape of Grösse size Form und of object to be der Greifobjekte gripped

Mass and Masse und Eigenproperties schaften der of objects Objekte

Berechnung von Kräften Calculation of forces und Momenten, die and moments während der Bewegung which occur during des Teils im Raum 3D motion of object auftreten

Geschwindigkeit Speed and und Beschleuniacceleration of gung robotdes perRoboaxis ters je Achse

Calculation of Berechnung der contact pressure an Flächenpressung at gripping point der Griffstelle

Definitionder of drive Festlegen Antriebsparameters parameter für Greiferfor gripper drive antrieb

Material and Werkstoff und surface of des Oberfläche gripping object Greifobjekts

Bewegungsfolge Axis motion insequence den Achsen

Shape and number of Form und Anzahl der gripper components, Elemente, Auswahl des choice ofund gripper and Greifers der Kinekinematics of gripper matik der Arbeitscomponents elemente des Greifers

Berechnung der auf den Calculation of forces Greifer wirkenden acting on gripper during Kräfte bei Bewegungen robot motion des Roboters

Determination Bestimmen des of gripper gearing Greifergetriebes

Kontrollrechnung und Checking calculation Optimierung der and optimisation Greiferkonstruktion of gripper design

Gripper flange

Greiferflansch and type of und Energieart power source

Selection of type Auswahl des Typs des of gripper drive Greiferantriebs

Design Konstruktion of gripper connection Greiferanschluß

Avoid placing unnecessarily high requirements on the gripper technology, since this will increase purchase and operating costs. Rather than have a robot continually returning to a waiting position, it is better to reduce its speed slightly. With complicated workpiece shapes, the procedure must start somewhat earlier with a search for suitable gripping surfaces on the object. This process is shown in Fig. 8-2. We must be aware that any change in grip-ping points will have effects on the dynamic behaviour of the gripper and must be recalculated for a given gripper. If a workpiece has recesses or shoulders which can be used to create a positive-locking connection with the gripper, these should be used. This will allow the gripping force to be reduced.


71

Fig. 8-2: Generation of characteristic gripper data, starting with the gripper location

Determination Greifzoneof gripping zone ermitteln

Finding a gripping Griff-Fläche surfacesuchen on the object

Calculation of Berechnung: --Force Kraft --Moment Moment --Moment of inertia Trägheitsmoment

Provisional vorläufiger location of gripper Greifort

Surface properties beschaffenheit

Oberflächen-

Überprüfen Checking of der Greiferdaten gripper data

Estimation of Abmessungen von workpiece dimensions Werkstückabsätzen

Relocation of der Verschieben gripper plane Greifebene

Study of gripper und Greifervarianten variants and Feststellung der suitability of these Eignung

Dokumentieren der Documentation Ergebnisse of results

A number of attempts are being made to produce programs which will allow the automatic planning of handling systems, including grippers. The procedure used here, too, is to identify all possible gripping surface pairs on the object to be manipulated (relating to two-jaw grippers, i.e. parallel surfaces). The contactfree regions (gripping zone) are then defined. We then determine all the grippers able to work within the defined gripping zones. There must be adequate coincidence between the object gripping surfaces and gripper surfaces (gripper jaws). The decision as to whether a gripper is able to grip a workpiece by the pair of gripping surfaces selected is then taken in a process which takes into account various physical and other parameters. Once all possible types of grippers have been identified, an optimisation process is started to identify the best gripper (type and variant). The most practical method would of course be a comprehensive simulation system in which a gripper could execute all the required motion sequences on the computer screen in the form of an animation. This would allow continuous output of all major parameters, with automatic signalling whenever limit values were exceeded. It would also allow cases to be identified in which even slight changes in speed or acceleration would offer an advantage.

72


Step-by-step gripper selection

The first step towards finding the right gripper is to undertake a comprehensive description of the task for which the gripper is to be used. This will often involve the combination of several components in an assembly station and will require severval grippers. The question then is whether to use a gripper changer system or not. Gripper systems which can be matched to individual assembly operations make cycle times shorter and thus result in faster flexible assembly systems. The time required for gripper changing must, however, be short, and only an automatic changer system can ensure this. The determining variables are the range of workpiece variants and the batch sizes involved. As a rule of thumb, automatic gripper changer systems will be economically viable if the number of geometrically different component variants per batch is 5 or more or the production time per batch is up to 2 hours. The flow chart shown in Fig. 8-3 can be used to select an individual gripper. Each activity can be correlated to typical questions. These can help in a discussion with users to arrive at a binding list of technical characteristics. Intuitive selection of a gripper on the basis of gripping force alone, as is still done in many cases today, can easily lead to error and should be avoided at all costs.


73

Fig. 8-3: The main steps in the selection of grippers

Start Start

Preguntas Typische típicas Fragen

Determination of Bedingungen all conditions (mass, size, shape) applying to the Abklärung aller (Masse, Größe, Form) am Greifobjekt, gripped object,the motion sequence and any limiting factors der Bewegungssequenz und einschränkender Randbedingungen originating in the process or its environment von Prozeß und Umgebung

1 to 5 5 1 bis

Definition of gripper principle: Single, Einzel-, multible Mehrfachgreifer, or special grippers, Festlegung des Greifprinzips: holding system, kinematics or force fields Sondergreifer, Haltesystem, Kinematik bzw. Kraftfelder

6 6tobis 12 12

Determination of necessary forces, forces occuring and sowie loads der dabei Ermitteln der notwendigen und auftretenden Kräfte to which workpiece is subjectzu ertragenden Belastungen vom Werkstück

1313tobis 16 16

no nein

Can the load beBelastungssituation successfully handled in in jeder every Kann die direction of motion? Bewegungsrichtung beherrscht werden ?

17 21 21 17tobis

yes ja ListZusammenstellung of other important requirements, such asAnforderungen accuracy, sonstiger wichtiger wie z.B. connection conditions,Anschlußbedingungen, overload protection, deviation of gripping Genauigkeit, Überlastschutz, points and monitoring devices Greifpunktverlagerungen und Kontrollen

22 2626 22tobis

DesignGestaltung of gripper jaws, sensor equipment, media Ausstattung, der Greiferbacken, sensorische throughfeeds and mounting Medienführung und Befestigung

27 32 32 27tobis

no nein

Does the concept theAnforderungen requirements Entspricht das conform Konzepttoden as expressed in theAufgabenstellung problem description? gemäß ?

33 3535 33tobis

yes ja Evaluation and selection of gripper model Bewertung und Typauswahl (monetär sowie nichtmonetär) (financial and nonfinancial factors)

Outsourcing, special or in-house Realisierung durchexternal Zukauf,manufacture Fremdvergabe oder Eigenbau

36 3939 36tobis

40 40

Stop Stopp

Let us now consider the questions: 1. Are the object properties, especially mass, size, fragility and surface quality, sufficiently well known or are further tests required? 2. Is there good access to the gripped object (gripper freedom “feed channel”)? 3. Has the gripper application (handling cycle) been defined in a detailed and binding way or are changes likely? 4. Is a single gripper required to handle both blanks and ready-machined workpieces (radical changes in shape during working cycle)? 5. Are all working conditions known (pressure, temperature, object condition, cycle time, dust generation, oil mist, humidity, coefficient of friction, mass, etc)?

74


6. Can the object be held by a force-locking connection, or is positive locking or a combination of the two methods also possible? 7. What principle is to be used, clamping or adhesion (fluidic, magnetic, frictional)? 8. Are the object gripping surfaces (and forbidden zones) specified or can they still be changed? 9. Is the workpiece being gripped at its centre of gravity to avoid moments? 10. Would a gripper changer system or turret gripper be worthwhile for the handling of several different workpieces? 11. Is a positive-locking connection provided in the direction of highest acceleration? 12. Is the workpiece position at the pick-up point in the magazine the same as at the machining point? 13. Is it necessary also to make allowance for process forces, such as occur during assembly operations, etc.? 14. Are high-friction or patterned gripper jaws advisable? 15. Are the grippers and the motion devices to which they are connected dimensioned for peak forces and moment (e.g. in an emergency stop situation)? 16. Can the workpiece withstand the intended contact pressure or must larger or additional contact areas be provided? 17. Is it possible to dangerous situations (uncontrolled release of workpiece) to result from a high-speed emergency stop)? 18. Has a sufficient safety factor been incorporated into the calculated gripping force required? 19. Must the gripping force be precisely limited to prevent damage to the workpiece? 20. Is a device required in order to maintain gripping force (double non-return valve)? 21. Is it advisable to fit a collision- and overload-protection unit? 22. Is the achievable accuracy sufficient in order to provide a reliable solution for the task in question? 23. Can the clearance contour of the gripper when open (or closed with a workpiece) cause a risk of collision in the gripper's environment? 24. What kind of centring effect (during pick-up and alignment) is the gripper expected to provide? 25. What triggering method (e.g. directional control valve combination) should be recommended to users of pneumatic grippers? 26. Is the gripper control system required to be event-triggered (by the presence of the object to be gripped)? 27. Are the gripper fingers (lever arms) as short as possible? 28. Is it necessary to monitor the finger positions (open, closed) with sensors? 29. Particularly in the case of assembly operations, is it advisable to provide assembly mechanisms and/or force sensors? 30. Are several sets of gripper jaws (with facility for quick changing) to be provided?


75

31. Are adapter plates required for the mechanical connection of the gripper, and are these commercially-available designs? 32. Are the gripper jaws required to compensate for parallelism errors at the gripping surfaces of the object? 33. Is the achievable time sequence (for pick-up, transportation and release) acceptable? 34. Will the gripper provide the expected service life under the operating conditions in question? 35. Are all operating values for the gripper within permissible limits? 36. Will a standard gripper, possibly with special jaws, be suitable, or will a special solution be required? 37. Are the conditions of supply and guarantee as expected? 38. Do auxiliary devices (pressure devices, gripper magazine) need to be considered? 39. What level of cleaning, servicing and maintenance will be required? 40. Has a satisfactory gripper been found or should the problem be referred to a gripper specialist? The Festo gripper selection tool (GST) can be used to make a selection on the basis of technical/physical parameters. The input parameters are as follows: • • • • • • • • •

Distance between centre of gravity of object and gripper Mass of object and fingers Mass moment of inertia of gripper finger and distance to centre of gravity Gripping motion type (internal, external) Direction of acceleration and maximum acceleration value Coefficient of friction between finger and object Operating pressure in compressed air supply network Safety factor Eccentricity of centre of gravity of object.

The result of this is a recommended gripper size, with an indication of the degree of utilisation of maximum capacity in percent. This provides a starting point for optimisation. It is possible, for example, to move to the next smallest size of gripper by lengthening the closing times (if the process permits this) if this is the only parameter which exceeds the permissible limit. An additional program is available for the determination of mass moments of inertia. Programs of this kind do not of course provide all the answers but they relieve users of time-consuming calculations and provide more time for an examination of all the other parameters. Technical/physical parameters governing fit and clearances have first priority in nay case in the gripper selection process.

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9 Suction grippers – abhorred by nature

Air as a medium

In the Middle Ages, vacuum was very much an unknown quantity. It was not until Otto von Guericke of Magdeburg conducted his famous experiment with hemispheres and teams of horses that a first insight was gained into this strange empty state which nature was said to abhor. People were, however, aware of such things as the 45-centimetre scars seen on the skin of whales, caused by the suction cups of giant octopuses. These cups can be 10 to 15 centimetres in diameter and are located on the octopuses' tentacles, which are up to 15 metres long. Today, suction cups are in widespread use in industry as an inexpensive and simple automation tool. This is the first of three articles dealing with these devices.

Vacuum technology operates with a flowing compressible medium – air. Vacuum is said to be present in a space in which the air is diluted, resulting in a pressure significantly lower than the surrounding (atmospheric) pressure. This principle, applied to suction cups, means that there is a pressure difference between the interior of the suction cups and the surrounding air. Atmospheric pressure presses the lips of a suction cups against a workpiece. The suction cup is thus a means to create the boundary of a pressure zone. Fluctuations in atmospheric pressure mean variations in holding force. For every 100 metres increase in altitude above sea level, atmospheric pressure falls by 12.5 mbar. This pressure is 1013 mbar at sea level, falling to 763 mbar at an altitude of 2,000 metres above sea level. Suction cups are a popular and simple solution for repetitive gripping applications of the “pick up, move, set down” type, provided that the workpieces in question have flat non-porous surfaces. A further advantage is that suction cups can be used with non-magnetic materials such as glass, ceramics and wood. We can make a general distinction between 2 types of suction-cup applications: • Large suction area and small pressure difference The advantage here is that the holding force can be built up quickly and that there is little deformation of soft flexible workpieces. In the case of slightly porous materials, air is not drawn through these. • Small suction area and large pressure difference This means high gripping forces as the suction cups used become smaller. This allows the clearance radius of manipulators to be made smaller, which is often a decisive factor when space is limited. Fig. 9-1 shows the most important functions of a suction cup. Not all these functions will be used in every application.


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Fig. 9-1: The most important functions and properties in relation to suction grippers

1

2 1 2 3 4 5 6 7 8

Vacuum line Pressure switch Angular freedom Quick exhaust Vertical freedom Quick interchangeability Generation of holding force Workpiece contact sensor

3

4 5

6 7

8

Suction cups are suitable for a large number of handling operations, such as sorting, feeding, clamping, turning and stacking, and are used as grippers with lifting devices, balancers, feed devices, stacking systems, packing machines and production lines. Suction cups are particularly convenient when workpieces have the following features: • • • • •

78

Awkward dimensions Susceptible to deformation Non-magnetic Surfaces sensitive to scratching (ground, polished, painted) Undulating but non-porous surfaces.


Sizing suction cups

The purpose of this is to define the vacuum in the suction chamber and the size of the suction area in such a way that these compensate reliably for all the forces occurring during manipulation operations. In the case of slow motions, such as the movement of suction-held workpieces on a balancer, it is sufficient to consider static forces. With high-speed motions, dynamic forces must also be considered. Fig. 9-2 illustrates the relevant force conditions. The following applies as a general principle: F = (po – pu) · A · n3 · η · z ·

1 S

The terms used in the above are as follows: A = Theoretical area of suction cup. F = Working load; weight force of gripper object; total load acting on suction bond. n3 = Coefficient of deformation. Very soft lips (bell-shaped suction cups) deform strongly as vacuum builds up, which may reduce the effective suction area. n3 = 0.9 to 0.6. po = Atmospheric pressure; dependent on altitude above sea level. pu = Pressure in seal suction chamber. S = Safety factor to guard against detachment of workpiece. A state of equilibrium alone is not sufficient – the gripped object must be pressed against the suction cup with a certain force. S = 2 to 3. z = Number of suction cups. η = Efficiency of system, including leakage allowance. Fig. 9-2: Force conditions with vertically-moving suction cup

FS

pu A

F

po

With high-speed motions, allowance must also be made for forces resulting from weight, mass moment of inertia and centrifugal force. This results in different lines of action for the overall force. Furthermore, the centre of gravity of the gripped object may not coincide with the centre of the suction cup. Fig. 9-3 shows the resulting typical load cases and the calculation of the required suction cup force FS.


79

The force F is always the force resulting from all static and dynamic effects, including allowance for superimposed motions. Fig. 9-3: Typical force situations at suction cup

1

FS

v

F Sum of all forces producing detachment and shift FS Force produced by vacuum n1 Safety coefficient against shift

S

2

Fs ≥ n1 · F

F

FS

v

Fs ≥ F(n1 · cosα + n2/µ · sinα) µ FX S

3

n2 Coefficient of protection against shift µ Coefficient of friction (suction cup/workpiece)

α FZ

F

FS

v

Fs ≥ n1 · Fz + n2/µ · Fx

Fs ≥ n1 · k1 · F k1 = 1 + r/R

S r

F 4

k1 Coefficient of eccentricity of line of force action r Distance between force action and suction-cup axis R External radius of suction cup

R

FS

v µ

Fs ≥ n1 · k1 · Fz + n2/µ · Fx or Fs ≥ F(n1 · k1 · cosα + n2/µ · sinα)

S FX

r FZ 5

α

F

FS µ Fy

S R

Fs ≥ n1 · k1 · Fz + n2/µ · k2 · Fx or Fs ≥ F(n1 · k1 · cosα + n2/µ · k2 ·sinα) k2 = 1 + r/R + Fz/Fy · µ

F

α

r

α Angle between force action and vertical S Centre of gravity of gripped object

k2 Coefficient of eccentricity of line of force action

FZ v

6

Fs ≥ n2 · F/µ S

FS

µ

80

Special case of (2) with α = 90° With a horizontal suction-cup axis, the holding force remains less than 50% of the value for a vertical axis

F


In cases of lateral motion and where the suction surface is positioned vertically, we must consider a further variable – the coefficient of friction µ. This can be taken as µ = 0.5 for clean dry glass, stone and plastic, falling to µ = 0.1 to 0.4 with damp and oily surfaces. Other sources quote the following guide values: Type of suction cup

Type of surface

Coefficient of friction with peak-to-valley height Ra = 0.05 µm Ra = 1.5 µm

Rigid

Oil-free

0.85

—

Slightly deformable

Oil-free

0.45

0.65

Rigid/slightly deformable

Lubricated with drilling emulsion

0.15

0.35

Rigid

Lubricated with coolant

0.05

0.25

Slightly deformable

Lubricated with coolant

0.025

0.15

Notwithstanding this, exercise caution; the coefficient of friction can fluctuate widely, like a person’s blood pressure. In the case of oiled or greased sheet metal, which often requires handling during shaping processes, problems may be encountered due to the fact that the specified coefficient of friction between the steel and rubber no longer applies. The coefficient will be considerably lower, since in most cases the lips of the suction cup will not penetrate the oil film. In situations of this kind, it is the law of fluid friction which applies and not Coulomb’s law of friction. Tests should always be made in cases of this kind before a suction cup is selected. To select a suction cup, proceed as follows: 1. Identify all the external forces acting on the suction cup. These will include weight, inertia and centrifugal forces. Examples, assistance and notes on this calculation can be found in [1] to [3]. 2. Determine the suction force FS, to be generated by vacuum in accordance with the force situation shown in Fig. 9-3. The load may vary during the handling sequence, e.g. if the attitude of the suction surface changes from horizontal to vertical. Calculation should always be carried out on the basis of the worst load case. 3. Choose a vacuum operating pressure as appropriate to the vacuum generator. Always try to choose an economic pressure. It is possible, for example, by increasing the vacuum from pu = – 0.6 bar to pu = – 0.9 to boost force by a factor of 1.5, but energy consumption will rise by a factor of 10. A vacuum of pu = – 0.7 bar will generally be used. 4. Calculate the suction cup size from the required suction area. If a standard suction cup smaller than the necessary size has been chosen, calculate the number of cups required. Distribution of the holding force among several suction cups makes a handling system more reliable.


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5. Define the suction time (evacuation time). This is calculated from the volume to be evacuated (suction cups, lines) and the performance curve for the vacuum generators, or in other words the volume to be evacuated divided by the evacuation flow rate per unit time.

How can the necessary vacuum be achieved?

Suction cups used in handling systems operate with rough vacuum, which ranges from 105 to 102 Pa. Other ranges include fine, high and ultra-high vacuum. 70% vacuum is the value generally used with suction cups. This means 0.7 bar vacuum or 0.3 bar absolute pressure. We will present 4 types of vacuum generation in this article (Fig. 9-4). These are: • Vacuum pumps and blowers • Vacuum generators operating on the venturi principle (ejectors) • Adhesive suction cups • Pneumatic cylinders.

Fig. 9-4: Methods of producing a vacuum a) Rotary pump or other type of pump b) Vacuum generator c) Adhesive suction cup d) Piston suction system

a)

b)

c)

d)

The use of vacuum pumps has the following advantages: • Higher vacuum is possible • Low operating costs • Low noise level. The disadvantages are the higher purchase cost and the cost of further accessories, such as air reservoirs. Some companies, for example light-bulb manufacturers, use not only a central compressed-air supply but also a central vacuum supply. In these cases, decentral vacuum generators are not required.

82


Vacuum blowers produce only a relatively low vacuum, as can be seen from the comparison in Fig. 9-5. They do, however, have a large suction capacity, making them useful in cases where it is necessary to compensate for the porosity of workpieces. Fig. 9-5: Comparison of performance of typical vacuum generators

-0,9 -0,8 -0,7

Vakuumpumpe Vacuum pump

Vacuum Vakuum in barin bar

-0,6 -0,5

Ejector Ejektor

-0,4 -0,3 -0,2

Vacuum blower Vakuumgebäse

-0,1 0 0

100

200

300

400

500

600

700

800

900

1000

Suction capacity Saugvermögen in l/min in l/min

Venturi-type vacuum generators have the following advantages: • Very simple design, with no moving parts and low purchase cost • No additional equipment required; fast response time • Extremely reliable. The disadvantages are the higher operating cost resulting from the consumption of compressed air and the need for silencers. These vacuum generators must be sized for peak load, since no reservoir is used. Suction air is generated in the ejector as compressed air passes through the restricted cross-section at the drive nozzle. This restriction produces an increase in flow velocity. Following this, the air expands and exits via the receiver nozzle. If the exhaust-air duct is shut off (Fig. 9-6b), an ejector pulse effect is produced. The venture principle is named after Giovanni Battista Venturi (1746 to 1822), an Italian physicist. Venturi's main work was concerned with hydrodynamics and hydraulics and he invented the nozzle with flow restrictor which bears his name today. This nozzle is also used as a measuring nozzle to determine flow rate in accordance with the Bernoulli formula.


83

Fig. 9-6: Mode of operation of venturi nozzle, with shut-off valve to produce ejector effect

2

1

3 4

a) Suction b) Ejection

5 1 2 3 4 5 6

Shut-off valve Receiver nozzle Driver nozzle Compressed air supply Suction cup Workpiece

6

a)

b)

Ejectors can be multi-stage and can be operated in parallel. In comparison with a single-stage ejector, a multi-stage (multi-chamber) ejector is a series connection of several vacuum generators (Fig. 9-7). Series circuits permit the rapid suction of large quantities of air (shorter evacuation times). The advantage of this arrangement is thus the high suction capacity. Fig. 9-7: Vacuum generators in a series circuit p Pressure V Vacuum

p

V

Adhesive suction cups are merely pressed onto the workpiece to force out the air they contain; a vacuum is then created by the resilience of the suction cup material or by weight forces. It is virtually impossible to compensate for leakage losses. The suction surfaces must be smooth and non-porous. Piston suction systems are occasionally used with automatic assembly machines. These systems produce vacuum and an ejector pulse in the same line in alternation in synchronisation with the machine cycle. The piston stroke and timing are stored in the form of a control cam. It is even possible to generate vacuum using a solenoid, as with the patented suction cup shown in Fig. 9-8. As the solenoid picks up, the volume of the chamber below the suction cup increases, creating a vacuum which holds the workpiece in place.

84


Fig. 9-8: Solenoid-actuated suction cup

Vacuum circuits

In order to make a suction cup work, it is necessary to connect up a number of other components. Fig. 9-9 shows a typical circuit based on an ejector. A vacuum switch is used to monitor the vacuum and detect whether the necessary vacuum has been reached after the start of suction. Only then does the handling device continue its motion sequence. Pressure data can also be used for “vacuum management”, i.e. to switch off the vacuum generator for short period to save energy. Vacuum switches are also used to generate an alarm in the case of an undesired pressure drop, for example in the case of balancers and small hoists where operators are working in the vicinity of the load.

Fig. 9-9: Example of a vacuum circuit based on an ejector 1 Directional control valve for compressed air supply 2 Ejector 3 DCV to control exhaust air (switchover to ejection) 4 Silencer 5 Filter 6 Pressure switch for vacuum 7 Suction cup 8 Distributor

1

2

3

4

p

5

6

8

7

The line diameter in the vacuum circuit should not be undersized, since this will increase flow resistance, but should not be oversized that suction times become too long. We see in nature that trees need to feed vital liquids to the very last leaf tip, which has resulted in the evolution of appro-priate distribution systems.


85

A similar task faces technicians who need to size the lines of a suction system. They must consider the flow resistance in the tubing of the system. If we imagine a tree fitted with suction cups, as shown in Fig. 9-10, we must size the tubing in accordance with the factors shown. Each further branch should be made smaller by a factor of 1.42. Fig. 9-10: Correct choice of tubing diameter is important in the distribution of suction air

1

1 Suction cup 2 Line

0,35.D 0,5.D

D Tubing diameter

2 0,71.D

D

Publications

86

[1] Automatisieren mit Vakuum (“Automation With Vacuum”; 4th edition), published by FESTO Pneumatic Esslingen [2] Greifer für die Handhabungstechnik (“Grippers For Handling Systems”; brochure) by FESTO Pneumatic Esslingen 1996 [3] Vakuum-Greifer und -Saugdüsen einfach und schnell auswählen (“Fast And Easy Selection Of Vacuum Grippers And Generators”; slide rule), produced by FESTO Pneumatic Esslingen


10 Suction cups for every application

Large number of application parameters

Suction cups are the active components which create the contact between a handling device and the workpiece to be handled. There are many different workpieces and gripping applications, and an equally diverse range of suctioncup variants, with differences of size, material, geometry, Shore hardness and design. We will investigate some of these variants in the following.

Familiar suction-cup materials include perbunan (buna-N), silicone, polyrethane and neoprene. Natural rubber is also used. In certain applications, there may be a requirement for suction cups which do not mark workpieces, for example for use with polished plate glass or polished metal workpieces. One method is to use textile hoods under the suction cups or textile laminates. There are also fluorine rubber suction cups which are non-marking. Shore hardness values (in accordance with DIN 535051) lie in the range of 30° to 90°. The choice of suction cups is greatly influenced by the intended application and the associated loads presented by the workpiece and environment. Particularly important are properties such as resistance to abrasion, oil resistance (chemical resistance), suitability for use with food and short- or long-term temperature resistance. With standard quality rubber, elementary sulphur is often used in conjunction with vulcanisation accelerators. It is possible for some free sulphur to remain which then reacts with the workpiece. Only sulphur-free elastomers should therefore be used to handle metals. Workpiece temperature in general can vary between – 50 and 250° C. Anything over 70° C can be regarded as a special case and will usually require special materials. At temperatures below zero, the hardness of suction cups may increase, making the cups virtually rigid and preventing adequate adaptation to the surface of the workpiece. The elasticity of suction cups means that handling applications in general cannot achieve positioning accuracies of better than ±1.0 mm. Additional technical measures are therefore required if the positioning error is to be reduced further. Normal suction-cup diameters range from 1 to 630 mm (flat suction cups).


87

Suction-cup shapes and designs

If we could bring the suction cups available from every manufacturer together in one place, we would have a collection as garish as any eastern bazaar. Let us, however, first consider the different shapes and designs of suction cups. As shown in Fig. 10-1, these are as follows: • Bellows suction cups Suitable for slightly curved, inclined, easily deformable and uneven surfaces. Provides a slight lifting motion; compensates for height differences; can have up to 6.5 pleats. Small diameters of this type are Suitable for thin materials. Bellows suction cups with a large number of pleats are sometimes fitted with an internal or external support spring to provide additional rigidity. • Flat suction cups In general terms universal suction cups, Suitable for non-porous flat and slightly curved surfaces; able to transmit high vertical forces. • Deep suction cups Good adaptation to round surfaces (Fig. 10-5) and profile sections. Should not be used for flat surfaces, since rigidity is low and wear is rapid. • Ribbed suction cups Suitable for flat and unstable surfaces. The ribs across the mouth of the cups prevent thin materials from being drawn into the cups and make these more resistant to lateral deformation. This type of suction cup is also useful for use with vertical surfaces, since the ribs provide increased friction when the workpiece is in contact with these after being picked up. Since the lips of the suction cups do not flex very much, virtually 100% of the effective suction area is maintained. The more rigid design means on the one hand that suction cups can be produced in larger sizes without a support plate but on the other means that the suction cups cannot grip objects with any pronounced curve. • Suction cups with cellular rubber seal These provide a good seal with uneven and heavily-textured surfaces, such as corrugated sheet metal, textured glass, concrete slabs, fireproof bricks, etc. Not good for applications with vertical workpieces. • Oval suction cups Good for long, narrow or slightly-curved workpieces. Can be used for “spiders” (large grippers with a large number of suction cups spread over their area) in the automobile industry; typical features are metal baseplates and narrow flexible sealing lips.

88


• Double-lip suction cups A seal is provided by a combination of a sealing lip and sealing ring. This gives great elasticity. When operated at close to maximum load, these suction cups may alternate in a relatively uncontrolled way between their inner and outer sealing lips. • Self-adhering suction cups These are not connected to an external vacuum source. A vacuum is created by the suction cup itself as it presses against a workpiece. There is no compensation for leakage losses. A hand-lever valve is used to break the vacuum. As the cup is pressed against a workpiece, its volume is reduced, causing air to be displaced. As the cup springs back into shape, a vacuum is created under the suction cup. The level of this vacuum is, however, hard to define, since the deformation force can vary widely. • Lifting suction cups Combination of a piston system and a suction cup. Once a workpiece has adhered to the suction cup, it rises away from the workpiece stack. A lifting suction cup can therefore make a separate lifting axis unnecessary. Strokes of up to 50 mm are normally available. This type of suction cup is used to handle cut cardboard, paper, foil pieces, thin sheet metal, packaging items, etc. Fig. 10-1: A small selection of the major types of suction cup 1 2 3 4 5 6 7 8 9 10 11

Bellows suction cup Flat suction cup Deep suction cup Ribbed suction cup Profile suction cup Suction cup with cellular rubber seal Lifting suction cup Oval suction cup with metal plate Double suction cup Double-lip suction cup Self-adhering suction cup

1

2

5

6

9

10


3

4

7

8

11

89

Fig. 10-2 shows the results of a comparative study [1] of flat suction cups without metal reinforcement (A), flat suction cups with metal plates and small flexible sealing lips at the extreme edges (B), double-lip suction cups (C) and bellows suction cups (D). The aim was to find the maximum trans-mittable vertical and horizontal forces, the flexible vertical stroke produced by a vertical force and the residual volumetric flow rate between the suction cup and the workpiece. These involve relationships, in certain cases very complex relationships, between the shape and material of the suction cup and the surface properties of the workpiece. Despite its relatively simple design, type A exhibits good results for all criteria. Its comparatively large flexibility in the vertical direction as the vertical force increases, makes it suitable for use in all but a few applications. Type B allows very high vertical forces, since the vacuum chamber stays in shape even under high vacuum thanks to spacers and the small narrow sealing lips. In the case of type C, the double seal results in a very low residual volumetric flow rate. The complex seal system, however, takes up more space, thus reducing the effective diameter of the suction cup. Type D is characterised by the low maximum vertical forces which it can transmit, its lack of geometrical stability under the action of lateral forces, and its very large elastic vertical stroke. This rules this type out for a variety of handling applications. Fig. 10-2: Evaluation of various types of suction cups Very good Ideal Suitable Suitable in certain cases — Not applicable

Criteria Design

Transmittable vertical force

Transmittable horizontal force

Flexible vertical stroke

Residual volumetric flow rate

A

B

C

D

—

90


—

Freedom of movement of suction cups

The ideal workpiece surface for suction cups is one which is perfectly flat and in particular non-porous. In many cases, however, workpiece surfaces are not flat, which means that suction cups must be compliant or adjustable on their vertical axis and in their angular attitude. There are various ways of achieving this (Fig. 10-3). In the simplest cases, bellows suction cups can be used. They offer a certain degree of angular compliance. Multi-axis freedom of movement is required mainly in cases involving large irregularly-shaped workpieces, such as are typically encountered in the automobile industry.

Fig. 10-3: Freedom of movement of suction cups 1 Bellows suction cup 2 Spring-loaded flat suction cup 3 Angle adaptability provided by ball-and-socket head 4 Longitudinal freedom of movement through suction cup's own mass 5 Fixed angle setting 6 Longitudinal and angular freedom of movement through double joint and longitudinal guide

1

2

3

4

5

6

Spring-loaded suction cups can also cushion the impact of workpiece contact and compensate for height differences. The spring tension also offers the advantage that the suction cup comes into contact with the workpiece before the handling device stops. This reduces the time taken to build up the required vacuum in the end position.


91

Ball-and-socket brackets also reduce the bending forces which occur during the handling of movable objects. Another method which has been suggested of achieving flexibility is a matrix of individual suction cups which are able to execute a large vertical motion (Fig. 10-4). The suction cups can then adapt to a given surface as they are lowered onto an indi-vidual workpiece or a pile of workpieces. When in position, all the rods are clamped into place, allowing the handling device to store the workpiece shape temporarily and lift a layer of individual workpieces. Fig. 10-4: Suction-cup array fitted to rods allowing longitudinal movement and used to pick up workpieces of constantly varying contours [2]

Let us at this point once again mention deep suction cups (bell-shaped), which have the adaptability to handle concave and convex workpieces very well, as shown in Fig. 10-5. Fig. 10-5: Deep suction cups can adapt well to curved surfaces a) Gripping a convex body b) Gripping on a concave surface

a)

92


b)

A special type of vacuum gripper is shown in Fig. 10-6 which in a way combines a bellows with the sleeve principle of a rolling diaphragm, resulting in a very large freedom of movement. This is intended for use with objects whose geometry, position and orientation vary continually within certain limits. Suction cups with numerous pleats are often used in the food industry. Fig. 10-6: Vacuum gripper with very large freedom of movement of suction components 1 Suction cup 2 Workpiece 3 Magazine plate

1 2 3

Non-rigid sheet workpieces have always been considered difficult to handle. “Normal” grippers are defeated by foils, due to the “choking” effect which occurs. Fig. 10-7 shows this. The foil material is drawn into the suction cup and finally lies directly over the suction hole. This means that the gripped area is very small. The workpiece cannot be held and falls away from the suction cup. Fig. 10-7: Standard suction cups are not very suitable for use with thin foil material

1

2

3


4

93

The answer is to use numerous individual suction cups, each operating at a low vacuum. Even better are low-pressure grippers whose active surface consists of porous material which can let air through. Special plates are also available with a large number of fine suction holes. Components of this kind can be combined to form large units, as shown in Fig. 10-8. One disadvantage of course is that the gripper is awkward due to its size. Fig. 10-8: Low-pressure gripper equipped with porous plastic or perforated plates

Ensuring a correct gripping position

Particularly with soft-lipped and bellows suction cups, it is often necessary to take measures to ensure a correct gripping position. These measures can be as follows: • Precise alignment and stopping of workpiece while contact is established by the gripper. The aim of this is to prevent slippage during pick-up. • Fitting of internal positioning devices • Attachment of external positioning aids (Fig. 10-9). We accordingly use centring aids, insertion guides and support stops. In the case of electronic components for fitting to PCBs, which require gripping from above, it can improve accuracy to grip the components a second time after they have been aligned mechanically.

94


Fig. 10-9: Positioning aids and stops for use with vacuum grippers a) Fine positioning during gripper contact b) Stop to reduce displace ment during lateral motion c) Alignment by springloaded stops before gripper contact 1 2 3 4 5 6 7 8 9 10 11

Centring mandrel Spring Suction cup Workpiece Magazine feed Arm Support Vacuum connection Roller conveyor Tapered guide Spring-loaded guide wedge

Workpiece-controlled activation of suction air

1

v

2 3

6 4

5 7

a)

b) 8 10 11

9

c)

The needless discharge of suction air means a waste of energy and usually also indicates a gripper malfunction. Attempts are therefore made to equip suction cups in such a way that they activate vacuum generation only when they reach the workpiece surface. There is a further problem: When several suction cups are used in an array, it may occur that not all the suction cups are covered by the workpiece, for example during the handling of packages of varying sizes in positions which are not always precisely defined. Any suction cups which remain uncovered must be deactivated in order to prevent the vacuum from collapsing. The basic concept is illustrated in Fig. 10-10.

Fig. 10-10: Automatic deactivation of uncovered suction cups 1 2 3 4 5

1 2

Vacuum Basic body Airborne ball bearing Suction cup Workpiece

3 4 5


95

The flow of incoming air closes the uncovered suction cup. Flow-activated valves, triggered by flow losses, can also be used with permeable surfaces such as perforated plates to deactivate any uncovered suction device. Valves of this kind are commercially available and are generally equipped with a filter to prevent dirt particles from entering the vacuum circuit. In the case of the Festo vacuum efficiency valve ISV-..., a spring-loaded “float” is used as a shut-off device. Provided that a vacuum starts to build up under the suction cup, a ring seal will open and release the required suction cross-section. Fig. 10-11 shows an application in the form of a circuit diagram. Fig. 10-11: Circuit diagram for a suction head fitted with vacuum efficiency valves

1 2

1 2 3 4

Vacuum generator Exhaust air Distributor Vacuum efficiency valve with filter 5 Flat suction cup

3

4

5

It also takes some organisation to activate a suction cup at the right moment. Sensor valves are often used for this purpose, as are proximity sensors. Fig. 10-12 shows a number of variants. Sensor valves are installed in the vacuum line and act directly on this, while external sensors supply an electrical signal which is used to activate directional control valves. With the solution shown in Fig. 10-12b, the handling device travels towards a stack whose height is of course constantly changing. Once the workpiece has been reached (topmost sheet), the lowering motion is stopped and the vacuum is activated. In the case of Fig. 10-12d, a “positive signal” triggers the vacuum by actuating a directional control valve.

96


Fig. 10-12: Activating the vacuum

2

1 5

a) Integrated electrical sensor b) External electrical sensor c) Vacuum sensor valve in suction line d) Integrated inductive sensor 1 2 3 4 5 6 7 8 9 10

Vacuum connection Electrical sensor Sealing lip Stack of workpieces Limit switch Flat suction cup Sensor valve Suction bore Inductive sensor Workpiece held by vacuum

Ejector systems

6 3 4

b)

a)

8 7

9

10

d)

c)

Rapid ejection of workpieces from suction cups is just as important for fast machining cycles as a fast pick-up. There are various ways of achieving this. If a vacuum generator is used to produce the vacuum, it has become standard practice to fill a small reservoir with compressed air during vacuum generation. When the compressed-air supply to the generator is switched off, vacuum generation ceases and at the same time the compressed-air reservoir discharges abruptly. This creates a positive pressure in the suction chamber, ejecting the workpiece from the suction cup (Fig. 10-13).

Fig. 10-13: Circuit diagram for a vacuum generator with an ejector pulse system p Input pressure V Vacuum

p

V

If it is necessary to pressurise long supply lines from a vacuum pump to the suction cup, the ejection process of course takes much longer. Here, too, however, it is possible to provide a “short-circuit” to atmosphere. This is shown in Fig. 10-14. As the suction cup contacts and holds the workpiece, the head retracts. Once the air suction is switched off, it only requires a slight fall in pressure for the spring to cause the head to advance. This exposes the bypass 10 Suction cups for every application

97

hole, which accelerates the creation of a pressure equilibrium. The speed of response when triggered depends on the size of the bypass cross-section in the valve or any other equivalent opening leading to atmosphere. These openings should therefore be as large as possible. Fig. 10-14: Vacuum head with “shortcircuit” hole to atmosphere

It is of course also possible to switch straight from suction air to compressed air, and this is done in practice. An example of this is shown in our last illustration, Fig. 10-15. By the way, precise ejection at the desired point is particularly important with fragile or very light workpieces, since these could otherwise stick to the suction cup momentarily and then fall at a greater height from the handling device during its return stroke and possibly suffer damage. Fig. 10-15: Circuit diagram for a suction gripper with a vacuum generator and compressedair ejector system p Supply pressure V Vacuum

p

V

Publications

98

[1] Braun, D.: Industrieroboter - Auslegung von pneumatischen Flächengreifern (“Robots industriales: Dimensionado de ventosas de sujeción neumáticas”), publicado por Verlag T+V Rheinland, Cologne 1989 [2] Tella, R.; Birk, J; Kelley, R.: Una ventosa de vacío adaptada al contorno, 10º Simposio Internacional de Robots Industriales, Tagungsband, Milán 1980


11 Suction cups in handling technology

Exploiting the properties of suction cups

The range of applications of standard and special suction cups is very wide, covering everything from sanitary porcelain and strips, planks and panels to foodstuffs. The processes involved generally form part of a medium or large scale production operation. Suppliers of suction cups usually offer a complete selection of individual units and modular systems. Apart from suction cups themselves, systems of this kind include vacuum generators, valves, tubing and piping, instrumentation, control equipment and flexible mounting systems. We will consider some selected examples of applications below.

It is always surprising to see how suction-cup applications can be modified. This will be demonstrated by bellows suction cups and the examples shown in Fig. 11-1. It is, for example, possible to exploit the angular flexibility of suction cups with inclined workpiece surfaces or alternatively to work with different levels of vacuum to produce bowing of thin sheet-metal workpieces. It is also possible to bring an inclined workpiece surface into a horizontal position as it is picked up by providing leveling stops on the gripper. Conversely, workpieces which are normally straight can be inclined if this is required in packing or magazining operations.

Fig. 11-1: Some typical applications of bellows suction cups 1 Picking up inclined workpieces 2 Leveling of an inclined formed part 3 Limiting suction force 4 Highly-flexible suspension for undulating workpieces in random orientation 5 Picking up stepped workpieces 6 Gripping formed parts with undulating surface 7 Picking up flat workpieces from magazine 8 Separating and picking up flat workpieces

2

1

3

4

5 p1

p1

p2

p2

p1 > p2 6

7

8

The last example concerns a situation which is frequently encountered and which we will now study more closely.


99

Feeding of thin panels and sheet-metal material

It is not easy to grip thin sheet-metal material, since sheets can stick to a stack over their entire area and burred edges can become tangled. Plastic panels may also stick together due to electrostatic charges. It is therefore necessary to prevent two workpieces from being picked up at the same time. Fig. 11-2 shows a number of ways of achieving this [1].

Fig. 11-2: Picking up thin panels with a suction cup

1 2

a) Undulating effect b) Air nozzle to assist separation 1 2 3 4

Suction cup Blast nozzle Indexing motion Lifting device to raise stack

3 4 a)

b)

In Fig. 11-2a, the suction cups do not merely lift the panel but first generate undulations along the length of the panel to detach any second panel which may be adhering to the underside of the first. Only then is the panel lifted. Each lifting cylinder must accordingly be controlled separately. “Peeling” effects can also be produced by combining spring-loaded suc-tion cups with a non-springloaded suction cup at the edge of the panel. In this case, the panel is lifted at the edge while still being held down at other points by the spring force of the suction cups. It is also possible to equip flat suction cups with a separator insert, which is a fixed support within the suction chamber. When vacuum is present, the panel picked up by the suction cup bends slightly around the separator insert, due to the upward motion of the soft suction-cup lips. This effect can be exploited with thin sheet metal up to approx. 3 mm. When panels are picked up which are slightly porous, such as chipboard, a “through-suction” effect may be encountered, also resulting in two panels being lifted from a stack at the same time. The remedy here is to increase the suction area (by using more suction cups) and reduce the vacuum level. Problems may also be experienced with magnetic grippers when picking up thin sheet-metal workpieces, since field lines may pass right through the first workpiece and pick up a second workpiece as well. In order to solve this problem, combination grippers have been developed, as shown in Fig. 11-3. The workpiece is first gripped by a suction cup and slightly lifted by this.

100


The magnet is now activated, significantly increasing the holding force. This higher force allows high-speed manipulations to be carried out. Fig. 11-3: Combination gripper for handling thin ferritic sheet-metal workpieces

V 1

1 Electromagnetic coil 2 Suction-cup lip made of soft cellular rubber

2

V Vacuum

200 mm

Double lifting of small blanks can be rectified by using a second suction cup to remove the second workpiece, which is held less firmly than the first workpiece and can thus be “vacuumed” away and set down at another place. The first gripper can swivel away from the second suction cup or else rotate, as shown in Fig. 11-4. The two grippers rotate synchro-nously in opposition. The suction forces are adjusted to different levels. A second workpiece is picked up along with the first, carried along and then ejected into a set-down tray. Fig. 11-4: “Vacuuming” second workpieces away with a rotating suction cup 1 Blanks magazine 2 Suction cup for second workpiece 3 Set-down tray for second workpieces 4 Rotary gripper 5 Feed device for production machine

1

2 3

4 5

A further combination of physical effects is shown in Fig. 11-5. It can be used only with ferromagnetic workpieces, since it utilizes “spreader magnets” at the sides of the stack of metal sheets. The magnetic fields of these produce repulsion forces within the stack which cause the top sheets to float (peel away). This reduces the risk of two sheets being picked up at once and allows the suction cup to make gentler contact with the workpiece. The number, 11 Suction cups in handling technology

101

arrangement and cross-section of these permanent magnets is governed by the thickness and size of the sheet-metal workpieces. In the case of metal strips, it is sufficient to have a spreader magnet positioned at the ends where the suction cup is applied to the strip. Fig. 11-5: Workpieces removed from stack using a suction cup and spreader magnets

2 mm

1 1 Suction cup 2 “Floating” sheet at top of stack 3 Permanent magnet 4 Stack of sheet-metal work pieces

2 3

4

Here is another feed solution based on suction cups. In duplication systems, the device shown in Fig. 11-6 is used to prevent double pick-ups. The magazine outlet features a width restrictor which causes workpieces to become bowed as they are removed from the magazine. Any second workpiece adhering to the first is detached and remains in the magazine. Fig. 11-6: Forced bowing of thin blanks at magazine outlet prevents double pick-ups

1 1 2 3 4

Vertical magazine Toothed insert Spring-loaded ratchet Suction cup

2

3 4

102


The bowing effect is used in many other adapter feed devices. The feed station shown in Fig. 11-7 also features a suction cup which holds the sheet-metal workpieces at their centre. The dead weight of the workpieces causes these to sag. After the sagging phase during lifting, the sheet-metal workpiece springs into the pick-up roller slot. The rollers grip the metal workpiece and convey it outwards, whilst the suction cup simultaneously detaches from the workpiece and returns to its “home” position. Fig. 11-7: Feeding station for thin sheet metal 1 2 3 4 5

1

Lifting cylinder Suction Pick-up rollers Contact point Workpiece stack

2

3

4

5


103

Distribution and feeding with suction cups

The number of applications which fall under this heading is colossal. We can therefore do no more than show just a few examples, which may prove useful for your own applications. As is well known, suction cups with soft lips do not operate very accurately. Furthermore, the workpiece in question may be displaced when the suction cup “springs” into action if it is not specially guided. This is a disadvantage, but one which does not become apparent in many applications, since the workpiece is precisely centered by other technical means at its destination, particularly in the case of feed operations. This can be seen in the example of a tub filling and closing machine shown in Fig 11-8. A tub released by a distributor device is picked up by vacuum and set down on a rotary table. The suction cup passes through the workpiece carrier on the table in order to do this. The tub aligns itself precisely on the table. It would in theory also be possible to allow the workpieces to fall into their carriers by gravity, but this would not provide sufficient reliability for automated operation. If workpieces are allowed to move at random even for a short time, there can be no guarantee of accurate movement. Even if the result is acceptable 99 times out of 100, this is not good enough.

Fig. 11-8: Feed system on a packing machine 1 2 3 4 5 6 7

Vertical magazine Distributor Suction cup Rotary indexing table Lifting cylinder Tub (workpiece) Production machine

1 2

3 4 6 5 7

Fig. 11-9 shows the feeding of food containers on a filling line. Retaining brushes on the magazine ensure that only one container is removed at a time. The gripper arm carries a suction cup and is able to reach down between the two conveyor belts and set down the deep-drawn foil container. Only when this has been transported onwards can the arm swivel back into its pick-up position. A rotary pneumatic cylinder can be used as a drive or, as shown here, a rackand-pinion gear unit with a linear cylinder.

104


Fig. 11-9: Feeding of food containers 1 2 3 4 5 6 7 8 9 10 11

1

Retaining brush Stack of containers Magazine rod Spindle-driven lift ram Limit switch to monitor feed Double conveyor belt Lateral guide Suction-air line Gripper arm drive Swivel arm Suction cup

2

3 4 11 10

5 6

7

8 9

Suction cups are also frequently used in combination with other grippers to provide auxiliary functions. This is illustrated in the example, Fig. 11-10 showing the stacking of spools of textile thread. These spools are gripped internally by a mandrel gripper and placed on a pallet. The auxiliary function to be provided by the handling device, is to insert a separator board between each layer. The device picks up the separators from another stack via suction cups which are advanced into their working position specifically for this operation. In this way, it is possible to operate without a gripper-changing system. Combinations of this kind can be produced relatively easily by using pneumatic cylinders with hollow piston rods. Fig. 11-10: Multi-layer stacking of textile thread spools, using a combination gripper 1 2 3 4 5 6 7 8

Gripper connection Lifting cylinder Hollow piston rod Separator Mandrel gripper Gripper thread spool First stack layer Transport pallet

1

2

5

3 4 6 7 8


105

Fig. 11-11 shows a glass bulb feeder system. The bulbs enter the feed conveyor by gravity and are transferred to a pick-up position. These workpieces, which are fragile and curved on all sides, can be reliably picked up via suction gripper and fed into the machine in a 2-second cycle. If it is not possible to align light workpieces sufficiently well by gravity, it may be necessary to provide an aid. This can take the form of mechanically-operated V-shaped jaws at the pick-up station. A mechanical radial gripper can provide a suitable basic module for a device of this kind. Fig. 11-11: Feeding station for glass bulbs 1 Swivel magazine 2 Handling device with suction cup 3 Forked carrier in feed chain 4 Glass bulbs 5 Filling area 6 Magazine rail

5 1 6

2

3 4

Publications

[1] Hesse, S.: Atlas der modernen Handhabungstechnik (“Atlas Of Modern Handling Technology”), published in German by Vieweg Verlag, Wiesbaden 1995

106


7

8

Fig. 1-1: 12 List of illustrations

Fig. 1-2: Fig. 1-3: Fig. 1-4: Fig. 1-5: Fig. 1-6: Fig. 1-7: Fig. 1-8: Fig. 2-1: Fig. 2-2: Fig. 2-3: Fig. 2-4: Fig. 2-5: Fig. 2-6: Fig. 2-7: Fig. 2-8: Fig. 3-1: Fig. 3-2: Fig. 3-3: Fig. 3-4: Fig. 3-5: Fig. 3-6: Fig. 3-7: Fig. 3-8: Fig. 3-9: Fig. 3-10: Fig. 4-1: Fig. 4-2: Fig. 4-3: Fig. 4-4: Fig. 4-5: Fig. 4-6: Fig. 4-7:

Division of a workpiece into gripping zone (G), clamping zone (S) and set-down zone (A) . . . . . . . . . . . . . . . . . . . . 10 How can a workpiece be picked up? . . . . . . . . . . . . . . . . . . . . . . . . . 10 The right choice of gripping point can affect the positioning error during assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Feeding a clamping device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Types of point loading resulting from gripping. . . . . . . . . . . . . . . . . 13 Unambiguous pick-up points ensure reliable gripping . . . . . . . . . . 14 Gripper devices which close in an arc may cause a shift of the gripping centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Centre deviation resulting from workpiece form errors . . . . . . . . . . 15 Using a 3-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Multi-point gripper for long workpieces . . . . . . . . . . . . . . . . . . . . . . 17 Multi- workpiece gripper for transfer of complete rows of workpieces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Multi-workpiece gripper for assembly operations . . . . . . . . . . . . . . 19 Multiple suction cup grippers for assembly operations . . . . . . . . . . 20 Methods of holding a workpiece (example: ball bearing) . . . . . . . . 21 Gripping principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Gate feeder using a parallel jaw gripper. . . . . . . . . . . . . . . . . . . . . . 23 3-Point gripper combined with swivel/linear unit . . . . . . . . . . . . . . 24 The human hand can execute motions with 6 degrees of freedom (according to Bejczy) . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Double gripper designed as crown turret . . . . . . . . . . . . . . . . . . . . . 25 Shaft gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Handling module for assembly of small workpieces . . . . . . . . . . . . 27 Handling unit with suction cup and semi-rotary actuator . . . . . . . . 28 Inverting workpieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Tripple gripper installed on a special machine with double stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Picking up ferromagnetic sheets from a stack using a suction cup/lifting module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Simple specimen shaker made from standard components . . . . . . 31 Some of the subsystems of a mechanical gripper . . . . . . . . . . . . . . 33 The contour at the gripping point of the workpiece determines the jaw shape used 1, 2 or 3 . . . . . . . . . . . . . . . . . . . . . 33 Gripper jaws with compliant surfaces. . . . . . . . . . . . . . . . . . . . . . . . 34 Jaw shape with centring effect for scissor tong grippers . . . . . . . . . 35 Gripping several workpieces simultaneously using a pressure distributor to compensate for tolerances . . . . . . . 36 Jaws of a parallel gripper for 3 diameter ranges. . . . . . . . . . . . . . . . 37 Gripper jaws with specially shaped multiple gripping surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

12 List of illustrations

107

Fig. 4-8: Fig. 4-9: Fig. 4-10: Fig. 5-1: Fig. 5-2: Fig. 5-3: Fig. 5-4: Fig. 5-5: Fig. 5-6: Fig. 5-7: Fig. 5-8: Fig. 5-9: Fig. 5-10: Fig. 6-1: Fig. 6-2: Fig. 6-3: Fig. 6-4: Fig. 6-5: Fig. 6-6: Fig. 6-7: Fig. 6-8: Fig. 6-9:

Fig. 6-10: Fig. 7-1: Fig. 7-2: Fig. 7-3: Fig. 7-4: Fig. 7-5: Fig. 7-6: Fig. 7-7: Fig. 7-8: Fig. 7-9: Fig. 7-10:

108

Variants of jaws for parallel grippers . . . . . . . . . . . . . . . . . . . . . . . . 38 Mobile gripper jaws lift the workpiece out of the V-shaped recess in the magazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 The type of approach affects the required opening . . . . . . . . . . . . . 40 The law of interacting forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Forces acting on gripped objects (state of rest) . . . . . . . . . . . . . . . . 42 Plan view of 2 gripper situations . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Calculation of contact forces for a gripper with a V-jaw on one side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Forces at the parallel jaw gripper with V-jaw for workpieces . . . . . . 45 Force situations during gripper motion . . . . . . . . . . . . . . . . . . . . . . 46 Example of a handling task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Examples of torque created as a result of a gripped workpiece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Eccentric forces acting on a gripper finger . . . . . . . . . . . . . . . . . . . . 50 Gripping force FG as a function of gripper stroke h . . . . . . . . . . . . . 51 Characteristic data for grippers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 General model of a handling operation . . . . . . . . . . . . . . . . . . . . . . 54 A joining mechanism with combined lateral and angular compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Situation, in which an overload of the gripper and handling device can occur if no compensation is provided . . . . . . . 56 Picking up a workpiece from a magazine pallet . . . . . . . . . . . . . . . . 56 Gripper combined with a pressing element . . . . . . . . . . . . . . . . . . . 58 Simple joining mechanism for vertical assembly . . . . . . . . . . . . . . . 58 Collision protection with adjustable parameters for a gripper showing reaction capability . . . . . . . . . . . . . . . . . . . . . 59 When working with three-dimensional object configurations, consideration must be given to the clearance contour of the gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Specifications of load capacity for industrial robots . . . . . . . . . . . . 61 Approximate correlation between gripped objects and gripper types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Radial gripper holding a sheet metal workpiece . . . . . . . . . . . . . . . 64 IC’s have splayed pins, which are aligned to the desired spacing during the gripper motion . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Example of twin workpiece gripper as a special use of a parallel jaw gripper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Handling a length of bar material with a 3-point gripper . . . . . . . . . 65 Gripping a rectangular workpiece . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 A 4-point gripper created by combining 2-point grippers . . . . . . . . 67 Gripper module for flexible assembly . . . . . . . . . . . . . . . . . . . . . . . . 68 Assembly gripper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Sequence for assembly within a gripper . . . . . . . . . . . . . . . . . . . . . 69


Fig. 8-1: Fig. 8-2: Fig. 8-3: Fig. 9-1: Fig. 9-2: Fig. 9-3: Fig. 9-4: Fig. 9-5: Fig. 9-6: Fig. 9-7: Fig. 9-8: Fig. 9-9: Fig. 9-10:

Fig. 10-1: Fig. 10-2: Fig. 10-3: Fig. 10-4:

Fig. 10-5: Fig. 10-6: Fig. 10-7: Fig. 10-8: Fig. 10-9: Fig. 10-10: Fig. 10-11: Fig. 10-12: Fig. 10-13: Fig. 10-14: Fig. 10-15:

Mutually influential factors and basic variables relating to the selection of grippers from the technical point of view. . . . . . 71 Generation of characteristic gripper data starting with the gripper location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 The main steps in the selection of grippers . . . . . . . . . . . . . . . . . . . 74 The most important functions and properties in relation to suction grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Force conditions with vertically moving suction cup . . . . . . . . . . . . 79 Typical force situations at suction cup . . . . . . . . . . . . . . . . . . . . . . . 80 Methods of producing a vacuum. . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Comparison of performance of typical vacuum generators . . . . . . . 83 Mode of operation of Venturi nozzle with shut-off valve to produce ejector effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Vacuum generators in a series circuit . . . . . . . . . . . . . . . . . . . . . . . . 84 Solenoid actuated suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Example of a vacuum circuit based on an ejector . . . . . . . . . . . . . . 85 Correct choice of tubing diameter is important in the distribution of suction air . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 A small selection of the major types of suction cups . . . . . . . . . . . . 89 Evaluation of various types of suction cups . . . . . . . . . . . . . . . . . . . 90 Freedom of movement of suction cups . . . . . . . . . . . . . . . . . . . . . . . 91 Suction cup array fitted to rods allowing longitudinal movement and used to pick up workpieces of constantly varying contours. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Deep suction cups can adapt well to curved surfaces . . . . . . . . . . . 92 Vacuum gripper with very large freedom of movement of suction components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Standard suction cups are not very suitable for use with thin foil material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Low pressure gripper equipped with porous plastic or perforated plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Positioning aids and stops for use with vacuum grippers . . . . . . . . 95 Automatic deactivation of uncovered suction cups . . . . . . . . . . . . . 95 Circuit diagram for a suction head fitted with vacuum efficiency valves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Activating a vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Circuit diagram for a vacuum generator with an ejector pulse system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Vacuum head with “short-circuit” hole to atmosphere . . . . . . . . . . 98 Circuit diagram for a suction gripper with a vacuum generator and compressed air ejector system . . . . . . . . . . . . . . . . . 98


109

Fig. 11-1: Fig. 11-2: Fig. 11-3:

Some typical applications of bellows suction cups . . . . . . . . . . . . . 99 Picking up thin panels with a suction cup . . . . . . . . . . . . . . . . . . . 100 Combination gripper for handling thin ferritic sheet metal workpieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Fig. 11-4: “Vacuuming” second workpieces away with a rotating suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Fig. 11-5: Workpieces removed from stack using a suction cup and spreader magnets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Fig. 11-6: Forced bowing of think blanks at magazine outlet prevents double pick-ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Fig. 11-7: Feeding station for thin sheet metal . . . . . . . . . . . . . . . . . . . . . . . . 103 Fig. 11-8: Feed system on a packing machine . . . . . . . . . . . . . . . . . . . . . . . . 104 Fig. 11-9: Feeding of food containers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Fig. 11-10: Multi-layer stacking of textile thread spools using a combination gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Fig. 11-11: Feeding station for glass bulbs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

110


2-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3-dimensional axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

13 List of special terms

A

Accuracy of gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Adapter rail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Adhesive suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Alignment effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Angle gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36, 51 Angular compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Application area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Assembly gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Assembly mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Axial alignment error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Axis gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

B

Ball-and-socket head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Bellows suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 99

C

Centre deviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Centring aid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Changing system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Characteristic curve for gripping force . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Characteristic data for grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Checklist for grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Clamping force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Clamping marks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Clamping safety margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Clamping zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Clearance contour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Coefficient of friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 80 Collision-protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 Combination gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38, 66, 100, 105 Compensate for tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Compliance device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Contact force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 43 Contact sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Crown turret . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Cushioned stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

D

Deceleration force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Deep suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 92 Degrees of freedom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Degrees of transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Diameter of the interference circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Distribution with suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Double gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Double-lip suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Duplex machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


111

112

E

Eccentric force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Ejector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Ejector system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 External gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

F

Feed channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60, 65 Feed gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Feeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Feeding station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103, 106 Final effector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Force conditions with suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Force-locking connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Friction force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Friction locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

G

Geometrical error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Gripped object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Gripper finger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Gripper fingers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Gripper jaws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 37 Gripper module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Gripper pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Gripper selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Gripper selection tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76 Gripper system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66 Gripper types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Gripper working area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Gripping centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Gripping force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41, 43 Gripping point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 17 Gripping stroke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Gripping surface pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Gripping zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 72 Guide wedge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

H

Hand axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 27 Hand-joint sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Handling module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Handling of sheet metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Handling operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 High point loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

I

Inertia force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Interference circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Internal gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 IRCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

J

Jaw shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Jaw-type gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

K

Knee-lever gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51


L

Lateral compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Lifting suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Load-bearing capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Low-pressure gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

M

Magazine pallet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Maximum load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Moulding jaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Multiple suction-cup gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Multi-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Multi-stage ejector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Multi-workpiece gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

N

Nominal load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Non-slip covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Normal force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

O

Opening safety margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Oval suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Overload of gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

P

Parallel-jaw gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 23, 26 Peak-to-valley height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Pendulum jaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Pick-and-place device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Piston suction system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Placing error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Planned gripper application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Plastic covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Plug gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Point loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Position compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Positioning aid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Positioning error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 14 Positive-locking connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Positive-locking gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Pressure device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Pressure distributor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Pressure per unit area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Pressure plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Pulse effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Q

Quick exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

R

Radial gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 62, 64 RCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 Repetition accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Ribbed suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Rubber covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Running freedom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

S

Safety factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Scissors-type gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14


113

Securing function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Self-adhering suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Sensor, inductive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Sequence gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Service life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Set-down zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Shaft gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Short-stroke axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Shut-off device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Single-stage ejector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Specimen shaker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Spreader magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Spring-loaded gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Standard gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Stepped track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77, 96 Suction-cup array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Suction-cup shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Suction-cup/lifting module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Swivel/linear unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

114

T

TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Third law of motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Three-finger gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Three-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Thruster device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Tool centre point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 35 Triple turret gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Turning workpieces over . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Turret gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Twin-workpiece gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Two-finger gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 43 Type of approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

U

Universal jaws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Uses of grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

V

Vacuum blower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Vacuum circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Vacuum efficiency valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Vacuum generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Vacuum management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Vacuum switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Vacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Vacuum pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Venture-type vacuum generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Venturi nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Vertical magazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Vice-type gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 V-jaw gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

W

Wide-range gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14


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