Mechatronics
“If you cannot be a star in the Sky, at least be a lamp in your Home… !” By:- Swamy Vivekananda
Dr.N.V. Raghavendra
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Mechatronics
Prepared by: Dr. N.V.Raghavendra Dept. of Mechanical Engineering National Institute of Engineering, Mysore.
MECHATRONICS UNIT 1
Dr.N.V. Raghavendra
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Mechatronics
MECHATRONICS
The integration of electronic engineering, electrical engineering, computer technology and control engineering with mechanical engineering is increasingly forming a crucial part in the design, manufacture and maintenance of a wide range of engineering products and processes. The term mechatronics
describes this integrated approach.
Why Mechatronics ? • In recent years, the application of micro-electronics and computers in the design and manufacturing sector has significantly improved functionality, quality and productivity of mechanical products • Integrated embedded technology has become integral part of automation • Automation and control represent a broad area with diverse applications, such as, manufacturing processes and equipments, process control, robotics, home automation, office automation, and so on. • Mechatronics has enabled high level of flexibility and sophistication in products and processes.
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Multidisciplinary Scenario
Multidisciplinary Scenario • In figure 1, one can distinguish between the traditional and current curriculum scenario which are separated by an axis • The figure shows how the traditional electrical and mechanical disciplines have given birth to new disciplines, which further encouraged many other branches to emerge • The engineering disciplines are now converging rather than diverging, because of requirements of inter-disciplinary knowledge • The engineering filed is being radically altered with the advent of – digital technology, low cost VLSI chips, embedded technology, control networking systems (filedbus technology), microcontrollers, advanced software tools (CAD/CAM, OO-based, artificial neural networks, fuzzy logic, etc), and so on.
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Multidisciplinary Scenario • Advanced technological designs are highly complex and of inter-disciplinary nature involving synergetic integration of mechatronics, photonics, computronics and communication • Studies have shown that the productivity of an industry can increase upto 40% by employing engineers with interdisciplinary skills
• Some typical mechatronics platforms: space shuttles, air crafts, industrial machines, automobiles, robots, material transfer equipments, etc
Origin of Mechatronics • The term mechatronics originated in Japan in the late 1970s to describe design of electro-mechanical products • The field has been driven in recent times by rapid progress in the field of microelectronics • Major areas where rapid developments are taking place are: • Motion control • Robotics • Automotive systems • Intelligent control • Actuators and sensors • Modeling and design • System integration • Manufacturing • Micro devices and optoelectronics • Vibrations and noise control
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Evolution of Mechatronics
• Mechatronics has evolved through four stages during its development to the present state:
1. Primary level mechatronics 2. Secondary level mechatronics 3. Tertiary level mechatronics 4. Quaternary level mechatronics
Evolution of Mechatronics Primary level mechatronics • This level encompasses input/output (I/O) devices such as sensors and actuators that integrate electrical signaling with mechanical action at the basic control level • Electrically controlled fluid valves and relay switches are two examples Secondary level mechatronics • Integrates microelectronics into electrically controlled devices • Sometimes, these products are stand-alone • Example: a cassette tape player
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Evolution of Mechatronics Tertiary level mechatronics • The mechatronic systems at this level are called smart systems • The control strategy uses microelectronics, microprocessors, and other application specific integrated circuits as bits and pieces for control realisation • A microprocessor based electrical motor used for actuation purpose in industrial robots is an example of such systems
Evolution of Mechatronics
Quaternary level mechatronics • This level attempts to improve smartness a step ahead by introducing intelligence and FDI (fault detection and isolation) capability into the systems • Artificial neural network and fuzzy logic try to capture some of the intellectual capabilities of the intelligence
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Scope of Mechatronics
Integrated design issues in Mechatronics • Mechatronics is a design philosophy and an integrated approach to engineering design • An important characteristic of mechatronic devices and systems is their built-in intelligence, which results through a combination of precision mechanical and electrical and real-time programming integrated with the design process • The integration within a mechatronic system is performed through the combination of hardware and software. • Hardware integration results from designing the mechatronic system as an overall system and bringing together the sensors, actuators, and microcomputers into the mechanical system.
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Integrated design issues in Mechatronics • Software integration is primarily based on advanced control functions • Figure below illustrates how the hardware and software integration takes place
Mechatronics Design Process • Product design has inherent complexity due to the multidisciplinary nature of the design process • The Mechatronic design approach applies concurrent engineering concepts instead of the traditional sequential approach • The mechatronic design process consists of three phases: a) modeling and simulation, b) prototyping and c) deployment • Because of their modularity, mechatronic systems are well suited for applications that require reconfiguration
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Mechatronics Design Process
Mechatronics Design Process Hardware-in-the-loop simulation
• In the prototyping step, many of the noncomputer subsystems of the model are replaced with actual hardware • Sensors and actuators are also put in their respective places • The resulting model is part mathematical and part real • This process of fusing and synchronising model, sensor, and actuator information is called real-time interfacing or hardwarein-the-loop simulation
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Advanced Approaches in Mechatronics • Recent developments in Mechatronics are creating opportunities in intelligent manufacturing
• Sensor-based manufacturing systems are becoming order of the day • The new approach is towards the design of intelligent autonomous inspection systems as well as intelligent decision making systems that perform tasks automatically, without human intervantion • Mechatronic technology used in manufacturing will impact new equipment as well as some retrofit applications
Advanced Approaches in Mechatronics
Intelligent Supervisory Control Structure
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Advanced Approaches in Mechatronics
Model based Monitoring System
Advanced Approaches in Mechatronics
Mechatronic System with Open Architecture Platform
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Advanced Approaches in Mechatronics Major advanced mechatronic application: • Autonomous production cells with image-based object recognition
• Integrated supervisory systems with multi-process control capability and shared databases from CAD drawings • FMS with off and on-line programming • Bio-robotics • Endoscopic and orthopedic surgery • Magnetically levitated vehicles • Robotics in nuclear and space applications
Sensors and Transducers The term sensor is used for an element which produces a signal relating to the quantity being measured. For example, in an electrical resistance temperature element, the quantity being measured is temperature and the sensor transforms an input of temperature into a change in resistance.
The term transducer is often used in place of the term sensor. Transducers are defined as elements that when subject to some physical
change experience a related change. Transducers also convert signals in one form into another.
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Performance Terminology 1. Range and span 2. Error 3. Accuracy 4. Sensitivity 5. Hysteresis error 6. Non-linearity error 7. Repeatability 8. Stability 9. Dead band time 10.Resolution 11.Output impedance
Performance Terminology 1. Range and span: • The range of a transducer defines the limits between which the inputs can vary • The span is the maximum value of the input minus the minimum value • A load cell for the measurement of forces might have a range of 0 to 50 KN and a span of 50 KN 2. Error: • Error = measured value – true value 3. Accuracy • It is the extent to which the value indicated by a measurement system might be wrong • It is thus the summation of all the possible errors that are likely to occur, as well as the accuracy to which the transducer has been calibrated
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Performance Terminology 4. Sensitivity:
• It is the relationship indicating how much output you get per unit input • For example, a resistance thermometer may have a sensitivity of 0.5 ohms/0 C • This term is also frequently used to indicate the sensitivity to inputs other than that being measured, i.e., environmental changes, such as temperature changes in the environment 5. Hysteresis error:
Transducers can give different outputs from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or continuously decreasing change. This effect is called hysteresis.
Performance Terminology 6. Non-linearity error: • For many transducers a linear relationship between the input and output is assumed over the working range, i.e., a graph of output plotted against input is assumed to give a straight line.
• Few transducers however, have a truly linear relationship and therefore, errors occur as a result of the assumption of linearity. • The error is defined as the maximum difference from the straight line.
• Various methods are used for the numerical expression of the nonlinearity error.
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Performance Terminology
a) Using end-range values
b) Best straight line for all values
Performance Terminology
• The error is generally quoted as a percentage of the full range output • For example, a transducer for the measurement of pressure might be quoted as having a non-linearity error of ± 0.5% of the full range
c) Best straight line through Zero point
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Performance Terminology 7. Repeatability / reproducibility: • Ability of a transducer to give the same output for repeated applications of the same input value • Repeatability = (max. – min. values given) X 100 full range
• For example, a transducer measuring angular velocity can be said to have a repeatability of ± 0.1% of the full range at a particular angular velocity
Performance Terminology 8. Stability: • It is the ability to give the same output when used to measure a constant input over a period of time • The term drift is often used to describe the change in output that occurs over time • The drift may be expressed as a percentage of the full range output 9.
Dr.N.V. Raghavendra
Dead band time: • It is the range of input values for which there is no output • The dead band time is the length of time from the application of an input until the output begins to respond and change • For example, bearing friction in a flow meter using a rotor might mean that there is no output till input has reached a particular velocity threshold
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Performance Terminology 10.Resolution: • The resolution is the smallest change in the input value that will produce an observable change in the output • For a wire-wound potentiometer the resolution might be specified as, say, 0.50 11 Output impedence: • When a sensor giving an electrical output is interfaced with an electronic circuit it is necessary to know the output impedence since this impedence is being connected either in series or parallel with that circuit • The inclusion of the sensor can thus significantly modify the behaviour of the system to which it is connected
Static and Dynamic Characteristics
• Static characteristics are the values given when steady-state conditions occur, i.e., when the transducer has settled down after having received some input • Dynamic characteristics refer to the behaviour between the time that the input value changes and the time that the value given by the transducer settles down to the steady-state value • Dynamic characteristics are stated in terms of the response of the transducer to inputs in particular forms, such as step input, ramp input or sinusoidal input
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Static and Dynamic Characteristics 1. Response time: This is the time which elapses after a constant input, is applied to the transducer up to the point at which the transducer gives an output corresponding to some specified percentage, e.g., 95% of the value of input. 2. Time constant: This is the 63.2 % response time. The time constant is a measure of the inertia of the sensor and so how fast it will react to changes in its input; the bigger the time constant slower will be its reaction to a changing input signal.
Static and Dynamic Characteristics
3. Rise time: This is the time taken for the output to rise to some specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value 4. Settling time: This is the time taken for the output to settle to within some percentage, e.g., 2% of the steady-state value
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Mechatronics
Static and Dynamic Characteristics 4. Settling time: Consider the following data which indicates how a thermometer reading changed with time.
The steady-state value is 550 C and therefore, 95% of 55 is 52.250 C, the 95% response time is about 228 secs.
Displacement, Position and Proximity
1. Displacement sensors are concerned with the measurement of the amount by which some object has been moved 2. Position sensors are concerned with the determination of the position of some object with reference to some reference point
3. Proximity sensors are a form of position sensor and are used to determine when an object has moved to within some distance of the sensor. They are essentially devices which give on-off outputs
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Displacement, Position and Proximity Sensors
Considerations for selection: • The size of the displacement, or how close the object is before it is detected • Whether the displacement is linear or angular • The resolution required
• The accuracy required • What material the measured object is made up of • Contact or non-contact type • The cost
Potentiometer Sensor
It consists of a resistance element with a sliding contact which can be moved over the length of the element Such elements can be used for linear or rotary displacements, the displacement being converted into a potential difference The rotary potentiometer consists of a circular wire-wound track or a film of conductive plastic over which a rotatable sliding contact can be rotated
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Potentiometer Sensor With a constant input voltage Vs between terminals 1 and 3, the output voltage V0 between terminals 2 and 3 is a fraction of the input voltage This fraction depends on the ratio of the resistance R23 between terminals 2 and 3 compared with the total resistance R13 between terminals 1 and 3 V0/Vs = R23/R13 If the track has a constant resistance per unit length, i.e., per unit angle, then the output is proportional to the angle through which the slider has rotated. Hence, angular displacement can be converted into a potential difference
Potentiometer Sensor An important effect to be considered with a potentiometer is the effect of a load RL connected across the output. The resistance RL is in parallel with the fraction x of the potentiometer resistance Rp.
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Mechatronics
Strain-gauged Element • The electrical resistance strain gauge is a metal wire, or a metal foil strip of semiconductor material which is wafer like and can be stuck onto surfaces like a postage stamp • When subject to strain, its resistance R changes, the fractional change in resistance dR/R being proportional to the strain Є
• i.e., dR/R= GЄ…. Where G is a constant of proportionality and termed the gauge factor • The gauge factor is normally supplied by the manufacturer of the strain gauges from a calibration made of sample strain gauges taken from a batch
Strain-gauged Element • One from of displacement sensor has strain gauges attached to flexible elements in the form of cantilevers, rings or U-shapes. • The change in resistance is a measure of the displacement or deformation of the flexible element • Such gauges have linear displacement of the order of 1 mm to 30 mm and have a non-linear error of about ± 1% of full range
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Capacitive Element • The capacitance C of a parallel plate capacitor is given by C = ЄrЄ0A/d where Єr is the relative permittivity of the dielectric between the plates, Є0 a constant called the permittivity of free space, A is the area of overlap between the two plates and d the plate separation • Capacitive sensors for the monitoring of linear displacements might thus take the forms shown in the adjoining figure • In case (a), if separation d is increased by a displacement x, then the capacitance becomes:
Differential Transformers
Linear Voltage Differential Transformers, generally abbreviated as LVDT Consists of 3 coils symmetrically spaced along an insulated tube. The central coil is the primary coil and the other two are identical secondary coils which are connected in series in such a way that their outputs oppose each other A magnetic core is moved through the central tube as a result of the displacement being monitored
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Differential Transformers When there is an ac input to the primary coil, alternating emfs are induced in the secondary coils
With the magnetic core in central position, emf induced in each coil is same. They are so connected that their outputs oppose each other, the net result being zero output When the core is displaced from the central position, there is a greater amount of magnetic core in one coil than the other. The result is that a greater emf is induced in one coil than the other. Therefore, there is a net output from the two coils. Greater the displacement, more is the net output voltage.
Differential Transformers
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Differential Transformers
…eqn(1)
Differential Transformers
With this form of output, the same amplitude output voltage is produced for two different displacements. To give an output voltage which is unique to each value of displacement we need to distinguish between where the amplitudes are same but there is a phase difference of 1800.
A phase sensitive demodulator, with a low pass filter, is used to convert the output into a d.c. voltage which gives a unique value for each displacement.
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Mechatronics
Differential Transformers
A rotary variable differential transformer (RVDT) can be used for the measurement of rotation, and it operates on the same principle as the LVDT The core is a ‘cardioid’ shaped piece of magnetic material and rotation causes more of it to pass into one secondary coil than the other The range of operation is typically ±40% with a linearity error of about ±0.5% of the range
Eddy Current Proximity Sensors If there is a metal object in close proximity to an alternating magnetic field, then eddy currents are induced in it, and the eddy currents themselves produce a magnetic field As a result impedence of the coil changes and so the amplitude of the ac current The figure shows the basic form of such a sensor which can be used for nonmagnetic but conductive materials These sensors are small in size, relatively inexpensive, highly sensitive and high in reliability
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Optical Encoders An encoder is a device that provides a digital output as a result of a linear or angular displacement Two types of position encoders: incremental and absolute In incremental type shown in adjoining figure, a beam of light passes through slots in a disc and is detected by a suitable light sensor When the disc is rotated, a pulsed output is produced by the sensor with the number of pulses being proportional to the angle through which the disc rotates
Optical Encoders
The angular position of the disc, and hence the shaft rotating it, can be determined by the number of pulses produced since some datum position The inner track is used to locate the home position The other two tracks enable the determination of direction of rotation The resolution is determined by the number of slots on the disc
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Optical Encoder: Absolute type
This gives an output in the form of a binary number of several digits, each such number representing a particular angular position The rotating disc has many concentric circles of slots and sensors to detect the light pulses The slots are arranged in such a way that the sequential output from the sensors is a number in the binary code
Optical Encoder: Absolute type
Typical encoders tend to have up to 10 or 12 tracks The number of bits in the binary number will be equal to the number of tracks With 10 tracks there will be 10 bits and so the number of positions that can be detected is 210, i.e., 1024, and a resolution of 360/1024 = 0.350.
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Optical Encoder: Absolute type Binary and Gray codes
In the normal form of binary code, change from one binary code to the next can result in more than one bit changing Due to misalignment, one of the bits may change fractionally before the others, which leads to false counting In gray code, only one bit changes in moving from one number to the next
Proximity Switches
Roller operated
Lever operated
Cam operated
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Proximity Switches Reed Switch
It is a non-contact proximity switch It consists of two magnetic switch contacts sealed in a glass tube When a magnet is brought close to the switch, the magnetic reeds are attracted to each other and close the switch contacts The reed switch is commonly used for checking closure of automatic doors It is also used in tachometers which involve the rotation of a toothed wheel past the reed switch. If one of the teeth has a magnet attached to it, every time it passes the switch it momentarily closes the contacts and produces an electrical pulse in the associated circuit
Proximity Switches Photo-electric sensor
Photo-sensitive devices can be used to detect the presence of an opaque object by it breaking a beam of light, or infrared radiation falling on such a device or by detecting light reflected back by the object
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Hall Effect Sensor A current flowing in a conductor is like a beam of moving charges and will be deflected from its straight line path when a magnetic field is applied on it This effect was discovered by E.R.Hall in 1879 and is called the hall effect
Consider electrons moving in a conductive plate with a magnetic field applied at right angles to the plane of the plate as shown in the adjoining figure As a consequence, electrons are deflected to one side of the plate and that side becomes negatively charged while the opposite side becomes positively charged This charge separation produces an electric field in the material
Hall Effect Sensor Where V is the transverse potential difference, B is the magnetic flux density at right angles to the plate, I is the current through it, t the plate thickness and KH a constant called the hall coefficient Thus, if a constant current source is used with a particular sensor, the hall voltage is a measure of the magnetic flux density
Hall effect sensors are generally supplied in an integrated circuit with the necessary signal processing capability Hall effect sensors are immune to environmental contaminants and can be used under severe service conditions
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Hall Effect Sensor
There are two basic forms of hall effect sensors, linear where the output varies in a reasonably linear manner with the magnetic flux density, and threshold where the output shows a sharp drop at a particular magnetic flux density Hall effect sensor has the advantage of being able to operate as a switch that can operate up to 100 KHz repetition rate, cost less than electro-mechanical switches
Hall Effect Sensor Hall effect sensors can be used to sense position, displacement and proximity if the object being sensed is fitted with a small permanent magnet It can be used to sense the level of fuel in an automobile fuel tank, as shown in adjoining figure A magnet is attached to a float and as the level of fuel changes, the float distance from from the hall sensor also changes The result is a hall voltage output which is a measure of the distance of the float from the sensor and hence the level of fuel in the tank
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Tactile Sensor A tactile sensor is a particular form of pressure sensor It is used on the ‘finger tips’ of robotic hands to determine which hand has come into contact with an object They are also used for ‘touch display’ screens where a physical contact has to be sensed One form of tactile sensor uses piezoelectric polyvinylidene fluoride (PVDF) film. Two layers of the film are used and are separated by a soft film which transmits vibrations
The lower PVDF film has an alternating voltage applied to it and this results in mechanical oscillations of the film (the piezoelectric effect in reverse)
Tactile Sensor
The intermediate film transmits these vibrations to the upper PVDF film As a consequence of the piezoelectric effect, these vibrations cause an alternating voltage to be produced across the upper film When pressure is applied to the upper PVDF film its vibrations are affected and the output alternating voltage is changed
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Mechatronics
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