me3101 design report final

NATIONAL UNIVERSITY OF SINGAPORE

ME 3101 Mechanical Systems Design I In-house Design Project – Champion Stacker

Project SKY HWK

Group Members (Group 8) Foong Yi Wen Kenneth Stephen Wilson Law Wei Seng Low Jiamin Sheryl Shi Hong Sheng Yew Teik Kheng (Group Leader)

[U076701Y] [U076722A] [U076706H] [U076734X] [U076631N] [U076684E]

Group Tutor Prof Li Xiaoping Date of Submission th 6 November 2009

Table of Contents Introduct Introduction ion ................................... .................................................... .................................. ................................. .................................. ................................... ................... .. 5 Objective Objective ................................. .................................................. .................................. .................................. .................................. .................................. ...................... ..... 5 Key Challenges.. Challenges................... .................................. .................................. .................................. .................................. ................................... ............................ .......... 5 Design Requirem Requirements ents ............................... ................................................. ................................... .................................. ................................... ......................... ....... 6 Idea Generatio Generation n .................................. ................................................... ................................... .................................. .................................. ............................... ............. 6 Grabbin Grabbing g Mechanism...... Mechanism....................... ................................... .................................. ................................. .................................. ............................... .............. 6 Robotic Robotic Hand .................................. ................................................... .................................. .................................. ................................... ............................ .......... 6 Scooper Scooper................................... .................................................... .................................. ................................. .................................. ................................... ................... .. 7 Plate Plate gripper gripper ................................ ................................................. ................................... .................................. .................................. ............................... ............. 7 Driving Driving Mechanism Mechanism ............................... ................................................. ................................... .................................. ................................... ......................... ....... 8 Tracked Tracked Wheels Wheels ................................. .................................................. .................................. .................................. ................................... ......................... ....... 8 Different Differential ial Drive............................... ................................................. ................................... .................................. ................................... ......................... ....... 9 Navigatio Navigation n Methods Methods ................................. .................................................. .................................. .................................. ................................... .................... .. 10 Retro-re Retro-reflecti flective ve tape ................................... ................................................... .................................. .................................. ............................... ............... 10 Programmi Programming ng .................................. ................................................... .................................. .................................. ................................... .......................... ........ 10 Final Final Design................................. .................................................. .................................. .................................. .................................. .................................. .................... ... 10 Description of Overall Design .............................................................................................. 11 Design Specific Specification ation.................................. ................................................... .................................. .................................. ................................... .................... .. 11 Assembly Assembly Drawings Drawings............................... ................................................. ................................... .................................. ................................... ....................... ..... 13 Machine Machine Operation Operation............................... ................................................. ................................... .................................. ................................... ....................... ..... 17 Detailed Functionalities of Mechanism................................................................................ 19 Grabbin Grabbing g Mechanism...... Mechanism....................... ................................... .................................. ................................. .................................. ............................. ............ 19 Lifting Lifting Mechanism Mechanism ................................. .................................................. .................................. .................................. ................................... ....................... ..... 21 Driving Driving Mechanism Mechanism ............................... ................................................. ................................... .................................. ................................... ....................... ..... 22 Stepper Stepper Motor Motor ................................. .................................................. ................................... .................................. .................................. ............................. ........... 24 Wheels Wheels ................................. .................................................. .................................. .................................. ................................... .................................. ...................... ...... 25 Navigatio Navigation n Mechanism Mechanism................................. .................................................. .................................. .................................. ................................... .................... .. 25 Navigatio Navigation n Pseudocode Pseudocode .................................. .................................................. .................................. ................................... ............................... .............. 25 Main Pseudocodes Pseudocodes ............................... ................................................. ................................... .................................. ................................... ....................... ..... 26 Control Control System ................................ ................................................. ................................... .................................. .................................. ............................. ........... 45 2|P a g e

Microcontroller (PIC18F4520) ...................................................................................... 45 DC motor controller (Dual H-Bridge Junior 2) ............................................................... 46 Stepper Motor Controller (JS Motor Board) ................................................................. 48 Photo-reflective Sensor ................................................................................................ 48 Guiding Vanes and Bump Switches............................................................................... 50 Side Rollers Rollers .................................. ................................................... ................................... .................................. .................................. ............................. ........... 52 Design Analysis Analysis ................................. ................................................. .................................. ................................... .................................. ............................... .............. 53 Stabili Stability ty Analysis Analysis ................................. .................................................. .................................. .................................. ................................... .......................... ........ 53 Stress and Deflection Analysis ......................................................................................... 53 Material Material Selection Selection .................................. ................................................... .................................. .................................. ................................... .......................... ........ 57 Aluminium Aluminium ................................. ................................................. .................................. ................................... .................................. ............................... .............. 57 Mild Steel................................................. .................................................................. .................................. .................................. .................................. ................. 58 Cost Analysis Analysis .................................. ................................................... .................................. .................................. .................................. .................................. ................. 59 Cost of Product Product............................................ ............................................................ .................................. ................................... .................................. ................. 59 Cost of Prototype Prototype .................................. ................................................... .................................. .................................. ................................... ....................... ..... 60 Manufacturing Process........................................................................................................ 61 Main Body .................................. ................................................... .................................. .................................. .................................. .................................. ................. 61 Grabbin Grabbing g Mechanism...... Mechanism....................... ................................... .................................. ................................. .................................. ............................. ............ 66 Construction Procedure and Testing ................................................................................... 77 Control Control system ................................ ................................................. ................................... .................................. .................................. ............................. ........... 77 Movement Movement.................................. ................................................... .................................. .................................. .................................. .................................. ................. 79 Grabbin Grabbing g Mechanism...... Mechanism....................... ................................... .................................. ................................. .................................. ............................. ............ 80 Lifting Lifting Mechanism Mechanism ................................. .................................................. .................................. .................................. ................................... ....................... ..... 81 Navigatio Navigation n system ................................. .................................................. .................................. .................................. ................................... ....................... ..... 82 Entire Entire Structure Structure .................................. ................................................... .................................. .................................. ................................... .......................... ........ 83 Further Further testing testing ................................. .................................................. ................................... .................................. .................................. ............................. ........... 83 Strengths and weaknesses .................................................................................................. 84 Suggested Suggested Improvement Improvementss................................... ................................................... .................................. .................................. ............................... ............... 88 Photoresis Photoresistors tors ................................. .................................................. ................................... .................................. .................................. ............................. ........... 88 Linear Linear Actuator Actuator ................................ ................................................. ................................... .................................. .................................. ............................. ........... 90 Conclusio Conclusion n .................................. ................................................... .................................. .................................. .................................. .................................. .................... ... 90 Appendix A1 – A1 – Machine Machine Assembly Drawing ..................................................................... 91 3|P a g e

Appendix A2 – A2 – Subassemblies Subassemblies Drawings .......................................................................... 92 Appendix B1 – B1 – Calculations Calculations for Grabbing Mechanism ...................................................... 99 Calculation of Gripping Force ....................................................................................... 99 Calculation of Torque for Grabbing Mechanism Motor .............................................. 100 Friction due to movement of Sliding Connection ........................................................ 100 Power Screw Calculations .......................................................................................... 101 Appendix B2 – B2 – Calculations Calculations for Lifting Mechanism ......................................................... 102 Appendix B3 – B3 – Calculations Calculations for Driving Mechanism ....................................................... 104 Appendix B4 – B4 – Stability Stability Analysis....... Analysis........................ ................................... .................................. .................................. ........................... ......... 107 Appendix C1 – C1 – Specification Specification of Sanyo Denki 103H546-0440 .................. ......... .................. .................. ................ ....... 109 Appendix C2 - Schematic Diagram and Manual for JS Motor Board................................ 110 Appendix C3- Datasheet of QRB1134 IR Photoreflector ................................................. 111 Bibliogr Bibliography aphy ................................... .................................................... .................................. ................................. .................................. ................................. ............... 115

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Introduction Objective In this project, our group aims to design and produce an autonomous machine to collect blocks from designated locations and stack them on a raised platform within the competition arena.

Key Challenges The time limit is set at 3 minutes for a maximum number of blocks to be placed on the platform. Some of our key concerns include: 

Power required for the machine’s movement and picking up the blocks



Speed and navigation of the machine required to place as many blocks as possible on the platform within the time limit l imit



Designing the machine to fit within the space constraint of 180mm X 180mm and



To produce a machine that is cost effective and within budget

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Design Requirements In designing our machine, we have identified 4 key mechanisms needed for our machine to enable it to perform the tasks mentioned: a) Grabbing mechanism to grab and hold on to the blocks b) Lifting mechanism to lift the grabbing mechanism together with the blocks c) Driving mechanism to move the machine around the competition arena d) Navigation mechanism to guide the machine’s movement around the arena

Idea Generation After several brainstorming sessions, we had more than 10 ideas for each of the mechanism. In this section, we will present some of the ideas w e had for our machine and how we arrive at the final design.

Grabbing Mechanism Robotic Hand Mounted on the machine, a robotic hand simulates a human arm with multiple degrees of freedom to grab the blocks.

PROS 

  



Both grabbing and lifting mechanisms are integrated into one hand High degree of freedom Secure grip Highly accurate method with the accompaniment accompaniment of good programming Aesthetically pleasing

CONS 









Difficult to manufacture and programme Only able to collect one block at a time Multiple motors needed (motor needed for every joint) unless a hydraulic system is used, but it is not cost effective Need for counterbalance to ensure stability Expensive if purchased from shop

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Scooper Similar to a bulldozer, this mechanism scoops up 3 blocks against the wall and stacks them .

PROS 

 

CONS

Efficient method because it can collect 3 blocks at a time Easy to manufacture Minimal number of motors required (as there are lesser moving parts)

 





Requires good alignment High friction must be overcome to scoop beneath the blocks High bending moment on the mechanism due to the long arm Long mechanism may cause the machine to exceed the size limit

Plate gripper For this mechanism, we have identified 2 key methods a) Double side clamping plates b) Swivel front plate with fixed back plate a) Double Side Clamping Plates

Two parallel plates that clamp the 3 blocks together.

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b) Swivel plate with fixed backing

A swivel plate is hinged at the end of the top plate to clamp the blocks against the fixed backing.

PROS 

  



CONS

Strong grip that can be further enhanced by installing rubber lining on the plates Easy to manufacture Low accuracy required Efficient method because it can pick up multiple blocks at a time Only one motor is required



Issues with aligning to the wall

Conclusion – Grabbing Grabbing Mechanism After weighing the pros and cons, a swivel plate with fixed backing is the best in terms of manufacturability, manufacturability, grip, efficiency and minimising the number of m otors required.

Driving Mechanism Tracked Wheels Multiple wheels are linked by a chained track. The

driving

force

is

generated

by

two

independent motors, which rotate the track wheels

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Differential Drive One input is geared to drive two outputs. The differential allows for each wheel to rotate at different speeds

Tracked Wheels PROS CONS 

Good grip in uneven terrain







High friction from sliding during turning Bulky and heavy mechanism Poor grip on smooth surface

Differential Drive PROS CONS 

Able to take corners well as different wheels can rotate at different speeds





The differential gear reduces overall torque Both motors must rotate at the same speed to move in a straight line which is difficult to achieve

Conclusion – Driving Driving Mechanism We have decided to use a hybrid system of two wheels connected to independent motors. This makes use of the benefit of the differential drive system which enables the machine to turn sharp corners as both wheels are on independent axles. It also avoided the downside of the tracked wheel system, which is heavier and more prone to navigational errors due to sliding.

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Navigation Methods Retro-reflective tape This method utilises the retro-reflective tape in the arena. The photo-reflective sensor generates a distinct high or low voltage depending on whether it senses a reflective or nonreflective surface.

PROS 

The machine follows a fixed path and is

CONS 

less likely to go out of a planned route r oute 

Less accumulation of error because the

Slow alignment due to adjustment and feedback



A longer distance is required if the

retro-reflective tape serves as a reference

machine is going to follow only the tape,

route

thus taking a longer time. 

Unable to ensure blocks are placed at the exact same spot (stacked on each other) for subsequent depositing trips

Programming A designed set of algorithm to direct the machine to move in a specific manner.

PROS 

More freedom of movement



Faster (lack of feedback eliminates need

CONS 

Accumulation Accumulation of errors

to follow a standard route) 

Able to deposit at the same point for different depositing trips

Conclusion – Navigation Navigation Mechanism We will be using a hybrid of both, since programming provides freedom and speed in movement but reflective tape can serve as a guide to prevent accumulation of errors. This system gives more accuracy and reliability.

Final Design In the end, we decided on a swivel plate with fixed backing for the block collection, a forklift system for raising the blocks, a differential gear system for movement and a hybrid of retroreflective tape and direct programming for naviga tion.

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Description of Overall Design Design Specification

Min Machine Height

=

230.00mm

Min Machine Length

=

179.29mm

Max Machine Height

=

315.00mm

Max Machine Length

=

225.70mm

Estimated Weight

=

1017.23g

(Refer to Appendix A1 for the Detailed Machine Assembly Drawing) (Refer to Appendix A2 for Detailed Machine Subassembly Drawings)

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Centre of Mass

Using Solidworks, we are able to find the centre of m ass of the machine Center of mass with respect to the blue dot and the axes is as depicted X = 23.60 Y = 28.33 Z = -55.12

The Pink Axis Shows the Center of Mass and Moment of Inertia.

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Assembly Drawings

Item No. 1

2 3

PARTS LIST Item Description Lifting Tamiya Mechanism 70115 R/C Forklift Main Body Aluminium Grabbing Aluminium Mechanism

Quantity 1

1 1

1

2

3

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PARTS LIST Item No. 1

Item

Description

Quantity

Stepper Motor

Sanyo Denki 103H546-0440 Speed Run Robot Wheels 8mm Pololu Ball Caster with 3/8” Metal Ball Fairchild QRB1134 IR Photoreflector

2

2

Wheel

3

Caster Caster Ball Bearing Wheel Photo-reflective Sensor Bump Switch DC Motor Controller Stepper Motor Controller Microcontroller Microcontroller Programmer Bolt and Nut Main Body

4 5 6 7 8 9 10 11

Dual H-Bridge Junior 2 JS Motor Board

10 11

2

2 2 2

1

2 1 1

PICkit 2 Programmer

1 1

Mild Steel Aluminium

4 1

9 8 7

6

3 2

5

4

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Item No. 1 2 3 4 5 6

PARTS LIST Item Description Descript ion Motor Tamiya Plasma Dash Motor Worm Gear Plastic Spur Gear Plastic Power Mild Steel Screw Mast Aluminium Al uminium Fork Aluminium

Quantity 1 1 1 1 1 1

6

5 1 2

4

3

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Item No. 1

10

1

2 3 4

2 9

3

5

8 6 7

4 8 9

10

6

PARTS LIST Item Descriptio n Motor with Tamiya Plasma Spur Gear Dash Motor Spur Gear Plastic Power Screw Mild Steel Fixed Plate Aluminium and Mild Steel Rubber Rubber Lining Swivel Plate Aluminium Springloaded Hinge Sliding Aluminium and Connection Mild Steel Plastic Plastic (High Padding Density Polyethylene) Side Roller Aluminium

Quantity 1 1 1 1 2 1 2 1 1

1

7

5

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Machine Operation Our machine is designed to be able to carry 6 blocks at once and place them onto the platform. Our target for the machine is to stack as many as 18 blocks in 3 trips within the time limit (3 minutes). To achieve this, our machine is programmed to collect the blocks in the following manner: a) The machine will move to the side wall of the arena and align itself sideways with the wall. b) Then it will move towards the first 3 blocks and push it until the blocks touches the next set of 3 blocks. c) After that, it will grab the 6 blocks using its grabbing mechanism and lift the blocks to the required height using the lifting mechanism. d) The machine will then proceed to the platform and deposit the 6 blocks onto the platform. e) After depositing the blocks, the machine will make a second trip (followed by a third) to collect another 6 blocks and place it on top of the 6 blocks previously deposited on the platform. Refer to the next page for a diagram of how the machine operates.

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st

st

Machine moves towards 1 3 blocks

Machine pushes 1 3 blocks and approaches the next 3 blocks

Forklift mechanism lifts the 6 blocks

Machine swivel plate clamps the 6 blocks

Machine delivers the 6 blocks to the

Machine repeats the cycle and stacks s tacks 12

platform

and finally 18 blocks

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Detailed Functionalities of Mechanism In this section, we will describe in detail the functionalities of each mechanism and the different components that make up the mechanism.

Grabbing Mechanism Sliding Connection

DC Motor

Power Screw

Fixed Plate

Rubber Lining

Swivel plate

Plastic Padding Spring-loaded Hinge

Threaded Connection

We have designed a grabbing mechanism which utilises a swivel plate that is able to flip up or down. When the swivel plate flips up, the grabbing mechanism is opened to allow the first set of 3 blocks to enter the grabbing mechanism. Then, the fixed plate will push the blocks until it reaches another set of 3 blocks. After that, the swivel plate will flip down and grab the 6 blocks altogether. To open the grabbing mechanism, the DC motor will first turn the power screw, which will in turn cause the threaded sliding connection to move forward. As it moves forward, the end of the power screw gets further and further away from the swivel plate. As a result, the swivel plate will swing up due to the spring-loaded hinges installed. The function of the hinges is to keep the swivel plate in the open o pen position.

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Spring-loaded hinge To close the grabbing mechanism, the DC motor will then reverse its rotation. Therefore, the power screw will also turn in the other direction and cause the threaded sliding connection to move backward. As it moves backward, the end of the power screw touches the swivel plate and pushes it down slowly to a closed position. As the swivel plate and the power screw are frequently in contact and rubbing against each other, the swivel plate is prone to wear and tear. Therefore, to protect the swivel plate, a replaceable plastic padding is placed at the point of contact of the power screw and the swivel plate. Besides that, rubber lining will also be added to the inner surface of the swivel plate and fixed plate to provide a higher coefficient of friction to grab the blocks. From our calculations, the grabbing force required to hold the 6 blocks is 1.714N. The torque required to provide this amount of force is 7.685 mNm. The Plasma Dash Motor used can provide a torque of 1.962 mNm. Hence, the gear ratio required would be 3.92 which is approximately approximately 4. (Refer to Appendix B1 for detailed calculations for the grabbing mechanism)

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Lifting Mechanism

To minimize the number of parts that have to be manufactured, we decided to purchase a forklift model and use certain parts from it for our lifting mechanism. The model that we will be using is the Tamiya 70115 R/C Forklift. This lifting mechanism is driven by a DC motor at the base, where gears are used to transfer the rotational motion from the motor to the vertical power screw. The power screw will in turn transmit the rotational motion into a linear up or down motion of the mast. The fork is then lifted using the chain connected to the mast. The grabbing mechanism will be connected to the fork so that our machine can lift the blocks up and place them onto the platform. To ensure that our lifting mechanism has enough power to lift the weight of the grabbing mechanism and six blocks, we will change the stock motor to another model with a higher torque and rpm (Tamiya Plasma Dash Motor). From our calculations, the torque required for the motor to lift the grabbing mechanism and the blocks is 0.302 mNm. The Plasma Dash Motor that we intend to use is able to provide a maximum torque of 1.962 mNm. (Refer to Appendix B2, for f or detailed calculations for the lifting mechanism)

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Driving Mechanism

As mentioned, we have decided to use the differential drive system for our machine due its mechanical simplicity. A differential drive is a two-wheeled drive system with independent actuators for each wheel. Using this system, the two driving wheels on the machine can move at different speeds and directions. This can be achieved by connecting the two wheels 1

on each side of the machine to their own motors m otors . The following table shows the various movement modes that can be achieved using this driving mechanism (Astolfo, Ferrari, & Ferrari): Left Wheel

Right Wheel

Machine’s Movement

Stationary

Stationary

Rests stationary stationary

Stationary

Forward

Turns counter clockwise, pivoting around the left wheel

Stationary

Backward

Turns clockwise, pivoting around the left wheel

Forward

Stationary

Turns clockwise, pivoting around the right wheel

Forward

Forward

Goes forward

Forward

Backward

Spins clockwise in place

Backward

Stationary

Turns counter clockwise, pivoting around the right wheel

Backward

Forward

Spins counter clockwise in place

Backward

Backward

Goes backward

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Other than the usual ninety-degree cornering, turning of any radius and angle can also be achieved by using different combinations of rotational speed and direction of motors. For our machine, we will be connecting 2 Sanyo Denki Model 103H546-0440 stepper motors to the wheels to deliver the required torque to drive the machine. From our calculations, the torque required to drive our machine is 1.983 mNm and the maximum torque that the stepper motor can provide is 147 mNm. Thus, the stepper motor will be able to provide more than enough torque to drive our machine. (Refer to appendix B3 for detailed calculations on the driving mechanism) Perhaps one drawback of the differential drive system is that the machine may not be able to move in a perfectly straight line due to the different efficiencies and rotating speeds of the motors. Therefore, we decided to use the steppers motors which are highly accurate to minimize such a problem. Besides that, our machine will also be equipped with photoreflective sensors to detect the retro-reflective tape so that it can follow the tape and not deviate from its intended path.

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Stepper Motor

After much consideration, we decided to use 2 stepper motors to drive our wheels due to the following reasons (Industrail Circuits Application Note: Stepper Motor Basics): a) Stepper motor provides precise positioning and repeatability of movement since it has an accuracy of 3% to 5% of a step and this error is non cumulative from one step to the next. This high level of accuracy is crucial to our machine as we cannot afford to have huge accumulation of positional errors because such errors will lead to an undesirable outcome, which is, not having any block stacked. 2

b) Stepper motor functions well in an open loop system . Being able to work without positional feedback information (from multiple sensing and feedback devices) allows us to stay within the constraints of our tight budget. c) Stepper motor has an excellent response to starting, stopping or reversing. d) It is also reliable because unlike normal DC motors, there are no contact brushes in the stepper motor. Therefore, its lifespan is only dependent on the lifespan of the bearing, which can be changed easily. This will be more economical in the long run. e) The use of a stepper motor also eliminates the need for gear reduction because the stepper motor itself can provide sufficient enough torque and speed to drive the machine. (Refer to Appendix C1 for the datasheet of Sanyo De nki 103H546-0440)

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Wheels

Rear Aluminium Wheels

Caster Wheel

Using large diameter wheels will give our machine low torque but high velocity. From our calculations, calculations, the stepper motor can provide more than enough torque to drive the machine. We are more concerned with the speed of the machine because it has to cover a large distance within the time limit. Therefore, we decided to use large diameter wheels that are available on the market. In this case, we will be using a set of 51.3 mm diameter wheels that are specially designed for the Sanyo Denki Model 103H546-0440 stepper motor. These wheels are very light because they made of aluminium and precision engineered. Also, with a width of 8 mm, wear and tear on the wheels can be minimized as a wide wheel will cause 3

increased resistance while rotating on the arena surface . As for the front caster wheels, we will be using the Polulu ball caster with 3/8” metal ball due to its small size and reasonable price (Palmisano, 2009).

Navigation Mechanism Navigation Pseudocode To show how our machine will complete the task given within the time limit, we have designed a pseudocode which will be implemented into the programming of the machine.

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Main Pseudocodes Number 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Pseudocode Open Grabber Move Forward Curve Right Push Forward Close Grabber + Lift Reverse Curve Right Rotate Anti-Clockwise Move Forward Open Grabber + Reverse + Lower Grabber Rotate 90 Anti-Clockwise Move Forward Curve Right Push Forward Close Grabber + Lift Reverse Curve Right Rotate Anti-Clockwise Move Forward Open Grabber + Reverse + Lower Grabber Rotate 90 Anti-Clockwise Move Forward Curve Right Push Forward Close Grabber + Lift Reverse Curve Right Rotate Anti-Clockwise Move Forward Open Grabber

Phase

Remark

Movement Phase 1

Attack Phase 1

st

1 6 Blocks

Delivery Phase 1

°

Movement Phase 2

Attack Phase 2

nd

2 6 Blocks

Delivery Phase 2

°

Movement Phase 3

Attack Phase 3

rd

3 6 Blocks

Delivery Phase 3

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Calculations for Movement Phase and Attack Phase 216

D

360

166.5

C Machine Grabber

150 radius

Machine

O

Body

66

210 A

B

80

100

LEGEND All dimensions in mm unless otherwise stated and light tape is 50mm wide O is the center of rotation for the machine ABCD is the perimeter of the machine from the plan view Calculation for arc length travelled   ℎ  = 9 3 × = 146. 146.08 08 

2

st

Calculation of distance remaining till contact with 1 3 blocks

  = 400 − 216 − (   = 166. 166.5 5 

35 ) 2

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PSEUDOCODE 1 – OPEN OPEN GRABBER [MOVEMENT PHASE 1]

INITIALIZE POWER

GRABBER TO OPEN BY 90 °  

Grabber motor on [2.00 sec] Grabber motor off

PSEUDOCODE 2 – MOVE MOVE FORWARD [MOVEMENT PHASE 1] DIFFERENTIAL MOTORS 



Both motors forward for 290mm [2.90 sec] Both motors off

Machine moves forward by 290mm

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PSEUDOCODE 3 – CURVE CURVE RIGHT [MOVEMENT PHASE 1] DIFFERENTIAL MOTOR LEFT 

Forward motion for arc of radius 150mm [1.46 sec]

DIFFERENTIAL MOTOR RIGHT 


MOVE IN A CURVE 

 



Move in a curve for a distance of 146.1mm [ 1.46 sec] Horizontal distance moved = 150mm st Horizontal distance remaining to the 1 3 blocks = 166.5mm Motors off

Machine moves in an arc of radius r adius 93mm (to the centre of machine)

Horizontal distance remaining to st the 1 3 blocks = 166.5mm

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PSEUDOCODE 4 – PUSH PUSH FORWARD [ATTACK PHASE 1] DIFFERENTIAL MOTORS Both motors forward for 166.5mm [1.67 sec] Motors )continue for 186.687mm [ sec] forward motion 

Machine moves forward until contact with blocks



ST

PUSHING OF 1 3 BLOCKS 





st

Contact with 1 3 blocks after 166.5mm and pushing begins st 1 light tape is detected after 42.5mm of pushing st 1 3 blocks are pushed for a total of 165mm [1.65 sec]

st

1 light tape

nd






st

nd

1 3 blocks gets in contact with 2 3 blocks after 165mm of pushing Pushing of a total of 6 blocks begins for 77.5mm [0.78 sec] nd 2 light tape is detected and both motors are turned off

nd

2 light tape st

1 light tape

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PSEUDOCODE 5 – CLOSE CLOSE GRABBER + LIFT [ATTACK PHASE 1] GRABBER TO CLOSE BY 90 ° 



Grabber motor on in reverse [2.00 sec] Grabber motor off

LIFT GRABBER BY 36mm  

Forklift motor on [3.00 sec] Forklift motor off

PSEUDOCODE 6 – REVERSE REVERSE [ATTACK PHASE 1] DIFFERENTIAL MOTORS 

 

Both motors in reverse for 150mm [1.50 sec] st 1 light tape detected Both motors off Distance between light tapes 150

nd

2 light tape st 1 light tape

nd


Machine reverses for 150mm

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Calculations for Delivery Phase Overall Movement 600

450

336

393 From the diagram, Radius of arc for left motor Radius of arc for right motor Radius of arc (through centre)

157

= 450 mm = 336 mm = 393 mm

Detailed Diagram B C x x D A

450 550 C’ x x D’

From close-up, where θ is the angle between the back of the machine and the vertical Position of centre of arc, OC = 396.17 (sin(θ + θC) i + cos( θ + θC) j) Position of light sensor, OD = 405.52 (sin(θ + θD) i + cos(θ + θD) j) -1 where θC = tan (50/393) θC =7.25 °

32 | P a g e

For blocks to be aligned with platform, θ + θD = 90 – 7.25 θ = 90 – 7.25 = 82.75 ° °

°

°

Arc length travelled, | CC’| |CC’| = 396.17 x π / 180 x 82.75 = 572.17mm

To realign with the platform, a rotation on the spot for 82.75 anti-clockwise is performed. °

Arc length travelled for rotation Arc = 57 x π / 180 x 82.75 = 82.32mm Distance remaining to platform Distance = 550 – 550 – radius radius of arc – arc – distance distance to pusher = 550 – 550 – 396.17 396.17 - 50 = 103.83mm

PSEUDOCODE 7 – CURVE CURVE RIGHT [DELIVERY PHASE 1] DIFFERENTIAL MOTOR LEFT 




MOVE IN A CURVE 

 



Move in a curve for a distance of 572.2mm [5.72 sec] nd rd th 2 , 3 and 4 light tapes are detected Both motors off once the curve is completed and machine detects the th 4 light tape Motors off

nd

2 light tape

rd

3 light tape th

4 light tape

33 | P a g e

PSEUDOCODE 8 – ROTATE ROTATE ANTI-CLOCKWISE [DELIVERY PHASE 1]

DIFFERENTIAL MOTOR LEFT 

DIFFERENTIAL MOTOR RIGHT

Backward motion for arc of radius 57mm [0.82 sec]




ROTATE ON THE SPOT  

 

Rotate 82.75 Both wheels move in an arc of 82.32mm [0.82 sec] Machine now faces the platform Motors off

Machine rotates 90 anti-clockwise °

°

Machine now faces the platform

PSEUDOCODE 9 – MOVE MOVE FORWARD [DELIVERY PHASE 1] DIFFERENTIAL MOTORS 





Both motors forward for 103.83mm [1.04 sec] Both switches in contact with platform Motors off

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PSEUDOCODE 10 – OPEN GRABBER + REVERSE + LOWER GRABBER [DELIVERY PHASE 1]

GRABBER TO OPEN BY 90 °   

Grabber motor on [2.00 sec] Grabber motor off 6 blocks are placed on platform

DIFFERENTIAL MOTORS 



Both motors in reverse for 275mm [2.75sec] Both motors off

Machine reverses for 275mm

LOWER GRABBER   

Forklift motor in reverse [3.00 sec] Forklift lowered by 36mm Forklift motor off

35 | P a g e

Calculations for Movement Phase 2 Overall Movement Arc of radius 150

Light tape of width 50

150

100

150

125

200

275

PSEUDOCODE 11 – ROTATE 90° ANTI-CLOCKWISE [MOVEMENT PHASE 2] DIFFERENTIAL MOTOR LEFT 







Rotate 90 Both wheels move in an arc of 89.54mm [0.90 sec] Motors off °


 

Both motors forward for 300mm [3.00 sec] th 5 light tape is detected after 175mm Motors off th

5 light tape

36 | P a g e





MOVE IN A CURVE 

 

Move in a curve for a distance of 146.1mm [1.46 sec] Horizontal distance moved = 150mm Horizontal distance remaining to the nd 2 3 blocks = 7.5mm

th

6 light tape

PSEUDOCODE 14 – PUSH PUSH FORWARD [ATTACK PHASE 2] DIFFERENTIAL MOTORS Both motors forward for 250mm [2.50 sec] for 186.687mm ) [ sec]st Contact with 1 3 blocks after 7.5mm and pushing begins th 6 light tape is detected after 42.5mm Machine continues pushing motion st 1 3 blocks are pushed for a total of 165mm [1.65 sec] 





 

th

6 light tape

Machine moves forward, comes into contact with 3 blocks, and continues forward motion

nd






st

nd

1 3 blocks gets in contact with 2 3 blocks after 165mm of pushing Pushing of a total of 6 blocks begins for 77.5mm [0.78 sec] th 7 light tape is detected and both motors are turned off

th

7 light tape

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PSEUDOCODE 15 – CLOSE CLOSE GRABBER + LIFT [ATTACK PHASE 2]

GRABBER TO CLOSE BY 90 ° 



Grabber motor on in reverse [2.00 sec] Grabber motor off




 

Both motors in reverse for 550mm [5.50 sec] th th th 8 , 9 and 10 light tape detected Both motors off

Distance between light tapes 550

nd


th

10 light tape

th

8 light tape

th

9 light tape Machine reverses for 550mm

38 | P a g e







MOVE IN A CURVE 







Move in a curve for a distance of 572.2mm [5.72 sec] th th th 11 , 12 and 13 light tapes are detected Both motors off once the machine th detects the 13 light tape and the curve is completed Motors off

th

11 light tape

th

12 light tape th

13 light tape









 



°

Machine now faces the platform 39 | P a g e







PSEUDOCODE 20 – OPEN GRABBER + REVERSE + LOWER GRABBER [DELIVERY PHASE 2] GRABBER TO OPEN BY 90 °   

Grabber motor on [2.00 sec] Grabber motor off 6 blocks are placed on top of previous 6 blocks

DIFFERENTIAL MOTORS 



Both motors in reverse for 275mm [2.75 sec] Both motors off Machine reverses for 275mm

LOWER GRABBER   

Forklift motor in reverse [6.00 sec] Forklift lowered by 71mm Forklift motor off

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PSEUDOCODE 21 – ROTATE 90° ANTI-CLOCKWISE [MOVEMENT PHASE 3] DIFFERENTIAL MOTOR LEFT 









Rotate 90 Both wheels move in an arc of 89.54mm [8.95 sec] Motors off °






Both motors forward for 300mm [3.00 sec] th 14 light tape is detected after 175mm Motors off

th

14 light tape





MOVE IN A CURVE 

 

Move in a curve for a distance of 146.1mm [1.46 sec] Horizontal distance moved = 150mm Horizontal distance remaining to the th 15 light tape = 50mm

th

15 light tape

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PSEUDOCODE 24 – PUSH PUSH FORWARD [ATTACK PHASE 3] DIFFERENTIAL MOTORS Both motors forward for 577.5mm [5.78 sec] for 186.687mm ) [ sec] th 15 light tape is detected after 50mm th 16 light tape is detected after another 200mm rd Contact with 3 3 blocks after 157.5mm and pushing begins th 17 light tape is detected after 42.5mm 

 

th

th

15 light tape

17 light tape th 16 light tape





Machine moves forward, passing lig ht tape 15, 16 and 17

GRABBING OF FINAL 6 BLOCKS 

 

rd

3 3 blocks gets is pushed for 165mm and comes into contact with final 3 blocks 6 blocks are pushed for 5mm Both motors off

PSEUDOCODE 25 – CLOSE CLOSE GRABBER + LIFT [ATTACK PHASE 3] GRABBER TO CLOSE BY 90 ° 

Grabber motor on in reverse [2.00 sec]



42 | P a g e




Both motors in reverse for 822.5mm [8.23sec] th th th st nd 18 , 19 , 20 , 21 and 22 light tapes detected nd

Distance to 22 light tape 822.5

th

st

18 light tape

st

19 light tape

21 light tape

th

22 light tape

th

20 light tape





MOVE IN A CURVE 







Move in a curve for a distance of 572.2mm [5.72 sec] th th th 23 , 24 and 25 light tapes are detected Both motors off once the machine th detects the 25 light tape and the curve is completed Motors off

rd

23 light tape th

24 light tape th

25 light tape

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



 



°

Machine now faces the platform







PSEUDOCODE 30 – OPEN OPEN GRABBER [DELIVERY PHASE 3] GRABBER TO OPEN BY 90 °   

Grabber motor on [2.00 sec] Grabber motor off 6 blocks are placed on top of previous 12 blocks

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Control System To make our machine autonomous, we will install a microcontroller unit (MCU) on our machine. Basically, MCU is the “brain” that controls the actions of the machine. To do so, the MCU will first receive feedback from the various sensors and process the data. It will then make decisions based on the programmed code and generate signals to various actuators to perform the required tasks. For our machine, we will be using the MCU provided by the school, which is the PIC18F4520 (Peripheral Interface Controller) by Microchip. The programming of the MCU will be done using the PICKit 2 Microcontroller Programmer to input the codes based on the pseudocode that we have programmed. Two bump switches and two photo-reflector sensors will be connected to the MCU to provide feedback to the system. Besides that, the MCU will also be connected to DC motor controller (Dual HBridge Junior 2) and a stepper motor controller (JS Motor Board) so that the MCU will be able to control the motors installed in the machine.

Microcontroller (PIC18F4520) The PIC18F4520 is a low-power microcontroller that uses the high-speed 8-bit CMOS FLASH technology. It utilizes a 16-bit program word architecture and incorporates an advanced RISC architecture with 32 level-deep stack, 8x8 hardware multiplier, and multiple internal and external interrupts. This MCU is chosen because it is the most popular architecture for new 8-bit designs where the programming is done in C. This particular model comes in 40 pins and it supports both 3V and 5V applications. The pin configurations configurations of the PIC18F4520 are shown below. below. (The datasheet for PIC18F4520 is available for download at http://ww1.microchip.com/downloads/en/DeviceDoc/39631a.pdf

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DC motor controller (Dual H-Bridge Junior 2) Using this controller, we will be able to connect the two DC motors used for the grabbing mechanism and lifting mechanism. The operating current for the controller is 3A or 6A and it requires a voltage between 4.8V and 9.6V. This motor controller offers quick motor direction change and also allows braking in neutral position. The board layout of the motor controller is shown below:

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An H-bridge is an electronic circuit that allows voltage to be applied across a load in either direction to allow the DC motors to run forward a nd backward. An H-bridge is built with four switches. The structure of a H-bridge (highlighted in red) is shown below:

When the switches S1 and S4 are closed and S2 and S3 are opened, a positive voltage will be applied across the motor. Therefore, by opening S1 and S4 switches and closing S2 and S3 switches, the motor operation is reversed because the voltage is reversed. However, switches S1 and S2 should never be closed at the same time because it will cause a short circuit on the input voltage source. The same applies to the switches S3 and S4. Besides reversing polarity, the H-Bridge can also be used to 'brake' the motor by shorting its terminal, or to let the motor 'free run' to a stop by disconnecting it from the circuit. The following table summarizes the operation of H-Bridge (Wikipedia, n.d.): S1 S2 S3 S4 Result

1 0 0 1 Motor moves right 0 1 1 0 Motor moves left 0 0 0 0 Motor free runs 0 1 0 1 Motor brakes 1 0 1 0 Motor brakes (http://en.wikipedia.org/wiki/H-bridge http://en.wikipedia.org/wiki/H-bridge))

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Stepper Motor Controller (JS Motor Board)

( http://www.active-robots.com/products/motorcon/dual-stepper-motor-driver-system.shtml http://www.active-robots.com/products/motorcon/dual-stepper-motor-driver-system.shtml )

The JS Motor Board is a dual stepper motor driver board that can drive two stepper motors (9V to 25V) independently. Two SLA7024M (Sanken) chips are used as drivers, allowing a current of 1.5A to be delivered to each motor. It also has a Phase Selection option that enables us to select either a 2 phase or 1-2 phase driving method. In addition, it also has separate control signals and adjustable current limits via the use of potentiometers (Active Robots, 2009). (Refer to Appendix C2 for the schematic diagram and manual for the controller)

Photo-reflective Sensor

In this stacking competition, our machine will navigate the arena according to predetermined steps written in programming codes as well as by following the lines of retroreflective tape on the arena.

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To direct our machine along the tape, we have decided to use photo-reflective sensors together with a microprocessor that deciphers the feedback information. We will be using the QRB1134 IR photoreflector due to its capability to detect the tape more effectively with its built-in daylight filter that reduces interferences from ambient lighting. The line detecting system of our machine will make use of two QRB1134 IR photoreflectors strategically placed 45mm apart at the bottom of the machine’s main body. The following figure shows the bottom view of our machine and locations where the sensors will be installed.

Photoreflective sensors

Bottom view of the machine

With respect to the machine’s machine’ s central axis, Sensor 1 will be placed to its right while Sensor 2 to its left (from top view), with a distance of about 45mm between them. Two sensors are needed to ensure that the center of the machine is aligned with the central axis of the 50 mm wide retro-reflective tape. In other words, the machine should always be centered on the retro-reflective tape to ensure precision in positioning. To achieve this, both the photoreflective sensors should always detect the reflected infrared light from the tape. I f Sensor 2 does not receive any signal but Sensor 1 does, then it means that the machine has deviated to the left and it should be steered right and vice versa. (Refer to Appendix C3 for the datasheet of QRB1134 IR photoreflector)

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Guiding Vanes and Bump Switches

The main body of our machine will have guiding vanes and bump switches installed to help the machine to position itself in front of the platform before releasing the blocks. As slight positional error is inevitable, our machine might approach the platform at a slightly deviated angle or off-centered position. To ensure that our machine will stack the blocks up in a precise manner, these vanes and switches will guide the machine to always stop at the same position in front of platform before releasing the blocks. The function of the guiding vanes is to ensure that the machine is not off-centered from the platform. Furthermore, the bump switches are installed to ensure that the machine is in the correct position with respect to the platform before releasing the blocks. The following figures (next page) depict the functionalities of the guiding vanes together with the bump switches.

50 | P a g e

Machine approaches platform at incorrect angle. If either or both the bump switches are not turned on, the machine will continue to move forward to align itself with the platform. Guiding vane improves positioning accordingly as long as forward motion continues.

Both the bump switches are in contact with the platform and are turned on. Machine is now in the correct position to deposit blocks. On a side note, due to dimensional and geometrical constraint caused by the side rollers, the grabbed blocks will not be centered on our machine, but slightly offset to the left. Therefore, we will make one side of the guiding vane tapered (see figure1) so that the blocks will be offset back to the center when they are deposited onto the platform.

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Side Rollers

As mentioned, our machine will be programmed such that it collects the blocks by aligning itself against the wall of the competition arena. Therefore, it is very important to reduce the friction between our machine and the wall in order to reduce unnecessary energy lost. Installing rollers on the side of our machine greatly reduce this friction and thus, improving the machine efficiency. Other than reducing friction, the rollers are also equally useful in the aligning phase. When the machine reaches the wall at an incidence angle, the rollers will automatically align the machine to the wall as long as forward motion continues. As mentioned before, the installation of the side rollers will cause some constraint to our machine. As a result, the grabbed blocks will be slightly offset from the central axis of our machine. However, we will make one of the guiding vanes tapered to offset the blocks back to the center.

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Design Analysis Stability Analysis Using Solidworks, we worked out the center of mass of our machine and found that it lies behind the caster wheels. Therefore, our machine will be stable at rest and will not topple. Besides that, we also considered the dynamic stability of our machine when it is on the move. In the worst case scenario, our machine is the most unstable when it is carrying 6 blocks and suddenly comes to a stop. Therefore, we did some calculations based on this scenario to determine if our machine will topple under this situation. The calculations show that our machine will not topple under any movement phase. (Refer to Appendix B4 for detailed calculations for the stability analysis)

Stress and Deflection Analysis In order to assess the stress distribution and the deflection of the critical parts of our machine, we perform finite element analysis (FEA) on the parts using Solidworks. This analysis is important to ensure that none of the parts will fail when it is loaded.

Swivel Plate on Grabbing Mechanism One of the parts of concern is the swivel plate on the grabbing mechanism. From the result of the FEA performed on our first proposed swivel plate, it is shown that the point where the load is being applied will deflect as much as 0.001801 mm. At the same time, highest stress experienced by the plate is 1.631 MPa. This gives a safety factor of around 16.9 when aluminium’s aluminium’s yield strength of 27.57 MPa is taken into i nto consideration.

Stress distribution on swivel plate

Displacement analysis of swivel plate 53 | P a g e

Based on the result shown above, although our first proposed swivel plate looks promising, we have decided to strengthen it by adding a rib to the center of the plate. This is essential because we cannot afford to have our grabbing mechanism failing during the competition and not able to collect any blocks at all. From the following figures, it can be seen that with the additional rib, our improvised swivel plate’s safety factor is now 24.7 with maximum stress of 1.118 MPa experienced. Also, deflection has been reduced to 0.0005934 mm. These improvements have justified our decision of installing the additional rib to the swivel plate.

Stress distribution on new swivel plate

Displacement analysis of new swivel plate

54 | P a g e

Main Body As about 1/3 of the machine weight will be resting on the main body, it is also important to assess the stress distribution in the main body frame to ensure that it does not fail. Again, the result result obtained from FEA shows that our machine’s main body will not fail and the deflection due to the weight on it (0.006711 mm) is also negligible as depicted in the following figures.

Stress distribution on main body

Displacement analysis of main body

Lifting Mechanism (Fork) The fork that joins the lifting mechanism and the grabbing mechanism together is also region of high stress. Therefore, we will also analyze this part using FEA.

Fork

Fork

55 | P a g e

Assuming that the total weight of all the blocks and the grabbing mechanism is being applied on and evenly distributed between the two forks that join the two mechanisms together (see figure above), the following figures show the stress distribution in each of the fork as well as its deflection.

Stress distribution on fork

Displacement analysis of fork

From the result of FEA, the maximum deflection of the fork is 0.1268 mm at its free end. We consider this maximum deflection to be acceptable and will not have any significant effect on our grabbing mechanism. Besides that, the maximum stress experienced by the fork is also well below the yield strength of the aluminium used to make it.

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Material Selection Most of the parts of our machine that need to be manufactured will be made out of aluminium. However, some specific areas that are subjected to high wear and tear will be made out of mild steel, which has high hardness and stiffness.

Aluminium Aluminium is chosen for most of the parts because it is very strong, light, resistant to corrosion, and affordable. More importantly, it is very easy to cut, shape, drill, and bend using the machines available in our labs. Although, aluminium is not as cheap or strong as steel, it has a greater strength to weight ratio. This means that for an equivalent mass of aluminium and steel, aluminium is much stronger. Besides that, less material can be used to achieve the same strength required. Thus the cost of using aluminium may not necessary be higher. Although aluminium can rust like most metal, it differs in its ability to quickly form an oxidized layer which serves as a protective coating against more rusting. But there are some occasions where rust could still potentially present a problem when the material is subjected to physical wear and tear. This oxide layer could be scraped off, resulting in aluminium rust. There are a few ways to solve this problem. Firstly, the aluminium can be coated with Iridite. Iriditing is a cheap and fast process that coats the aluminium in a hard protective oxidized layer that prevents rust. It can be done at a machinist shop, or at home with an iriditing kit. Two other common coatings used to resist corrosion are black oxidization and galvanization. Black oxidation like iriditing, coats a hard layer on top of aluminium to resist corrosion due to wear. Galvanization is coating of a layer of z inc over the metal for corrosion prevention. Corrosion will affect the zinc coating preferentially because zinc is more anodic than aluminium and thus leaving aluminium rust free. Due to the softness of aluminium, it is often easily deformed or scratched due to wear. Anodization which is a process that coats aluminium with an extremely hard protective layer helps to solve this problem. However, because it is also expensive and highly specialized, it may not be very viable for our project. Therefore, we decided to use mild steel for parts that are prone to wear and tear. Aluminium can be worked using just a bandsaw and drill press for cutting and drilling. However, we will be using milling to work on aluminium because it can yield a clean cut surface. Furthermore, our ME workshops are also equipped with Milling Machines. To join several pieces of aluminium together, we can weld them directly using Gas Tungsten Arc Welding, Shielded Metal Arc Welding or Gas Metal Me tal Arc Welding. Perhaps another alternative material that can be considered for our machine is aluminium alloy. Generally, aluminium alloys are harder, stronger and more resistant to corrosion. 57 | P a g e

However, they carry the disadvantage of being harder to machine or formed to the desired shape and dimensions because of these properties too. Besides, aluminium alloys which are heat treated are very susceptible to losing their superior qualities when subjected to high heat in welding.

Mild Steel

Power Screw Threaded Connection

For moving components such as the power screw and the supports, mild steel is used as it is a lot harder, making it more resistant to wear while being in contact with other components. It is cheap, strong and can be bended or welded. Mild steel ty pically contains a maximum of 0.25% Carbon, 0.4%-0.7% manganese, 0.1%-0.5% Silicon and some traces of other elements such as phosphorous, lead or sulphur. However, mild steel is also heavier and harder to machine compared to aluminium. Nonetheless, this should not be problem for our machine because most of these parts are small (little contribution to weight) and only minimal machining is required.

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Cost Analysis Cost of Product The cost of product is the cost of manufacturing the machine (not the prototype) including assembly and manufacturing costs. This cost is estimated based on the mass production of the machine. The prices for the components will be cheaper because they are purchased in bulk from distributor. Electrical Components Description Sanyo Denki 103H546-0440 Tamiya Plasma Dash Motor

No. 1. 2. 3. 4.

Item Stepper Motor DC Motor Bump Switch Photo-reflective Sensor

5.

Power Supply

6. 7.

DC Motor Controller Stepper Motor Controller Microcontroller Microcontroller Unit PIC18F4520 Microcontroller PICkit 2 Microcontroller Programmer Programmer Wire

8. 9. 10.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Quantity 2 2 2 2

Fairchild QRB1134 IR Photoreflector HW1288 Switching Mode 1 Multi Power Supply 15A Dual H-Bridge Junior 2 1 JS Motor Board 1

Non Electrical Components Grabbing Mechanism Aluminium and Mild Steel Lifting Mechanism Aluminium and Plastic Main Body Aluminium Wheel Speed Run Robot Wheels 8mm Caster Ball Bearing Pololu Ball Caster with 3/8” Wheel Metal Ball Bolt, Nut and Washer Mild Steel Side Rollers Aluminium Power Screw Mild Steel Spur Gear Plastic Worm Gear Plastic

1 1

Price (SGD) 48.00 20.00 0.30 6.00

40.00 35.00 55.00 65.00 60.00

2.00 Subtotal 329.30 1 1 2

35.00 20.00 25.00 30.00

2

6.00

4 2 2 3 1 Subtotal Total

2.00 6.00 2.00 0.90 0.30 127.2 456.5

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Cost of Prototype The cost of prototype is the cost for assembling and manufacturing the prototype model (not the mass produced product) not including components such as the motor controllers, microcontroller unit and microcontroller programmer since they are provided free of charge. This cost is estimated based the production of one model only. Therefore, the cost of the components will be higher because they are purchased in small quantities from retailers. Furthermore, Furthermore, our budget to produce the prototype is limited to SGD400.00.

Electrical Components Description Sanyo Denki 103H546-0440 Tamiya Plasma Dash Motor

No. 1. 2. 3. 4.

Item Stepper Motor DC Motor Bump Switch Photo-reflective Sensor

5.

Power Supply

6. 7.

DC Motor Controller Stepper Motor Controller Microcontroller Microcontroller Unit PIC18F4520 Microcontroller PICkit 2 Microcontroller Programmer Programmer Wire

8. 9. 10.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Quantity 2 2 2 2

Fairchild QRB1134 IR Photoreflector HW1288 Switching Mode 1 Multi Power Supply 15A Dual H-Bridge Junior 2 1 JS Motor Board 1

Non Electrical Components Grabbing Mechanism Aluminium and Mild Steel Lifting Mechanism Tamiya 70115 R/C Forklift Main Body Aluminium Wheel Speed Run Robot Wheels 8mm Caster Ball Bearing Pololu Ball Caster with 3/8” Wheel Metal Ball Bolt, Nut and Washer Mild Steel Side Rollers Aluminium Power Screw Mild Steel Spur Gear Plastic Worm Gear Plastic

1 1

Price (SGD) 78.00 30.00 0.60 10.00

70.00 Provided Provided Provided Provided

3.00 Subtotal 191.60 1 1 2

35.00 72.00 30.00 42.50

2

10.50

4 2 1 3 1 Subtotal Total

4.00 8.00 2.00 3.00 1.00 208.00 399.60

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Manufacturing Process Main Body

Top Piece - Mill an aluminium sheet to achieve the th e following dimensions.

Bottom Piece - Mill an aluminium sheet to achieve the following dimensions. From the

upper right hand corner of the work piece, we have to mill off 5mm x 3mm x 2mm of material from the sheet.

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Left Piece - Mill an aluminium sheet to achieve the following dimensions. From the upper

right hand corner of the work piece, we have to mill off 8mm x 42mm x 2mm of material from the sheet.

Right Piece - Mill an aluminium sheet to achieve the following dimensions. From the upper

right hand corner of the work piece, we have to mill off 8mm x 42mm x 2mm of material from the sheet. From the lower left hand corner of the work piece, we have to mill off 3mm x 28mm x 2mm of material from the sheet.

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Back Piece - Mill an aluminium sheet to achieve the following dimensions. From the lower

right hand corner of the work piece, we have to mill off 5mm x 30mm x 2mm of material from the sheet.

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Extreme Back Piece - Mill an aluminium sheet to achieve the following dimensions. Next, we

will need to drill 4 holes of diameter 3mm into the work piece with the following dimensions for the holes.

Guiding Vane – Mill – Mill an aluminium block to achieve the following dimensions. From the top

hand corner, mill off a right angle triangle of 8mm x 5mm x 8mm of material from the block.

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Final Assembly - Weld all the pieces together in the following manner to make the main

body

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Grabbing Mechanism Swivel Plate

Obtain an aluminium block with dimensions mentioned below. For all the work pieces to be machined, it is be advisable to obtain a block that is slightly larger than the said dimensions. This is to avoid having blocks that are too small to be machined into the final product.

Mark a line on the block such that the line is diagonal of the smallest cross sectional area as seen in the diagram below.

Using an angle mill or a modified clamp, mill the unwanted material off the aluminium block. The yellow line refers to the portion of the aluminium block that is to be milled off. The isometric view of the shape that we are interested in is also showed in the following diagram.

66 | P a g e

Mark a line such that the line will make a 50° a ngle with the horizontal datum.

Using an angle mill or a modified clamp, mill the aluminium block so as to achieve the following shape.

Mark a line such the final aluminium work piece will have a thickness of 2mm.

Using an angle mill or a modified clamp, mill the aluminium work piece so as to achieve the following shape.

67 | P a g e

Next, Mill 3 pieces of aluminium with the following dimensions.

st

Weld the 3 triangular pieces to the 1 work piece at specific distances of 0mm, 53mm and st

106mm from the Right Face of the 1 work piece. The final product will be as seen in the below diagram.

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Sliding Connection

Mill an aluminium sheet to achieve the following dimensions.

From the upper left and right hand corner of the work piece, we have to remove 1mm x 36mm x 2mm of material from the material.

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The yellow portion shows the portion of material that needs to be removed. The material that needs to be removed will be milled off. The following diagram shows the work piece after material removal.

We will need to leave a width of 5mm for the work piece. As such, we will need to mill off the work piece according to the following diagram.

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Next, we will need to mill off the work piece as shown in the following diagram.

Mill a mild Steel block with the following dimensions.

Drill a 5mm hole to the mild steel block in which the center of the circle is 4.5mm from the top of the block. The drilled block is shown in the following diagram. Using a screw thread device, we will be able to obtain screw thread that is of 5mm Coarse-Pitch Metric Thread. (Budynas & Nisbett, 2008)

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st

Once we have obtained the threaded block, weld the block on top of the 1 portion of the work piece to complete the sliding connection.

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Fixed Plate

Mill 3 pieces of aluminium sheet of thickness 2mm to achieve the following dimensions.

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Weld the 3 pieces of aluminium together to make the following work piece.

Mill 2 pieces of alumimum sheets so as to achieve the following dimensions. Using a Turning Machine, bore a hole of 5mm diameter. The centre of the hole is to be 4.5mm from the top of the aluminium sheet. The final product should be as seen in the diagrams below.

st

Using the 2 similar pieces that is Milled, weld the 2 pieces to the 1 work piece. Weld the pieces such that they are 11mm and 22mm away from the front of the work piece. As such, the following work piece will be obtained.

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Mill 2 small aluminium blocks to obtain the following dimensions.

Next, we need to mill off 6mm x 2 mm x 12mm of material off the small block. The final product is as shown in the diagram below.

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st

Using the 2 similar pieces that is milled, weld the 2 pieces to the 1 work piece. Weld the pieces such that they are of in line with the left and right lower corners as shown in the diagram. As such, the following completed fixed plate will be obtained.

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Construction Procedure and Testing This section will describe the steps we intend to take in the manufacture of our prototype. Various tests will be carried out as an integral part of the process to ensure its functionality and reliability. These tests, conducted throughout the developmental stages, are necessary because the actual performance may deviate from the desired performance, due to factors that we might have overlooked or were unable to foresee during the design stage. As such, we expect to continually modify and calibrate our machine until the desired performance for every component as well as the entire machine is achieved.

Control system

Microchip PIC microcontroller (Wikipedia, n.d.) (http://en.wikipedia.org/wiki/File:Microchip_PIC24HJ32GP202.jpg)

We will be using a PIC augmented microcontroller. Firstly, the input voltage from the power source has to be stepped down to 5V for the PIC to function. In addition, a reset button and an oscillator must be connected to it. For testing, initial wiring of the PIC to the reset button and the oscillator to the various sensors and actuators will be done on a breadboard.

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(Surplustronics, 2009) PICkit2 Programmer board (Surplustronics, (http://www.surplustronics.co.nz/shop/productimages/KS0050.jpg)

A PICkit2 programmer board will be linked to a computer via a USB cable and used to program the PIC. The software we intend to use is MPLAB v7.60 which uses C language. Because transfer of the chip between the programmers board and the breadboard will happen frequently, we intend to protect the fragile pins of the PIC with a 40-pin socket. s ocket.

IC socket 40-pin (Digikey.com) (http://rocky.digikey.com/weblib/Mill-Max/Web%20Photos/110-13-640-41-001000.jpg)

After programming the PIC, it will interface with the sensors and actuators of the robot. Since the sensors and actuators have integrated circuitry, the task at hand is to connect the respective pins together, e.g. power supply pins to the battery, control pins to the respective sensor and actuator pins etc. It should be noted that the PIC will have some pins serving as input pins to take in electrical signals (converted from physical stimulus by the sensors) and output pins providing electrical signals (to be converted into physical responses). The connections should be made to be fairly robust and able to withstand repetitive mechanical motion and vibrations. A PCB (Printed Circuit Board) board, which consists of metal lines fabricated onto a plastic board, serves as a stable platform for making such connections. The design and layout of the PCB will be done using a program, and the design file generated will be used by the fabrication machine to manufacture the PCB itself. Following that, an IC socket mount is soldered onto the PCB to secure the PIC, and a number of individual connectors are arranged at strategic locations on the PCB to allow for optimal 78 | P a g e

(and shortest) connections to the sensors, actuators and power source. This reduces the chance of noise generated within the wires. To control the actuators, the PIC is pre-programmed to respond to different conditions, depending on the sensor input. This will be an independent and automated process controlled by the PIC. Moreover, a power source s ource will provide a constant power supply to the PIC, sensor and actuators. This electrical signal, originating from a common source, is also channeled using the metal strips fabricated on the PCB. For additional safety to both the user and the electronic components, a power IC may be used to regulate and ensure a stable voltage regardless of the ambient temperature or other mitigating factors. Since the current logical process of the machine is not visible during its operation, LED lights will be placed on the PCB to indicate which ‘mode’ it is in, corresponding to the state diagram of the logic design. For instance, using 4 LEDs, if the machine is at rest, we can output a ‘0000’, i.e. no LEDs are on. If a machine is idling, its state is ‘0001’, i.e. the left most LED is on. In this case, we can set aside a maximum of 16 possible states. If more states are needed, more LEDs can be implemented on the PCB. Once the circuits are setup and tested individually, the integrated test will involve the operation of multiple components, e.g. power source and back wheels, power source and castor forklift etc. Once they are tested to work together, a final test is performed with the machine running autonomously. If the tests are successful, additional tests can be carried out in more stringent conditions, e.g. under the sun, in a windy or wet environment. This is to determine the overall performance capability and limits of performance.

Movement Our machine is mainly driven using the differential drive system and supported by the two caster ball bearing wheels. wheels . The testing for machine movement will require placing a weight which ideally represents exactly that of the rest of the machine and the blocks, onto the body. Both stepper motors will then be allowed to run, to determine if the torque provided is sufficient to drive the wheels of the machine forward. The distance travelled and the time taken by the machine will be measured to determine its travelling speed. Because our machine requires a certain degree of precision and stability, factors such as the initial starting movement and braking will be closely observed to ensure that its movements are not excessively jerky.

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Grabbing Mechanism For testing of the grabbing mechanism, the motor will first be initiated to drive the power screw connected to the threaded top sliding plate. The DC motor will provide enough driving force to the power screw to move the sliding plate, as well as to push the swivel plate (with its spring-loaded hinges) downwards. If necessary, the fitting of the sliding connection should be adjusted such that the movement of the top sliding plate is observed to be smooth, stable, and in the horizontal plane only. Furthermore, if the tip of the power screw is found to cause excessive friction with the plastic padding it rubs against, the tip of the screw might have to be smoothened. A spirit level will also be placed on the top plate to ensure that the grabbing mechanism is horizontal. This is important for the proper grabbing and depositing of the blocks. In the second testing phase, the grabbing mechanism will attempt to firmly clamp the 6 blocks placed in the manner as depicted on the specification of the actual arena (see details in the Pseudocode section). Preferably, these blocks will be the actual blocks used in the competition to ensure accuracy of the test. The entire grabbing process will then be tested to simulate the actual grabbing procedure. This ensures the stability of the grabber and the adequacy of the clamping strength of the swivel plate and frictional force provided by the rubber lining. Should any of the blocks slip or fall out at any time, testing should be repeated with an increased clamping force, or a replacement of the rubber lining with higher coefficient of friction. During the testing of the fabricated grabbing mechanism and before the attachment to the lifting mechanism, attention should be paid to ensure that the grabbing mechanism does not deform excessively under loading. In I n addition, if any critical components are found to be poorly attached during movement simulation, additional steps should be taken to secure them. The next step would be to programme the time required to raise the platform into the PIC microcontroller.

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Lifting Mechanism

Tamiya Plasma Dash Motor (Mini4WD,

n.d.)

Tamiya TAM70115 Remote-Controlled forklift (eTamiya, 2008) ( http://www.etamiya.com/shop/images/tamiya_forkids/70115.jpg http://www.etamiya.com/shop/images/tamiya_forkids/70115.jpg ) ( http://mini4wd.pokedream.com/info/motors/plasma_dash.jpg http://mini4wd.pokedream.com/info/motors/plasma_dash.jpg )

We intend to modify the ready-made Tamiya TAM70115 Forklift used for our lifting mechanism so that it meets the requirements of our machine. Firstly, the forklift mechanism will be removed from the original body. Next, the fork would be trimmed such that the grabbing mechanism can fit onto it. In addition, we intend to replace the stock motor in the forklift with the Tamiya Plasma Dash Motor . This motor would ideally provide the torque and rpm required for our lifting mechanism. For the testing of the lifting mechanism, the final grabbing mechanism would have to be first attached to the underside of the fork (refer diagram). The motor will then be initiated and tested to ensure that sufficient power is provided by the motor to lift the grabbing mechanism together with the 6 blocks. Fork

Grabber mechanism

In addition, the structure of the lifting mechanism will have to be strong and sturdy enough to withstand the weight of the grabbing mechanism together with the 6 blocks. Hence, tests would have to be carried out to ensure that the mechanism retains its stability over repeated testing. In a case where the structure of the forklift is found to fail or deform excessively, the parts involved would have to be replaced or reinforced by adding ribs or increasing its thickness. 81 | P a g e

A stopwatch will then be used to record the time required to lift the grabbing mechanism to the desired height. This information will be programmed into the PIC microcontroller to control the speed of the motor. The time required for the platform to return to its original position will also be recorded. When the lifting mechanism has been tested to perform as desired, it will be mounted onto the body of our machine.

Navigation system The components that make up the navigation system include the photo-reflective sensor, bump switches, guiding vanes and side rollers. r ollers.

Fairchild QRB1134 IR Photoreflector (Active Robots, 2009) (http://www.active-robots.co.uk/fairchild-qrb1134-ir-photoreflector-p-626.html)

The photo-reflective sensor that we have chosen to use is the Fairchild QRB1134 IR Photoreflector due due to its high sensitivity, required area of optimum response (~5mm radius) as well as suitable output voltage (~5V). (~5V) . The sensors’ sensitivity sensit ivity must first be calibrated on the breadboard by changing the connecting resistors and testing the voltage output with a voltmeter. The phototransistor in the sensor responds to radiation from the emitting diode when a reflective object passes within its field of view. Hence, the sensor should generate a distinct high or low voltage depending on whether it senses a reflective or non-reflective surface. This calibration will be performed both under indoor lighting and daylight conditions, and their respective values stored in the PIC microcontroller. A physical latch switch connected to one of the PIC’s port will be used to indicate the lighting condition and hence which value to be used during the actual competition. To confirm the accuracy of the logic, the photo-reflective sensor, and both the motors of the wheels should be mounted onto the breadboard and tested. To do this, both a black and white surface will be placed over the sensor one at a time, and the reaction of the motors will be observed to see if it navigates along the white and black surfaces as required.

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When both bump switches are activated, the circuit will be closed and a voltage will be generated and detected at the microcontroller to activate the forklift motor and deactivate the wheel motors. Hence, to test the bump switches, we will similarly connect it to a breadboard and calculate the voltage reading using a voltmeter. When these values are obtained, they can then be input into the PIC. The various components of the navigation system w ill then be fitted onto the specified parts of the body. Next, the system will be tested out, with particular attention paid to the turning phase, such that optimum angle at which the machine turns can be determined. In addition, the positioning of the side rollers to aid in its turning can also be adjusted for them to work better. The machine will be required to position itself in front of the platform with the help of the guiding vanes and the bump switches. Hence, at this stage, we will be looking into the effect of both the components and possibly improve on their designs.

Entire Structure Finally, after every component has been tested and the required modifications have been made for it to work, the assembly will be done, after which there will be several additional tests to ensure that the machine performs according to the design. Firstly, we will examine the weight distribution of the machine to verify that it remains stabile, even as when it is in motion, especially in critical movement phases. In a case where it is not, we intend to rearrange the components or add counter weights to achieve stability. Besides that, there should not be any undesirable interference between the different components.

Further testing In the final phase of testing, we will attempt to construct the competition arena according to the specification, specification , and study the machine’s performance in it. At this point, we will be looking at ways to fine tune and enhance the performance of our machine. This could be done by increasing its speed, reliability or accuracy, and perhaps looking into whether an increase in the voltage supply to the motors is a vi able option. If the need arises, we might explore the possibility of our machine taking an alternative path, or employing a different strategy altogether.

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Strengths and weaknesses Component Grabbing Mechanism

Strengths 



Grabbing from the top is a creative approach as

Weaknesses 

The sliding connection may be prone to

compared to the conventional side grabbing approach.

misalignment due to vibration. Therefore,

This design is able to collect 6 blocks at one go, thus

the parts have to be machined accurately.

increasing the efficiency and speed in blocks co llection.



Friction generated at the supports will result in some wear and tear.



Motor mounted on top of the grabbing mechanism will increase its weight. This may cause undesirable deflection on the fork of the lifting mechanism. This effect is minimized by mounting the motor to the back as much as possible.

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Navigation





The navigation system of our machine is less susceptible



To stack the 6 blocks on top of one

to accumulation of errors which will affect the accuracy

another, our machine will have to deposit

of our machine’s positioning.

This is because our

the blocks at the same position on the

machine uses the wall acting as an alignment guide,

platform every time. Therefore, our

photo-reflective sensors to check its immediate position,

machine has to travel extra distance

and bump switches and guiding vanes to position itself

instead of approaching the platform from

around the platform.

the

Compared to navigating the arena using retro-reflective

necessary to ensure the precision in

tape only, which is slow due to continuous correction

stacking up the blocks.

nearest

side.

However,

this

is

from feedback loops, our machine uses a fast hybrid navigation which avoids long routes with precise programming. At the same time, it checks for important intermediate locations using its photo-reflective sensors 

While collecting the blocks, this approach is fast since it does not use the conventional route of following the retro-reflective tape directly in the front of the 3 blocks. Instead, our machine will push the first 3 blocks until the blocks touch the next 3 blocks before grabbing all the 6 blocks at one go.

External Power Supply





Eliminates the need for recharging or replacement of



Distance from power source needs to be

batteries. This will save cost in the long run and

taken into account to ensure that the wire

minimizes wastage.

is sufficiently long.

Voltage can be stepped up or down to suit the machine’s



Wires might get in the way of the machine 85 | P a g e

requirement.

while it is moving or it might get



A steady voltage and current can be s upplied.

entangled in moving parts.



No battery on the machine can significantly reduce the



Proper covers for gears are necessary to avoid entanglement. Careful securing of

weight of the machine.

wires is also required to prevent it from getting in the way during movement. Differential Drive System



Lightweight.



Able to turn on the spot.



Faster and lesser energy losses in comparison to tracked



Inability to travel in a straight line due to the difference in the motors’ efficiencies.

wheels. Multiple Sensors & Switches



Ensures accuracy in the machine’s positioning.



Helps to ensure that the position and alignment is correct before grabbing or releasing the blocks.





Installation of sensors incurs additional cost.



Ensuring

desired

feedback

is

time

consuming.

Checking of position each time also minimizes the accumulation of errors.

Parallel Guiding Vanes





Ensures alignment with platform for precise depositing of



Requires design to incorporate some

blocks.

allowance or space in front of the main

One side is tapered to offset the grabbed blocks back to

body, thus the size of the body needs to

the center of the grabbing mechanism

be shorter to ensure that the size of the machine is within the specification.

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Plastic Padding



The plastic padding is used to protect the swivel plate of



the grabbing mechanism from wear and tear. 

Replacement of plastic padding uses additional resources.

It is also easily replaceable, thus there is no need to change the whole grabbing mechanism altogether.

Rubber Lined Grabbing Plates

Side Rollers





Ensures better grip on the blocks by increasing the



Design of grabber mechanism has to allow

coefficient of friction between the blocks and the

for some thickness due to the rubber

grabbing mechanism.

lining.

The side rollers reduce friction between machine and



The side rollers will introduce dimensional

wall, thus improving the machine efficiency. It also allows

constraint to our machine, as it will cause

our machine to align itself with wall smoothly after

the grabbed blocks to be slightly offset

turning.

from the central axis of the grabbing mechanism. 

However, the side rollers are essential in reducing friction and side alignment. Besides, we managed to offset the blocks back to the center using a tapered guiding vane on the main body.

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Suggested Improvements Photoresistors Currently, our machine is designed to stop its grabbing movement by estimating and preprogramming the time required for the motor to close the swivel plate into a 90° fit.

Open clamp position

90° fit position

Instead of using pre-programmed timings, we can look into incorporating photoresistors, which can accurately deactivate the grabber motor so that it does not overrun and cause damage to the moving parts. A photoresistor is a resistor whose resistance decreases with increasing light intensity. intensity. Thus we will consider consider two resistances, one at ambient ambient lighting, and one with no light (fully covered by sliding connection). These two resistances would result in two distinct voltages generated, generated , allowing the motor to be either “on” or “o ff”. The photoresistor comes into use when the machine is grabbing the blocks. As the swivel plate is being lowered, this causes the sliding connection to move towards the photoresistor.

Direction of motion When the swivel plate reaches the 90 ° fit, the sliding connection covers the photoresistor entirely. The photoresistor will then generate high resistance (no light condition), lowering the potential and causing the motor to be switched off (refer to picture below).

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Motor off

Photoresistor covered by sliding

When the swivel plate is moving towards the fully opened position, the sliding connection is now moving away from the photoresistor. When the sliding connection is fully opened, the resistance decreases to the ambient lighting value.

Direction of motion

The logic for the switch is now reversed, i.e when the resistance decreases back to the ambient value (indicating the sliding connection is fully opened), the motor is now turned off.

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Linear Actuator

Linear Actuator (DirectIndustry, 2008) (http://news.directindustry.com/press/lim-tec-beijing-transmission-equipment-co-ltd/lim-tec-company-linearmotion-expert-56033-32354.html)

We can also consider the use of a miniature linear actuator to replace the power screw used in the grabbing mechanism. A linear actuator will ideally incur less energy losses. It should also eliminate the need for a complex design f or the grabbing mechanism, thus saving space and materials besides increasing the ease of manufacturability of the mechanism. However, the cost of a miniature DC electric linear actuator is about USD 70-80 (E-Motion LLC, 2006). If used in our design, we will most likely exceed our given budget. Therefore, unless we can find a way to reduce the cost for the other components, using a linear actuator in our grabbing mechanism is currently not cost-feasible.

Conclusion The machine we have designed has been thoroughly analyzed to ensure that it meets the desired specifications and is able to efficiently achieve the objective of block collection. The machine employs an unconventional creative approach in grabbing the blocks, uses an efficient and accurate hybrid navigation system to maneuver around the arena and has safety features to ensure the blocks are placed precisely. The next phase of this project would then be to manufacture, programme and rigorously test the machine to ensure it achieves the designed objective.

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Appendix A1 – Machine Machine Assembly Drawing

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Appendix A2 – Subassemblies Subassemblies Drawings

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Appendix B1 – Calculations Calculations for Grabbing Mechanism Calculation of Gripping Force

Vertical equilibrium, 2 f = 6mg f = 3mg = 1.2 N μR = 1.2 ; μ = coefficient of friction = 0.7 (between rubber & wood) R = 1.714 N  Gripping force needed to hold blocks in place

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Calculation of Torque for Grabbing Mechanism Motor Spring with reference from (Budynas & Nisbett, 2008) Due to its relatively lower price, spring made of A 227 steel is chosen. Specifications of the spring are as follows: Spring outer diameter = 3 mm Wire diameter, d = 0.8 mm Number of turns = 12 Based on formulas and values taken from tables, Strength of spring, σ allowed = 0.78 x Sut ; Sut = tensile strength m = 0.78 x A / d 0.19 = 0.78 x 1783 / 0.8 = 1450.97 MPa To calculate the tension/bending moment in spring, a few values are first obtained, Spring mean diameter, D = 3 mm – mm – 0.8 0.8 mm = 2.2 mm C = D/d = 2.75 2 2 Spring stress-correction factor, K = (4C + C – C – 1) 1) / (4C – 4C) – 4C) = 1.6623 3 From spring bending equation, σ = 32KFr / πd , Bending moment in spring, M y = Fr 3 = σ πd / 32K = 43.875 Nmm = 0.043875 Nm

Force required to bend spring, F

=

My / r

= =

0.043875 / (8.24 x 10 ) 5.325 N

-3

Friction due to movement of Sliding Connection f 2

=

μR

;

= = =

0.6 x mg 0.6 x 0.06 x 10 0.36 N

μ

= =

coefficient of friction 0.6

Hence, Hence, total load = ∑Force = 1.714 + 2 x 5.325 + 0.36 = 12.72

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Power Screw Calculations with reference from (Budynas & Nisbett, 2008) Specifications Specifications of screw used: Screw mean diameter, d m = 3 mm Screw lead, l = 1.5 mm Amount of torque needed to provide efficient force to grab blocks, TL = (Fdm / 2) (πfd m – l) / (πd m + fl) = (12.72 x 0.003 / 2) x 0.004155 / 0.01032 = 0.007685 Nm = 7.685 mNm

Name

Rpm (with load)

Rpm (without load)

Torque

Power Consumed

Plasma Dash

25000

29000

20 g.cm

4100

Based on the above figure, the motor of our choice provides a torque of 20 g.cm, which is equivalent to 1.962 mNm. As such, in order to have our grabbing mechanism functions well, a gear ratio of 7.685 / 1.962 = 3.92 should be used to provide sufficient torque to the worm gear.

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Appendix B2 – Calculations Calculations for Lifting Mechanism Due to time constraint, it is desired that our machine’s lifting mechanism can raise the grabbed wooden blocks for 100 mm in seven seconds. Thus, raise speed = 0.1 / 7 -1 = 0.0143 ms x 60 = 0.857 m/min The screw of the forklift has Acme thread and a pitch of 1.5 mm, -3 So, screw speed = 0.857 / 1.5 x 10 = 571.43 rpm

Name

Rpm (with load)

Rpm (without load)

Torque

Power Consumed

Plasma Dash

25000

29000

20 g.cm

4100

Based on the above figure, the motor speed under load is 25000 rpm. Speed ratio = 25000 / 571.43 = 43.75 = Gear ratio Total load on screw of the forklift

= = =

weight of six blocks + weight of grabber (6 x 0.04 + 0.3) x 9.81 5.2974 N

By referring to mechanical handbook Shigley’s Mechanical Engineering Design (Budynas & Nisbett, 2008), Screw torque, TR = (Fdm / 2) (πfd m sec α + l ) / (πd m – f – f l sec α) dm f l α TR

Collar torque, T c

= = = = = = = = =

pitch diameter 3 mm coefficient of friction 0.6 lead 1.5 mm 14.5° 7.9461 x 7.341 / 8.495 6.866 mNm

= = = =

Ff cdc / 2 ; f c 0.6 5.2974 x 0.6 x 4 / 2 6.357 mNm

=

friction coefficient

Total torque = ∑T i = 6.866 + 6.357 = 13.223 mNm 102 | P a g e

Hence, Tmotor = = =

Tscrew / speed ratio 13.223 / 43.75 0.302 mNm

Again, from the previous figure, motor Plasma Dash can provide torque of magnitude 20 g.cm which is equivalent to 1.962 mNm. This gives a safety factor of 1.962 / 0.302 = 6.5, which is good enough to convince us that our lifting mechanism can definitely be driven by Tamiya Plasma Dash motor.

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Appendix B3 – Calculations Calculations for Driving Mechanism Calculation of Torque for Differential Wheels (Stepper Motor)

Due to our machine’s capability capabi lity of carrying six six wooden blocks at one go, given three minutes, our team aim to have a total of eighteen blocks stacked by making three trips to the centre raised platform. To have everything done within the time limit, each trip needs to be completed in average of one minute. The third trip, trip , being the trip with longest distance covered, is thus being analysed as the wheel-driving stepper motors are supposed to drive the machine at a higher velocity and acceleration to have the task done. The following f igure depicts the path that will be taken by our machine in its third trip, trip , collecting those orangecoloured blocks along the way.

From the dimensions given, total distance covered in this trip is approximately 2700mm. Excluding operating time of mechanisms such as c losing of clamp, rising of the grabbed blocks et cetera, we expect our machine to have 30 second as its travelling time. Hence, travelling speed, v

= =

2.7 / 30 -1 0.09 ms

However, due to the fact that this is the average velocity, we need to cater time for acceleration and deceleration of the machine. Hence, it would need to travel at a higher -1 speed e.g. 0.1 ms .

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-2

Desired acceleration, a = 0.1 ms With this acceleration, it takes 1 second for our machine to reach top speed. Since wheel diameter = Number of wheel revolution = = = Hence, motor speed, ω = =

51.3 mm, 2700 / wheel circumference 2700 / 161.2 16.75 = 16.75 x 2π / 30 3.509 rads-1 33.51 rpm

Free Body Diagram of Machine

Note: Q and P are the action-reaction forces between wheels and machine body In this analysis, only level ground will be considered as the arena is assumed to be perfectly horizontal with zero inclination. Considering caster bearing wheel, Take moment about centre, F2r = Iα ; 2 F2 = Ia / r ; = (2/5) ma

α = Isphere =

a/r 2 2mr / 5

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= = =

2/5 x 0.01 x 0.1 -4 4.0 x 10 N P2

Considering main machine body, Taking horizontal kinematics, 2 (P1 – P – P2) = Mtotala P1 = Mtotala / 2 + P2 -4 = 1.5 x 0.1 / 2 + 4.0 x 10 = 0.0754 N = F1 Considering rear wheel, Taking moment about centre, –F –F1r + M = Iα ; M

= = = =

α Idisk

= =

a/r 2 mr / 2

F1r + Iα ; -3 0.0754 x 51.3 x 10 / 2 + mra / 2 -3 1.934 x 10 + 0.04 x 0.02565 x 0.1 / 2 -3 1.985 x 10 Nm Torque =

1.983 mNm

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Appendix B4 – Stability Stability Analysis

Forward velocity of machine, u

=

0.1 ms

-1

Assuming that the machine takes 0.5s to come to complete rest, a = -0.1 / 0.5 -2 = -0.2 ms From vertical equilibrium, Na + Nb = total weight = Wb + W = 1.44 x g Na + Nb = 14.13 N By taking moment about A, -Wtotal x 0.0455 + Nb x 0.0505 = Nb =

ma x 0.068 13.12 N

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Hence, Na

= = =

Wtotal – N – Nb 14.13 – 14.13 – 13.12 13.12 1.01 N

The fact that N a > 0 indicates the rear wheels are still in contact with the ground and it can be concluded that the machine will not topple when being forced to a stop while travelling -1

at a speed of 0.1 ms .

Justification: -3

-3

Counter-clockwise Counter-clockwise moment about B = =

53 x 10 x Wb + 50.5 x 10 Na 0.176 Nm

Clockwise moment about B

W x 17 x 10 0.204 Nm

= =

-3

Since clockwise moment > counter-clockwise moment, the machine will not topple over, as concluded. Also, since the machine will not topple in the phase w hen it is moving at the highest speed rd (the 3 and final trip to the raised platform), it can be induced that our machine will be stable throughout the blocks collecting and stacking process.

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Appendix C1 – Specification of Sanyo Denki 103H5460440

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Appendix C2 - Schematic Diagram and Manual for JS Motor Board

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Appendix C3- Datasheet of QRB1134 IR Photoreflector

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