Manipulator and Appendage Design Tips for Robotics Competitions

1. Manipulator and Appendage Design Andy Baker Sept. 2018

2. Andy Baker • President and Owner, AndyMark • FIRST Mentor since 1998 • Mechanical Engineer, University of Evansville, ‘91 • FIRST Championship WFA winner, 2003 28

3. Types of Manipulators • Conveying and Gathering • Ball (or object) Handling • Ball shooting • Winches • Turrets • Collectors • Rollers • Gripper • Latch • Vacuum • Lifts • Articulating Arms • Telescoping • Scizzor 27

4. Collectors • Horizontal Roller • High roller rpm, producing > robot’s driving speed • Wide • Funneling mechanism • Outside or inside? 26

5. Collectors • Horizontal Roller • Outside • Better collecting ability • Easier to break • Inside • Protected • Weight saving • Not defensible 25

6. Grippers • Grabbing a scoring object (not a field component) • Grab 1 item • aka pick and place, aka touch it, grab it • Speed to grab < 1 second • Holding force > 2x object weight • Designed-in sensors detect object presence • Lightweight (usually on an arm or lift system) • Robust: bend, not break • Easily repairable (make many spares) • High coefficient of friction w/ game piece, > 1 24

7. Grippers 23

8. Latches • Grab field component (hanging bar, bridge, wall, etc.) • Sometimes not legal! • Failsafe, secure latch • Often letting go after the match, but this needs to be done quickly • Use a smart mechanism • Spring loaded (preferred) • Sensor met and automatic command given • Use a manual lever, switch to let go 22

9. Articulating Arms Shoulder Elbow Wrist 21

10. D Arm: Forces, Angles & Torque 10 lbs Example: Lifting at different angles • Torque = Force x Distance • Same force, different angle, less torque 10 lbs < D 20

11. Arm: Power Example • Same torque w/ Twice the Power results in Twice the Speed • Power = Torque/ Time • Be conservative: design in a safety factor of 2x or 4x 10 lbs 10 lbs 125 Watts, 100 RPM 250 Watts, 200 RPM 19

12. Arm: Design Tips • Lightweight Materials: tubes, thin wall sheet • Design-in sensors for feedback & control • limit switches and potentiometers • Linkages help control long arms • KISS • Less parts to build or break • Easier to operate • More robust • Use off-the-shelf items • Counterbalance • Spring, weight, etc. 18

13. Notes: Chain & belt tensioning challenge Watch robot rules for expanding outside frame perimeter Anatomy of an Arm Aluminum or plastic structural tubing 4 bar Motor / gearbox position choices Push rod Position and motion control sensors Counterweight or spring / latex tubing Game object sensors 17

14. Four Bar Linkage • Pin loadings can be very high • Watch for buckling in lower member • Counterbalance if you can • Cg moves forward & backward (watch tipping) • Limited rotation • Keeps gripper in known location 16

15. Telescoping Lifts • Extension Lift • Motion achieved by stacked members sliding on each other • Scissor Lift • Motion achieved by “unfolding” crossed members 15

16. Extension Lift Considerations • Drive cables up AND down, or add a cable recoil device • Segments must move freely • Cable lengths must be adjustable • Minimize slop and free-play • Maximize segment overlap • 20% minimum • more for bottom, less for top • Stiffness and strength are needed • Heavy system, overlapping parts • Minimize weight, especially at the top 14

17. Extension - Rigging Cascade Continuous 13

18. Slider (Stage3) Stage2 Stage1 Base Extension: Continuous Rigging • Cable Goes Same Speed for Up and Down • Intermediate Sections sometimes Jam • Low Cable Tension • More complex cable routing • The final stage moves up first and down last 12

19. Slider (Stage3) Stage2 Stage1 Base Extension: Continuous Internal Rigging • Even More complex cable routing • Cleaner and protected cables 11

20. Slider (Stage3) Stage2 Stage1 Base Extension: Cascade Rigging • Upward and Downward Moving Cables Have Different Speeds • Different Cable Speeds Can be Handled with Different Drum Diameters or Multiple Pulleys • Intermediate Sections Don’t Jam • Much More Tension on the lower stage cables • Needs lower gearing to deal with higher forces 10

21. Purchased Elevator Lift Systems • Many COTS versions available now 9

22. Scissor Lifts • Advantages • Minimum retracted height - can go under field barriers • Disadvantages • Tends to be heavy to be stable enough • Doesn’t deal well with side loads • Must be built very precisely • Stability decreases as height increases • Loads very high to raise at beginning of travel • I do not recommend this! 8

23. Arm vs. Lift 7

24. Conveying & Gathering • Conveyor - device for moving multiple objects, typically within your robot • Continuous Belts • Use 2 at same speed to avoid jamming • Individual Rollers • Best for high traction balls, which can jam 6

25. Conveyors Why do balls jam on belts? • Sticky and rub against each other as they try to rotate along the conveyor Solution #1 • Use individual rollers • Adds weight and complexity Solution #2 • Use pairs of belts • Increases size and complexity Solution #3 - Use a slippery material for the non-moving surface (Teflon sheet works great) 5

26. Ball System Tips • More control is better • Avoid gravity feeds – these WILL jam • Try to reduce “random” movements • Not all Balls are created equal • Balls tend to change shape • Building adaptive/ flexible systems • Speed vs. Volume • Optimize for the game and strategy • The more capacity, the better 4

27. Ball Shooting Systems • Secure shooting structure = more accuracy • Feed balls individually, controlling flow • Turret allows for aiming • Sensors detect ball presence, direction • Types: • Single Wheel • Double Wheel • Catapult 3

28. Winches & Lifts • Raise wheels with articulating arm • Raise robot with winch (and lock it up) • Design in a lock to prevent back drive 2

29. Turret • Position fine tuning • Must be VERY robust • Difficult to repair • Slow moving is ok • Limited rotation (90-180 max) • Belt / chain / gear drive • Not for quick 1x scoring • Good for stream shooting • Good for placing over blocker 1

30. Thank you! 1