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

11. Arm: Power Example 125 Watts, 100 RPM 250 Watts, 200 RPM
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. Anatomy of an Arm Aluminum or plastic structural tubing 4 bar
Notes:
Chain & belt tensioning challenge
Watch robot rules for expanding outside frame perimeter
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 Scissor 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. 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
Slider
(Stage3)
Stage2
Stage1
Base 12

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

20. 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
Slider
(Stage3)
Stage2
Stage1
Base 10

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

22. Scissor Lifts Advantages Disadvantages I do not recommend this!
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 Feature Arm Lift Reach over object Yes No
Fall over, get up
Yes, if strong enough
Go under barriers
Yes, fold down
Maybe, limits lift height
Center of gravity (Cg)
Not centralized
Centralized mass
Small space operation
No, needs swing room
How high?
More articulations, more height (difficult)
More lift sections, more height (easier)
Complexity
Moderate
High
Powerful lift
Combination
Insert 1-stage lift at bottom of arm  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 ( max)
Belt / chain / gear drive
Not for quick 1x scoring
Good for stream shooting
Good for placing over blocker 1

30. Thank you! 1