1. Building competitive manipulators
Greg Needel
Mentor teams: 131, 1511

2. Strategy, Strategy, Strategy!
Read the rules
Outline the game objectives
Look for the “gimmie” robot design
Try small simulations
Whatever you choose STICK WITH IT!

3. Types of Manipulators Articulating Arms Telescoping Lifts Grippers
Latches
Ball Systems

4. Arm: Forces, Angles & Torque
Example #1 - Lifting
Same force, different angle,
less torque
10 lbs
< D
10 lbs D

5. Torque Torque = Force x Distance
10 lbs
< D
Torque = Force x Distance
The farther something is from the pivot the harder it is to lift
Power is more important then Torque

6. Power Power = Force x Distance / Time
Power = Torque x Rotational Velocity
Power (FIRST def.) – how fast you can move something

7. Arm: Power Example Same torque, different speed 10 lbs 10 lbs
0.2 HP, 200 RPM Motor w/ 1” sprocket OR
100 RPM w/ 2” sprocket
0.1 HP, 100 RPM Motor w/ 1” sprocket

8. Arm Design
“Arm”: device for grabbing & moving objects using members that rotate about their ends
Think of your materials (thin wall is good)
Every Pivot has to be engineered (less is more)
Linkages help control long arms.
Use mechanical advantage (it is your friend)
Think of the drivers (pivots on pivots are hard)
Operator Interface (keep this in mind)

9. Arm Advice K.I.S.S. doesn’t mean bad Feedback Control is HUGE
Potentiometers, encoders, limits
Automatically Take Action Based on Error
Design-in sensors from the start of design
Think outside the box.
Off the shelf components are good (andymark.biz, banebots.com )

10. Four Bar Linkage
Pin Loadings can be very high Watch for buckling in lower member Counterbalance if you can Keep CG aft

11. 4 bar linkage example :

12. Arm Example: 234 in 2001

13. Arm Example: 330 in 2005

14. Arm Example: 1114 in 2004

15. Telescoping Lifts
Extension Lift
Scissor Lift

16. Extension

17. Extension Lift Considerations
Should be powered down AND up
If not, make sure to add a device to take up the slack if it jams
Segments need to move freely
Need to be able to adjust cable length(s).
Minimize slop / free-play
Maximize segment overlap
20% minimum
more for bottom, less for top
Stiffness is as important as strength
Minimize weight, especially at the top

18. Extension - Rigging
Cascade
Continuous

19. 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

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

21. Extension: Cascade Rigging
Up-going and Down-going 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
I do not prefer this one!
Slider
(Stage3)
Stage2
Stage1
Base

22. Team 73 in 2005, Team 340 in 2004

23. Scissor Lift

24. Scissor Lift Considerations
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 recommend you stay away from this!

25. Team 158 in 2004

26. Arm vs. Lift Feature Arm Lift Reach over object Yes No
Fall over, get back up
Yes, if strong enough
Go under barriers
Yes, fold down
No, limits lift potential
Center of gravity (Cg)
Can move it out from over robot
Centralized mass over robot
small space operation
No, needs swing room
How high?
More articulations, more height (difficult)
More lift sections, more height (easier)
Complexity
Moderate
High
Accumulation
1 or 2 at a time
Many objects
Combination
Insert 1-stage lift at bottom of arm
<-

27. Braking: Prevent Back-driving
Ratchet Device - completely lock in one direction in discrete increments - such as used in many winches
Clutch Bearing - completely lock in one direction
Brake pads - simple device that squeezes on a rotating device to stop motion - can lock in both directions
Disc brakes - like those on your car
Gear brakes - applied to lowest torque gear in gearbox
Note : any gearbox that cannot be back-driven alone is probably very inefficient

28. Power Summary BUT, no power transfer mechanisms are 100% efficient
All motors can lift the same amount (assuming 100% power transfer efficiencies) - they just do it at different rates
BUT, no power transfer mechanisms are 100% efficient
Inefficiencies (friction losses, binding, etc.)
Design in a Safety Factor (2x, 4x)

29. Grippers Gripper (FIRST def) grabbing game object How to grip
How to hang on
Speed
Control

30. How to grip Pneumatic linkage grip Motorized grip Roller grip
1 axis
2 axis
Motorized grip
Roller grip
Hoop grip
Pneumatic grip

31. Pneumatic linear grip
Pneumatic Cylinder extends & retracts linkage to open and close gripper
254 robot: 2004, 1-axis
968 robot: 2004, 1-axis
Recommended

32. Pneumatic linear grip
Pneumatic Cylinder, pulling 3 fingers for a 2-axis grip
60 in 2004
Recommended

33. Motorized Linear Grip Slow More complex (gearing) Heavier
Doesn’t use pneumatics
49 in 2001
Not
recommended

34. Roller Grip Slow Allows for misalignment when grabbing Won’t let go
Extends object as releasing
Simple mechanism
45 in 98 and 2004
Recommended

35. Hoop grip Slow Needs aligned Can’t hold on well 5 in 2000 Not
recommended

36. Pneumatic Grip Vacuum: generator & cups to grab Slow Not secure
Not easy to control
Simple
Problematic
Not recommended

37. Hang on! Friction: High is needed (over 1.0 mu)
Rubber, neoprene, silicone, sandpaper
Force: Highest at grip point
Force = multiple x object weight (2-4x)
Linkage, toggle: mechanical advantage
Extra axis of grip = More control

38. Speed Quickness covers mistakes Fast Not fast Quick to grab
Drop & re-grab
Fast
Pneumatic gripper
Not fast
Roller, motor gripper, vacuum

39. Grip control Holy grail of gripping:
Get object fast
Hang on
Let go quickly
This must be done under excellent control
Limit switches
Auto-functions
Ease of operation

40. Latches
Spring latches
Hooks / spears
Speed & Control

41. Latch example: 267 Pneumatic Latch 2001 game Grabs pipe
No “smart mechanism”

42. Latch example: 469 Spring-loaded latch Motorized release
Smart Mechanism
2003

43. Latch example: 118 Spring-loaded latch Pneumatic release
Smart mechanism
2002

44. Latching advice
Don’t depend on operator to latch, use a smart mechanism
Spring loaded (preferred)
Sensor met and automatic command given
Have a secure latch
Use an operated mechanism to let go
Be able to let go quickly
Pneumatic lever
Motorized winch, pulling a string

45. Ball Systems Accumulator = rotational device that pulls objects in
Types:
Horizontal tubes - best for gathering balls from floor or platforms
Vertical tubes - best for sucking or pushing balls between vertical goal pipes
Wheels - best for big objects where alignment is pre-determined

46. Conveying & Gathering
Conveyor - device for moving multiple objects, typically within your robot
Types:
Continuous Belts
Best to use 2 running at same speed to avoid jamming
Individual Rollers
best for sticky balls that will usually jam on belts and each other

47. Conveyors Rollers Use individual rollers Adds weight and complexity
Double Belts
Use pairs of belts
Increases size and complexity
Single Belt
- Use a slippery material for the non-moving surface

48. Ball System Tips More control is better
Avoid Gravity feeds
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

49. Roller example: 188

50. Accumulator example: 173 & 254

51. Questions? Thanks to: Andy Baker (45) Raul Olivera (111)

52. Extra Stuff Pneumatics vs. Motors Materials Shapes / Weights
Fabrication processes
Environment

53. Pneumatics vs. Motors Some, but not all important differences
Cylinders use up their power source rather quickly
the 2 air tanks we are allowed do not hold much
Motors use up very little of the total capacity of the battery
Cylinders are great for quick actuations that transition to large forces
Motors have to be geared for the largest forces
Our ability to control the position of mechanisms actuated by cylinders is very limited
We are not given dynamic airflow or pressure controls
We are given much more versatile electronic controls for motors
Since air is compressible, cylinders have built-in shock absorption
Cylinders used with 1-way valves are great for Armageddon devices - stuff happens when power is shut off
This could be good or bad - use wisely

54. Materials Aluminum, thin-wall tubing Polycarbonate sheet, PVC tubing
Fiberglass (used rarely, but advantages)
Spectra Cable
Stronger than steel for the same diameter
Very slippery
Easy to route
Needs special knots to tie
Can only get it from Small Parts and select other suppliers
Pop Rivets
Lighter than screws but slightly weaker - just use more
Steel and Aluminum available
Great for blind assemblies and quick repairs

55. Shapes
Take a look at these two extrusions - both made from same Aluminum alloy:
Which one is stronger?
Which one weighs more?
1.0”
0.8”
1.0”
0.8”
Hollow w/ 0.1” walls
Solid bar

56. Shapes, cont. The solid bar is 78% stronger in tension
The solid bar weighs 78% more
But, the hollow bar is 44% stronger in bending
And is similarly stronger in torsion

57. Stress Calculations It all boils down to 3 equations: Bending Tensile
Shear
Where:
 = Bending Stress
M = Moment (calculated earlier)
I = Moment of Inertia of Section
c = distance from Central Axis
Where:
 = Tensile Stress
Ftens = Tensile Force
A = Area of Section
Where:
 = Shear Stress
Fshear = Shear Force
A = Area of Section

58. Structural Shapes
I am willing to bet that none of our robots are optimized with respect to strength to weight ratios
We all have more material than we need in some areas and less than we need in others.
It would take a thorough finite element analysis of our entire robot with all possible loading to figure it all out
We only get 6 weeks!!
But, this does not mean we cannot improve

59. Fabrication Processes
Laser cutting causes localized hardening of some metals
Use this to your benefit when laser cutting steel sprockets
Cold forming causes some changes in strength properties
Some materials get significantly weaker
Be aware of Aluminum grades and hardness's
Welding - should not be a problem if an experienced welder does it