Compilation of approaches used for FIRST robotics game piece manipulating systems. Will briefly discuss pro’s, con’s and design trade offs for a number of different manipulator designs.

1. Manipulators for FIRST
FRC Robotics
FIRST Fare 2012
Bruce Whitefield
Mentor, Team 2471

2. Manipulate What ?
Game pieces come in many sizes and shapes

3. Hanging
Manipulate How ?
Game objectives change each year
Lifting Dumping
Throwing
Kicking Gathering

4. Stuff That Don’t Work
Know the possibilities Things that Manipulate
Look on line, at competitions
Manipulators that
Talk to mentors, teams
work for the game
This seminar
Know the Objectives
Game pieces
Game & robot rules
Your game strategy
Your
Design
Know your capabilities
Tools, Skills, Materials,
Manpower , Budget , Time Manipulators you
can build
Don’t be this team

5. Many Types of Manipulators
 Heavy lifting / Long reach Reoccurring
o Articulating Arms
o Parallel arms
themes in FIRST
o Telescoping Lifts games
 Grippers
o Rollers
o Clamps
o Claws FIRST definition for
 Collect and Deliver a manipulator:
o Accumulators & Conveyers
o Turrets
o Shooters & Kickers
o Buckets & Tables
Device that
 Power & Control moves the game
o Winches piece from where
o Brakes
o Latches it is to where it
o Pneumatics
o Springs and Bungee
needs to be
o Gears & Sprockets

6. Arms
Shoulder
Elbow
Wrist

7. Torque: Angle and Distance
 Torque = Force x Distance
o Same force, different angle = different torque
o Measure from the pivot point
o Motor & gearing must overcome torque W= 10 lbs
o Maximum torque at 90 degrees
Torque = W x D Torque = W x D/2
W=10 lbs
1/2 D
D

8. Power: Torque and Speed
 Power determines how fast you can move things
 Power = Torque / Time or Torque x Rotational Velocity
 Same torque with 2x Power = 2x Speed
 Or twice the arm length at same speed ….
3 ft
Power 10 lbs 20 lbs
10 lbs trade 30 ft-lbs 60 ft-lbs
offs
425 100 RPM 50 RPM
Power limits arm length. Watts 1.6 rps .83 rps
Probably don’t want a 10 ft arm 42.5 10 RPM 5 RPM
whipping around at 1000 RPM Watts .16 rps .08 rps
anyway…

9. Arm Design Tips
Lightweight Materials: tubes, thin wall
Watch elbow and wrist weight at ends of arm
Sensors for feedback & control
 limit switches, potentiometers, encoders
Linkages help control long arms
KISS your arm
 Less parts… to build or break
 Easier to operate
 More robust
Counterbalance
 Spring, weight, pneumatic, bungee…
Calculate the forces
 Watch out for Center of gravity
 Sideways forces when extended
Model the reach & orientation

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

11. Extension Lift Rigging
Continuous Cascade
 The final stage moves up
first and down last.
Previous stage speed plus
own speed
o Power vs. speed
equations apply. Don’t
underestimate the power.
o Often needs brakes or
ratchet mechanism

12. Cascade Rigging
 Up-pulling and down-pulling
cables have different speeds
 Different cable speeds can be Slider
handled with different drum (Stage3)
diameters or multiple Pulleys
 Intermediate sections don’t jam
Stage2
– active return
 High tension on the lower
stage cables Stage1
Base

13. Continuous Rigging
Slider
 Cable goes same speed for (Stage3)
up and down pulling cables
 Intermediate sections
sometimes jam
Stage2
 Lower cable tension
 More complex cable routing
Stage1
Base

14. Continuous Rigging Internal
 Pull-down cable routed back on
reverse route as pull-up cable
 Most complex cable routing Slider
 All stages have active return (Stage3)
 Cleaner and protected cables
Stage2
Stage1
Base

15. Extension Lift Design tips
 Use cables to drive 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 needed
 Heavy system, overlapping parts
 Minimize weight, especially at top

16. Arms vs. Lifts
Feature Arm Lift
Reach over object Yes No
Fall over, get up Yes, if strong enough No
Complexity Moderate High
Weight capacity Moderate High
Go under barriers Yes, fold down Maybe, limits lift height
Center of gravity () Cantilevered Central mass
Operating space Large swing space Compact
Adding reach Difficult. More Easier.
articulations More lift sections
Combinations Arm with extender Lift with arm on top

17. Combination Example:
 Continuous direct drive chain
runs stage 1 up and down
 No winch drum needed
 Cascade cable lifts slider stage
 Gravity return
 Got away with this only
because slider GOG very
well balanced on first stage
 Telescoping arm with wrist on
slider stage to reach over objects
2471 in 2011

18. Get a Grip
FIRST definition of a gripper:
Device that grabs a game object
…and releases it when needed.
Design Concerns Methods
Getting object into grip Pneumatic claws /clamps
Hanging on  1 axis
 2 axis
Speed of grip and release
Motorized claw or clamp
Position control
Rollers
Weight and power source Hoop grips
 If at end of arm
Suction

19. Claw or clamp
Pneumatic
One fixed arm
reduce weight of claw
Can make one or two
moving sides
768 in 2008

20. Pneumatic: 2 and 3 point clamps
 Pneumatic Cylinder
extends & retracts linkage
to open and close gripper
 Combined arm and gripper
 Easy to manufacture
 Easy to control
 Quick grab
 Limited grip force
 Use 3 fingers for 2-axis grip 968 in 2004
60 in 2004

21. Motorized clamp
Generally slower
 May be too slow for
frequent grips
 Okay if few grabs per
game of heavy objects
More complex (gearing
& motors)
Heavier
Tunable force
No pneumatics
49 in 2001

22. Suction Grips
 Needs vacuum generator
 Uses various cups to grab
 Slow
 Not secure
 Not easy to control
 Simple
 Subject to damage of
suction cup or game pieces
Not recommended for
heavy game pieces
Used successfully to hold
soccer balls for kickers
(Breakaway 2010)

23. Roller grips
 Allows for misalignment when
grabbing
 Won’t let go
 Extends object as releasing
 Simple mechanism
 Have a “full in” sensor
 Many possible variations
 Mixed roller & conveyer
45 in 2008
 Reverse top and bottom roller
direction to rotate object
148 in 2007

24. Counter Rotating Methods
Crossed round belts
 Many ways to achieve counter
rotating shafts. Here are few
configurations that can run off a
single motor or gearbox.
 Could also drive each shaft with
own motor
Counter
Stacked pulleys rotating
on single drive gearbox
shaft

25. Gripper design
Hang On!
 High friction needed
 Rubber, neoprene, silicone, sandpaper … but don’t damage game object
 Force: Highest at grip point
 Force = 2 to 4 x object weight
 Use linkages and toggles for mechanical advantage
 Extra axis of grip = More control
The need for speed
 Wide capture window
 Quickness covers mistakes
 Quick to grab , Drop & re-grab
 Fast : Pneumatic gripper. Not so fast: Motor gripper
 Make it easy to control
 Limit switches , Auto-functions
 Intuitive control functions
 Don’t make driver push to make the robot pull

26. Rotating Turrets
 Tube or post (recommended)
 Lazy Susan (not for high loads)
 Know when it is needed
o One Goal = good
o Nine Goals = not so good
o Fixed targets = good
o Moving targets = not so good
 Bearing structure must be solid
 Rotation can be slow
 Include sensor feedback
o Know which way it is pointing

27. Accumulator: Rotational device that collects objects
 Horizontal rollers: gathers balls from floor or platforms
 Vertical rollers: pushes balls up or down
 Wheels: best for big objects
 Can use to dispense objects out of robot
 Pointing the robot is aiming method in this case

28. Conveyers: For moving multiple objects
 Point to point within your robot
 When game involves handling
many pieces at once
Why do balls jam on belts?
- Stick 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)

29. Conveyer roller Examples
Rollers Belts Tower
Solution 1 Solution 2 Solution 3

30. More control is better
 Avoid gravity feeds – these are slow and will jam
 Direct the flow: reduce “random” movements.
Not all game objects are created equal
 Tend to change shape, inflation, etc
 Building adaptive/ flexible systems
 Test with different sizes, inflation, etc.
Speed vs. Volume
 Optimize for the game and strategy
 The more capacity, the better

31. Intake Rollers and Accumulators
173 2471

32. Secure shooting structure = more accuracy
Feed balls individually, controlling flow
Rotating tube or wheel
 One wheel or two counter rotating 1771 in 2009
 High speed & power: 2000-4000 rpm
 Brace for vibration
 Protect for safety
Turret allows for aiming
Sensors detect ball presence
& shot direction Note Circular
Conveyer. One
roller on inside
cylindrical rolling
surface outside

33. Use for dumping many objects
Integrate with your accumulator
and conveyer
Heavy ones may move too slow
Usually pneumatic powered but
can run with gear, spring or winch
488 in 2009

34. 2010
Many uses
 Hanging Robots: 2000, 2004, 2010
 Lifting Robots: 2007
 Loading kickers 2010
Great for high torque applications
Fits into limited space
Easy to route or reroute
Good Pull. Bad Push 2004

35. Capture
the
cable
Secure the cable routing
 Smooth winding & unwinding
 Leave room on drum for wound up cable
 Guide the cable
Return cable or spring
 Must have tension on cable to unwind
 Can use cable in both directions Guide
 Spring or bungee return the Maintain
 Gravity return (not recommended) cable Tension
 Calculate the torque and speed
 Use braking devices
Brake or ratchet

36. Sudden release of power
Use stored energy:
 Springs Bungee, Pneumatic
Design & test a good latch
mechanism
 Secure lock for safety
 Fast release
1 per game deployments
 2011 minibot release

37.  Hooking and latching devices used to
grab goals, bars, and other non-
scoring objects
 Hold stored power in place
 Spring or bungee power
 Several ways: Self latching wheel lock
 Hooks
 Locking wheels
 Rollers Basic Automatic Latch
 Pins
Bevel for self latching
 Springs Removable safety pin
Small release force
at end of lever
Strong pivot in
Spring line with large
return and force being held
stop

38.  Start design early. Latches tend to be afterthoughts but
are often a critical part of the operation
 Don’t depend on driver to latch, use a smart mechanism
 Spring loaded (preferred)
 Sensor met and automatic command given
 Use operated mechanism to let go, not to latch
 Have a secure latch
 Don’t want release when robots crash
 Be able to let go quickly
 Pneumatic lever
 Motorized winch, pulling a string
 Cam on a gear motor
 Servo (light release force only)
 Don’t forget a safety pin or latch for when you are
working on the robot

39. Look around see what works and does not work
Know your design objectives and game strategy
Stay within your capabilities
Design before you build
 Calculate the forces and speeds
 Understand the dimensions using CAD or a model
Keep it simple
Make it well
 Poor craftsmanship can ruin the best design
Analyze for failure points.
 Use to refine design and decide on spare parts
Have fun

40. Many thanks to teams and companies who made
materials for this presentation freely available on
web sites to help FIRST students.
 Andy Baker : Team 45
 Jason Marr : Team 2471
 Wildcats: Team 1510
 The Flaming Chickens: Team 1540
 Society of robots
http://www.societyofrobots.com/mechanics_gears.shtml
 Andy Baker’s original presentation and inspiration for this
seminar is available on line
 There are many examples and resources available. Be
sure to use them for your robot designs.

41. Appendix

42. Motor Power:
Assuming 100% power transfer efficiency:
All motors can lift the same amount they just do it at
different rates.
No power transfer mechanisms are 100% efficient
 Inefficiencies due to friction, binding, etc.
 Spur gears ~ 90%
 Chain sprockets ~ 80%
It adds up!
2 spur gears + sprocket =
 Worm gears ~ 70%
.9 x.9 x.8 = .65
 Planetary gears ~80%
Losing 35% of power to the drive train
 Calculate the known inefficiencies and then design in a
safety factor (2x to 4x)
Stall current can trip the breakers

43. Parallel Arms
• Pin loading can be very high
• Watch for buckling in lower arm
• Has limited range rotation
• Keeps gripper in fixed orientation

44. Scissor Lifts
 Advantages
 Minimum retracted height - can go
under field barriers
 Disadvantages
 Tends to be heavy when made
stable enough
 Doesn’t deal well with side loads
 Must be built very precisely
 Stability decreases as height
increases
 Stress loads very high at beginning
of travel
 Not recommend without prior
experience

45.  Pneumatic latch, solidly
grabs pipe
 Force and friction only
 No “smart mechanism”
 Spring-loaded latch
 Motorized release
 Smart Mechanism
469 in 2003

46. Parallel arm Fixed Arm
Jointed Arm

47.  Ratchet - Complete lock in one direction in discrete increments
 Clutch Bearing - Completely lock in one direction any spot
 Brake pads - Squeezes on a rotating device to stop motion - can lock in
both directions. Simple device
 Disc brakes - Like those on your mountain bike
 Gear brakes - Apply to lowest torque gear in gearbox
 Belt Brake- Strap around a drum or pulley
 Dynamic Breaking by motors lets go when power is lost.
 Use for control, but not for safety or end game
 Gearbox that cannot be back-driven is usually an inefficient one.