Bruce  Whitefield  Mentor,  Team  2471    

Manipulators for FIRST FRC Robotics

�  Game pieces come in many sizes and shapes

Game objectives change each year Manipulate How ?

Kicking

Lifting Dumping

Hanging

Throwing Gathering

Know the Objectives Game pieces Game & robot rules Your game strategy

Things that Manipulate

Manipulators that work for the game

Know the possibilities Look on line, at competitions Talk to mentors, teams This seminar

Know your capabilities Tools, Skills, Materials, Manpower , Budget , Time Manipulators you

can build

 Your  Design  

�  Heavy lifting / Long reach o  Articulating Arms o  Parallel arms o  Telescoping Lifts

�  Grippers o  Rollers o  Clamps o  Claws

�  Collect and Deliver o  Accumulators & Conveyers o  Turrets o  Shooters & Kickers o  Buckets & Tables

�  Power & Control o  Winches o  Brakes o  Latches o  Pneumatics o  Springs and Bungee o  Gears & Sprockets

Many Types of Manipulators Reoccurring

themes in FIRST games

FIRST definition for

a manipulator:

Device that moves the game piece from where

it is to where it needs to be

Shoulder  Elbow  Wrist  

Articulating Arms

�  Torque = Force x Distance o  Same force, different angle = different torque o  Measure from the pivot point o  Motor & gearing must overcome torque o  Maximum torque at 90 degrees

W=10 lbs

W= 10 lbs

1/2 D

D

Torque = W x D

Torque: Angle and Distance

Torque = W x D/2

�  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 ….

10 lbs Power  trade  offs  

10  lbs  30  ft-­‐lbs  

20  lbs  60  ft-­‐lbs  

425  Watts  

100  RPM  1.6  rps  

50  RPM  .83  rps  

42.5  Watts  

10  RPM  .16  rps  

5  RPM  .08  rps  

3 ft

Probably don’t want a 10 ft arm whipping around at 1000 RPM anyway…

Power: Torque and Speed

�  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% �  Worm gears ~ 70% �  Planetary gears ~80%

�  Calculate the known inefficiencies and then design in a safety factor (2x to 4x)

�  Stall current can trip the breakers

Motor Power:

It adds up! 2 spur gears + sprocket = .9 x.9 x.8 = .65 Losing 35% of power to the drive train

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

Parallel Arms

Parallel arm Fixed Arm

Jointed Arm

�  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

Arm Design Tips

� Extension Lifts �  Motion achieved by stacked members sliding on each other

� Scissor Lift �  Motion achieved by “unfolding” crossed members

Telescoping Lifts

Continuous Cascade

Extension Lift Rigging

�  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

�  Up-pulling and down-pulling cables have different speeds

�  Different cable speeds can be handled with different drum diameters or multiple Pulleys

�  Intermediate sections don’t jam – active return

�  High tension on the lower stage cables

Slider (Stage3)

Stage2

Stage1

Base

Cascade Rigging

�  Cable goes same speed for up and down pulling cables

�  Intermediate sections sometimes jam

�  Lower cable tension �  More complex cable routing

Stage2

Stage1

Base

Continuous Rigging Slider

(Stage3)

�  Pull-down cable routed back on reverse route as pull-up cable

�  Most complex cable routing �  All stages have active return �  Cleaner and protected cables

Slider (Stage3)

Stage2

Stage1

Base

Continuous Rigging Internal

�  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 are needed �  Heavy system, overlapping parts �  Minimize weight, especially at top

Extension Lift Design tips

�  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

Scissor 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 articulations

Easier. More lift sections

Combinations Arm with extender Lift with arm on top

Arms vs. Lifts

Combinations Can work: �  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

FIRST definition of a gripper: Device that grabs a game object …and releases it when needed.

Get a Grip

Methods �  Pneumatic claws /clamps

�  1 axis �  2 axis

�  Motorized claw or clamp �  Rollers �  Hoop grips �  Suction

Design Concerns �  Getting object into grip �  Hanging on �  Speed of grip and release �  Position control �  Weight and power source

�  If at end of arm

�  Pneumatic �  One fixed arm �  reduce weight of claw �  Can make one or two

moving sides

768 in 2008

Claw or clamp

�  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

968 in 2004

Pneumatic linear: 2 point clamp

�  Pneumatic Cylinder, pulling 3 fingers for a 2-axis grip

60 in 2004

Pneumatic linear 3 point 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

Motorized clamp

�  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 capture balls for kickers (Breakaway 2010)

Suction 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 �  Reverse top and bottom roller

direction to rotate object

45 in 2008

148 in 2007

Roller grips

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

Gripper design

�  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 �  Sensor feedback

o  Know which way it is pointing

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

�  Typically 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)

Conveyer: For moving multiple objects

Conveyer roller Examples

�  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

Intake Rollers and Accumulators

173 2471

�  Secure shooting structure = more accuracy �  Feed balls individually, controlling flow �  Rotating tube or wheel

�  One wheel or two counter rotating �  High speed & power: 2000-4000 rpm �  Brace for vibration �  Protect for safety

�  Turret allows for aiming �  Sensors detect ball presence & shot direction

1771 in 2009

Note Circular Conveyer. One roller on inside cylindrical rolling surface outside

�  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

�  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

�  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

2010

�  Secure the cable routing �  Smooth winding & unwinding �  Leave room on drum for wound up cable �  Guide the cable �  Must have tension on cable to unwind

�  Can use cable in both directions �  Spring or bungee return �  Gravity return (not recommended)

�  Calculate the torque and speed �  Use braking devices

Capture the cable

Maintain Tension

Guide the cable

Brake or ratchet

Retu

rn c

able

or

spring

�  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.

�  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: �  Hooks �  Locking wheels �  Rollers �  Pins �  Springs

Self latching wheel lock

Basic Automatic Latch

Strong pivot in line with large force being held

Spring return and stop

Small release force at end of lever

Bevel for self latching

�  Pneumatic latch, solidly grabs pipe

�  Force and friction only �  No “smart mechanism”

�  Spring-loaded latch �  Motorized release �  Smart Mechanism

469 in 2003

�  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)

�  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 in CAD or 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

�  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

�  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.

http://www.societyofrobots.com/mechanics_gears.shtml