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Bruce WhitefieldMentor, Team 2471
Manipulators for FIRST FRC Robotics
FIRST Fare 2012
Game pieces come in many sizes and shapes
Manipulate What ?
Game objectives change each yearManipulate How ?
Kicking
Lifting Dumping
Hanging
ThrowingGathering
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
Stuff That Don’t Work
Don’t be this team
Heavy lifting / Long reacho Articulating Armso Parallel armso Telescoping Lifts
Gripperso Rollerso Clampso Claws
Collect and Delivero Accumulators & Conveyerso Turretso Shooters & Kickerso Buckets & Tables
Power & Controlo Wincheso Brakeso Latcheso Pneumaticso Springs and Bungeeo Gears & Sprockets
Many Types of ManipulatorsReoccurring
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
ShoulderElbowWrist
Arms
Torque = Force x Distanceo Same force, different angle = different torqueo Measure from the pivot pointo Motor & gearing must overcome torqueo 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 thingsPower = Torque / Time or Torque x Rotational VelocitySame torque with 2x Power = 2x SpeedOr twice the arm length at same speed ….
10 lbsPower trade offs
10 lbs30 ft-lbs
20 lbs60 ft-lbs
425 Watts
100 RPM1.6 rps
50 RPM.83 rps
42.5 Watts
10 RPM.16 rps
5 RPM.08 rps
3 ft
Power limits arm length.
Probably don’t want a 10 ft arm whipping around at 1000 RPM anyway…
Power: Torque and Speed
Lightweight Materials: tubes, thin wall Watch elbow and wrist weight at ends of armSensors for feedback & control
limit switches, potentiometers, encodersLinkages help control long armsKISS 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 LiftsTelescoping Lifts
Continuous Cascade
Extension Lift RiggingExtension 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 RiggingCascade Rigging
Cable goes same speed for up and down pulling cables
Intermediate sections sometimes jam
Lower cable tensionMore complex cable routing
Stage2
Stage1
Base
Continuous RiggingContinuous RiggingSlider
(Stage3)
Pull-down cable routed back on reverse route as pull-up cable
Most complex cable routingAll stages have active returnCleaner and protected cables
Slider(Stage3)
Stage2
Stage1
Base
Continuous Rigging InternalContinuous Rigging Internal
Use cables to drive up AND down, or add a cable recoil device
Segments must move freelyCable lengths must be adjustableMinimize slop and free-playMaximize segment overlap
20% minimum More for bottom, less for top
Stiffness and strength neededHeavy system, overlapping partsMinimize weight, especially at top
Extension Lift Design tipsExtension Lift Design tips
FeatureFeature ArmArm LiftLiftReach over objectReach over object YesYes NoNo
Fall over, get upFall over, get up Yes, if strong enoughYes, if strong enough NoNo
ComplexityComplexity ModerateModerate HighHigh
Weight capacityWeight capacity ModerateModerate HighHigh
Go under barriersGo under barriers Yes, fold downYes, fold down Maybe, limits lift heightMaybe, limits lift height
Center of gravity ()Center of gravity () CantileveredCantilevered Central massCentral mass
Operating spaceOperating space Large swing spaceLarge swing space CompactCompact
Adding reachAdding reach Difficult. More Difficult. More articulations articulations
Easier. Easier.
More lift sectionsMore lift sections
CombinationsCombinations Arm with extenderArm with extender Lift with arm on topLift with arm on top
Arms vs. LiftsArms vs. Lifts
Combination Example: 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
FIRST definition of a gripper:
Device that grabs a game object
…and releases it when needed.
Get a GripGet a Grip
MethodsPneumatic claws /clamps
1 axis2 axis
Motorized claw or clampRollersHoop gripsSuction
Design ConcernsGetting object into gripHanging onSpeed of grip and releasePosition controlWeight and power source
If at end of arm
PneumaticOne fixed armreduce weight of clawCan make one or two
moving sides
768 in 2008768 in 2008
Claw or clampClaw or clamp
Pneumatic Cylinder extends & retracts linkage to open and close gripper
Combined arm and gripperEasy to manufactureEasy to controlQuick grabLimited grip forceUse 3 fingers for 2-axis grip
968 in 2004968 in 2004
Pneumatic: 2 and 3 point clampsPneumatic: 2 and 3 point clamps
60 in 200460 in 2004
Generally slower May be too slow for
frequent grips Okay if few grabs per
game of heavy objects
More complex (gearing & motors)
HeavierTunable forceNo pneumatics
49 in 200149 in 2001
Motorized clampMotorized clamp
Needs vacuum generatorUses various cups to grabSlowNot secureNot easy to controlSimpleSubject to damage of
suction cup or game pieces
Not recommended for heavy game pieces
Used successfully to hold soccer balls for kickers (Breakaway 2010)
Suction GripsSuction Grips
Allows for misalignment when grabbing
Won’t let goExtends object as releasingSimple mechanismHave a “full in” sensorMany possible variations
Mixed roller & conveyer Reverse top and bottom roller
direction to rotate object
45 in 200845 in 2008
148 in 2007148 in 2007
Roller gripsRoller grips
Counter Rotating Methods
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
Crossed round belts
Counter rotating gearbox
Stacked pulleys on single drive shaft
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 Gripper design
Tube or post (recommended)Lazy Susan (not for high loads)Know when it is needed
o One Goal = goodo Nine Goals = not so good o Fixed targets = goodo Moving targets = not so good
Bearing structure must be solidRotation can be slowInclude sensor feedback
o Know which way it is pointing
Rotating TurretsRotating Turrets
Accumulator: Rotational device that collects objectsHorizontal rollers: gathers balls from floor or platformsVertical rollers: pushes balls up or downWheels: best for big objectsCan use to dispense objects out of robot
Pointing the robot is aiming method in this case
Point to point within your robotWhen 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)
Conveyers: For moving multiple objects
Conveyer roller Examples Conveyer roller Examples BeltsBeltsBeltsBelts TowerTowerTowerTowerRollersRollersRollersRollers
Solution 1Solution 1Solution 1Solution 1 Solution 3Solution 3Solution 3Solution 3Solution 2Solution 2Solution 2Solution 2
More control is betterAvoid gravity feeds – these are slow and will jamDirect the flow: reduce “random” movements.
Not all game objects are created equalTend to change shape, inflation, etcBuilding adaptive/ flexible systemsTest with different sizes, inflation, etc.
Speed vs. VolumeOptimize for the game and strategyThe more capacity, the better
Intake Rollers and Accumulators
173 2471
Secure shooting structure = more accuracy Feed balls individually, controlling flowRotating tube or wheel
One wheel or two counter rotating High speed & power: 2000-4000 rpm Brace for vibration Protect for safety
Turret allows for aimingSensors detect ball presence
& shot direction
1771 in 20091771 in 2009
Note Circular Note Circular Conveyer. One Conveyer. One roller on inside roller on inside cylindrical rolling cylindrical rolling surface outsidesurface outside
Use for dumping many objectsIntegrate with your accumulator
and conveyerHeavy ones may move too slowUsually pneumatic powered but
can run with gear, spring or winch
488 in 2009488 in 2009
Many uses Hanging Robots: 2000, 2004, 2010 Lifting Robots: 2007 Loading kickers 2010
Great for high torque applicationsFits into limited spaceEasy to route or rerouteGood Pull. Bad Push
20042004
20102010
Secure the cable routing Smooth winding & unwinding Leave room on drum for wound up cable Guide the cableMust have tension on cable to unwind
Can use cable in both directions Spring or bungee returnGravity return (not recommended)
Calculate the torque and speedUse braking devices
Capture Capture the the cablecable
Maintain Maintain TensionTension
Guide Guide the the cablecable
Brake or ratchetBrake or ratchet
Retu
rn c
able
or
spri
ng
Retu
rn c
able
or
spri
ng
Sudden release of powerUse stored energy:
Springs Bungee, PneumaticDesign & test a good latch
mechanismSecure lock for safetyFast release
1 per game deployments2011 minibot release
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 lockSelf latching wheel lock
Basic Automatic LatchBasic Automatic LatchBasic Automatic LatchBasic Automatic Latch
Strong pivot in Strong pivot in line with large line with large force being heldforce being held
Strong pivot in Strong pivot in line with large line with large force being heldforce being held
Spring Spring return and return and stopstop
Spring Spring return and return and stopstop
Small release force Small release force at end of leverat end of lever
Small release force Small release force at end of leverat end of lever
Bevel for self latchingBevel for self latchingBevel for self latchingBevel for self latching
Removable safety pinRemovable safety pinRemovable safety pinRemovable safety pin
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 latchDon’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
Look around see what works and does not workKnow your design objectives and game strategyStay within your capabilitiesDesign before you build
Calculate the forces and speeds Understand the dimensions using CAD or a model
Keep it simpleMake 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 45Jason Marr : Team 2471Wildcats: Team 1510The Flaming Chickens: Team 1540Society 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.shtmlhttp://www.societyofrobots.com/mechanics_gears.shtml
Appendix
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
Advantages Minimum retracted height - can go
under field barriersDisadvantages
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 LiftsScissor Lifts
Pneumatic latch, solidly grabs pipe
Force and friction onlyNo “smart mechanism”
Spring-loaded latchMotorized releaseSmart Mechanism
469 in 2003469 in 2003
Parallel arm Fixed Arm
Jointed Arm
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.