Manipulator and Manipulator and Appendage Design in FTCAppendage Design in FTC

Andy Baker

Sept. 2015Sept. 2015

Why should you listen to me?Why should you listen to me?Andy Baker

• President and co-owner, AndyMark• Founded in 2004• Crown Supplier to FIRST• andyb@andymark.com

• FIRST Mentor• FRC mentor: 1998-current• FLL mentor: 2012-current• FTC mentor: 2009

• Mechanical Engineer, University of Evansville, ‘91• FIRST Championship WFA winner, 2003• Husband, father of 3 teenage daughters

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Types of ManipulatorsTypes of ManipulatorsArticulating ArmsTelescoping LiftsGrippersLatchesTurretsBall Handling SystemsShootersWinches

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Articulating ArmsArticulating ArmsShoulderElbowWrist

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Arm: Forces, Angles & TorqueArm: Forces, Angles & TorqueExample: Lifting at different anglesTorque = Force x DistanceSame force, different angle, less torque

10 lbs

10 lbs

< DD

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PowerPowerPower = Torque/ Time

ORPower = Torque x Rotational VelocityPower (FIRST definition) – how fast you can move

something

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Arm: Power ExampleArm: Power ExampleSame torque w/ Twice the Power results in Twice the SpeedPower = Torque/ TimeBe conservative: design in a safety factor of 2x or 4x

125 Watts, 100 RPM

250 Watts, 200 RPM

10 lbs10 lbs

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Arm: Design TipsArm: Design Tips

Lightweight Materials: tubes, thin wall sheetDesign-in sensors for feedback & control

limit switches and potentiometers

Linkages help control long armsKISS

Less parts to build or break Easier to operate More robust

Use off-the-shelf itemsCounterbalance

Spring, weight, etc.

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Four Bar LinkageFour Bar Linkage

•Pin loadings can be very high•Watch for buckling in lower member•Counterbalance if you can•Keep CG aft•Limited rotation•Keeps gripper in known location

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Telescoping LiftsTelescoping LiftsExtension Lift

Motion achieved by stacked members sliding on each other

Scissor Lift Motion achieved by “unfolding” crossed members

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Extension Lift ConsiderationsExtension Lift ConsiderationsDrive cables up AND down, or add a cable recoil deviceSegments must move freelyCable lengths must be adjustableMinimize slop and free-playMaximize segment overlap

20% minimummore for bottom, less for top

Stiffness and strength are neededHeavy system, overlapping partsMinimize weight,

especially at the top

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Extension - RiggingExtension - Rigging

Continuous Cascade

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Extension: Continuous RiggingExtension: Continuous Rigging

Cable Goes Same Speed for Up and Down

Intermediate Sections sometimes Jam

Low Cable TensionMore complex cable

routingThe final stage moves up

first and down last

Slider(Stage3)

Stage2

Stage1

Base

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Extension: Continuous Internal RiggingExtension: Continuous Internal Rigging

Even More complex cable routing

Cleaner and protected cables

Slider(Stage3)

Stage2

Stage1

Base

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Extension: Cascade RiggingExtension: Cascade RiggingUp-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 cablesNeeds lower gearing to deal

with higher forces

Slider(Stage3)

Stage2

Stage1

Base

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Scissor LiftsScissor LiftsAdvantages

Minimum retracted height - can go under field barriers

DisadvantagesTends to be heavy to be stable

enoughDoesn’t deal well with side loadsMust be built very preciselyStability decreases as height

increasesLoads very high to raise at

beginning of travel

I do not recommend this!

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Arm vs. LiftArm vs. Lift

FeatureFeature ArmArm LiftLiftReach over objectReach over object YesYes NoNo

Fall over, get upFall over, get up Yes, if strong enoughYes, if strong enough NoNo

Go under barriersGo under barriers Yes, fold downYes, fold down Maybe, limits lift heightMaybe, limits lift height

Center of gravity (Cg)Center of gravity (Cg) Not centralizedNot centralized Centralized massCentralized mass

Small space operationSmall space operation No, needs swing roomNo, needs swing room YesYes

How high?How high? More articulations, more More articulations, more height (difficult)height (difficult)

More lift sections, more More lift sections, more height (easier)height (easier)

ComplexityComplexity ModerateModerate HighHigh

Powerful liftPowerful lift ModerateModerate HighHigh

CombinationCombination Insert 1-stage lift at Insert 1-stage lift at bottom of armbottom of arm

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GrippersGrippers

Gripper (FIRST definition):Device that grabs a game object

Motorized gripRoller gripHang onDesign advice

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Roller GripRoller GripAllows for misalignment when

grabbingWon’t let goExtends object as releasingSimple mechanismHave a “full in” sensorSlow

Recommended

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Hang on!Hang on!High friction is needed

over 1.0 muRubber, neoprene, silicone, sandpaper… but, don’t damage game object

Force: Highest at grip pointForce = multiple x object weight (2-4x)Use linkages and toggles for mechanical advantage

Extra axis of grip = More control

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Gripper Design AdviceGripper Design AdviceGet object fast

Quickness covers mistakesDrop & re-grab

Hang onLet go quicklyMake this easy to control

Limit switchesAuto-functionsEase of operation

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Latching AdviceLatching AdviceDon’t depend on operator to latch, use a smart

mechanismSpring loaded (preferred)Sensor met and automatic command given

Have a secure latchUse an operated mechanism to let goBe able to let go quickly

Servo release

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Ball SystemsBall Systems

Accumulator: rotational device that collects objects Horizontal tubes: gathers balls from floor or platforms Vertical tubes: pushes balls between vertical goal pipes Wheels: best for big objects

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Conveying & GatheringConveying & GatheringConveyor - device for moving multiple objects, typically within

your robotContinuous Belts

Use 2 at same speed to avoid jamming

Individual Rollers Best for high traction balls, which can jam

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ConveyorsConveyors

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)

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Ball System TipsBall System Tips

More control is betterAvoid gravity feeds – these WILL jamTry to reduce “random” movements

Not all Balls are created equalBalls tend to change shape Building adaptive/ flexible systems

Speed vs. VolumeOptimize for the game and strategyThe more capacity, the better

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Ball Shooter SystemsBall Shooter Systems

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

One wheel or two Protect for safety

Turret allows for aiming Sensors detect ball presence

& shot direction

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Winches & LiftsWinches & Lifts Raise wheels with articulating arm

Raise robot with winch (and lock it up)

Thank you!

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