FIRST RoboticsFIRST RoboticsDrive TrainsDrive Trains

Dale YocumDale YocumRobotics Program DirectorRobotics Program Director

Catlin Gabel SchoolCatlin Gabel SchoolTeam 1540Team 1540

OverviewOverview Traction overviewTraction overview Review popular drive trainsReview popular drive trains

2 wheel2 wheel 4 wheel4 wheel 6 wheel6 wheel 8 wheel8 wheel MecanumMecanum TreadsTreads Swerve +Swerve +

TransmissionsTransmissions Innovation FIRSTInnovation FIRST AndyMarkAndyMark BaneBotsBaneBots

WheelsWheels Innovation FIRSTInnovation FIRST AndyMarkAndyMark SkywaySkyway

Final TipsFinal Tips

Coefficient of FrictionCoefficient of FrictionMaterial of robot wheels

Soft “sticky” materials have higher COFHard, smooth, shiny materials have lower COF

Shape of robot wheelsWant wheel to interlock with surface for high COF

But not this way!

Always test on playing surface

Traction BasicsTraction BasicsTerminologyTerminology

The coefficient of friction for any given contact with the floor, multiplied by the normal force, equals the maximum tractive force can be applied at the contact area.

normalforce

tractiveforce

torqueturning the

wheel

maximumtractiveforce

Normal Force(Weight)

Coefficientof friction= x

weight

Source: Paul Copioli, Ford Motor Company, #217

Traction FundamentalsTraction Fundamentals“Normal Force”“Normal Force”

weightfront

The normal force is the force that the wheels exert on the floor, and is equal and opposite to the force the floor exerts on the wheels. In the simplest case, this is dependent on the weight of the robot. The normal force is divided among the robot features in contact with the ground.

normalforce(rear)

normalforce(front)

Source: Paul Copioli, Ford Motor Company, #217

Traction FundamentalsTraction Fundamentals“Weight Distribution”“Weight Distribution”more weight in back

due to battery andmotors

front

The weight of the robot is not equally distributed among all the contacts with the floor. Weight distribution is dependent on where the parts are in the robot. This affects the normal force at each wheel.

morenormalforce

lessnormalforce

less weight in frontdue to fewer partsin this areaEXAMPLE

EXAMPLEONLYONLY

Source: Paul Copioli, Ford Motor Company, #217

Weight Distribution is Not Weight Distribution is Not ConstantConstant

arm position inrear makes the weightshift to the rear

front

arm position in frontmakes the weightshift to the front

EXAMPLEEXAMPLEONLYONLY

normalforce(rear)

normalforce (front)

Source: Paul Copioli, Ford Motor Company, #217

How Fast?How Fast? Under 4 ft/s – Slow. Great pushing power Under 4 ft/s – Slow. Great pushing power if enough if enough

traction.traction. No need to go slower than the point that the wheels loose No need to go slower than the point that the wheels loose

traction, usually around 6 ft/sec with 4 CIMstraction, usually around 6 ft/sec with 4 CIMs 6-8 ft/s – Medium speed and good power. Typical of a 6-8 ft/s – Medium speed and good power. Typical of a

single speed FRC robotsingle speed FRC robot 9-12 ft/s – Fast. Low pushing force9-12 ft/s – Fast. Low pushing force Over 13ft/sec – Crazy. Hard to control, blazingly fast, Over 13ft/sec – Crazy. Hard to control, blazingly fast,

no pushing power.no pushing power. CIMs draw 60A+ at stall but our breakers trip at 40A!CIMs draw 60A+ at stall but our breakers trip at 40A!

Base ChoicesBase ChoicesEverything is a Everything is a

compromisecompromise

Two Wheels – CastersTwo Wheels – Casters Pros:Pros:

SimpleSimple LightLight Turns easilyTurns easily CheapCheap

Cons:Cons: Easily pushedEasily pushed Driving less predictableDriving less predictable Limited tractionLimited traction

Some weight will always be Some weight will always be over non-drive wheelsover non-drive wheels

If robot is lifted or tipped If robot is lifted or tipped even less drive wheel even less drive wheel surface makes contact.surface makes contact.

4 Standard Wheels4 Standard Wheels Pros:Pros:

Simpler than 6 wheelSimpler than 6 wheel Lighter than 6 wheelsLighter than 6 wheels Cheaper than 6 wheelsCheaper than 6 wheels All weight supported by drive All weight supported by drive

wheelswheels Resistant to being pushedResistant to being pushed

ConsCons Turning! (keep wheel base short)Turning! (keep wheel base short) Can high center during climbsCan high center during climbs

Bigger wheels = higher COGBigger wheels = higher COG

4 Wheels With Omni Wheels4 Wheels With Omni Wheels Pros:Pros:

Same as basic four wheelSame as basic four wheel Turns like a dream but not Turns like a dream but not

around the robot centeraround the robot center Cons:Cons:

Vulnerable to being Vulnerable to being pushed on the sidepushed on the side

Traction may not be as Traction may not be as high as 4 standard wheelshigh as 4 standard wheels

Can still high center = Can still high center = bigger wheelsbigger wheels

6 Wheels6 Wheels Pros:Pros:

Great traction under most Great traction under most circumstancescircumstances

Smaller wheelsSmaller wheels Smaller sprockets = weight Smaller sprockets = weight

savingssavings Turns around robot centerTurns around robot center Can’t be easily high centeredCan’t be easily high centered Resistant to being pushedResistant to being pushed

Cons:Cons: WeightWeight More complex chain pathsMore complex chain paths

Chain tensioning can be funChain tensioning can be fun More expensiveMore expensive

Note: Center wheel often lowered about 3/16”

8 Wheels8 WheelsPros:• Allows for small

wheels and low CG• Climbs like a tank

Cons: • Complex chain

paths• Heavy, lots of

bearings and chains

8 Wheels8 Wheels

Team 177

MecanumMecanum Pros:Pros:

HighlyHighly maneuverable maneuverable Might reduce complexity Might reduce complexity

elsewhere in robotelsewhere in robot Simple Chain Paths (or no chain)Simple Chain Paths (or no chain) RedundancyRedundancy Turns around robot centerTurns around robot center

Cons:Cons: Lower tractionLower traction Can high centerCan high center Not great for climbing or pushingNot great for climbing or pushing Software complexitySoftware complexity Drift dependant on weight Drift dependant on weight

distributiondistribution Shifting transmissions Shifting transmissions

impracticalimpractical Autonomous challengingAutonomous challenging More driver practice More driver practice necessarynecessary ExpensiveExpensive

Holonomic DriveHolonomic Drive

2047’s 2007 Robot

TreadsTreads

Pros:Pros: Great tractionGreat traction Turns around robot centerTurns around robot center Super at climbingSuper at climbing Resistant to being pushedResistant to being pushed Looks awesome!Looks awesome!

ConsCons Not as energy efficientNot as energy efficient High mechanical complexityHigh mechanical complexity Difficult for student-built teams to Difficult for student-built teams to

makemake Needs a machine shop or buy them from Needs a machine shop or buy them from

Outback ManufacturingOutback Manufacturing Turns can tear the tread off and/or Turns can tear the tread off and/or

stall motorsstall motors

Swerve/CrabSwerve/Crab Wheels steer independently or as a Wheels steer independently or as a

setset More traction than MecanumMore traction than Mecanum Mechanically Complex!Mechanically Complex! Adds weightAdds weight

Wild SwerveWild SwerveBased on Wildstang DesignBased on Wildstang Design

Wild SwerveWild Swerve

8.4 lbs per wheel (less motor and chain)

TransmissionsTransmissions

AndyMark ToughboxAndyMark Toughbox

12.75:1 RatioOptions for 6:1 and 8.5:1Long shaft option2.5 lbs (options for -.85lbs)Encoder optionOne or two CIMs$88 (two came in last year’s kit)

Toughbox MiniToughbox Mini

12.75:1 RatioOptions for 6:1 and 8.5:1Long shaft option1.95lbs (options for -.56lbs)Encoder optionOne or two CIMs$90

Toughbox NanoToughbox Nano

12.75:1 RatioOptions for 6:1 and 8.5:1Long shaft option1.9 lbs (options for -.28lbs)Encoder optionOne CIM$78

BaneBotsBaneBots

Many gear ratios 3:1- 256:1Long shaft options$1032.5 lbsAvoid dual CIMsPlanetary not quite as efficientOrder Early!

AndyMark Gen 2 ShifterAndyMark Gen 2 Shifter

11:1 & 4:1 Ratios3.6 lbsOne or two CIMsServo or pneumatic shiftingTwo chain pathsEncoder included$350

AndyMark SuperShifterAndyMark SuperShifter

24:1 & 9:1 standard ratios + optionsMade for direct drive of wheels4 lbs without pneumatics. (-.6 option)One or two CIMsServo or pneumatic shiftingDirect Drive ShaftIncludes encoder$360

WheelsWheels

Wheels are a CompromiseWheels are a Compromise(Like everything else)(Like everything else)

Coefficient of frictionCoefficient of friction You can have too much traction!You can have too much traction!

WeightWeight DiameterDiameter

Bigger equals better climbing and grip but Bigger equals better climbing and grip but also potentially higher center of gravity, also potentially higher center of gravity, weight, and larger sprockets.weight, and larger sprockets.

Forward vs lateral frictionForward vs lateral friction

Wheel TypesWheel Types Conveyer belt coveredConveyer belt covered Solid PlasticSolid Plastic PneumaticPneumatic

MechanumMechanum

OmniwheelsOmniwheels

AndyMark.bizAndyMark.biz

Innovation FIRSTInnovation FIRST

Skyway Skyway

Tips and Good PracticesTips and Good PracticesFrom Team 488From Team 488

Three most important parts of a robot are drive train, drive train and drive train.Three most important parts of a robot are drive train, drive train and drive train. Good practices:Good practices:

Support shafts in two places. No more, no less.Support shafts in two places. No more, no less. Avoid long cantilevered loadsAvoid long cantilevered loads Avoid press fits and friction beltsAvoid press fits and friction belts Alignment, alignment, alignment!Alignment, alignment, alignment! Reduce or remove friction everywhere you canReduce or remove friction everywhere you can Use lock washers, Nylock nuts or Loctite EVERYWHEREUse lock washers, Nylock nuts or Loctite EVERYWHERE

Tips and Good Practices: Tips and Good Practices: Reparability (also from 488)Reparability (also from 488)

You will fail at achieving 100% reliabilityYou will fail at achieving 100% reliability Design failure points into drive train and know Design failure points into drive train and know

where they arewhere they are Accessibility is paramount. You can’t fix what Accessibility is paramount. You can’t fix what

you can’t touchyou can’t touch Bring spare parts; especially for unique items Bring spare parts; especially for unique items

such as gears, sprockets, transmissions, such as gears, sprockets, transmissions, mounting hardware, etc.mounting hardware, etc.

Aim for maintenance and repair times of <10 Aim for maintenance and repair times of <10 min.min.

So Which is “Best”So Which is “Best”2008 Championship Division Winners and Finalists2008 Championship Division Winners and Finalists

14 Six Wheel14 Six Wheel2 Six Wheel with omnis2 Six Wheel with omnis2 Four wheel with omnis2 Four wheel with omnis2 Mecanum2 Mecanum2 Crab Drive2 Crab Drive1 Four wheel rack and pinion!1 Four wheel rack and pinion!

2010 Championship Division Winners and Finalists2010 Championship Division Winners and Finalists2 Four Wheel2 Four Wheel5 Six Wheel5 Six Wheel10 Eight Wheel10 Eight Wheel2 Nine Wheel (148, 217 partnership)2 Nine Wheel (148, 217 partnership)1 Mecanum1 Mecanum3 Crab Drive3 Crab Drive1 Treads1 Treads

Drive Teams Make the Drive Teams Make the DifferenceDifference

A great drive team can make a A great drive team can make a average robot great.average robot great.

A weak drive team will make a great A weak drive team will make a great robot average (or worse). robot average (or worse).

Drive teams need practice, rest, and Drive teams need practice, rest, and freedom from other distractions at freedom from other distractions at the competition.the competition.

QuestionsQuestions