FIRST RoboticsFIRST RoboticsDrive TrainsDrive Trains

Dale YocumDale Yocum

Robotics Program DirectorRobotics Program Director

Catlin Gabel SchoolCatlin Gabel School

OverviewOverview Traction overviewTraction overview Review popular drive trainsReview popular drive trains

2 wheel2 wheel 4 wheel4 wheel 6 wheel6 wheel MecanumMecanum TreadsTreads

TransmissionsTransmissions Innovation FIRSTInnovation FIRST AndyMarkAndyMark BaneBotsBaneBots

WheelsWheels Innovation FIRSTInnovation FIRST AndyMarkAndyMark SkywaySkyway

Final TipsFinal Tips

Coefficient of FrictionCoefficient of Friction

Material of robot wheelsSoft “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!

Surface Material and conditionAlways test on carpet

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

tractiontraction 5-7 ft/s – Medium speed and power. Typical of a single 5-7 ft/s – Medium speed and power. Typical of a single

speed FRC robotspeed FRC robot 8-12 ft/s – Fast. Low pushing force8-12 ft/s – Fast. Low pushing force Over 13ft/sec – Crazy. Hard to control, blazingly fast, no Over 13ft/sec – Crazy. Hard to control, blazingly fast, no

pushing power.pushing power. Remember, many motors draw 60A+ at stall but our Remember, many motors draw 60A+ at stall but our

breakers trip at 40A!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 dive wheel even less dive 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 Turning! (keep wheel base

short)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”

Xbot’s Six Wheel VariantsXbot’s Six Wheel Variants

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 themNeeds a machine shop or buy them

Turns can tear the tread off and/or stall Turns can tear the tread off and/or stall motorsmotors

997

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

Don’t try this at home!Don’t try this at home!

TransmissionsTransmissions

AndyMark ToughboxAndyMark Toughbox

Came in last year’s kit12.75:1 RatioOptions for 6:1 and 8.5:1Long shaft option2.5 lbsOne or two CIMs$98

BaneBotsBaneBots

Many gear ratios 3:1- 256:1

Long shaft options

$107

2.5 lbs

Don’t drive to the limit!

Avoid dual CIMs

Order 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 wheels4.6 lbsOne 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 Bigger equals better climbing and grip but also potentially higher center of but also potentially higher center of gravity, weight, and larger sprockets.gravity, 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”

Depends on the challengeDepends on the challenge

2008 Championship Division Winners and 2008 Championship Division Winners and FinalistsFinalists14 Six Wheel drive14 Six Wheel drive

2 Six Wheel with omnis2 Six Wheel with omnis

2 Four wheel with omnis2 Four wheel with omnis

2 Mecanum2 Mecanum

2 Serve/Crab drive2 Serve/Crab drive

1 Four wheel rack and pinion!1 Four wheel rack and pinion!

QuestionsQuestions