first robotics drive trains dale yocum robotics program director catlin gabel school
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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
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