Robot Chassis and Robot Chassis and Drivetrain FundamentalsDrivetrain Fundamentals

Andy Baker, Team 45Andy Baker, Team 45

John Neun, Team 20John Neun, Team 20

2006

I am not JohnI am not JohnV-Neun (sorry!)V-Neun (sorry!)

John NeunJohn NeunSenior Development Senior Development Engineer Engineer

Albany InternationalAlbany International

Mentor on team 20, Mentor on team 20, the Rocketeersthe Rocketeers

Andy BakerAndy Baker

TechnoKats team leader (#45)TechnoKats team leader (#45) Sr. Mechanical Engineer: Delphi Sr. Mechanical Engineer: Delphi

CorporationCorporation Co-Owner: AndyMark, Inc. (Co-Owner: AndyMark, Inc. (

www.andymark.bizwww.andymark.biz)) 2003 Championship Woodie Flowers 2003 Championship Woodie Flowers

Award WinnerAward Winner

What is most important?What is most important?

1.1. Drive BaseDrive Base2.2. Drive BaseDrive Base3.3. Drive Base*Drive Base*

* - stolen from Mr. Bill Beatty (team 71)* - stolen from Mr. Bill Beatty (team 71)

ObjectivesObjectives

Review “Base” DesignReview “Base” Design ChassisChassis

StructureStructure GeometryGeometry MaterialMaterial ExamplesExamples

DrivetrainDrivetrain WheelsWheels MotorsMotors TransmissionsTransmissions ExamplesExamples

fearfear

Chassis DesignChassis Design Review principles of chassis designReview principles of chassis design Examine trade-offsExamine trade-offs MaterialMaterial WeightWeight

Chassis FunctionChassis Function

Provide platform for everythingProvide platform for everything StrongStrong StableStable Well laid out and accessibleWell laid out and accessible LightLight

Resist, defend against shockResist, defend against shock

WeightWeight Develop a weight budget and stick to it!Develop a weight budget and stick to it! Start coarse: chassis = 60 lbs, tower = 60 lbsStart coarse: chassis = 60 lbs, tower = 60 lbs Tip:Tip: parts far from the floor should be the lightest parts far from the floor should be the lightest Refine:Refine:

ie Chassisie Chassis FrameFrame WheelsWheels GearboxGearbox ControlsControls

Trade-offTrade-off How many ½ inch diameter holes in .100 Al are needed How many ½ inch diameter holes in .100 Al are needed

for 1 pound? for 1 pound?

200!200!

CGCG Keep it Low!!Keep it Low!!

ddspreadspreadsheetsheet

spreadspreadsheetsheet

Given the will, any Given the will, any configuration can workconfiguration can work

GeometryGeometry StrengthStrength SpaceSpace AccessibilityAccessibility

ExampleExample

BumpersBumpers

Kit Chassis Kit Chassis (pictures available at www.innovationfirst.org)(pictures available at www.innovationfirst.org)

Advantages: lightweight, quick to build, uses standard partsAdvantages: lightweight, quick to build, uses standard parts Disadvantages: may not fit your design, requires added structure (that will most likely be put on Disadvantages: may not fit your design, requires added structure (that will most likely be put on

anyway)anyway)

T-slot styleT-slot style

Advantages: quick to build, standard parts, easy to create tension Advantages: quick to build, standard parts, easy to create tension and to add fastening pointsand to add fastening points

Disadvantages: heavy, expensiveDisadvantages: heavy, expensive

Welded Aluminum Tube & Welded Aluminum Tube & PlatePlate

Advantages: lightweight, strength, fits your designAdvantages: lightweight, strength, fits your design Disadvantages: takes time, requires skill, non standard partsDisadvantages: takes time, requires skill, non standard parts

Unique Drive BasesUnique Drive Bases

Advantages: fits your design, uniqueAdvantages: fits your design, unique Disadvantages: takes much time, requires skill, non standard partsDisadvantages: takes much time, requires skill, non standard parts

Chassis MaterialsChassis Materials Aluminum ExtrusionAluminum Extrusion

1/16” – 1/8”: usable but will dent and bend1/16” – 1/8”: usable but will dent and bend T-slot: use 1” sized profiles or higherT-slot: use 1” sized profiles or higher

Aluminum plates and barsAluminum plates and bars 3/16” – ¼” used often3/16” – ¼” used often

Plastic SheetPlastic Sheet Spans structures, provides bracingSpans structures, provides bracing Polycarbonate (LEXAN, etc.) NOT Acrylic (Plexiglas, etc.)Polycarbonate (LEXAN, etc.) NOT Acrylic (Plexiglas, etc.)

WoodWood Lightweight and easy to useLightweight and easy to use Will splinter and fail but can be fixedWill splinter and fail but can be fixed

Steel Tube and AngleSteel Tube and Angle Strong, but heavy, 1/16” wall thickness is plenty strongStrong, but heavy, 1/16” wall thickness is plenty strong

luckluck

Drivetrain DesignDrivetrain Design Review basicsReview basics Examine trade-offsExamine trade-offs Formulas for modeling and designFormulas for modeling and design Sample CalculationsSample Calculations

Drivetrain: #1Drivetrain: #1 What must the robot do?What must the robot do?

SpeedSpeed ForceForce ManeuverabilityManeuverability

Game rules and team strategy: Game rules and team strategy: set specsset specs

Drivetrain Foundation Drivetrain Foundation BasicsBasics PhysicsPhysics

Force = mass x acceleration (pounds)Force = mass x acceleration (pounds) Frictional force = constant x Normal forceFrictional force = constant x Normal force Torque = force x distance (foot-pounds)Torque = force x distance (foot-pounds) Power = force x velocity (HP, watts)Power = force x velocity (HP, watts)

= amps x volts= amps x volts Work = power x time (HP-hour)Work = power x time (HP-hour) Efficiency = (power out)/(power in)Efficiency = (power out)/(power in)

Principles of DC MotorsPrinciples of DC Motors Principles of Gear TrainsPrinciples of Gear Trains

ReductionReduction Mechanical advantageMechanical advantage

WheelsWheels Provide contact with groundProvide contact with ground

DriveDrive TractionTraction SteeringSteering Support and stabilitySupport and stability

Wheel FrictionWheel Friction Theory: F = kNTheory: F = kN

Frictional force has no dependence on contact Frictional force has no dependence on contact areaarea

HOMOGENEOUS, 2 dimensional surfacesHOMOGENEOUS, 2 dimensional surfaces

Drive direction vs. lateral frictionDrive direction vs. lateral friction

NN

FF

Steering wheelsSteering wheels ““Car steering:” complexCar steering:” complex ““Tank steering:” simpleTank steering:” simple

Wheels skateWheels skate

Tank Tank SteeringSteering

Hi CGHi CG Short wheelbaseShort wheelbase ““Bouncy” wheelsBouncy” wheels Solutions:Solutions:

Smaller Dia. WheelsSmaller Dia. Wheels Use wider Frame (see Chris Use wider Frame (see Chris

Hibner’s white paper on Hibner’s white paper on www.chiefdelphi.comwww.chiefdelphi.com))

Use Omni-wheels (Use Omni-wheels (www.andymark.bizwww.andymark.biz))

6 Wheel Drive6 Wheel Drive

Teams can purchase these Teams can purchase these treaded wheels at…treaded wheels at…

www.andymark.biz

www.innovationfirst.com

Crab or Swerve SteeringCrab or Swerve Steering

Tank Tread DriveTank Tread Drive

Fall Over Drive BasesFall Over Drive Bases

MotorsMotors

Fixed population Fixed population of choicesof choices Range of speed and torqueRange of speed and torque Specifications readily availableSpecifications readily available

DC motors with speed controlled via PWMDC motors with speed controlled via PWM Last year’s motors:Last year’s motors:

Use these numbers, but DON’T Use these numbers, but DON’T assume they are all true. For assume they are all true. For instance, the Fisher-Price motor instance, the Fisher-Price motor could not be operated at 12 could not be operated at 12 volts, and was later volts, and was later recommended to run at 6 volts.recommended to run at 6 volts.

Max Motor LoadMax Motor Load

TTLL = Torque from load = Torque from load

IIMM = Maximum current draw (motor limit) = Maximum current draw (motor limit)

TTss = Stall torque = Stall torque

IIFF = Motor free current = Motor free current

IISS = Motor stall current = Motor stall current

Calculate the Max Motor Calculate the Max Motor LoadLoad

Current Draw vs. Load Torque1 Chiaphua Motor

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5

Load Torque (N*m)

Mo

tor

Cu

rren

t D

raw

(A

mp

)

stallstall

Free Free speedspeed

Torque = Stall torque - {speed x (stall torque/free speed)}Torque = Stall torque - {speed x (stall torque/free speed)}

•Determine maximumDetermine maximumdrive train load fromdrive train load from““wall push”wall push”

•Motor running Motor running characteristicscharacteristics

Max torque per Max torque per current limitcurrent limit

Gearbox Design ProcessGearbox Design Process

•Pick motor Pick motor •(load vs amps)(load vs amps)

•Pick wheel config.Pick wheel config.•no. of wheelsno. of wheels•materialmaterial•diameterdiameter

Calculate required gearCalculate required gearratio from motor andratio from motor and

output torquesoutput torques

Calculate speedCalculate speed& acceleration& acceleration

Running characteristicsRunning characteristicsCurrent limitsCurrent limits

IterateIterate

First, choose “Motion” Objective: Robot Speed 13 fps, full speed within 10 feetFirst, choose “Motion” Objective: Robot Speed 13 fps, full speed within 10 feet

Transmission Goal: Transmission Goal: Translate Motor Motion and Translate Motor Motion and Power into Robot MotivationPower into Robot Motivation

MotorMotor Speed (rpm)Speed (rpm) TorqueTorque

RobotRobot Speed (fps)Speed (fps) WeightWeight

First Step: First Step: Pushing against a wall…Pushing against a wall…

Objective: Determine maximum load limit Objective: Determine maximum load limit (breakaway load for wheels)(breakaway load for wheels)

System must withstand max loadSystem must withstand max load Run continuously under maximum loadRun continuously under maximum load Not overload motorsNot overload motors Not overload circuit breakersNot overload circuit breakers (Not break shafts, gears, etc.)(Not break shafts, gears, etc.)

Suboptimum – ignore limit (risk failure)Suboptimum – ignore limit (risk failure)

Pushing against a wall…Pushing against a wall… Known Factors:Known Factors:

Motor UsageMotor Usage Motor CharacteristicsMotor Characteristics

Wheel FrictionWheel Friction Max Motor Load (at 40 amps)Max Motor Load (at 40 amps)

Solve For:Solve For: Required Gear RatioRequired Gear Ratio

Robot WeightRobot WeightMotor specsMotor specsFrictional coef.Frictional coef.

SpeedSpeedaccelerationaccelerationGear RatioGear Ratio

Calculate the Gearbox LoadCalculate the Gearbox LoadFind Required Gearbox Find Required Gearbox RatioRatio

Friction between wheel Friction between wheel and carpet acts as a and carpet acts as a “brake”, and provides “brake”, and provides gearbox load.gearbox load.

Find torque load per Find torque load per gearbox.gearbox.

Now Solve for Required Now Solve for Required Gear RatioGear Ratio

LoadMax Motor

LoadGearbox RatioGear

WeightWeight

no. of wheelsno. of wheels

Frictional Frictional forceforce

Check Robot SpeedCheck Robot Speed How fast will the robot go with this How fast will the robot go with this

required gear ratio?required gear ratio?

Remember Units!!!Remember Units!!!

Loss Speed * RatioGear * RPMMotor RPMOutput

Conversion Unit *nceCircumfera Wheel*

RPMOutput Velocity Robot

Be Careful!Be Careful!

Is this fast enough?Is this fast enough? Major Design Compromise…Major Design Compromise… Is this speed fast enough?Is this speed fast enough? No?No?

Decrease Gearbox LoadDecrease Gearbox Load Increase Gearbox PowerIncrease Gearbox Power Live with the low speed…Live with the low speed… Design two speeds! Design two speeds!

Low speed/high forceLow speed/high force High speed/low forceHigh speed/low force

Risk failureRisk failure Design is all about tradeoffsDesign is all about tradeoffs

Secondary AnalysisSecondary AnalysisPlotting AccelerationPlotting Acceleration

Calculate Motor Current Draw and Robot Calculate Motor Current Draw and Robot Velocity over time (during robot acceleration).Velocity over time (during robot acceleration).

Time to top speedTime to top speed Important to show how drivetrain will perform (or Important to show how drivetrain will perform (or

NOT perform!)NOT perform!) If a robot takes 50 feet to accelerate to top speed, it If a robot takes 50 feet to accelerate to top speed, it

probably isn’t practical!probably isn’t practical! Performance on flat floor is VASTLY different on a Performance on flat floor is VASTLY different on a

ramp (2003 example)ramp (2003 example)

Plotting AccelerationPlotting Acceleration

Voltage to resting motorVoltage to resting motor Start at stall condition (speed = 0)Start at stall condition (speed = 0) Stall torque Stall torque initial acceleration initial acceleration

Robot acceleratesRobot accelerates Motor leaves stall conditionMotor leaves stall condition Force decreases as speed increases.Force decreases as speed increases.

Instantaneous Motor TorqueInstantaneous Motor Torque

When Motor RPM = 0, When Motor RPM = 0,

Output Torque = Stall TorqueOutput Torque = Stall Torque When Motor RPM = free speedWhen Motor RPM = free speed

Output Torque = 0 (in theory)Output Torque = 0 (in theory) (.81)(.81)

Torque Stall RPMMotor * )Speed Free

Torque Stall(- TorqueMotor

Gearbox (reduction) basicsGearbox (reduction) basics

Chain, beltChain, belt Gear Ratio = N2/N1Gear Ratio = N2/N1

Spur gearsSpur gears Gear Ratio = N2/N1Gear Ratio = N2/N1

N1N1 N2N2

N2N2N1N1

Gearbox Torque OutputGearbox Torque OutputRobot Accelerating ForceRobot Accelerating Force

Efficiency * Ratio Gear * TorqueMotor TorqueGearbox

)Radius Wheel

TorqueGearbox (*2 Forceon Accelerati

Instantaneous Instantaneous Acceleration and VelocityAcceleration and Velocity

Instantaneous Acceleration (dependant on Instantaneous Acceleration (dependant on robot velocity, as seen in previous equations).robot velocity, as seen in previous equations).

The instantaneous velocity can be numerically The instantaneous velocity can be numerically calculated as follows:calculated as follows:

MassRobot

ResistanceFriction - Forceon Accelerati on Accelerati

t)(* V V 112 (thanks, Isaac)(thanks, Isaac)

Velocity vs. TimeVelocity vs. Time

The numerical results can be plotted, as The numerical results can be plotted, as shown below (speed vs. time):shown below (speed vs. time):

Robot Velocity vs. Time

0

1

2

3

4

5

6

7

8

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Time (s)

Ro

bo

t V

elo

city

(ft

/s)

Current Draw ModelingCurrent Draw Modeling

The current drawn by a motor can be The current drawn by a motor can be modeled vs. time too.modeled vs. time too.

Current is linearly proportional to torque Current is linearly proportional to torque output (torque load) of the motor.output (torque load) of the motor.

Current Free Load Torque *Torque Stall

Current Free -Current Stall DrawCurrent

Current Draw vs. TimeCurrent Draw vs. Time The numerical results can be plotted, as The numerical results can be plotted, as

shown below:shown below:

Gearbox Current Draw vs. Time

0

50

100

150

200

250

0 1 2 3 4 5

Time (s)

Cu

rren

t D

raw

(A

mp

)

It’s just a It’s just a little volts little volts & amps& amps

What does this provide?What does this provide?

Based on these plots, one can see how Based on these plots, one can see how the drivetrain will perform.the drivetrain will perform. Does Does current drawcurrent draw drop below “danger” drop below “danger”

levels in a short time?levels in a short time?

How long does it take robot to How long does it take robot to accelerate accelerate to top speed?to top speed?

Are things okay? NO?!?Are things okay? NO?!?

How can performance be increased?How can performance be increased? Increase Drivetrain PowerIncrease Drivetrain Power

Use Stronger MotorsUse Stronger Motors Use Multiple MotorsUse Multiple Motors

Increase Gear Ratio (Reduce top speed)Increase Gear Ratio (Reduce top speed) Is this acceptable?Is this acceptable?

Adding Power – Multiple Adding Power – Multiple MotorsMotors

Combining Motors Together – Not Voodoo!Combining Motors Together – Not Voodoo! 2 Motors combine to become 1 “super-motor”2 Motors combine to become 1 “super-motor” Match motors at free speedMatch motors at free speed

Matching does not have to be exactMatching does not have to be exact Sum all characteristicsSum all characteristics

Motor Load is distributed proportional to a ratio of free Motor Load is distributed proportional to a ratio of free speed.speed.

2 of the same motor is easy!2 of the same motor is easy! 4 Chiaphua Motors4 Chiaphua Motors

Multiple Speed Multiple Speed DrivetrainsDrivetrains Allows for multi-speed setup using max Allows for multi-speed setup using max

motor power:motor power: 1 “pushing” speed & 1 “cruising” speed1 “pushing” speed & 1 “cruising” speed 1 “cruising” speed & 1 “very fast” speed1 “cruising” speed & 1 “very fast” speed

Shift-on-the-fly allows for accelerating Shift-on-the-fly allows for accelerating through multiple gears to achieve high through multiple gears to achieve high speeds.speeds.

Shifting optimizes motor power for Shifting optimizes motor power for application at hand.application at hand.

www.andymark.bizwww.andymark.biz sells 2-speed sells 2-speed transmissions for FIRST applications.transmissions for FIRST applications.

Take necessary Take necessary precautionsprecautions

The big picture…The big picture…

These calculations are used to design a These calculations are used to design a competition drivetrain.competition drivetrain.

Rather than do them by hand, most Rather than do them by hand, most designers use some kind of tool.designers use some kind of tool. Excel SpreadsheetExcel Spreadsheet Matlab ScriptMatlab Script Etc…Etc…

And then…And then…

This is a starting pointThis is a starting point Iterate to optimize resultsIterate to optimize results TestTest

Use your imaginationUse your imagination Infinite speedsInfinite speeds Multiple motorsMultiple motors Many gearsMany gears This isn’t the “end all” method.This isn’t the “end all” method.

•Determine maximumDetermine maximumdrive train load fromdrive train load from““wall push”wall push”

•Motor running Motor running characteristicscharacteristics

Max torque per Max torque per current limitcurrent limit

Gearbox Design ProcessGearbox Design Process

•Pick motor Pick motor •(load vs amps)(load vs amps)

•Pick wheel config.Pick wheel config.•no. of wheelsno. of wheels•materialmaterial•diameterdiameter

Calculate required gearCalculate required gearratio from motor andratio from motor and

output torquesoutput torques

Calculate speedCalculate speed& acceleration& acceleration

Running characteristicsRunning characteristicsCurrent limitsCurrent limits

IterateIterate

Set “Motion” Objective: Robot Speed 13 fps, full speed within 10 feetSet “Motion” Objective: Robot Speed 13 fps, full speed within 10 feet

AutomationAutomation

Spreadsheet to do drivetrain design atSpreadsheet to do drivetrain design at www.team229.orgwww.team229.org

Calculation ExampleCalculation Example

Motor NamePeak

Power (W)

Free Speed (RPM)

Stall Torque (N*m)

Stall Current (Amp)

Free Current (Amp)

Gearbox Ratio

Atwood Chiaphua Motor

321 5500 2.22 107 2.3

Fisher Price - Johnson (2005) (No Gearbox)

407 24000 .647 148 1.5

FP w/Gearbox 407 193 80 148 1.5 124:1

Globe Motor (With Gearbox)

50 100 19 21 .82 117:1

Van Door Motor 69 75 35 40 1.1

Nippon Window Motor (2002)

22 92 9.2 24.8 3

Jideco Window Motor (2005)

18.5 85 8.33 21 3

Remember:Remember:It’s no big deal!It’s no big deal!

Thanks!Thanks!

Ken PattonKen Patton

Paul CopioliPaul Copioli

““Robot System Drive Robot System Drive Fundamentals”Fundamentals”

Questions?Questions?