Presentation on theme: "Drive Systems Presented By:"— Presentation transcript:
1Drive Systems Presented By: Ben Heaivilin – Lead Advisor: Team 1764 (5 years)Jon Nelson – Mentor; Industrial Engineer - HoneywellRachel Lindsay – Student Team 1764
2Overview Importance Fundamental Considerations Types of Drive Systems TractionPower and Power TransmissionPractical & Realistic ConsiderationsCredits
3Importance The best drive train… is more important than anything else on the robotmeets your strategy goalscan be built with your resourcesrarely needs maintenancecan be fixed within 4 minutes
4Fundamental Considerations Know your resourcesCost, Machining Availability, Parts, Expertise, etcKeep it simple (KISS)Easy to design and buildGets it up and running quickerEasier to fixGet it RunningFind out what is wrongPractice for DrivingTime for Fine-TuningGive software team TIME to work
5Types of Drive Train Drive Train Decision Depends on: Team StrategyAttributes neededSpeed, Power, Pushing, Climbing, Maneuverability, Acceleration, Accuracy, Obstacle Handling, Reliability, Durability, Ease of ControlResources availableMust sacrifice some attributes for others. No one system will perform all the above functions
6Good Features to have to attain proper attributes High Top SpeedHigh PowerHigh Efficiency/Low LossesCorrect Gear RatioAccelerationLow InertiaLow MassPushing/PullingHigh TractionObstacle HandlingGround ClearanceObstacle "Protection”Drive Wheels on FloorAccuracyGood Control CalibrationCorrect Gear RatioClimbing AbilityHigh TractionGround ClearanceReliability/ DurabilitySimpleRobustGood Fastening SystemsEase of ControlIntuitive ControlHigh ReliabilityManeuverabilityGood Turning Method
7What types of Drives are Available? 2 Wheel Drive4 Wheel Drive with 2 Gearboxes4 Wheel Drive with 4 Gearboxes6 Wheel Drive with 2 GearboxesTank Drive and TreadsOmni-directional Drive SystemsMecanumHolonomic / KilloughCrab/SwerveOther
8Usage in FRC? 2008 Championship Division Winners and Finalists 14 Six Wheel drive2 Six Wheel with omni’s2 Four wheel with omni’s2 Mecanum2 Swerve/Crab drive1 Four wheel rack and pinion!Remember: Drives Game Dependent
102 Wheel Drive Pros (+) Cons (-) Easy to Design Easy to Build Light WeightInexpensiveAgileEasy TurningFastCOTS PartsCons (-)Not Much PowerDoes not do well on rampsPoor PushingSusceptible to spin outs.Able to be pushed from the sideDriven WheelsGearboxGearboxMotors can be driven in front or rearPosition of Driven Wheels:Near Center of Gravityfor most tractionFront Drive for MaxPositioning3) Lose Traction if weight not over wheelsCasterorOmni
124 Wheel Drive – 2 Gearboxes (AKA Tank Drive – no treads) Pros (+)Easy to DesignEasy to BuildMore PowerfulSturdy and stableWheel OptionsOmni, Traction, OtherCOTS PartsCons (-)Not AgileTurning can be difficultAdjustment NeededSlightly SlowerDriven WheelsPosition gearboxes anywhere as needed for mounting and center of gravityPosition of Wheels:Close together = better turningSpread Apart = Straighter drivingChain or beltDriven Wheels
144 Wheel Drive – 4 Gearboxes Pros (+)Easy to DesignEasy to BuildPowerfulSturdy & StableMany OptionsMecanum, Traction, Omni, ComboCOTS PartsCons (-)HeavyCostlyTurning may or may not be difficultOptions4 traction+ Pushing, Traction, Straight- TurningAll Mecanum; 2 traction & 2 Omni+ Mobility- Less traction, Less pushingDriven WheelsGearboxGearboxTypes of wheels determine whether robot has traction, pushing ability, and mobilityIf all traction wheels, keep wheel base short; difficult to turn.Driven WheelsGearboxGearbox
166 Wheel Drive – 2 Gearboxes Pros (+)Easy to Design & BuildPowerfulStableAgileTurns at center of robotPushingHarder to be high CenteredCOTS PartsCons (-)Heavy & CostlyTurning may or may not be difficultChain pathsOptionalSubstitute Omni Wheel sets at either endTraction: Depends on wheelsPushing = Great w/ traction wheelsPushing = Okay w/ Omni2 Ways to be agile:Lower Contact on Center WheelOmni wheels on back, front or bothCenter wheel generally larger or lowered 1/8” - 1/4”Rocking isn’t too bad at edges of robot footprint, but can be significant at the end of long arms and appendagesThis is theGOLD STANDARDfor FIRST
18Lower track at center slightly to allow for better turning. Tank Drive/TreadsPros (+)Climbing Ability(best attribute)Great TractionTurns at CenterPushingVery StablePowerfulCons (-)Energy EfficiencyMechanical ComplexityDifficult for student build teamsTurns can tear off treadsWEIGHTExpensiveRepairing broken treads.Lower track at center slightly to allow for better turning.
20Omni-directional drives “Omnidirectional motion is useless in a drag race… but GREAT in a mine field”Remember, task and strategy determine usefulness
21For best results, independent motor drive for each wheel is necessary. Omni: MecanumPros (+)Simple MechanismHigh ManeuverabilityImmediate TurnSimple Control4 wheel independentSimple mounting and chainsTurns around Center of robotCOTS PartsCons (-)Braking PowerOK PushingSuspension for teeth chatteringInclinesSoftware complexityDrift (uneven weight distribution)ExpenseMotor(s)Motor(s)For best results, independent motor drive for each wheel is necessary.Motor(s)Motor(s)
26Omni: Holonomic / Killough 4-wheel drive needs square base for appropriate vector additionPros (+)Turns around Center of robotNo complicated steering methodsSimultaneously used 2D motion and rotationManeuverabilityTruly Any Direction of MotionCOTS partsCons (-)Requires 3-4 independently powered motorsWeightCostProgramming Skill NecessaryNO BrakeMinimum Pushing PowerTeeth Chattering (unless dualies)ClimbingDrifting (Weight Distribution)3-wheel drive needs separated 120 degrees for appropriate vector addition
33Other Drive Systems N Wheel Drive (More than 6) 3 Wheel Drive Not much better driving than 6 wheel DriveImproves climbing, but adds a lot of weight3 Wheel DriveAtypical – Therefore time intensiveTeam 16 – Bomb SquadLighter than 4 wheel driveBall DriveRack and Pinion / Car Steering
36Coefficient of Friction Coefficient of Friction is Dependent on:Materials of the robot wheels/beltsShape of robot wheels/beltsMaterials on the floor surfaceSurface Conditions
37Materials of the robot wheels/belts High Friction Coefficient:Soft Materials“Spongy” Materials“Sticky” MaterialsLow Friction Coefficient:Hard MaterialsSmooth MaterialsShiny MaterialsIt is often the case that “good” materials wear out much faster than “bad” materials - don’t pick a material that is TOO good!
38Shape of robot wheels/belts Shape of wheel wants to “interlock” with the floor surface.
39Materials on the floor surface This is NOT up to you.Know what surfaces you are running on:Carpet,“Regolith”Aluminum Diamond PlateOtherFollow rules about material contactToo Much TRACTION for surface
40Surface Conditions Surface Conditions In some cases this will be up to youGood:Clean Surfaces“Tacky” SurfacesBadDirty SurfacesOily SurfacesDon’t be too dependent on the surface condition since you can’t control it.BUT… Don’t forget to clean your wheels
41Traction Basics Terminology maximumtractiveforceCoefficientof frictionxNormal Force(Weight)torqueturning thewheel=weighttractiveforcenormalforceThe 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.Source: Paul Copioli, Ford Motor Company, #217
42Traction Fundamentals “Normal Force” weightfrontnormalforce(rear)normalforce(front)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. Therefore: Adding more wheels DOES NOT add more traction -Source: Paul Copioli, Ford Motor Company, #217
43Traction Fundamentals “Weight Distribution” more weight in backdue to battery andmotorsless weight in frontdue to fewer partsin this areaEXAMPLEONLYfrontmorenormalforcelessnormalforceThe 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.Source: Paul Copioli, Ford Motor Company, #217
44Weight Distribution is Not Constant arm position inrear makes the weightshift to the reararm position in frontmakes the weightshift to the frontEXAMPLEONLYfrontnormalforce(rear)normalforce (front)Source: Paul Copioli, Ford Motor Company, #217
46How Fast? Under 4 ft/s – Slow. Great pushing power if enough traction. No need to go slower than the point that the wheels loose traction5-7 ft/s – Medium speed and power. Typical of a single speed FRC robot8-12 ft/s – Fast. Low pushing forceOver 13ft/sec – Crazy. Hard to control, blazingly fast, no pushing power.Remember, many motors draw 60A+ at stall but our breakers trip at 40A!
47Power Motors give us the power we need to make things move. Adding power to a drive train increases the rate at which we can move a given load or increases the load we can move at a given rateDrive trains are typically not “power-limited”Coefficient of friction limits maximum force of friction because of robot weight limit.Shaving off .1 sec. on your ¼-mile time is meaningless on a 50 ft. field.
48MORE Power Practical Benefits of Additional Motors Drawbacks Cooler motorsDecreased current draw; lower chance of tripping breakersRedundancyLower center of gravityDrawbacksHeavierUseful motors unavailable for other mechanisms
49Power Transmission Method by which power is turned into traction. Most important consideration in drive designFortunately, there’s a lot of knowledge about what works wellRoller Chain and SprocketsTiming BeltGearingSpurWormFriction Belt
50Power Transmission: Chain #25 (1/4”) and #35 (3/8”) most commonly used in FRC applications#35 is more forgiving of misalignment; heavier#25 can fail under shock loading, but rarely otherwise95-98% efficientProper tension is a necessity1:5 reduction is about the largest single-stage ratio you can expect
51Power Transmission: Timing Belt A variety of pitches availableAbout as efficient as chainFrequently used simultaneously as a traction deviceTreaded robots are susceptible to failure by side-loading while turningComparatively expensiveSold in custom and stock length – breaks in the belt cannot usually be repaired
52Power Transmission: Gearing Gearing is used most frequently “high up” in the drive trainCOTS gearboxes available widely and cheaplySee previous slidesDriving wheels directly with gearing probably requires machining resourcesSpur GearsMost common gearing we see in FRC; Toughboxes, NBD, Shifters, Planetary Gearsets95-98% efficient PER STAGEAgain, expect useful single-stage reduction of about 1:5 or less
53Power Transmission: Gearing Worm GearsUseful for very high, single-stage reductions (1:100)Difficult to backdriveEfficiency varies based upon design – anywhere from 40% to 80%Design must compensate for high axial thrust loading
54Power Transmission: Friction Belt Great for low-friction applications or as a clutchApparently easier to work with, but requires high tension to operate properlyUsually not useful for drive train applications
56Transmissions / Gearbox Transmission Goal:Translate Motor Motion and Power into Robot MotivationMotor:Speed (RPMs)Torque (ft-lbs or Nm)RobotSpeed (feet per second [fps])Weight
57Transmissions – AM ToughBox AndyMark ToughBoxStandard KOP2 CIMs or 2 FP with AM Planetary GearBoxOverall Ratio: 12.75:1Gear type: spur gearsWeight: 2.5 poundsOptionsRatio 1: 5.95:1Ratio 2: 8.45:1Weight Reduction$88.00
58Tranmissions – AM GEM500 GEM500 Gearbox Gear Ratios $120.00 Planetary Style1 CIM or 1 FP with Planetary GearboxWeight: 2.4 poundsOutput Shaft: 0.50”Gear RatiosEach stage has a ratio of 3.67:1.Base Stage: 3.67:1Two Stages: 13.5:1Three Stages: 45.4:1Four Stages: 181.4:1$120.00
59Transmissions – AM Planetary AM Planetary Gearbox AM-0002Same Mounting and Output as the CIM!For Fischer Price Mabuchi MotorAccepts Globe & CIM w/AlterationsWeight = 0.9 lbsGear ReductionSingle Stage: 3.67: 1Matches CIM… sort of$88.00With FP Installed: $117.00
60Transmissions – BB P80 Series BaneBots Planetary GearBoxMax Torque: 85ft-lbsAvailable with or without motorGear Ratios3:1 4: 1 9:112:1 16:1 27.136:1 48:1 64:181:1 108:1 144:1192:1 256:1$ $157.25
61Transmissions – BB P80 Dual BaneBots Planetary GearBoxMax Torque: 85ft-lbsAvailable with or without motorGear Ratios4: 1 12:1 16:136:1 48:1 64:1108:1 144:1 192:1256:1$ $199.75
62Shiftable Transmission: AndyMark (AM) Super Shifter am-0114Available from AndyMarkPurchased as setCost with Shipping$ EACH
63Shifting Transmissions: NDB Nothing But DeWaltsTeam Modifies DeWalt XRP DrillPurchase Pieces and AssembleCOST with Shipping:$ EACH!
64Compare SS and NBD 2 speed Interface with Gear Reduction Super Shifter AMNothing but dewalts2 speedInterface with2 CIMs2 AM Planetary GearboxGear Reduction67:117:1Shifts on the flyServoPneumatic (Bimba series)3 speedInterface with 1:Chiaphua (CIM)Fischer PriceGlobe MotorGear Reduction47:1, 15:1, 12:1Shifts on the flyServo only
65Compare SS and NBD Weight: 3.6 lbs w/o motors Size with: Comes with: Super Shifter AMNothing but dewaltsWeight: 3.6 lbs w/o motorsSize with:CIM: 6” x 4.25” x 8.216FP Mod: 6” x 4.25” x ”Comes with:Optical EncoderServo Shifter12 tooth #35 chain output sprockets per shaftOptional to purchase4:1 high/low ratioWeight: < 2 lbs w/o motorsSizeCIM: 9.5” x 4” x 3”Other: Varies on useDoes not come withServoServo ShifterEncoderMounting plates
66Custom Gearboxes Many teams build their own gearboxes Built to suit Can be very ruggedCan include single or multiple motorsEasier to add custom and Advanced featuresShift, Encoders, Straffing, etc.
67Basic Custom GearboxTwo 1/4” aluminum plates to mount shafts, separated by either four posts or two more aluminum platesMotor(s) bolted into back plateSprockets and chain to wheels
68Basic Custom Gearbox: Power Transmisison KeywaysStrongHard to machine KeywayPinsEasy to machineWeakerSet ScrewsAvoid if possibleLoctite and Knurled if usedBoltsVery effective for large gears/sprockets
70Remember…Most first teams overestimate their ability and underestimate reality.
71Reality CheckRobot top speed will occur at approximately 80-85% of max speed.Max speed CIM = 5600 rpms (NO LOAD)Reality: 5600 x 0.85 = 4760 rpmsFriction is a two edged swordAllows you to push/pullDoesn’t allow you to turnYou CAN have too much of it!Frequent for 4WD Systems
72Tips and Good Practices Most important consideration, bar none.Three most important parts of a robot are, famously, “drive train, drive train and drive train.”Good practices:Support shafts in two places. No more, no less.Reduces FrictionCan wear out faster and fail unexpectedly otherwiseAvoid long cantilevered loadsAvoid press fits and friction beltingAlignment, alignment, alignment!Reduce or remove friction almost everywhere you can
73Tips and Good Practices You will probably fail at achieving 100% reliabilityGood practices:Design failure points into drive train and know where they areAccessibility is paramount. You can’t fix what you can’t touchBring spare parts; especially for unique items such as gears, sprockets, transmissions, mounting hardware, etc.Aim for maintenance and repair times of <4min.TIMEOUTS!Alignment, Alignment, Alignment….AlignmentUse lock washers, Nylock nuts or Loctite EVERYWHERE
74Tips and Good Practices Only at this stage should you consider advanced thingamajigs and dowhatsits that are tailored to the challenge at handStairs, ramps, slippery surfaces, tugs-of-war“Now that you’ve devised a fantastic system of linkages and cams to climb over that wall on the field, consider if it’d just be easier, cheaper, faster and lighter to drive around it.”
75Credits AndyMark, Inc. BaneBots.com FIRST Robotics Drive Systems; Andy Baker, President: AndyMark, Inc.FIRST Robotics Drive Trains; Dale YocumFRC Drive Train Design and Implementation; Madison Krass and Fred Sayre, Team 448Mobility: Waterloo Regional; Ian MakenzieRobot Drive System Fundamentals – FRC Conference: Atlanta, GA 2007Ken Patton (Team 65), Paul Copioli (Team 217)