FIRST Robotics Drive Trains Dale Yocum Robotics Program Director Catlin Gabel SchoolTeam 1540, The Flaming Chickens
Coefficient of FrictionMaterial of robot wheels Soft “sticky” materials have higher COF Hard, smooth, shiny materials have lower COFShape of robot wheels Want wheel to interlock with surface for high COFAlways test on playing surface But not this way!
Traction Basics Terminology maximum Coefficient Normal Force torque turning the tractive = of friction x (Weight) wheel force weight tractive normal force forceThe coefficient of friction for any given contact with the floor, multipliedby the normal force, equals the maximum tractive force can be appliedat the contact area. Source: Paul Copioli, Ford Motor Company, #217
Traction Fundamentals “Normal Force” weight front normal normal force force (rear) (front)The normal force is the force that the wheels exert on the floor, and isequal and opposite to the force the floor exerts on the wheels. In thesimplest case, this is dependent on the weight of the robot. The normalforce is divided among the robot features in contact with the ground. Source: Paul Copioli, Ford Motor Company, #217
Traction Fundamentals “Weight Distribution”more weight in backdue to battery and less weight in frontmotors due to fewer parts EXAM in this area PL ONLY E front more less normal normal force force 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. Source: Paul Copioli, Ford Motor Company, #217
Weight Distribution is Not Constantarm position inrear makes the weightshift to the rear arm position in front makes the weight shift to the front EXAM PL ONLY E front normal normal force force (front) (rear) Source: Paul Copioli, Ford Motor Company, #217
Two Wheels – Castersn Pros: n Simple n Light n Turns easily n Cheapn Cons: n Easily pushed n Driving less predictable n Limited traction n Some weight will always be over non-drive wheels n If robot is lifted or tipped even less drive wheel surface makes contact.
4 Standard Wheelsn Pros: n Simpler than 6 wheel n Lighter than 6 wheels n Cheaper than 6 wheels n All weight supported by drive wheels n Resistant to being pushedn Cons n Turning! (keep wheel base short) n Can high center during climbs n Bigger wheels = higher COG
4 Wheels With Omni Wheelsn Pros: n Same as basic four wheel n Turns like a dream but not around the robot centern Cons: n Vulnerable to being pushed on the side n Traction may not be as high as 4 standard wheels n Can still high center = bigger wheels
6 Wheelsn Pros: n Great traction under most circumstances n Smaller wheels n Smaller sprockets = weight savings n Turns around robot center n Can’t be easily high centered n Resistant to being pushedn Cons: n Weight n More complex chain paths n Chain tensioning can be fun n More expensive Note: Center wheel often lowered about 3/16”
8 Wheels Pros: • Allows for small wheels and low CG • Climbs like a tank Cons: • Complex chain paths • Heavy, lots of bearings and chains
Mecanumn Pros: n Highly maneuverable n Might reduce complexity elsewhere in robot n Simple Chain Paths (or no chain) n Redundancy n Turns around robot centern Cons: n Lower traction n Can high center n Not great for climbing or pushing n Software complexity n Drift dependant on weight distribution n Shifting transmissions impractical n Autonomous challenging n More driver practice necessary n Expensive See one at http://www.youtube.com/watch? v=xgTJcm9EVnE
Treadsn Pros: n Great traction n Turns around robot center n Super at climbing n Resistant to being pushed n Looks awesome!n Cons n Not as energy efficient n High mechanical complexity n Difficult for student-built teams to make n Needs a machine shop or buy them from Outback Manufacturing n Turns can tear the tread off and/or stall motors
Swerve/CrabØ Wheels steer independently or as a setØ More traction than MecanumØ Mechanically Complex!Ø Adds weight
How Fast?n Under 4 ft/s – Slow. Great pushing power if enough traction. n No need to go slower than the point that the wheels loose traction, usually around 6 ft/sec with 4 CIMsn 6-8 ft/s – Medium speed and good power. Typical of a single speed FRC robotn 9-12 ft/s – Fast. Low pushing forcen Over 13ft/sec –Hard to control, blazingly fast, no pushing power.n CIMs draw 60A+ at stall but our breakers trip at 40A!
CIMple Box 4:67:1 One or two CIMs 1.4 lbs Came in last year’s kit
BaneBots Many gear ratios 3:1- 256:1 Long shaft options $103 2.5 lbs Avoid dual CIMs Planetary not quite as efficient Order Early!
CIMple TransmissionsConverts Fisher Price or similar into a CIM…around 5:1 ratio.
AndyMark Gen 2 Shifter 11:1 & 4:1 Ratios 3.6 lbs One or two CIMs Servo or pneumatic shifting Two chain paths Encoder included $350
AndyMark SuperShifter 24:1 & 9:1 standard ratios + options Made for direct drive of wheels 4 lbs without pneumatics. (-.6 option) One or two CIMs Servo or pneumatic shifting Direct Drive Shaft Includes encoder $360
WormBox 16:1 Accepts CIM motor $119.00 1.16 lbs
Wheels are a Compromise (Like everything else)n Coefficient of friction n You can have too much traction!n Weightn Diameter n Bigger equals better climbing and grip but also potentially higher center of gravity, weight, and larger sprockets.n Forward vs lateral friction
Tips and Good Practices From Team 488Ø Three most important parts of a robot are drive train, drive train and drive train.Ø Good practices: Ø Support shafts in two places. No more, no less. Ø Avoid long cantilevered loads Ø Avoid press fits and friction belts Ø Alignment, alignment, alignment! Ø Reduce or remove friction everywhere you can Ø Use lock washers, Nylock nuts or Loctite EVERYWHERE
Tips and Good Practices: Reparability (also from 488)Ø You will fail at achieving 100% reliability Ø Design failure points into drive train and know where they are Ø Accessibility is paramount. You can’t fix what you can’t touch Ø Bring spare parts; especially for unique items such as gears, sprockets, transmissions, mounting hardware, etc. Ø Aim for maintenance and repair times of <10 min.
Drive Teams Make the Differencen A great drive team can make a average robot great.n A weak drive team will make a great robot average (or worse).n Drive teams need practice, rest, and freedom from other distractions at the competition.n Drive team shouldn’t be the emergency repair crew.
Team 1114 Kitbot on Steroidshttp://www.simbotics.org/media/videos/presentations
So Which is “Best”?2010 Championship Division Winners and Finalists 2 Four Wheel 5 Six Wheel 10 Eight Wheel 2 Nine Wheel (148, 217 partnership) 1 Mecanum 3 Crab Drive 1 Treads2011 Championship Division Winners and Finalists