FRC Drive Train Design and Implementation Presented by: Madison Krass, Team 488 Fred Sayre, Team 488 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference Questions Answered Who are we? What is a drive train? Reexamine their purpose What won’t I learn from this presentation? No use reinventing the wheel, so to speak Why does that robot have 14 wheels? Important considerations of drive design Tips and Good Practices All in 40 minutes or less. We hope. 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference Who Are We? Madison 2008 is 10th season with FIRST Lead Design Mentor for Team XBot Fred – 2008 is 6th season with FIRST Keeps Madison in line 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference What is a drive train? Components that work together to move robot from A to B. Focal point of a lot of “scouting discussion” at competitions, for better or for worse. It has to be the most reliable part of your robot! That means it probably should be the least complicated part of your robot – unless you’re awesome. 2008 FIRST Robotics Conference

This presentation is not… a math lesson. Ken Patton’s presentation will rock your world. a tutorial. Access to resources greatly affects what sort of work you can do, so there is no single solution that is best for all teams unbiased. We call it like we see it. Your mileage may vary. 2008 FIRST Robotics Conference

Why does that robot have 14 wheels? Design your drive to meet your needs Different field surfaces Inclines and steps Pushing or pulling objects Time-based tasks Omnidirectional motion is useless in a drag race but great in a minefield. 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference Important Concepts Traction Double-edged sword Power More is better? Power Transmission This is what makes the wheels on the bus go ‘round and ‘round. Common Designs 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference Traction Friction with a better connotation. Makes the robot move Keeps the robot in place Prevents the robot from turning when you intend it to Too much traction is a frequent problem for 4WD systems Omniwheels mitigate the problem, but sacrifice some traction 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference Power 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 rate Drive 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. 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference More Power Practical Benefits of Additional Motors Cooler motors Decreased current draw; lower chance of tripping breakers Redundancy Lower center of gravity Drawbacks Heavier Useful motors unavailable for other mechanisms 2008 FIRST Robotics Conference

Power Transmission Method by which power is turned into traction. Most important consideration in drive design Fortunately, there’s a lot of knowledge about what works well Roller Chain and Sprockets Timing Belt Gearing Spur Worm Friction Belt

Power 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 otherwise 95-98% efficient Proper tension is a necessity 1:5 reduction is about the largest single-stage ratio you can expect

Power Transmission: Timing Belt A variety of pitches available About as efficient as chain Frequently used simultaneously as a traction device Treaded robots are susceptible to failure by side-loading while turning Comparatively expensive Sold in custom and stock length – breaks in the belt cannot usually be repaired

Power Transmission: Gearing Gearing is used most frequently “high up” in the drivetrain COTS gearboxes available widely and cheaply Driving wheels directly with gearing probably requires machining resources Spur Gears Most common gearing we see in FRC; Toughboxes, NBD, Shifters, Planetary Gearsets 95-98% efficient per stage Again, expect useful single-stage reduction of about 1:5 or less

Power Transmission: Gearing Worm Gears Useful for very high, single-stage reductions (1:100) Difficult to backdrive Efficiency varies based upon design – anywhere from 40% Design must compensate for high axial thrust loading

Power Transmission: Friction Belt Great for low-friction applications or as a clutch Apparently easier to work with, but requires high tension to operate properly Usually not useful for drive train applications

Common Drive Train Styles Skid Systems 2WD, 4WD, 6WD, 6WD+ Tank Treads/Belting Holonomic Systems Swerve/Crab Mecanum 2008 FIRST Robotics Conference

Two Wheel Skid | Four Wheel Skid The Good Cheap; Kitbot is 2WD More easily controlled Pretty simple to build Very simple to build Better traction The Bad Easily spins out Turning in place is more difficult Difficulty with inclines Loses traction when drive wheels leave floor Compromise between stability and maneuverability 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference 6 Wheel Skid Typically, one wheel is offset from the others to minimize resistance to turning Rocking creates two 4WD systems, effectively Typical offset is 1/8” – ¼” Rock isn’t too bad at edges of robot footprint, but can be significant at the end of long arms and appendages One or two sets of omniwheels can be substituted for offset wheels. 2008 FIRST Robotics Conference

2008 FIRST Robotics Conference 6+ Wheel | Tank Tread In the real world, we’d add more wheels to distribute a load over a greater area. Not a historically useful concept in most FRC games, Maize Craze possibly being an exception Simply speaking, traction is not dependent upon surface area Deformation plays a role in reality Diminshing returns Mechanically complex and expensive for marginal return 2008 FIRST Robotics Conference

Holonomic Drive Systems Allow a robot to translate in two dimensions and rotate simultaneously Two major mechanical systems Swerve/Crab Mecanum/Omni 2008 FIRST Robotics Conference

Holonomic Drive Systems: Swerve/Crab Naming isn’t standardized. I use them interchangeably. Most FRC drives of this type are not truly holonomic That requires wheels that are driven and steered independently

Holonomic Drive Systems: Mecanum/Omni Uses concepts of vector addition to allow for true omnidirectional motion No complicated steering mechanisms Requires four independently powered wheels COTS parts this system accessible to many teams

Tips and Good Practices KISS – Keep it Simple, Stupid We’re trying to get RRRR into the lexicon Reliability Reparability Relevance…ability Reasonability

Tips and Good Practices: Reliability! 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. Avoid long cantilevered loads Avoid press fits and friction belting Alignment, alignment, alignment! Reduce or remove friction almost everywhere you can

Tips and Good Practices: Reparability! You will probably fail at achieving 100% reliability Good practices: Design failure points into drive train and know where they are Accessibility is paramount. You can’t fix what you can’t touch Bring spare parts; especially for unique items such as gears, sprockets, transmissions, mounting hardware, etc. Aim for maintenance and repair times of <10 min.

Tips and Good Practices: Relevance…ability…! Only at this stage should you consider advanced thingamajigs and dowhatsits that are tailored to the challenge at hand Stairs, ramps, slippery surfaces, tugs-of-war Before seasons start, there’s a lot of bragging about 12 motor drives with 18 wheels; after the season is over, not as much

Tips and Good Practices: Reasonability! 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. FRC teams – especially rookies – grossly overestimate their abilities and, particularly, the time it takes to accomplish game tasks.

Resources ChiefDelphi Internet forum watched by the best of the best A lot of static, but patience yields great results http://www.chiefdelphi.com FIRST Mechanical Design Calculator by John V-Neun http://www.chiefdelphi.com/media/papers/1469 FIRST Robotics Canada Galleries http://www.firstroboticscanada.org/site/node/96