FRC Drive Train Design and Implementation

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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

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

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

19. 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

20. 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

21. 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

22. 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

23. 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

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

25. 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

26. 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.

27. 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

28. 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.

29. 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