FRC Drive Train Design and Implementation Presented by

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

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

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

What is a drive train? Ø Components that work together to move robot from A to B. Ø Focal point of a lot of “scouting discussion” at competitions, for better or for worse. Ø It has to be the most reliable part of your robot! Ø 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… Ø a math lesson. Ø Ken Patton’s presentation will rock your world. Ø a tutorial. Ø Access to resources greatly affects what sort of work you can do, so there is no single solution that is best for all teams Ø unbiased. Ø We call it like we see it. Your mileage may vary. 2008 FIRST Robotics Conference

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

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

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

Power Ø Motors give us the power we need to make things move. Ø 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 Ø Drive trains are typically not “power-limited” Ø Coefficient of friction limits maximum force of friction because of robot weight limit. Ø Shaving off. 1 sec. on your ¼-mile time is meaningless on a 50 ft. field. 2008 FIRST Robotics Conference

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

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

Power Transmission: Chain Ø #25 (1/4”) and #35 (3/8”) most commonly used in FRC applications Ø #35 is more forgiving of misalignment; heavier Ø #25 can fail under shock loading, but rarely otherwise Ø 95 -98% efficient Ø Proper tension is a necessity Ø 1: 5 reduction is about the largest single-stage ratio you can expect

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

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

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

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

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

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

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

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

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

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

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

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

Tips and Good Practices: Reliability! Ø Most important consideration, bar none. Ø Three most important parts of a robot are, famously, “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 belting Ø Alignment, alignment! Ø Reduce or remove friction almost everywhere you can

Tips and Good Practices: Reparability! Ø You will probably fail at achieving 100% reliability Ø Good practices: 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.

Tips and Good Practices: Relevance…ability…! Ø Only at this stage should you consider advanced thingamajigs and dowhatsits that are tailored to the challenge at hand Ø Stairs, ramps, slippery surfaces, tugs-of-war Ø 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! Ø 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. Ø FRC teams – especially rookies – grossly overestimate their abilities and, particularly, the time it takes to accomplish game tasks.

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