FRC Drive Train Design and Implementation

Presented by:

Madison Krass, Team 488 Fred Sayre, Team 488

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

Who Are We?

 Madison  2008 is 10 th season with FIRST  Lead Design Mentor for Team XBot  Fred –  2008 is 6 th season with FIRST  Keeps Madison in line

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

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

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

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

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  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

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

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