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

2. 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

3. 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

4. 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

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

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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. 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

9. 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

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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
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

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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

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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
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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
FIRST Mechanical Design Calculator by John V-Neun
FIRST Robotics Canada Galleries