1. FRC Drive Train: Design and Implementation
Originally created by:
Madison Krass, Team 488
Fred Sayre, Team 488
Modified by:
Mike Mellott, Teams 48 & 3193

2. Questions Answered What is a Drive Train?
Re-examine their purpose
What won’t I learn from this presentation?
No use reinventing the wheel…
Why does that robot have 14 wheels?
Important considerations of drive design
Tips and Good Practices

3. 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

4. Why does that robot have 14 wheels?
Design your drive train to meet your needs
Different field surfaces
Inclines and steps
Pushing or pulling objects
Time-based tasks
Omni-directional motion (yes, driving sideways!)
Useless in a drag race
Great in a minefield

5. Important Concepts Traction Power Power Transmission Wheel Size
Double-edged sword
Power
More is better…but not always
Power Transmission
This is what makes the wheels on the bus go ‘round and ‘round
Wheel Size
Common Designs

6. Traction Friction with a better connotation Makes the robot move
Also keeps the robot in place
Prevents the robot from turning when you intend it to turn
Too much traction is a frequent problem for 4WD systems
Omni-wheels mitigate the problem, but sacrifice some traction
Wait…what’s an Omni-wheel?

7. Traction This is an Omni-wheel:
Rollers are attached around the circumference, perpendicular to the axis of rotation of the wheel
Allows for omni-directional motion

8. 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 0.1 seconds on your ¼-mile time is meaningless on a 50-ft. field

9. More Power Practical Benefits of Additional Motors Drawbacks
Decreased current draw
Lower chance of tripping breakers
Motors run cooler
Redundancy (in case one fails)
Lower center of gravity
Drawbacks
Heavier
Useful motors unavailable for other mechanisms

10. 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
Friction Belt
Timing Belt
Gears
Spur
Worm

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

12. Power Transmission: Friction Belt
Great for low-friction applications or as a clutch
Easy to work with, but requires high tension to operate properly
Usually not useful for drive train applications
Belt will slip under too much load

13. Power Transmission: Timing Belt
A variety of pitches available
About as efficient as chain
Frequently used simultaneously as a traction device (i.e. tank treads)
Comparatively expensive
Sold in custom and stock lengths
Broken belts cannot usually be repaired

14. Power Transmission: Gears
Gearing is used most frequently “high up” in the drive train
COTS gearboxes available widely and cheaply
Driving wheels directly with gearing requires manufacturing precision
Spur Gears
Most common gearing we see in FRC (Tough-boxes, Shifters, Planetary Gearboxes)
95-98% efficient per stage
Again, expect useful single-stage reduction of about 1:5 or less

15. Power Transmission: Gears
Worm Gears
Useful for very high, single-stage reductions (1:20 to 1:100)
Difficult to back-drive
Efficiency varies based upon design – anywhere from 40 – 90%
Design must compensate for high axial thrust loading

16. Wheel Size Smaller wheel “pros” Larger wheel “pros”
Less gear reduction needed
Lower friction
Less weight
Larger wheel “pros”
Lower RPM for same linear velocity (robot travel speed)
Less tread wear…less frequent tread replacement
Larger sprocket to wheel ratio, which means less tension on drive chains

17. Common Drive Train Styles
Tank/Skid Systems: Left and right half of drive train are controlled independently (a.k.a. tank steering)
2WD, 4WD, 6WD, More than 6WD
Tank Treads, Half-Track
Holonomic Systems: Allow a robot to translate in two dimensions and rotate simultaneously
Swerve/Crab
Mecanum
Killough (Omni-drive)
Slide
Linkage

18. Skid/Tank Drive Systems: 2-Wheel Skid
The Good
Cheap
Very simple to build
The Bad
Difficulty with inclines and uneven surfaces
Looses traction when drive wheel are lifted from the floor
Easily spins out (non-driven wheels are typically Omni-wheels or casters), meaning low traction

19. Skid/Tank Drive Systems: 4-Wheel Skid
The Good
More easily controlled
Far better traction (than 2WD)
Easy to build
The Bad
Turning in place more difficult
Compromise between stability and maneuverability
Wheel footprint must be wider than length (or equal) to reduce stress on motors during turns

20. Skid/Tank Drive Systems: 6-Wheel Skid
Standard drive train in FRC
Stable footprint
Good power distribution
Agility must be designed
Lower contact point on center wheels (1/8” – 1/4”), creating two 4WD systems
Rocking isn’t too bad at edges of robot footprint, but can be significant at the end of tall robots and long arms
Replace front or rear pair of wheels with Omni-wheels
No need to lower center wheels, making for a much more stable base

21. Skid/Tank Drive Systems: More than 6-Wheel Skid
In the real world, one would add more wheels to distribute a load over a greater area.
Historically, not a useful concept in most FRC games
The only reason to use this system is to go over things
Very powerful, very stable
Diminishing returns
Heavy, mechanically complex, and very expensive for marginal return

22. Skid/Tank Drive Systems: Tank Treads
Again, the only reason to use this system is to go over things
Very powerful, very stable platform, not for speed
Heavy, mechanically complex, and very expensive for marginal return
Tread belts must be protected from side loads with extra wheel support
Typical belts cost $150 - $300 EACH (don’t forget spares)

23. Skid/Tank Drive Systems: Half-Track
One solution for a smooth, agile tank tread system
Still powerful, very stable platform
Still NOT made for high-speed lap driving
Not as expensive, not as mechanically complex
Tread belts must still be protected from side loads
Due to shorter-length treads, this is easier

24. Holonomic Drive Systems: Swerve/Crab
Wheel modules rotate on the vertical axis to control direction
Independently or chained together
Typically 4 high-traction wheels
Potential for high-speed agility
Very complex and expensive system to design, build, control and program
Can be difficult to drive

25. Holonomic Drive Systems: Mecanum
Rollers are attached to the circumference, but on a 45° angle to the axis of rotation of the wheel
Uses concepts of vector addition to allow for true omni-directional motion
No complicated steering mechanisms
Requires four independently-powered wheels
COTS parts make this system easily accessible but expensive

26. Holonomic Drive Systems: Killough (Omni)
Uses concepts of vector addition to allow for true omni-directional motion
No complicated steering mechanisms, fast turning
Requires four independently-powered wheels
No brakes
No pushing ability
Not good on inclines
Unstable ride without “dually” omni-wheels

27. Holonomic Drive Systems: Slide
Similar layout to 4-wheel drive with an extra wheel perpendicular to the others
Uses all omni-wheels to allow robot to translate sideways
Agile, easy to build and program
No pushing power
Extra motors, wheels, gearbox needed that cannot be used elsewhere

28. Holonomic Drive Systems: Linkage
Wheels can be mechanically rotated 90° simultaneously to allow for lateral movement
No “in-between” angles
Easy to control and program
Heavy, complex system to manufacture, space hog
Allows for very little ground clearance

29. Tips and Good Practices
“KISS” Principle – Keep it Simple, Stupid
More important are the Four R’s:
Reliability
Repair-ability
Relevance
Reasonability

30. Tips and Good Practices: Reliability!
The drive train is the most important consideration, period
Good practices:
Support shafts in two places…No more, no less
Bearings should be spaced 3-5 shaft diameters apart
Avoid long cantilevered loads
Avoid press fits
Alignment, alignment, alignment!

31. Tips and Good Practices: Reliability!
Good practices (con’t):
Keep things simple to start and add detail as the design develops
Balance the goal to minimize the number of components and component complexity with the number and complexity of manufacturing processes
Make your design repeatable first, and then tune it for accuracy
Triangulate parts and structures to make them stiffer
Avoid bending stresses—prefer tension and compression

32. Tips and Good Practices: Reliability!
Good practices (con’t):
Standardize components where possible
Bolts, washers, SAE/Metric, etc.
Reduce or remove friction where possible
Avoid sliding friction—use rolling element bearings
Avoid friction belting
If given a choice, use rotary motion over linear motion (less friction)
Using large sprockets with 35-series chain requires less tensioning
Space wheels/sprockets such that a whole number of chain links are needed to span the distance

33. Tips and Good Practices: Repair-ability!
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
gears, sprockets, transmissions, mounting hardware, etc.
Aim for maintenance and repair times of <10 minutes

34. Tips and Good Practices: Relevance!
Only at this stage should you consider advanced thing-a-ma-jigs and do-whats-its 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…not as much

35. 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

36. Food for thought…
It takes a lot of thought and knowledge to develop a design that requires little of either—that is the art of design!

37. Resources ChiefDelphi FIRST Mechanical Design Calculator (John V-Neun)
FIRST Mechanical Design Calculator (John V-Neun)
AndyMark
FIRST Robotics Canada Galleries