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