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

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.



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.


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.


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.


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


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


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.


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


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


Two Wheel Skid | Four Wheel Skid


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.


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


Holonomic Drive Systems

Allow a robot to translate in two dimensions and rotate simultaneously

Two major mechanical systems Swerve/Crab Mecanum/Omni


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

http://www.chiefdelphi.com/

http://www.chiefdelphi.com/

http://www.chiefdelphi.com/media/papers/1469

Documents

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