FRC Drive Train Design and Implementation

oregontrimmingAI and Robotics

Nov 2, 2013 (3 years and 5 months ago)

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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 10
th

season with FIRST


Lead Design Mentor for Team XBot


Fred




2008 is 6
th

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


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