FRC Drive Train:Design and Implementation

Originally created by:Madison Krass, Team 488

Fred Sayre, Team 488

Modified by:Mike Mellott, Teams 48 & 3193

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

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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

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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

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Important Concepts

Traction 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

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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 tractionWait…what’s an Omni-wheel?

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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

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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

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More Power

Practical Benefits of Additional Motors Decreased current draw

Lower chance of tripping breakersMotors run cooler

Redundancy (in case one fails) Lower center of gravity

Drawbacks Heavier Useful motors unavailable for other mechanisms

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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

SpurWorm

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Power Transmission: Chain

5:1 reduction is about the largest single-stage ratio you can expect

#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

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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

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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

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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 14

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

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Wheel Size

Smaller 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

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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

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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 18

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

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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

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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

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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)

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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

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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 24

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

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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

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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

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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

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Tips and Good Practices

“KISS” Principle – Keep it Simple, Stupid

More important are the Four R’s:ReliabilityRepair-abilityRelevanceReasonability

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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!

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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

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Tips and Good Practices:Reliability! Good practices (con’t):

Standardize components where possibleBolts, washers, SAE/Metric, etc.

Reduce or remove friction where possibleAvoid sliding friction—use rolling element bearingsAvoid friction belting If given a choice, use rotary motion over linear motion

(less friction) Using large sprockets with 35-series chain requires

less tensioningSpace wheels/sprockets such that a whole number of

chain links are needed to span the distance

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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

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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

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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

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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!

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Resources

ChiefDelphi http://www.chiefdelphi.com

FIRST Mechanical Design Calculator (John V-Neun) http://www.chiefdelphi.com/media/papers/1469

AndyMark http://www.andymark.com

FIRST Robotics Canada Galleries http://www.firstroboticscanada.org/site/node/96

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