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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?

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frc drive train design and implementation

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
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
what is a drive train
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
why does that robot have 14 wheels
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
important concepts
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
traction
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?
traction7
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
power
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
more power
More Power
  • Practical Benefits of Additional Motors
    • 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
power transmission
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

power transmission chain
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
power transmission friction belt
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

power transmission timing belt
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
power transmission gears
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
power transmission gears15
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

wheel size
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
common drive train styles
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
skid tank drive systems 2 wheel skid
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
skid tank drive systems 4 wheel skid
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
skid tank drive systems 6 wheel skid
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
skid tank drive systems more than 6 wheel skid
Skid/Tank Drive Systems:More than 6-Wheel Skid
  • Very powerful, very stable
  • Diminishing returns
    • Heavy, mechanically complex, and very expensive for marginal return

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

skid tank drive systems tank treads
Skid/Tank Drive Systems:Tank Treads
  • Tread belts must be protected from side loads with extra wheel support
  • Typical belts cost $150 - $300 EACH (don’t forget spares)

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

skid tank drive systems half track
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
holonomic drive systems swerve crab
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
holonomic drive systems mecanum
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

holonomic drive systems killough omni
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
holonomic drive systems slide
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
holonomic drive systems linkage
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

tips and good practices
Tips and Good Practices

“KISS” Principle – Keep it Simple, Stupid

More important are the Four R’s:

Reliability

Repair-ability

Relevance

Reasonability

tips and good practices reliability
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!

tips and good practices reliability31
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

tips and good practices reliability32
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

tips and good practices repair ability
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

tips and good practices relevance
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

tips and good practices reasonability
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

food for thought
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!

resources
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