FRC Drive Systems

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FRC Drive Systems

• by Luke Van Oort, 587 Team Captain

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Unusual Terms used in this Document

• Omni wheel A wheel that uses rolling elements
  to have a much lower coefficient of friction
  parallel to its axle than perpendicular to it.
• Mecanum wheel A wheel design with rolling
  elements at 45 degree angles relative to its axle.

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Unusual Terms used in this Document

• Swerve module A module that contains a wheel
  and a method for powering it the module is then
  mounted on bearings to allow for rotation.
• Ball caster (also called a ball transfer) A
  type of caster that uses a ball mounted in a
  holder. Ball casters provide equal resistance to
  motion in any direction, unlike shopping cart
  style casters which can unpredictably resist
  motion.

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Unusual Terms used in this Document

• Holonomic Technically means that every degree
  of freedom of a part/mechanism can be controlled.
  In the context of the FRC, it usually refers to
  drive systems that can translate left/right,
  forward/backwards, as well as rotate.

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Key to the Diagrams
Omni Wheel
Ball Caster
Tank Track
Normal Wheel
Mecanum Wheel
Swerve Module
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Tank Steered Systems
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Two Wheel Drives
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Centre Driven Wheel

• Pros
• Simple
• Highly Maneuverable
• Good Efficiency
• Cheap
• Cons
• Low Traction
• Easily Spun
• Can be difficult to drive

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Keys to a Successful Centre Driven Wheel 2WD

• Make sure to
• Use ball casters or plastic slides, shopping cart
  style casters tend to resist turning in odd
  manners

• Make sure not to
• Use in a manner where obstacle climbing or
  pushing are important.

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Front/Rear Driven Wheel

• Pros
• Maneuverable
• More pushing power than centre drive
• Good Efficiency
• Simple
• Cheap
• Cons
• Fishtails
• Low Traction
• Can be difficult to drive

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Keys to a Successful F/R Driven Wheel 2WD

• Make sure to
• Use ball casters or plastic slides, shopping cart
  style casters tend to resist turning in odd
  manners
• Put more weight over driven wheels, if you desire
  more traction

• Make sure not to
• Use in a manner where obstacle climbing or
  pushing are important.

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Four Wheel Drives
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Long 4WD without Omniwheels

• Pros
• High Traction
• Hard to push
• Hard to spin
• Relatively Simple
• Cons
• Poor maneuverability
• High current draw in turns

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Keys to Success

• Make sure to
• Gear motors for a large amount of torque,
  otherwise turning will be very difficult.

• Make sure not to
• Expect to turn rapidly.

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Wide 4WD without Omniwheels

• Pros
• High Traction
• Good Maneuverability
• Relatively Simple
• Cons
• Low front/back stability

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Keys to Success

• Make sure to
• Take care to avoid tipping when accelerating

• Make sure not to
• Put a lot of weight high up in your robot.

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4WD with Front/Rear Omniwheels

• Pros
• Good Traction
• Maneuverable
• Relatively Simple
• Cons
• Easily Spun

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Keys to Success

• Make sure to
• Use omniwheels with a lot of rollerstoo few
  roller results in a lot of vibration. Vibration
  causes bolts to loosen and just adds extra
  stresses to your robot.

• Make sure not to
• Expect to resist aggressive defence, you will be
  spun.

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4WD with Diagonally Placed Omniwheels

• Pros
• Good Traction
• Highly Maneuverable
• Relatively Simple
• More difficult to spin than front/rear omni
  layout
• Cons
• Easy to push sideways

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Keys to Success

• Make sure to
• Use omniwheels with a lot of rollerstoo few
  roller results in a lot of vibration.

• Make sure not to
• Use omniwheels and normal wheels of differing
  diameters. (This can be harder to avoid than it
  sounds)

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Six Wheel Drives
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Coplaner Six Wheel Drive

• Pros
• Decent Turning
• Decent ability to go straight
• Good Traction
• Cons
• Can be heavy
• Requires accurate manufacturing

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Keys to Success

• Make sure to
• Actually make it coplanar watch out for
  manufacturing errors
• Use plenty motors
• Try to keep centre of gravity in the centre of
  the robot

• Make sure not to
• Accidentally make centre wheels higherthen it is
  just a 4WD, but heavier

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Lowered Centre Wheel

• Pros
• Good turning
• Good traction
• Cons
• Can be heavy
• Can be hard to go straight

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Keys to Success

• Make sure to
• Avoid extreme frame warping (from welding, for
  example)
• Keep centre of gravity in the centre of the robot

• Make sure not to
• Lower centre wheel too much
• Lower centre wheel too little

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Corner Omni Wheels

• Pros
• Very good turning
• Good forward traction
• Cons
• Can be heavy
• Easy for defenders to spin
• Can be hard to go straight

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Keys to Success

• Make sure to
• Keep all wheels coplanar

• Make sure not to
• Expect to resist spinning

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Tank Treads
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Tank Style

• Pros
• High Traction
• Can have decent maneuverability
• Extremely good at climbing obstacles
• Cons
• Belts are expensive
• Belts often break
• Often is heavy

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Keys to Success

• Make sure to
• Lower the centre pulley to help turning
• Use strong belts
• Tension the belts well (VERY important)

• Make sure not to
• Make it hard to replace belts

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Split Track

• Pros
• High Traction
• Can have decent maneuverability
• Extremely good at climbing obstacles
• Belts break less often
• Cons
• Belts are expensive
• Heavier than one track

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Keys to Success

• Make sure to
• Lower the centre pulleys to help turning
• Use strong belts
• Tension the belts well (VERY important)

• Make sure not to
• Make it hard to replace belts

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Omnidirectional Drives
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Omni Wheel based Holonomic Drive

• Pros
• Extreme agility
• Low traction
• Cons
• Difficult to program
• Easy to push
• Complex to build
• Can be hard to drive

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Keys to Success

• Make sure to
• Keep all wheels in contact with the ground -gt
  ideally with a suspension
• Have PID loops controlling wheel speed.
• Have logical and effective controls
• Let your driver try it before you commit to the
  design. Driving a holonomic robot well is quite a
  challenge.

• Make sure not to
• Go open loop
• Expect to push well
• Expect to climb well

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Mecanum

• Pros
• Extreme agility
• Low traction
• Cons
• Difficult to program
• Easy to push
• Wheels are very complicated
• Can be complex to build (but not as bad a omni
  based holonomic drive)
• Can be hard to drive

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Keys to Success

• Make sure to
• Keep all wheels in contact with the ground -gt
  ideally with a suspension
• Have PID loops controlling wheel speed.
• Have logical and effective controls
• Arrange wheels so top rollers point inwards
  forming an X
• Let your driver try it before you commit to the
  design. Driving a holonomic robot well is quite a
  challenge.

• Make sure not to
• Go open loop
• Expect to push well
• Expect to climb well

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Swerve/Crab

• Pros
• Very good agility
• Good Traction
• Cons
• Extremely difficult to program
• Extremely hard to build
• Can be hard to drive

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Keys to Success

• Make sure to
• Keep all wheels touching the ground
• Use PID loops to control wheel velocity and
  module position
• Have logical and effective controls
• Let your driver try it before you commit to the
  design. Driving a swerve/crab robot well is quite
  a challenge.

• Make sure not to
• Support the module at only one spot (unless the
  bearing surface is REALLY big and rigid)