Drivetrain Selection

Purpose of a Drivetrain

  • Move around field
    • Typically 27' x 54' carpeted surface
  • Push/Pull Objects and Robots
  • Climb up ramps or over/around obstacles
  • Most important sub-system, without mobility it is nearly impossible to score or prevent points
  • Must be durable and reliable to be successful
  • Speed, Pushing Force, and Agility important abilities

Types of Wheels

  • "Traction" Wheels
    • Standard wheels with varying amounts of traction, strength & weight
      • Kit of Parts (KOP)
      • AndyMark (AM) or VEX Pro
      • Pneumatic
      • Slick
      • Custom
  • Omni
    • Rollers are attached to the circumference, perpendicular to the axis of rotation of the wheel
    • Allows for omni directional motion
  • Mecanum
    • Rollers are attached to the circumference, on a 45 degree angle to the axis of rotation of the wheel
    • Allows for omni directional motion

Types of Drivetrains

  • Tank
  • Swerve
  • Slide
  • Mecanum
  • Holonomic

Types of Drivetrains: Tank

  • Left and right wheel(s) are driven independently
  • Typically in sets of two (1-4 sets is common, sometimes higher)
    • Strengths
    • Simple & cheap to design, build, and program
    • Easy to drive
    • Potential for high speed and/or pushing force
  • Weaknesses
    • Slightly less agile than other drivetrains

Types of Drivetrains: Swerve/Crab

  • Wheels modules rotate on the vertical axis to control direction
  • Typically 4 traction wheels
  • Strengths
    • Potential for high speed and/or pushing force
    • Agile
  • Weaknesses
    • Very complex and expensive to design, build and program
    • Extra motors required to be able to rotate robot frame

Types of Drivetrains: Slide

  • Similar layout to tank drive, with an extra wheel(s) perpendicular to the rest
  • Must use all omni wheels
  • Strengths
    • Fairly easy and cheap to design, build, and program
    • Agile
  • Weaknesses
    • No potential for high pushing force
    • Extra wheel(s)/motor(s)/gearbox(es) required to allow robot translate sideways

Types of Drivetrains: Mecanum

  • Similar layout to tank drive, but each wheel must be driven independently
  • Must use 4 mecanum wheels
  • Strengths
    • Fairly easy to design & build
    • Agile
  • Weaknesses
    • No potential for high pushing force
    • Challenging to program and learn to drive well
    • Requires extra gearboxes
    • Wheels are expensive

Types of Drivetrains: Holonomic

  • 4 omni wheels positioned on 45 deg angle in the corners of the frame
  • Each wheel must be driven independently
  • Strengths
    • Agile
  • Weaknesses
    • No potential for high pushing force
    • Very challenging to program and learn to drive well
    • Requires extra gearboxes

Compare Drivetrains

  • Choosing the right drivetrain is critical to the success of an FRC robot
  • Several drivetrains to choose from
    • Each one has its own strengths and weaknesses
  • Important to quantitatively evaluate all options to ensure optimal solution is chosen
    • Best method to do this is a "Weighted Objectives Table"

Drivetrain Attributes

  • Agility
    • Ability to translate in the x and y axis as well as rotate about the z axis simultaneously
  • Strength
    • Push robots and/or game pieces
    • Resist defense from all sides of the drivetrain
  • Number of Motors
    • Number of motors allowed on an FRC robot is limited
    • Most drivetrains use 4 CIM motors to power wheels
    • Additional motors to rotate wheel modules or translate sideways may take away from motors for other robot functions
  • Programming
    • Ideally does not require sensor feedback (eg. wheel module angle)
    • Ideally does not require advanced algorithm to calculate individual wheel speed/power
  • Ease to Drive
  • Intuitive to control so little practice is required to be competitive
  • Just because some drivetrains have the ability to move sideways doesn’t mean the driver will use the ability
    • Often drivers end up turning the robot because it is more natural or going sideways feels (or actually is) slower
  • Traverse Obstacles
    • The ability of a drivetrain to traverse ramps, bumps or steps
  • Design
    • This is a very general heading. Sub headings grouped as there is a strong relationship between them
      • Cost
      • Ease to design (select components and choose dimensions)
      • Ease to manufacture
      • Ease to assemble
      • Ease to maintain/repair
      • Weight

Weighted Objectives Tables

  • Give each attribute of each drivetrain a relative score between 1 and 5
  • Weights are dependant on
    • Strategic analysis of the game (priority list)
    • Teams resources
 WeightTankSwerveSlideMecanHolo
Agility?35555
Strength?45111
Motors?51355
Program?51432
Drive?53321
Traverse?54431
Design?51443
  • Agility, Strength & Ability to traverse obstacles
    • Relative to #1 priory, reliability
      • 0 = not important or required
      • 10 = equally as important as reliability
  • Number of Motors
    • Depends on complexity of other robot features and ability to design with all motors
      • 0 = no other features/very strong ability to design with all motors
      • 10 = very complex/little ability to design with other motors
  • Programming
    • Depends on strength of programming team (# of students/mentors, experience, ect)
  • Ease to Drive
    • Depends on amount of available practice
      • 0 = have a full practice field and practice robot with committed drivers that train every day
      • 10 = no practice field/robot, no time in build season to practice
  • Design
    • How many students/mentors do you have?
    • How much experience do you have?
    • What tools are available to you (hand tools < bandsaw < mill)?
    • How many hours are your shop facilities available/will you use them?
    • How much money do you have?
    • Drivetrains with a low design score require significant resources to design a reliably
      • 0 = lots of experience, students, mentors, tools, money
      • 0 = The desired drivetrain has been used in a previous season or prototyped in the off season
      • 10 = No experience, few students, mentors, tools, money

Typical Weights for a Rookie or Low Resource Team

  • 5 - Agility
  • 5 - Strength
  • 5 - Number of Motors
  • 10 - Programming
  • 10 - Ease to Drive
  • 0 - Traverse Obstacles
  • 10 - Design
  • Resources are low, so it is more important to build a simple drivetrain that is easy to program and learn how to drive to ensure reliability.
  • The performance of the drivetrain (agility & strength) are not as important as reliability
  • The number of motors is not as important because additional features should be very basic and require few (or no) motors

Rookie/low Resource Team Weighted Table

  • Rookie/low resource team weighted table
  • Tank drivetrain much higher score than others
  • Slide drive second best
 WeightTankSwerveSlideMecanHolo
Agility53 (15)5 (25)5 (25)5 (25)5 (25)
Strength54 (20)5 (25)1 (5)1 (5)1 (5)
Motors55 (25)1 (5)3 (15)5 (25)5 (25)
Program105 (50)1 (10)4 (40)3 (30)2 (20)
Drive105 (50)3 (30)3 (30)2 (20)1 (10)
Traverse05 (0)4 (0)4 (0)3 (0)1 (0)
Design105 (50)1 (10)4 (40)4 (40)3 (30)
Total22593% (210)47% (105)69% (155)64% (145)51% (115)

Comparison of weighted tables for different resource teams

 RookieAverageStrong
Agility5810
Strength5810
Motors565
Program1073
Drive1073
Traverse000
Design1073

 TankSwerveSlideMecanHolo
Rookie93%47%69%64%51%
Average89%56%67%66%56%
Strong82%71%64%66%61%

When to choose a swerve drive

  • Strength & Agility equally as important as reliability
  • Lots of students/mentors
  • Access to advanced tooling
  • Large budget
  • Team has strong ability to use other motors for robot function
  • Team has practice field and practice robot
  • Team has used a swerve in a previous season, or prototyped one in the off season
 Swerve
Agility10
Strength10
Motors2
Program2
Drive2
Traverse0
Design2

 TankSwerveSlideMecanHolo
Swerve79%80%63%63%59%

When to choose a slide drive

  • Agility equally as important as reliability
  • Strength is not required (game has no interaction with opponents)
  • Team has practice field and practice robot
  • Team has used a slide in a previous season, or prototyped one in the off season
  • Lots of students/mentors
  • Team has strong ability to use other motors for robot function
 Slide
Agility10
Strength0
Motors1
Program3
Drive1
Traverse0
Design3

 TankSwerveSlideMecanHolo
Slide78%67%89%87%79%

When to choose a mecanum drive

  • Agility equally as important as reliability
  • Strength is not required (game has no interaction with opponents)
  • Team has practice field and practice robot
  • Team has used a mecanum in a previous season, or prototyped one in the off season
  • Strong programing ability
  • Lots of students/mentors
 Mecan
Agility10
Strength0
Motors5
Program2
Drive2
Traverse0
Design3

 TankSwerveSlideMecanHolo
Mecan82%60%83%88%82%

Designing a Tank Drivetrain

  • At this point we have concluded Tank-Style Drivetrain is usually the best option for all teams, regardless of the game or the teams resources
  • Why don’t all teams use Tank-Style Drivetrains?
    • Some (few) teams have a lot of resources
    • Trying new things to learn new skills/gain new experiences
      • Understanding this choice will make them less competitive
    • Improper strategic analysis of the game and evaluation of team resources
    • Improper analysis of strengths and weakness of various drivetrains
      • Omni directional drivetrains have a significant "cool factor" that distract teams