FRC Concepts:
Drivetrains
Team 1716 Redbird Robotics
Why are drivetrains important?
● It moves a robot from point A to point B
● Not all drivetrain designs are equal
o each have advantages and disadvantages, and not
all are entirely obvious
● A robot’s drivetrain is the most basic part of the robot
o It MUST be reliable
● A major consideration for pit scouting
Rules of Drivetrain Design
● Keep It Simple, Stupid
o
o
It’s a drivetrain, not a space station
Simple usually means reliable, too
● It Must Meet Our Needs
o
Field and operating conditions are the two primary
considerations for picking the type of drivetrain we
will be going with
How Power is Transmitted
● Motors turn
● Gearbox reduces speed,
increases torque*
● Chain, gears, or pulleys
transmit power to wheels
● Wheels turn
How a Drivetrain is
Controlled
Arcade
Tank
One joystick controls direction
Two joysticks control either side of
drivetrain
Why YOU need to know all this
● The engineering and scouting divisions need
you to know what you’re talking about
Common Drivetrain Styles
● “Skid” Systems
o
o
o
2WD, 4WD, 6WD, 6WD+
Tank Treads/Belting
Traditional/WCD
● Holonomic Systems
o
o
Swerve (Crab)
Mecanum
Two Wheel Skid
● Pros
o
o
o
Dirt cheap
Kitbot can be 2WD
Simple to build
● Cons
o
Weak, can’t climb inclines well
o Loses traction easily - if it gets into a pushing match with a robot,
driving wheels may get pushed up above the floor
o Noob tier
Four Wheel Skid
● Pros
o
o
Easier to control
Simple, more traction
● The Bad
o
o
o
Turning is more difficult (both wheels have traction)
A compromise between stability and maneuverability
Still noob tier
Six Wheel Skid
● Middle wheel is offset, essentially creating two 4WD
systems
● Pros
o Plenty of traction
o Minimized resistance to turning due to middle wheel offset
● Cons
o Drivetrain “rocks” slightly due to middle wheel offset
 Can be significant if robot has long arms and appendages
6+ Wheels/Tank Tread
● More wheels = more distributed load
o Pretty pointless in FRC
● Traction not dependent on surface area
o More wheels != more traction
● Pushover tier
o Complex and expensive for no good reason
o Tank treads only useful if field elements make maneuvering downright
diabolical
West Coast Drive
● Two wheels are driven directly by gearbox
● No outer chassis rails
● Wheels “cantilevered” outside the frame
o Meaning: the wheel axle is attached at only one point, that point being
the square metal tubing the axle is routed through
● Pros
o Easier to conduct maintenance on
● Cons
o Harder to implement as it requires precise machining
Holonomic Drive Systems
● Allows a robot to move in two dimensions,
instead of just forward and backward
o
o
This allows it to rotate simultaneously
It can also do this WITHOUT rotating
● Two major systems
o
o
Swerve/Crab
Mecanum/Omni
Swerve
● Each wheel can (at least theoretically) be independently
driven and steered
● In practice, arcade or tank drive is
utilized to simplify steering
● Pros
o Highest maneuverability
● Cons
o Complex to build, maintain,
code, and operate. Also quite heavy
Mecanum/Omni
●
●
●
●
Uses vector addition to allow for omnidirectional motion
No complex steering mechanisms
Requires four independent wheels
Mass-produced parts make this drivetrain accessible
(compared to swerve)
Four Guidelines of Drivetrain Design
● RRRR
o
o
o
o
Reliability
Repairability
Relevance
Resonability
Reliability
● Most important consideration
o
It’s the most important part of the robot
● Good practices
o
o
o
o
Wheels be aligned properly
Reduce or remove friction wherever possible
Components must be manufactured to a high
degree of quality and accuracy
Components must be durable (no wood or plastic)
Repairability
● It won’t be reliable 100% of the time, but it
should be close
● Good practices:
Accessibility
Have plenty of spare parts for quick repairs at
competition
o Maintenance and repair times should be quick (<10
min)
o
o
Relevance
● Everything on a drivetrain must be worth the
work and risk of including it
o
o
Driving around in circles doesn’t require tank treads
or swerve
Maneuvering around in tight spaces or climbing
bumps in the game field may make one of them
more worth it, however
Reasonability
● We have a limited amount of time and
money during the build season
o
o
We have to allot our resources very carefully, and
putting that time and money into something that
won’t offer a good return is a bad idea
this is the reason we didn’t go swerve last year (we’d
have to figure it out in only a few weeks)
Resources
www.chiefdelphi.com
FRC Drivetrain Design and Implementation
PPT:
http://www.chiefdelphi.com/media/papers/downl
oad/2256