So I was tasked with the selection of Shaker’s drive gearing. I initially did it very incorrectly, so we fixed it week 5 but I’ll explain the final drive gearing as if it were well thought out.

Anyway, we had some design criteria for the drivetrain this year in addition to the game specific stuff:

  • Must be COTS or fabricated off site
  • Must not cost too much (read this as shifters cost too much)
  • Preferred to use some AndyMark products (we already have a ton of them and they’re great)

Game specific criteria

  • Must resist pushing (high traction)
  • Should push a kit wheel drivetrain without tripping a circuit breaker
  • Should push a traction wheel drivetrain for a few seconds
  • Must turn on a dime
  • Must switch zones
  • Should go over the bump
  • Must be controllable and agile
  • Must go pretty fast

No one spelled this out that formally or at all on the team but it’s what we were going for.

With the constraints, we were limited to single speed drivetrains (DeWalts were voted down, and dog shifters cost upward of $300). We figured a max speed of about 8 FPS would be adequate (with acceleration getting from bump to bump in under 2 seconds, wall to wall in 3.5 or so). Anything faster would be a bit too hard to control for a driver that had never left regolith before. However, we also wanted to push. We decided the midfield would be the most important part of the field, and that we would need to fight for position under the return to pick up balls and kick them. Things like mecanum drive, omni drivetrains, etc. were just not acceptable as we could too easily be knocked out of alignment after grabbing a ball.

The main choice was between AndyMark Toughboxes and a 6 motor drivetrain using custom fabricated plates (thanks to RC from 1323). The 6 motor drive could be geared for 9 FPS and still basically be traction limited at 55 amps or so, which was perfectly adequate. Toughboxes taking only 2 CIMs each would be geared for a good clip but would not be traction limited so pushing against tougher robots would be limited. Ultimately because the latter drive could not be fabricated, and because we used the FPs for a vacuum system, we opted out of a custom drivetrain and used a standard Toughbox with no chain reduction to 6 inch plactions. 12.75:1

After the season I took the time to evaluate the robot from a drivetrain perspective. For the elims of our first regional onward we played the “front bot” so a few of our design considerations no longer really applied. We didn’t do nearly as much pushing, but we needed to resist being pushed a bit more. This made the single speed drivetrain less of a drawback. Our top speed in front was just fine as we could match most defensive robots; either they were slower because they wanted more traction, the same speed because they were a kitbot, or faster and we wouldn’t really get pushed by them. Driving skill helped here. We also resisted spinning and turning just fine. Overall, I’d say we lucked ourselves into a successful drive this year. If I did it again and knew we’d be in front the entire event, I’d have used 6 motors and geared it for around 9.5 FPS, but we did just fine where we were at.