You know you are addicted to FIRST when you are thinking about differential swerve while out doing yardwork…
So, there I was, out doing yardwork, thinking about differential swerve, when it occurred to me that the main reason to build a differential swerve is to have 8 motors applying power to the wheels. But a drivetrain with 8 CIM-class BLDC motors would most likely be highly traction limited if it was geared to the same gear ratio as a typical 4 motor drivetrain.
So, after the yardwork was done, I plugged some numbers into the ILITE drivetrain simulator. For a 4 NEO drivetrain, assuming 4 inch wheels and 150 lbs full-up robot weight and a 20 foot sprint distance, the optimal gear ratio for sprint time is about 5.5:1 or so. This ratio results in a theoretical free speed on 20 ft/s. I realize that this is on the high side, and there are some other factors that come into play when you select a gear ratio, but let’s use this one criteria (minimum sprint time over a 20 ft sprint) as a comparison point.
The first thing I did was just change the number of motors to 8, leaving all other factors the same. The time to 20 ft was improved to 1.4 seconds, but sure enough, the first 8 ft of the sprint was slip limited. Reducing the overall gear ratio to 4:1 improves the sprint time to 1 second and is no longer slip limited. This results in a theoretical free speed of 25 ft/sec. I think most of us who have been around for a few years would consider that speed a bit too close to plaid for comfort (i.e. ludicrous).
So, I am left with a dilemma. If we set the gear ratio of a differential swerve to get a free speed that we are comfortable with from our traditional 4 motor drivetrain experience, then I am not actually able to take advantage of 8 motors because at stall I am slip limited. If I reduce the overall gear ratio to the point where I am no longer slip limited at stall, I end up with a ludicrous speed that the drivers probably won’t be able to actually utilize.
I’m inclined to gear a diff swerve for the lowest speed that is just above slip (i.e. ~25 ft/s). My thought is that you can put a speed cap on the system (in code) if it turns out to be too much and you may actually be running the motors at a higher efficiency point of the motor curve when driving in the 16-18 ft/s range.