Gearing That Cannot Overcome Static Friction

[Tristan Lall]

Originally Posted by KenWittlief

If you have a transmission that shifts at the right points on the curve, your robot will have the best acceleration, pushing power, and top speed possible from what we are given to work with.

On the other hand, if you're slow to shift, or more precisely, if you can't go through a large spread of ratios quickly, you'll be spending too much time in neutral to be useful.

This is a bit of a problem with the DeWalt gearboxes, it seems. While the NBD actually shifts quite nicely (once the extra parts like the detent leaf spring and the clutch rod are removed), it takes too much time for a servo to work the action (and still have enough torque to move smoothly). It's also why a 6-speed based on (for example) 222's design might be a little tricky, because the locking grooves would be spending a disproportionate amount of their time in between gears.

If anyone's seen Volkswagen's new DSG gearbox, it addresses this issue by having two separate power paths, and alternating between them. This means that the left side is driving while the right is shifting (or already in gear and waiting to be engaged). When an upshift is commanded by the driver or engine computer, it just switches power paths. (If you wanted a downshift, and it's ready for an upshift, then there's an additional step, but it's still pretty fast for an automatic.)

It seems to me that to use these extra-high gears, you'd need to ensure that you shifted quickly in and out of them; they'd theoretically be useful for the high-speed dash at the end of a match, but only if you could accelerate quickly enough while running through the lower gears. The thing is, for a moderately efficient drivetrain and a 120 to 130 pound robot, you'd have to be gearing for something like 30 or 40 fps in order to be unable to move appreciably from a standing start (due to stall overcurrent). This means that in order to go that fast, you're probably going through so many ratios that the delay in shifting could represent a whole lot of coasting—in other words, you could have done the same job at a lower top speed, but equivalent average speed, with simpler gearing.

Also, 40 fps represents corner-to-corner dashes of less than 3 seconds, assuming prompt acceleration in low gear and fast shifts. Does the driver want to deal with that? I was quite happy to knock Sterilite boxes (and other robots) around on a wide-open field at 12 fps in 2003, but I'm told that gearing the 2004 robot for 16 fps in top gear made for a challenging test of skill for the driver. (This predates the current interpretations of the high-speed ramming rules. Things, especially in autonomous mode, were much more flexible at that point.)