Quote:
Originally Posted by Littleboy
Continuing on what IKE said, if you are going long and short distances a lot in one match, shifting may be helpful. You can have one gear maximized for long distances, and another gear for shorter distances. This way, no matter which distance you are going, it will usually be fairly efficient time-wise to get there.
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That's fantastic in theory. There are several issues that come in to play:
-Shift times/shift lurch
-Ratio spread
-Pushing
Pushing for long periods of time (e.g. pushing an entire alliance up a bridge for a triple) requires the robot to be traction-limited to 40a/motor or lower to avoid tripping the individual motor breakers. For a 4xCIM drivetrain, the highest speed you can get that at is around 5.5fps using high traction wheels on carpet. That's not a great speed to play most short games at.
Ratio spread of commercially-available transmissions limits your available choices in shifting. AM shifters are either 4:1 or 2.56:1 ratio spread, Vex ball shifter is 2.27:1. Since the vex ball shifter did not exist at the time, it was either 4:1 or 2.56:1 spread. To optimize a 2-speed for long/short gameplay (instead of driving/pushing), the ideal speeds for generalized FIRST games are somewhere in the vicinity of 8fps and 13fps, which is roughly a 1.6:1 ratio spread. You can't get that using parts from AndyMark.
Shift time and shift lurch are important when auto-shifting. Autoshifting can increase acceleration time. The general upshift logic looks for the point where the acceleration curves in low and high intersects, and shifts at or just before there. Downshifting uses totally different logic, with totally different goals, but it gets you back into low to accelerate. This is usually not a problem when pneumatically shifting, but we were servo shifting, and the shift time wasn't a huge problem, but the left and right sides would shift asynchronously and the robot would lurch to one side, which is not good. Our 2011 robot auto-upshifted, which we were happy with. It could also auto-downshift but we disabled that because it was too difficult to calibrate well with the driver interfaces we were using - A coastdown shift as we approached the rack would change the turning performance, which was not desired. I believe we could solve this issue with better calibration and the Halo-type drive we use now, but we weren't running Halo in 2011.
Conclusion: The real solution is a three speed transmission that can quickly shift between second and third. However, that's just too much work, so we'll stick with the AM or Vex 2-speeds and optimize the final drive to get the best driving performance in high gear, while keeping low traction limited.