basic question on using CIMs with TB Minis

[Kevin Ray]

When we were designing our shooters for both the alpha and beta bots we took the 14 or so mini CIMS that we had and measured their actual rpms, both forward and reverse. We found them to range from about 5650 rpm to 6440 rpm and it didn't matter whether it was forward or reverse. The only noticeable difference that we did discern was that 8 of them ran forward faster than reverse. Some of them were strangely different-- running 550 rpm faster in one direction vs the other.
We just used these numbers to mate a pair so that we were able to have the same speed (within 40 rpm, no load) with one forward and the other in reverse.

[Rafi Ahmed]

CIMs do have motor bias, as other motors do. Generally CIMs are pretty well balanced, but it has been known to happen. TBs, as far as I know, shouldn't cause bias "by design". I would check lubrication/assembly of them or motor controller calibration.

There are a lots of ways to control the outputs and make them match each other. "Drive Straight" code is very common for teams to use.

[zaphodb]

Thanks for all the replies. In the end I tracked it down to the Victor SP calibration. I noticed when moving forward/reverse that the red/green colors came on at different times. The VictorSP manual says that they are calibrated for the RoboRio by default, so could be there is still something I have to do in the RoboRio, but for now running through the calibration steps in the Victor SP manual fixes the issue.

For anyone curious, or running into this later here is the current manual:

http://content.vexrobotics.com/vexpr...e-20151209.pdf

Of course I unplugged the motors while running the calibration step.