When we try to drive backwards slowly the robot tries to turn as if it has a jag calibration problem, but we’ve calebrated many times, it doesn’t do this in any other direction. We’ve gone so far to replace jags and sidecar. Does any one have a clue as to why this might be happening?
One of your motors might have more directional bias than the others, or it could be a mechanical issue with one of the gearboxes. Put the robot up on blocks and run each motor individually at the same low power, and see if one of them turns more slowly.
It could be a mechanical issue you won’t see on blocks, even. Remember that mecanum wheels have significant sideload when you’re driving straight. One of your wheels might be getting shoved into something or you might be putting extra load on a questionable bearing when you’re driving backwards.