[seg9585]

Originally Posted by Alan Anderson

For what it's worth, I understood the pictures immediately, and I think they look correct.

How heavy is your robot, and how is the weight distributed? The symptom sounds like the back wheels are slipping a little on the carpet, which suggests that the robot is front-heavy.

The ultimate solution is going to be to recognize that controlling wheel speed precisely isn't good enough to always prevent unwanted rotation of a mecanum drivebase. You're going to have to add some way of measuring the actual robot motion, and in this case I think the appropriate sensor to do that is a gyro.

If the weight is not distributed evenly, for a given voltage the angular acceleration of the heavier weight on wheels will will be lower relative to the lighter wheel, mecanum roller slippage or not. I think using the PID loop to even out the wheel RPMs is the right stategy.
Having said that, I rarely use the built-in PID controllers WPI provides, usually just write my own control loop. I'm not sure why OP selected values of P=0.1, I=.001, D=1 . Recommend reducing D down to near zero while testing, as this would just produce a damper which could null out your rates to near zero despite your offset.