[Alan Anderson]

I am reluctant to offer suggestions when I can't get a straight answer to yes-or-no questions. I'll ask them again:

1) You want to control the drivebase motors in order to maintain a fixed position on the field, right?

2) Does it matter whether or not the robot gets turned in place?

I'm trying to find out if you care about the orientation of the robot (the direction it's facing) or if you just want to stay over the same patch of field carpet. There are several different ways to do it, and the most appropriate one depends on the details of your requirements.