[Alan Anderson]

That's most of an answer to my first question. If I understand you correctly, you want to control the drivebase motors in order to maintain a fixed position on the field, right?

Let's try for an answer to my other two questions now. What kind of drivebase do you have? Describe the number, type, and placement of wheels, number and type of motors, kind of gearbox, chain vs. belt, etc. Where do you have the encoders mounted? I'd rather give advice based on what you really need rather than assume things about what your robot is like.

Here's another question, now that I think you're talking about the drive motors: Does it matter whether or not the robot gets turned in place?