[Alan Anderson]

Using the built-in PID of a Talon SRX is easy for a random motor. There's a LabVIEW example project that shows you exactly how to do it. However, it gets kind of complicated when you want to use the Drive functions. Those functions are not really designed to give values outside the -1 to +1 range to the drive motors, but the Talons in PID mode want values that match the encoder resolution. It's also not obvious how to set the individual motors to PID mode when they've been opened as a 4 Motor Drive.

But it can be done. We had it working properly today (before the temporarily-mounted robot battery shifted and broke one of the encoder mounts).

Tuning a drivebase's PID parameters isn't trivial either. You can't do it with the robot on blocks and the wheels spinning freely. It's best done with the robot moving on the surface that it will actually be running on, though if you're clever you can get started by pressing a patch of carpet lightly against the wheels to give them some load.

If your problem is primarily with the robot not going straight, you might do better to use a gyro as the feedback for a PID controlling the rotation. Our robot's closed-loop mecanum motor speed control is not sufficient on its own to produce the desired motion, because the weight balance lets some wheels slip more than others.