[WesleyC]

Why use a case statement in a PD loop?

The gain I used on our robot is 5 or 6... however, the output of the PD loop is then sent to our drive code, which incorporates an exponential drive algorithm.

The best thing to do is to use a printf statement right before the drive motors (or, even better, AFTER) are set--this will enable you to read EXACTLY what the drive motors are getting.

Since this may not be possible with the robots having been already shipped, I'll give you a quick fix:

By now, since you've probably figured out a minimum value that DOES make the motors move the robot, set a MINIMUM speed limit. Something like...

//150 and 105 are just arbitrary numbers; replace them with the minimum speeds that turns your robot

//copy this code for each motor
if (motor_speed > 129 && motor_speed < 150)
{
motor_speed = 150;
}
else if (motor_speed < 126 && motor_speed > 105)
{
motor_speed = 105;
}

Do that for both motors, and it should limit their minimum speed. If they're getting a value a little off center it'll increase their speed to the value you define as a minimum. That stops them from "humming" and actually makes the robot move, even at very low speeds as defined by the PD control.