What about this paper?
http://www.chiefdelphi.com/media/papers/1823?
It explains the theory and then shows an example using a camera to track the green light, but that example is very similar to using a potentiometer to hold a mechanism in place. You are simply finding the difference between the desired and the actual values and multiplying the difference by a gain, the summation of the difference by a gain, and the change in the difference by a gain.