[KenWittlief]

its not clear what signals you are referring to. In a feedback system you normally have:

command: the signal that is telling the system what the operator wants: ie, desired distance from point of origin, or desired speed

control: the signal that is coming out of the PID controller, telling the motor or heater or actuator what to do at this instant

feedback: a sensor that is measuring what the system is doing at this instant - it could represent distance, speed, acceleration, torque, angle... depending on the type of sensor used

error: the error signal is the difference between the command and the feedback, what the operator is telling the system to do, and what the system is actually doing at this instant.

Its very easy to get confused, and to look at the distance sensor when you should really be looking at the error signal, or to take a velocity command input and compare it to a distance sensor feed back signal.

The best first step is to check all your units. Make sure you have velocity when you are working with velocity. If you start inverting signals you have changed the units.

The 2nd step is to make sure the polarity of your signals is correct. If you have negative feedback on a sensor the system goes berserk.

The third step is to realize the P I and D are all inter-related. You cant tweak the P to get the best response, then start adding I, then D. When you add I the P must also be tweaked, when you add D the P and I must be tweaked.

The best way I have seen this done empirically is to hit the system open loop with a step function (square waves), and measure the response. Then you can characterize the system and calculate the constants.

Turning the system on and twiddling the 'knobs' for P I and D will drive you crazy :^) All that math stuff: the bode plots, the pole/zero diagrams... it really is necessary unless you have a control system that can characterize the open loop response of the system by hitting it with step functions.