PID tuning

I've never used Zeigler-Nichols myself. It is certainly _a_ way, but I wouldn't use it for a final tuning unless my requirements were not very critical. It probably would be a good way to get a starting point for a stable system to tune further using other means, and I understand that there are a number of process-control auto tuners based on it.

Are you asking about your system dynamics, or how to set your proportional gain?

Initial values for proportional gain depend heavily on factors that can vary wildly depending on how your system is put together, so you're really on your own. Just try a value, then go up or down by factors of

For plotting, just capture your data set and feed it into a package that can plot. Matlab, MathCad, Maple and Mathmatica are all packages that you can buy. Octave, SciLab and 'R' are packages (or languages) that you can get for free. I've used both Octave and SciLab and they both seem decent.

If you know your motor characteristics and the characteristics of the load you're probably much better off modeling the system mathematically, doing an initial control system design by the numbers, then tuning it. Even for an unknown system I'm not sure that Z-N is the way to go -- I would lean more toward doing a system identification to get a model, then doing the control design by the numbers. I'd only tend toward Z-N if I knew my system to be severely nonlinear, I didn't have much time to spend, and my control requirements to be light.

For an alternative method of tuning a PID, further information about motor control (which may or may not apply in your case), and a paper on basic z-transform control system design (I assume you're doing this in sampled time) see my website and click the links on the left under "Articles".