Automatic PID control S7-300

I'm working on a temporary assignment where I work with a motor and a PID-control. I now have to increase my K-value in order to let the P-action fluctuate. When the P-action changes from a straight line into a nice sinus-shape I get my Pcritical and my Toscillation which I have to use in the Ziegler-Nichols formulas to then adjust the loop.
Now is my question, how do I find out when the P-action becomes a sinus? I thought about rising edges and measuring the time between them but of course there's the problem with transitioning and little oscillations due to the signal being analog.

Thanks in advance

Use a chart recorder to monitor PV.

The Ziegler-Nichols method may not be suitable for your application. Use the search facility on this forum and you will see that the subject has been discussed at great length.

To help forum members to help you maybe you could post some more information about your application e.g. what load your motor is controlling and how the feedback is measured etc.

Nick

I know Ziegler-Nichols isn't the best method but my promotor told me to use this one first in order to compare with other methods afterwards.
My motor isn't controlling a load, I'm just using it because it's a fairly fast proces and I get feedback fast. The feedback consist of an induction sensor which counts the number of turns the engine shaft makes through a little metal plate on it. I'm steering the engine with a frequency inverter (Siemens G110) using an analog signal from the plc (314C-2DP)

So this is a speed control application.

In your PLC program you will have converted the inductive sensor signal to a speed for use as feedback to your PID loop. Send this signal to an analogue output and connect it to a chart recorder. Alternatively, if you have an HMI, make a screen with a trend view on to show the motor speed.

Nick

I use the PID Control from Siemens to see the PV change, that is not the problem. The program has to be made so that it can detect Kcritical and Toscillation itself and then use it in the formulas of Ziegler-Nichols to adjust the PID-loop itself. But how can you detect whether or not your value changes along a sinus-shape function using the PLC?

Are we to take it then that you are developing and auto tuning PID control algorithm?

You first need to define "Kcritical" so that you know what to look for in your feedback signal.

Have a think about this. Let us assume that the signal is initially increasing in value. A sine wave continues to increase in value over time until it reaches maximum amplitude and then decrease in value until it achieves mimimum amplitude at which point it will start to increase again. If you log the feedback over time then you can carry out some analysis to determine amplitude of oscillation and period of oscillation. Determining whether it is actually a sine wave or some other shape might be more complex.

Just something to think about.

Nick