PID controller design: How to plot control signal?

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Matthias
Matthias 2016-7-14
评论： Suraporn Puangpanbut 2016-8-9
I'm using the Control System Toolbox in Matlab.
I have a plant P (an object of class idproc) and a PID controller C (class pid).
I can connect them using T = feedback(C*P, 1).
Now, I can use commands like step(T) to plot the response of the whole system to a step input.
What I would like to know is how to get access to the control signal that the PID controller came up with. So, given a particular input to the system in T, what is the control signal that the PID part of the system generated to yield the output plotted by step(T)?
Thanks, Matt
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Arkadiy Turevskiy
Arkadiy Turevskiy 2016-7-20
The transfer function from reference to controller effort is C/(1+CP), so you simply do: >>step(C/(1+C*P))
Also note that if you use PID Tuner app, it lets you plot all signals of interest in your loop by selecting options in drop down menus.
Suraporn Puangpanbut
Suraporn Puangpanbut 2016-8-9
Yes, @Arkadiy Turevskiy is right, I have tried it, thank you. But for matching the time vector, I think you should use the same timelenght,
y=step(C*P,1,t_final); mv=step(C,P,t_final);
or [y,t1]=step(C*P,1,t_final); [mv,t2]=step(C,P,t_final); if you want to get the time vector as well.

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回答（1 个）

Mischa Kim
Mischa Kim 2016-7-14
Matthias, for this simple configuration with C and P in the feed-forward path and unity feedback you can do the following:
The error signal in this configuration is the same as the output signal of a system that has C in the feed-forward path and P in the feedback path. In other words,
T1 = feedback(C,P)
e = step(T1)
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Matthias
Matthias 2016-7-14
编辑：Matthias 2016-7-14
Thanks for your answer! Unfortunately, I get the error:
Error using DynamicSystem/step (line 95)
Cannot simulate the time response of models with more zeros than poles.
As far as I understand, this may be because I use a complicated model for the Plant which cannot be inverted (I'm just piecing this together from an incomplete understanding of control theory, so please correct me if I misunderstood). I have real data that I'm trying to fit and the fit is best when I choose "underdamped pair + real pole" in the Plant Identification part of the PID Tuner.
However, there should, of course, be a well-defined control signal when driving the system forward even if the plant model is complicated and non-invertible. Is there another way to extract the control signal?
Suraporn Puangpanbut
Suraporn Puangpanbut 2016-8-9
T1 = feedback(C,P) e = step(T1)
The code should work well. The error you got comes from your total transfer function has more zero than pole, which is not casual, can not be implemented, try another controller structure.

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