What does the step response plot in PID tuner indicate ?

Hi everyone,

For example in the below Url, https://www.mathworks.com/help/slcontrol/gs/automated-tuning-of-simulink-pid-controller-block.html

what does the step response plot ( for reference tracking) signify? Does the step response plot given in this Url,denote the response of measured output speed to the reference speed or it shows the controller output response with respect to the reference speed ?

Answer 1

[Akbar]

"Reference tracking" is when the Controller Parameters are tuned to follow the reference. And there is another Approach too: "Disturbance rejection", in this case Controller is more focused on disturbance rejection rather than tracking the reference.

The plot Shows closed Loop step Response around the operating point. Where Input is reference and Output is Speed. As you see in the plot, plant step plot is different, Shows first order behaviour.

[Sagun Katta]

Thanks a lot Barak for the clarification. Please let me know how you made the step plot for plant. The response time slides shows, like how fast the controller responds to changes in Plant output ? What does step plot (for plant) first order behavior mean? I am trying to understand what are meant by operating points in the plant model from the Matlab codes in https://www.mathworks.com/matlabcentral/fileexchange/48019-blood-glucose-regulation-in-a-type-i-diabetic-in-simulink

Could you please help me in understanding the Insulin dynamics described in the above code ( from diabetic.m)

Have you performed a First Order plus dead time test (FOPDT) anytime in SIMULINK? Also have you performed tuning of PID controllers,?

Regards, Sagun Katta

[Akbar]

Just click on "add plot>>plant".

Yes, "Response time slider" is used to get the desired Performance by Tuning Controller Parameters.

By "first order behavior" i mean, there is no overshoot and no oscillation in the step response.

About operating Point. Real life plants are mostly nonlinear, you should use linear model in order to be able to design a Controller. To get a linear model you Need to choose one operating Point of your plant. For ex. one particular flow rate, temperature, pressure... And you analyze your plant only around that operating Point. In the case of PID tuner, linearization is done automatically.

I had a quick look at step_results.m file and saw a plot of Glucose and Insulin. As I understand, Glucose is a reference and Insulin should follow it. But in the generated plot i see that Insulin is sometimes more than Glucose (overshoot). So probably the Controller Needs to be tuned not to have overshoots.

You can get dead time just by looking at the step Response of plant. Tune Controller using PID tuner. In the figure above, tuned P and I values are shown at the bottom right, or click("Show parameters"). When you click "update block", then PID block in simulink will be updated with a new set of Parameters.