margin() is returning PM = inf when this doesn't match the visual plot.

EDIT: Running the same code through R2023a has solved the problem, but it did not work in R2022b.

I'm using Matlab to create a lead-lag controller with certain specifications (university course), and to find the compensator value, I wanted to use the margin() function to find and plot the gain and phase margins. However, Matlab calculates a phase margin of infinity, when I can see that the phase margin is approximately 18 in the plot itself. The plot also isn't showing the dotted lines anywhere on the phase subplot, which I haven't seen before.

Plot without the compensator, with a missing phase margin:

Similar plot with slightly different input transfer function:

Is there a reason that I'm not seeing the phase margin elements in the upper plot like I expected?

Input transfer function are below:

Cs = tf([alpha*Tz*K1,K1],[Tz,1]); % s-domain TF

Cz = c2d(Cs,T,'Tustin'); % z-domain TF using 'Tustin' method

Gs = tf(K,[Je,Be,0]);

Gz = c2d(Gs,T);

figure(8);

margin(Cz*Gz);

The constants used were generated in previous steps from Matlab:

alpha = 14.717, Tz = 6.901e-04, K1 = 1, K = 2.0329, Je = 7e-04, Be = 0.0061

[Paul]

Hi Emily,

Can you provide the inputs to the margin command that creates the upper plot? If the code is short enough, you can just copy/paste it into a new comment or as an edit to your question. Or, you can save the inputs to margin in a .mat file and use the paperclip icon in the INSERT portion of the ribbon to attach the .mat file to your question or a comment.

[Paul]

If still intereseted in pursuing this, please add the value of T to the Question. Also, if we really want to be exact, just save Cz and Gz to .mat file and attach it to the queastion using the paperclip icon.

Answer 1

[Emily Reid]

Running the unaltered code through R2023a seems to have fixed the plot, although I couldn't figure out any reason for the error in R2022b that I originally had.