Grid in MIMO nyquist diagram

Is it possible to costomize the grid in nyquistplot? Especially with MIMO Systems I have the problem, that no suitable grid is shown.

SISO System:

MIMO System:

Unfortunately I couldn´t find any costumization options for the grid and plotting the single SISO transfer functions in for example tiledlayout is not possible. Thank you for replying.

[Paul]

Hi Christian,

Are you showing the Nyquist plots for multiple systems on one plot?

Are the axes zoomed in? If not, why don't the plots make closed contours?

All of the examples in the doc for MIMO systems show that turning the grid on yields the same result, i.e., the M-circles aren't shown, and the doc makes no comment about that. Don't know if that means the result you're showing is a feature or a bug. It's probably a feature.

Finally, what information do you want to obtain from each Nyquist plot for each element of the 2x2 system?

Answer 1

[Star Strider]

I am not certain what you want to do or how much of the documentation you have explored.

Most of the Control System Toolbox and System Identification Toolbox plots listed in the documentation section on Customizing Response Plots from the Command Line have options that can be set. For the Nyquist plot specifically, see the documentation section on nyquistoptions. If you have already seen all of these, I have no further suggestions. Plots in those Toolboxes are characteristically difficult (although in many instances possible) to work with otherwise, and require a bit of what I call ‘handle spelunking’ to access and change. I believe most of the plots in those Toolboxes are subplot plots, not tiledlayout plots, although that could have changed in recent releases. (I have had no reason to explore that recently.)

It might be easier to help you if we had the system you are working with, the MATLAB release you are using, and a more detailed description of what you want to do and the result you want.

[Christian Haas]

Thank you for the answer. I managed to create my plots usind subplot and plotting every subsystem individualy. My problem was, that the grid option from the nyquistoptions doesn´t work propper, if applyed to a mimo system. Also I couldn´t find any handel for the grid, to change for exmaple the amplifications shown.

An example, where one could see the problem might be the following:

G_test=[tf(1,[1 1]) tf(1,[2 1]);tf(1,[3 1]) tf(1,[4 1]);];
opt = nyquistoptions;
opt.Grid = 'on';
figure;
nyquistplot(G_test(1,1),opt);

figure;
nyquistplot(G_test,opt);

I am using Maltab R2022a , but as shown above, the problem also ocures in 2023a.

[Star Strider]

What you want to do may not be possible, unfortunately.

I was able to determine that the detailed grid is (probably) fairly far down in the handle chain, specifically Axes.Children.Children.UserData, however that shows up as having the correct information (specifically ‘ctrluis.axesgrid’) and that apprently scales automatically to the size of the figure, since the contents of that entry seem to be the same for both the larger and smaller plots. When I access that structure from the larger plot and import it to the smaller plots, nothing changes. I have not proceeded further, since all of that structure’s properties are most likelly deeply hidden and likely could not be changed anyway.

My apologies for the delay — ‘handle spelunking’ generally takes a while.

[Christian Haas]

Thank you for the answer, no apologies needed. Using subplots I was capable of creating my figures. This is fine for me at the moment. Otherwise I´ll use the nyquist and plot commands to build my own figure.

[Star Strider]

My pleasure!

Away for a while today.

The only problem is not being able to export the nyquist or nyquistplot grids. Those are, as I mentioned, specific properrties of the Toolbox plots, and not easily replicated. I thought they would be ‘line’ objects and easily exported, however that is not an option.