How can I use allmargin() with non-unity feedback?

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Erik 2024-3-22

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评论： Sam Chak 2024-3-29

Hello!

I have a (MIMO) feedback system of which I would like to estimate the stability margins using allmargin(). Looking at the documentation of this function, it takes as input the loop gain L and assumes unity feedback, such that the system can be modelled as the figure below:

However, my system has some controller in the negative feedback path, such that the system looks as follows:

Is it possible to compute the stability margins of such a system using allmargin()?

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Paul 2024-3-22

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Hi Erik,

What's the loop transfer function, L(s), from which one could compute the stability margins (having already determined the closed-loop system is stable)?

In other words, are you asking about the math? Or do you know the math and are just asking how implement the math in Matlab?

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Erik 2024-3-29

Perfect, thanks! Sorry for the (ultimately) trivial question.

Sam Chak 2024-3-29

Hi @Erik

Personally, I have a preference for system (1), where the compensator is positioned on the feedback path. However, I acknowledge that system (2) may be more intuitive for most designers who prioritize driving the error to zero.

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