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margin command on a system with transfer function on reference

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I have the following system (see picture). When calculating gain and phase margins with 'margin' command, should I consider L=GK to be an open loop transfer function (i.e. should I call margin(GK) )? If so, what difference does the transfer function H applied on the reference r make to gain and phase margins?

Accepted Answer

Sriram Narayanan
Sriram Narayanan on 13 Aug 2015
It depends on what you are using the Transfer Function H for. H would typically be used as a prefilter or for feedforward control. So, it should not be shaping the crossover frequency and therefore, would not have an effect on the gain/phase margins. However, you should calculate gain/phase margins using the open-loop GK to meet desired robustness/disturbance rejection requirement and the prefilter H could help is adjusting the reference to meet reference tracking requirement.

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