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Problem in linearizing a Simulink model

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I am trying to use linmod to extract a linear model of a non linear controller. I first trim the model in SIMULINK using the "trim" command, and receive correct results for my trim states ("xInitial") and model inputs ("FCS_Input") for a known output of the system. When I use linmod('mymodel',xInitial,FCS_Input) with "xInitial" & "FCS_Input" calculated by the trim function, I receive a "D" matrix that is identically zero. My model obviously has some linear i/o transmissions so that D must have some non-zero elements in it. Could the result I'm getting be due to direct connection of some of the inputs to terminator blocks? (I do this because the inputs to the model are vectors, where only part of the components are actualy used by my controller)

Accepted Answer

Guy Rouleau
Guy Rouleau on 6 Dec 2013
Edited: Guy Rouleau on 6 Dec 2013
It is difficult to say without looking inside the subsystem in your model.
A few tips: - If you have Simulink Control Design, there is a diagnostic window that can help you understand how each block is linearized and see which block is problematic.
- With linmod, the only tip I can give it to linearize pieces by pieces to see which block linearizes as zero.

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