sisotool and bode problem

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Kiarash Gharani Khajeh 2019-6-6

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https://ww2.mathworks.cn/matlabcentral/answers/465766-sisotool-and-bode-problem

评论： Don Pickerel 2021-9-28

Hi everyone,

I have a problem and I cannot understand it at all and consequently I faced many problems. I appreciate if you could kindly help me.

I have a controller C and a open loop transfer function Go as attached. I am trying to find the closed loop transfer function.

1- First see what sisotool give to me which is what I expect and is the correct results (see Sisotool in the attachment).

2- Now, we know that the close loop transfer function equation is:

Gcloseloop=(C*Go)/(1+C*Go)

bode(Gcloseloop)

I have ploted this equation by bode(Gcloseloop) and suprisingly, I get what you can see in equation_bode picture in the attachment. As you can see it is compltely different with that of sisotool.

3-Now, if I use the following function, I get another result as you can see in seies-feedback picture in the attachement.

bode(feedback(series(C,Go),1)).

Can anyone says that why this happens and how can I get the same results of stage 1 by 2 and 3?

Many thanks in advance for your help.

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Don Pickerel 2021-9-28

I have a similar question. When I do an open loop bode in sisotool, I am getting a different result from "bode" (and evidently "allmargin" for the exact same transfer function.

I"m using r2019b.

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