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Inconsistent result with the documentation

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Take a look at a feedback system where G=1/s and H=1 where G is forward feedback and H backward feedback.
----O---------|G|----------->
^ |
|<----------|H|----|
According to the documentation,
sys = feedback(___,sign) returns a model object sys for a feedback loop with the type of feedback specified by sign. By default, feedback assumes negative feedback and is equivalent to feedback(sys1,sys2,-1). To compute the closed-loop system with positive feedback, use sign = +1.
It means T1 and T2 in the following code must be same
s = tf('s');
G = 1/s;
T1 = feedback(G,-1)
T2 = feedback(G,1,-1)
But they are not the same.

Accepted Answer

Walter Roberson
Walter Roberson on 9 Feb 2020
In the first one sys2 is -1 and the default negative feedback is used. In the second one sys2 is +1 and negative feedback is explicitly used. You are using different sys2 with the same feedback so the results are going to be different.
  2 Comments
Walter Roberson
Walter Roberson on 10 Feb 2020
In MATLAB documentation, when you see the function followed by ___ followed by a parameter, that means that you need to use one of the syntaxes before that point to replace ____ and then you use the parameter listed. Each of the syntaxes before that point has both sys1 and sys2, so you need both of those before sign
So valid would be any of
sys = feedback(sys1,sys2) %uses sign -1
sys = feedback(sys1,sys2,feedin,feedout) %uses sign -1
sys = feedback(sys1,sys2,'name') %uses sign -1
sys = feedback(sys1,sys2,sign) %uses sign you pass
sys = feedback(sys1,sys2,feedin,feedout,sign) %uses sign you pass
sys = feedback(sys1,sys2,'name',sign) %uses sign you pass

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