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How do you do Unity Feedback on Matlab? (Transfer Functions)

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Joel Okanta
Joel Okanta on 14 Jan 2021
Answered: Paul on 16 Jan 2021
Before you get angry, no I cannot post the picture or even do anything of that matter due to the fact that the editor for this Forum got changed, so I can only leave links now. No more image options.
Main Question:
I'm doing a task that involves mapping a Transfer function diagram on MATLAB then completing that block diagram by matlab functions (such as series, parallel, etc.) to reduce the block diagram. However, I am having a problem with doing unity feedbacks on MATLAB, unity feedback looks something like this: (Note that it's an "empty" line going back)
Where the line has no "H1" feedback signal or whatever, the line is empty and normal going back to a sumamtion point behind the main transfer function block. I initially tried to do F = feedback(F, F) since the signal goes back in itself, but it did not work and instead only worked when I did F = feedback(F, 1). (To confirm how I got the right answer I did a comparison by completing on paper and check online to see if it was correct; matlab was just the odd one out)
Can anyone explain why this is the case?

Answers (1)

Paul on 16 Jan 2021
Check the doc page for feedback()
doc feedback
In this case, the feedback line isn't "empty," it has a gain of 1 (or if you prefer H(s) = 1), which conventionally is not shown explicitly. So in the context of the feedback() command, sys1 = F and sys2 = 1 and you get the correct result.


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