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# View Root Locus of Nested Loop Using Simulink

Asked by Matthew on 5 Dec 2012

Hi,

I have a school project where I would like to use Simulink to model and tune my system. I have a relatively simple system that consists of two feedback loops, each with a gain. I'm using this tutorial

http://www.mathworks.com/help/slcontrol/ug/design-and-analysis-of-control-systems.html

to do the basic tuning, and I understand the basics. But for my system, I would like to tune both gains individually and be able to look at the root locus of the inner loop and the root locus of the outer loop simultaneously. Choosing the gains to be tuned is easy enough, but when I select the two root loci, they are both 5th order. The inner loop is only 3rd order and the outer loop is 5th order, so I would expect my root loci to reflect that. At this point, I don't know what the two loci represent since they are both of the 5th order.

Anyway, I hope I have given an adequate description of my problem, but I will gladly provide any more detail as necessary. I didn't want to bog down the question with unneeded specifics, but I'm not sure what may be relevant, so please ask for any needed clarifications.

## Products

My guess about why you are getting 5th order system for inner loop is that you did not specify loop opening properly to break the outer loop, when defining the inner loop.

Please take a look at this example, where the issue is explained in detail. You should be able to follow this example and use root locus plots instead of bode plots that are used in the example.

Hope this helps.

## 1 Comment

Matthew on 7 Dec 2012

Ah, perfect. Thank you very much.

The key piece of information was where it says to go to the "Architecture" tab under the SISO Design Task node and use the "Loop Configuration..." button. When configuring the loop in there, you can make it highlight the respective blocks and see if you're actually selecting what you want. For my case, I had to check a box in that window to get it to do what I wanted.

Thank you very, very much. This is an insanely powerful tool.