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# Simulink: High Freq. noise in sim. results

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Sophie on 11 Jun 2013
I am simulating a very simple control loop with a P-controller and a stable 1-pole plant (with a pole at -1.5*10^5) that is shown in the image.
I am expecting no oscillations in the output (because the plant is stable and the controller is a P-controller); however, I see high frequency oscillations in the output. What are they due to? How to avoid them?
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### Accepted Answer

Pourya Alinezhad on 11 Jun 2013
Edited: Pourya Alinezhad on 11 Jun 2013
hi.i draw your model in simulink and also modeled the system in matlab itself. the STEP()command in matlab obviously shows that the system is stable and has a soft first order response. but in simulink !! there are several solvers to solve the differential equations in sim model.in simulink menu click configuration parameters... change the solver type to be a stiff solver....and also change the values of tolerances to somewhat like 1e-5 .(already they are initiated to be auto) this solves the problem.
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### More Answers (2)

Sophie on 12 Jun 2013
Thank you, Pourya. Changing the solver to the stiff solver (ode15s) indeed helped.
Why is this system stiff? I thought that a system is stiff if it has one fast time constant and one slow time constant (and there are 2 different rates of decaying of the transients, as a result). However, my system has only 1 pole; why is it stiff?
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Pourya Alinezhad on 12 Jun 2013
this system is stiff because of fast raise of output. stiff equations have rapid changes in their response,so more data points are needed to solve them accurately in order to reach converge.

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Sophie on 19 Jun 2013
Ok, thanks.
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