MATLAB Answers


why don't these two transfer function models produce same open loop results?

Asked by John Hudak on 28 Jan 2015
Latest activity Answered by Mischa Kim
on 30 Jan 2015
I create a transfer function in simulink, drive it with a step input to look at open loop response, and observe the response on a scope. I create the same model in matlab, drive it with a step, and get a completely different response. I do not understand why. Here are the two models:
SImulink Step block (parameter range 0-1) Transfer Fcn block, 1/s^^2 + 10s +20 (Num coeff = 1 , Denon coeff = [1 10 20] ) Scope block
Matlab: s = tf('s'); P = 1/(s^2 + 10*s + 20); step(P)
I have varied the ode soln approach, checked parameter ranges and types, still no joy. The matlab approach produces the correct results btw. ANy insight is really appreciated.

  1 Comment

Can you attach your Simulink model? I get exactly the same results for both methods.

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1 Answer

Answer by Mischa Kim
on 30 Jan 2015

John, the only thing I can think of is the step time in the Simulink Step block. It's set by default to 1. Open the Step block, change the Step time to 0 and you should get the same result. I do:


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