Why is MATLAB and Simulink giving me different results when trying to find the closed-loop step response?

3 次查看（过去 30 天）
I
I 2016-4-13
评论： Rik 2019-8-2
for the unity feedback system G(s)= 30(s^2-5s+3)/(s+1)(s+2)(s+4)(s+5) determine the closed-loop step response
>> G = tf(conv([30],[1 -5 3]), conv(conv([1 1],[1 2]),conv([1 4],[1 5])))
G =
30 s^2 - 150 s + 90
---------------------------------
s^4 + 12 s^3 + 49 s^2 + 78 s + 40
Continuous-time transfer function.
>> H = feedback(G, 1)
H =
30 s^2 - 150 s + 90
----------------------------------
s^4 + 12 s^3 + 79 s^2 - 72 s + 130
Continuous-time transfer function.
>> step(H)
This is the code I use in MATLAB however, when I generate the model suing Simulink It's giving me a different answer
  2 个评论
Rik
Rik 2019-8-2
Flag by I:
Please delete this question as I do not want my profile publicly visible
Response:
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回答（2 个）

hashim khan
hashim khan 2017-11-22
I am facing the problem. the step response for one has no overshoot while the same feedback system has a 10% overshoot.
Kenny
Kenny 2017-11-27
编辑：Kenny 2017-11-27
For this particular closed loop feedback function, the denominator polynomial coefficients don't have all the same 'signs'. At least one coefficient is negative. So this suggests the system is unstable. So if you put in a unit step, the response is going to get out of control already.
Also, rather than type G(s)= 30(s^2-5s+3)/(s+1)(s+2)(s+4)(s+5), it would be better to express it like G(s)= 30(s^2-5s+3)/[(s+1)(s+2)(s+4)(s+5)], just to be clear about the order of the operations.

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