Warning: The closed-loop system is unstable.

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Dmitri McClane
Dmitri McClane 2019-11-13
回答： Star Strider 2019-11-13
I am getting this warning. It is saying that the function is unstable. The root locus plot indicates that the system is stable.
ctf =
10 s^3 + 1100 s^2 + 10000 s
-----------------------------------
s^4 + 210 s^3 + 11100 s^2 + 10000 s
Continuous-time transfer function.
>> rlocus(ctf)
>> [Gm]=margin(ctf)
Warning: The closed-loop system is unstable.
> In ctrlMsgUtils.warning (line 25)
In DynamicSystem/margin (line 65)
Gm =
Inf

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回答（2 个）

Walter Roberson
Walter Roberson 2019-11-13
There is no constant term in the denominator. There is a pole at 0, which is inside the unit circle, so it is unstable.
Star Strider
Star Strider 2019-11-13
The system has a pole-zero cancellation at the origin:
s = tf('s');
ctf = (10*s^3 + 1100*s^2 + 10000*s) / (s^4 + 210*s^3 + 11100*s^2 + 10000*s);
[p,z] = pzmap(ctf)
Use minreal to resolve it and produce a stable system:
ctf = minreal(ctf)
Gm = margin(ctf)

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