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2 DOF System is unstable? "Warning: The closed-loop system is unstable"

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I want to find the gain margin so I can calculate the parameters (kp,ki,kd) of ZN method. But I got this error.
m1 = 2500; % (kg)
m2 = 320; % (kg)
k1 = 80000; % (N/m)
k2 = 500000; % (N/m)
b1 = 350; % (N*s/m)
b2 = 15020; % (N*s/m)
% G(s) = (X1(s)-X2(s))/W(s)
num = [(0) (-m1*b2) (-m1*k2) (0) (0)];
den = [(m1*m2) (m1*b1+m1*b2+m2*b1) (m1*k1+m1*k2+m2*k1+b1*b2) (b1*k2+k1*b2) (k1*k2)];
G = tf(num,den);
Warning: The closed-loop system is unstable.
> In ctrlMsgUtils.warning (line 25)
In DynamicSystem/margin (line 65)**

Risposte (1)

Sebastian Castro
Sebastian Castro il 5 Apr 2016
Negative gain and phase margins means that the open-loop system is unstable.
You're right that a mass-spring-damper system shouldn't be unstable. Maybe there was some error in converting the system equations to that transfer function representation. Double-check the equations -- especially any minus signs that should have been positive, or vice-versa.
- Sebastian


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