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

I want to find the gain margin so I can calculate the parameters (kp,ki,kd) of ZN method. But I got this error.

%%Parameters
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

Warning: The closed-loop system is unstable. 
> In ctrlMsgUtils.warning (line 25)
  In DynamicSystem/margin (line 65)**

Answer 1

[Sebastian Castro]

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

[Bob]

I have Double-check the equations, I still get this error. Any other ideas?