The problem can be sorted out if we use Ziegler-Nichols Tuning of PID controller.
PID controller in simulink
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Hi everyone! I am working on PID controller. I am having a model in simulink which I have uploaded at http://tinypic.com/view.php?pic=15cyg2&s=6. The gains of PID are: kp=1,Kd=1,ki=1.Saturation block has limits equal to -30 & +30. The state-space model has A=[0 1 0 0; 0 3.5 -106.5 0.37;0 0 0 1;0 0.21 -6.3 1.59]; B=[0 0;48.06 46664; 0 0; 340.18 47.7]; C=[1 0 1 0]; D=[0 0] and initial condition equal to 0. The system should converge at 20 as input is 20. But the problem is firstly its not converging and secondly the input of PID which is error signal appears as it is; as output with no modification. Need urgent help?
Ryan G 2012-9-7
编辑：Ryan G 2012-9-7
I looked at the root locus with the PID and I don't believe this will be enough to adequately stabilize this system. Just for kicks I tried utilizing looptune to stabilize the system and was successful in finding a 4th order TF that would do it. Since I know nothing of your system other than the SS matrices it may or may not make any sense.
If you replace the PID with a Transfer fcn block this should stabilize the system:
num = [-1.19566835199456 0.150677296783384 -4.87470249290078 -5.9378741332245 -3.64167152790914e-05];
den = [1 32.4776016633891 994.732763803663 6558.83645377611 0.0401083722778298];
The resulting controller doesn't do a particularly good job but your system may not be easy to stabilize anyway.
What is the 2nd input and why don't you use it?