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From: Arkadiy Turevskiy on 9 Apr 2010 17:07
You always work with open loop in root locus.
so just do
>>C=(1+Td*s +1/(Ti*s); assume Kp=1
>>OL=C*P; %P is your plant
>>sisotool(OL)

use root locus plot in SISO Tool to find the gain that gives you good design. This is your Kp.



"Susan " <susanreneemueller(a)gmail.com> wrote in message <hpnt6b$s7h$1(a)fred.mathworks.com>...
> I changed my PID to
> s=tf('s')
> Contr_Kp_Ti_Td = Kp
>
> I plotted the CL loop TF with unity feedback where the OL = Contr_Kp_Ti_Td*Plant*Ki (where Ki is an additional integrator to cancel the zero at the origin).
>
> I found that a gain of zero results in the damping value of .785 which is as close as I can get to 0 .707, the value we want. A gain of zero means a PID=0. This is bad news b/c my system won't reach the desired set points without a controller.
>
> My advisor said to find the gain plotting the CL...but I also tried the OL which is what I thought was correct for RL. Anyway, I was able to get a damping value of .707 this way.
>
> Am I using the sisotool incorrectly or should I have been plotting the OL the whole time? The gain I found in the sisotool would be my new Kp, right?
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