I am involved in activities related to controllers tuning. These controllers have I-PD form.
I am trying to create the control system in simulink, by means of pid blocks, LTI System block and Check Step Response Characteristics.
The control system is about steam temperature control and the controller is responsible to command actuators whose role is to modulate water contribution to control output temperature (process variable), i.e. PID output increases if output temperature has to cool down, PID output decreases if output temperature has to rise.
Thsu, Plant has negative gain and controller is made to work as function of error computed as pv - sp (pv: process variable; sp: setpoint) .
The controller formula, implemented in the regulator I am working on, is I-PD:
MV(n)=MV(n-1) + dMV(n)
e(n) = PV(n)-SP(n);
dMV(n) =Kp*Ks*[ (PV(n)-PV(n-1)) + (Tc/Ti)*e(n) + (Td/Tc)*( PV(n)-2*PV(n-1)+PV(n-2) ) ];
I found in Simulink a way to have the I-PD structure , by PID Controller (2DOF) block where parameters b and c have to be set to 0.
The PID Controller (2DOF) uses error as Ref - y, that in my case would be setpoint-process variable.
What I cannot figure out is how to make it work with error as pv-sp. I tried connecting the output temperature to Ref input and the setpoint to the second input of the block, but when i try to tune i get infeasibility of fmincon for instability.
Can you give me a hint to represent this controller in simulink in order to use tuning utilities available?
Thanks a lot!
I attach a simulink model (2014b) I created but needs to be edited.