PID controller and process simulation with STEP7

Pandiani

Hello,
I have noticed that there are a lot of questions in which people ask about examples with PID blocks and even process simulation.
Attached, you'll find small but effective PLC program that represents real time process simulator. It can be used to simulate FOPDT (First Order Plus Dead Time) and SOPDT (Second Order Plus Dead Time) processes. FOPDT and SOPDT can be used to approximately describes most technological processes. More information about FOPDT can be found on www.controlguru.com. (look at heat exchanger example) SOPDT is very similar.
FOPDT can be described with the following transfer function: exp(-TD*s) *K/(Ts+1), where TD is dead time.
SOPDT uses representantion with damping and natural frequency. There are plenty materials on the Internet.
This program is my small contribution and I'm sure many will find it usefull. You can practice PID controller tunning (changing parameters or even enabling and disabling different forms P, PI or PID).

I used PLCSIM to simulate control loop . For example, if you want to use FOPDT simulation (after download and Run CPU) turn on M3.0 and M3.2 . Then in Setpoint field (MD20) enter desired value and observe how process output is changing (MD20). For SOPDT process value is MD30, but beside M3.1 and M3.3 you need to set MD25 as output from PID controler (since MD25 is input to SOPDT process).

For start, I recommend you to use FOPDT (default), set M3.0 and M3.2, then enter real number in Setpoint field. If you use PI controller (default), after some time PV (MD20) must reach desired value. If you disable integral component (I_SEL switch to Always OFF), then PV will not reach Setpoint and there will be always offset error.
You can also change parameters of process and parameters of controller.
I suggest you to use reasonable dead time and sample time values because I used static array with maximum assumed size for simulating dead time.

I hope you'll be able to open this project (File->Retrieve) because of known issue of STEP 7 with Regional Settings.

I'm also willing to share SCL source code, if someone is interesting.
This simulator is written by me with initial help from Peter Nachtwey, president of Delta Computer Systems.

I hope you'll find this useful.

All comments are useful.

Best regards

hdhosseini

Hello ,


PID controller (FB41) works based on error signal in
automatic mode .As we Know For most cases(Except
cases like THIS):

error:= Setpoint - process value



Monitor error signal and portions of LMN_P, LMN_I
and LMN_I in outcome LMN signal.Setpoint is the
constant part of equation.You should check what is
wrong with process value.

Best regards
Hamid Hosseini