PID Reset in Fanuc 90-30

Hi all,

I am trying to program a PID loop in a GE Fanuc 90-30 PLC. I am making pretty good progress with the majority of variables (PV, CV, SP) but I am having a bit of trouble in the actual application of the little guy.

My goal is to sum the PID's CV with a hard setpoint and to then output a modified signal. Because of the application, I need to turn the PID loop off and on at various times. Each time the loop is turned off, I need for the CV value to immediately go to zero so that the next time it is turned back on, the modified output will ramp back up or down as necessary using the PID. Typically I have had some sort of reset available. With the Fanuc's loop I cannot seem to find one.

What I am getting now is that when I turn off the PID, the CV value retains its last calculation and when I turn it back on it picks up where it last left off and this stored value is immediately added to the hard setpoint to create my modified signal.

Is there as reset I cannot find or am I just going about this all wrong?

If anyone has some help out there, I would appreciate it.

Steve

I haven't done PID Loops but assuming that the CV points to a register you could write a zero to that register whenever the PID Loop shuts off. That way when it is turned back on your CV starts at zero.

Yeah, I tried finding that register and doing just that, but I was not successful. There are 40 registers in GE's PID functin block and they don't give you any information on 35 of them.

Steve do you have access to GE Manual GFK-0467L or an infolink CD? Page 12-73 describes all the registers.

If not I am pretty sure it is available at the GE Fanuc Website.

I didn't before now. I just downloaded it and will let you know what I find.

Thanks for the lead.

Steve

All Right!

That manual gave me exactly what I needed. Below is directly from the manual:

In Automatic mode, the output Control Variable is placed in the Manual Command parameter
%Ref+13.
If power flow is provided to both Enable and Manual input contacts, the PID block is placed in
Manual mode and the output Control Variable is set from the Manual Command parameter
%Ref+13.

So, in a nutshell, I leave the PID loop on at all times but switch it back and forth between Automatic and Manual mode. When I go into Manual mode, I dump a zero into the %Ref+13 register which changes the CV value to a zero. Then, when I go back into Automatic, the zero value is then used as the starting point. With this arrangement, I can then always leave my modified signal as a sum of the hard setpoint and the PID's CV value.

Thanks for the help.

Steve