Logix 5K: PID Loop Integral Accumulator

[kdcui]

I'm having an issue with a PID loop I am looking at.

It is set up to control a temperature control valve. When not controlling the valve using the loop I place the loop into .SWM mode and give it a .SO of 0 if off, or a value from the SCADA based on what the user wants to control it at.

I've noticed while I'm in .SWM that the integral accumulator jumps to 159. The code is clearing out the setpoint while in SWM so I am guessing the PID loop uses the integral accumulator for some function.

However when I place the loop back into Auto, the accumulator remains at that high value and thus the output jumps to 100% (if it wasn't there already).

Obviously there is an issue in the way I am handling transfer from auto to SWM. I would like to be bumpless. Do I need to use the Tieback? Should there even be anything in the Tieback range in the Scaling tab? I always thought Tieback was only used while in .MO to set the output of the controller.

Thanks

 

[Peter Nachtwey]

When in manual mode we set the set point to the process variable so there is no error. The integrator output term is set to the manual output. I would bet Rockwell does the same thing. If you set the set point to the PV before entering auto mode you will not get the jump. If the set point is set to a high before entering then value then you should expect the control output to jump by the amount due to the proportional gain times the error.

If you don't want the control output to just then you must first subtract out the output due to the proportional gain times the error from the accumulated integrator output.

 

[Brownhat]

I would guess that the jump in the loop output is due to the loop setpoint returning from 0 to whatever it is supposed to be set to. Do you know why it's programmed to clear the setpoint while in SWM?