I normalize use FDB for my PID but it looks like it is not available in Ladder
see the following thread.
http://www.plctalk.net/qanda/showthread.php?t=61644
here is a bit of info on why to use ramping and what your are aiming for
if what you are looking for is on initial startup or when error is huge to have the system gradually move to the operator entered setpoint over a specified period, the a setpoint ramp would be the way to go.
I have never tried this but this is what I would attempt, others may have better suggestions.
On my condition, ie on startup and until I reach my setpoint, I would move the entered setpoint to a placeholder say FinalSetpoint to use in my calculation and feed the output of my ramp to the PIDE setpoint input. Otherwise just let the user entered setpoint go directly to PIDE.
Or put the PIDE in program mode and write to the leave the user entered SP as is and use as input to your calculation and feed the output of the Ramp function to the ProgramSP input. Take it out of program control
I would prefer the second method myself but depending on what faceplate you are using could cause issues, such as loss of operator input, ie operator put in too high of a setpoint program starts ramping but operator can't adjust the entered SP on faceplate.
I believe the stock AB PIDE faceplate will lock out user input when in program mode, but it can be modified, some do consider that bad practice if your changes aren't documented and someone tries to recreate the system using standard objects.
A few other side notes:
- Depending on how fast you want to execute your PID in relation to the other parts of the routine I would recommend putting your PID routine in a periodic task rather that using a timer to trigger it, it will guarantee its cycle time and IMHO is the proper place for PID on the Logix platform.
-Just some food for thought I would also check the minimum cycle time for your elements if that is what you are controlling with your SSRs, as in the higher %output you will get very quick on-off cycles ie at 49.99% you element will be on for 4.999 seconds cycle of for 0.001 second and turn back on. They may not like this very much.