PID Rung Review

[iadapt]

Could someone have a look at my PID loop rung and give me some input about the structure?

I am trying to regulate a group of heaters through one PID instruction to control a group of coils. But I am not sure this is correct.

 

[labeledas]

I guess my first question is you say group of heaters, are they all heating the same process, ie have the same PV.

If not you will have issues with some units be constantly over/under.

 

[iadapt]

Hello Labeledas,

Yes they all have the same heating process with one thermo-couple giving feedback.

 

[iadapt]

I need this process to also have a preheat. I believe I have that with the PID output at 20%. Am I correct?

 

[labeledas]

also is that multiplier for ZONE_HEAT_ON_TIME[1] 100 if so any PID output above 50% will result in the SSRs always being on, and act as a windup on your control.

 

[labeledas]

I need this process to also have a preheat. I believe I have that with the PID output at 20%. Am I correct?

That would depend on the process and what temp you want to preheat it to. with your current code you will have 40% duty cycle. normally when I hear preheat I think of a upstream heat exchanger preheating my fluid with a waste/used stream. or do you mean a slow warm up period before running the process.

Hopefully some one a lot smarter than me can chime in as well

 

[iadapt]

Yes regarding pre-heat it's a slow warm up that we're looking for

 

[iadapt]

The multiplier is at 100

 

[labeledas]

For a warm up I would just do a setpoint ramp

 

[iadapt]

How do you do a setpoint ramp labeledas?

 

[labeledas]

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.