PID Control loop with different setpoints

Hello all,

If I have a closed loop system/pid loop to control steam and have different setpoints to control the same loop. How should I go about it? Should I have a subroutine that my program is always scanning and have an OTE from another routine call that PID instruction, have a buffer for the setpoint and move it in or have different PID instructions controlling the same Control Variable Output. I hope my question is straight forward, it's a little fuzzy but hope it makes sense. Thanks!!!

Can you tell us what the set-points are. Two different pressures, two different temperature? Or one pressure and one temperature.

Is one the primary and the other just an override.

E.g. Temperature control with a overpressure protection.

@ Mickey

The set points are 140, 170 and 180 to heat up different chemicals with the same heat exchanger. And I have different routines for different logic for each chemical so I call it from different places. I know I'm able to use the output more than once ( so I think ) but I know it's not good to do so. I guess I can say they will all be primary depending on the function.

I'm not sure what your problem is. Is this a working system now?

Why can't you just change the setpoint and let the PID instruction do the work?

Or ramp to and from the different setpoints for a smooth transition.

@Mickey

No this is a new project. What I'm asking is this. In sorry if I wasn't clear. I have one heat exchanger to heat up the product/chemical. I have different routines for different products that have different logic. My question is should I have a subroutine to call the PID instruction and input the setpoint with a move instruction? Or have a PID instruction in each routine? It's my first control logix project so I'm a little lost on what is the purpose way to do so.

For instance: I have a PID tag Hotset_1, PV TempTrans_1, and CV Steam_Valve_1

Right now I programmed 3 routines that have a PID instruction with the same tags except the set point. Should I instead have an XIC instruction enabling an OTE and moving a setpoint into one PID induction rather than having one with the same tags in each routine?

I would only have one PID instruction. Use a MOV/Copy instruction to change the setpoint.

If you put the PID in a subroutine I would call the subroutine unconditionally.

Having more then one PID (all temperature control) controlling the same valve and in different conditional subroutines would be a troubleshooting nightmare.

Thanks