PID Setpoint jump

Darkzadow

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Join Date
Nov 2011
Location
Phily
Posts
94
Hi I am using a PID to control Air and gas in a combustion setting. When the temp reaches 30 degrees above my setpoint I set the air loop to 70% (operator suggested air setting)as opposed to the ratio from gas Setpoint. When the temp reaches within 10 degrees I set the Air setpoint back to what the gas ratio demands. I am noticing that the temperature is now swinging more than usual above and below Temp Setpoint.

Does my setpoint "jumping" effect the response of the PID air loop after I set it back to ratio SP? The jumps typically are around 10% air to 60% air and back as overtemp occurs.




Setup: Siemens 343(i think) controller. 2 loops. Loop1 temp sp= SP, temp feedback= PV, gas valve=CV. Loop2 sp is either ratio from gas or fixed sp for cooling, PV = air feedback, CV=air valve.
 
Hi there,

Any step change (and that includes changing the setpoint of course) in a closed loop system is going to cause some overshoot unless your loop is underdamped.

What I will often do is "ramp" a setpoint slowly if it needs changing. To do this I have a seperate variable for the operator adjustable setpoint and compare it to the PID's SP. I then use an ADD or SUBR function with a timer (not sure of equivalent in your Siemens PLC) to slowly increase / decrease the PID's SP until it matches the required setpoint. the ramp rate would depend on how fast your process is.

We use this in water treatment processes quite often to stop newbie operators from cranking the raw water flow rate up too quickly and causing clarifiers to go belly up.
 
Ok that sounds good. could I add a seperate loop with sp always at 70% and use a selection switch to select that loop cv output, with that loop set with a less harsh PID values?
 
Ok that sounds good. could I add a seperate loop with sp always at 70% and use a selection switch to select that loop cv output, with that loop set with a less harsh PID values?

You can do anything you want !

BUT : Be wary of using more than one loop - when one is active, the other will be accumulating integral error. Switching between them will always be troublesome.

One plant item - One PID... manipulate that PID as you wish.

You can change the P, I, and D terms dependant upon the switch setting.
 
Hi, I found a rate Limit block in my Siemens manual which I think will ramp my setpoints smoothly Thank you all for suggestions. I plan on playing with the control scheme more to update it from its 1940s design when I figure out how (straight from college with no experience in any of this). I also found a bit that will reset the accumulated integral error. I am still looking at switching PID loops and/or changing P,I,D terms on the fly. What is the effect of resetting the integral error when/if I change PID loops or settings?
 

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