SLC 5/04 PID Split Range Application

S

Thread Starter

SteveS

Has anyone successfully used a single SLC 5/04 PID instruction in a split range application? I want the control output to be 0% to 50% when PV > SP, and 50% to 100% when PV < SP. When PV = SP the control output should be 50%. It looks like the SLC PID instruction is "one-way" and I'd have to use 2 PID instructions, one direct- and one reverse-acting to achieve this. Any tips or
pointers?

Thanks,
Steve
 
Steve

Could you describe your application ? This does not sound like a split range application.

Regards
 
This is not split range control. What you've just described is simply proportional only control with the setpoint at the middle of the PV range and no manual reset applied. Think of this as a teeter-totter with the fulcrum in the middle. When the teeter-totter is dead level, the PV (on left side) and output(on right side) are both at mid scale (50%). If you move the
teeter-totter fulcrum left or right, you change the gain of the loop. Moving left increases gain and vice-versa. Hope this helps.
 
H
I "almost" agree with this message. I agree that it is simply proportional control. I disagree that there is no manual reset applied. The manual reset setting sets the controller output when SP = PV. So the manual reset in this
application must be 50% in order for the controller output to be 50% when SP = PV = 50%.

The equation for a proportional only controller is: Output = Gain * Error + Manual Reset, where
Error = SP - PV, if the controller is reverse acting, or Error = PV - SP, if the controller is direct acting.

Harold Wade
 
T
I think everyone so far missed your question. I suspect you either have a hydraulic servo valve spool position control, heat/cool application, an
accelerate/brake application, or a drag-up system. Regardless of the application, this can be done. The AB PID only supports forward (E=PV-SP) or reverse (E=SP-PV) acting but not both simultaneously. Here are two approaches you might take.

1) You will have to use two pid loops. Let both PIDs function in their full output range 0-16383 (you'll see why in a minute). Then scale the output of one to 50-100% and the other 0-50% of your CV, then use a LIM (limit test) to apply one or the other to your analog output address. Put the inactive loop in manual and zero (or full range) it's output when it is not in control so that when PV crosses SP and the loop switches back on you don't get a bump in the output. (thats why) Your going to have to work out the logic for this transition depending on your application, which you didn't state, but sometimes
you can't state the application in a forum like this, so I'm being general.

2)Alternatively, you can use just one PID if you toggle the /CM bit of your PID block with your ladder, but as you do you also have to switch to manual mode with your ladder, set the output for the transition, change the gains and limits if necessary, and do anything else applicable to the transition for your application, then switch back to auto. Set up some kind of a semaphore in your
ladder with the PID /DN bit to signal that the PID has executed at least once in manual mode before you switch back to auto. Also, OTU your math error bits befor the scan ends to avoid a nuisance fault.

I did something similar with the two PID block method in a hydraulic intensifier control aplication that had some very unique requirements, and I ended up with a very complex set of code with three cascaded loops, the inner-most loop in control of the servo valve spool position was forward & reverse acting in a similar manner. It took a couple of weeks to get that one working just right, but the result was fantastic, with pressure control better than 1 psi out of 20,000. Good luck.
 
J

Johan Bengtsson

What you describe is not what normally is called split-range (split range is when different ranges of the same PID output changes more than one
physical output)

What you describe is rather what you get with a PID controller with the I term shut off and an offset of 50%
Ie you want a P (or possibly a PD) controller.

/Johan Bengtsson

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