valve opening and closing

ddeshi2

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Join Date
Dec 2010
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Baton Rouge
Posts
89
Hi guys,

We are using a sliding gate valve with digital positioner to feed air to our system. The process requires air to be supplied slowly for every few minutes and I'm using PID loop with a Timer On Delay to slowly open the valve and ramp up to desired flow rates. It works fine. When the operator presses Stop Button, the same valve has to go to zero instantly. My question is, how can I achieve this? can I use different PID constants for the start up and stop procedures for the same PID loop (by using a MOV statement)? If so, which PID constant should I change to close the valve quickly - Kp, KI or Kd? I'm not using Kd in my start procedure. Please help!!!

Thanks,
Andy
 
It would be great to always mention what kind of PLC you are using.I'm assuming you are using a control valve to control your air. You said that your start up procedure is fine. You also said that when the Stop PB is pressed you would like the valve to go to zero instantly! so why don't you just MOV a value of 0, that way your are outputting only 0% to your control output? I don't know if it is safe to make it drop to zero because the sliding gate might slam down really quick if thats the kind of setup you have, so as a better suggestion you can just have a slow ramp time for the gate raising up, but for when the Stop is pressed still control it, but have it ramp down faster.

As far as PID's its usually best when your are controlling around a desired setpoint, if you need the Sliding gate to open/close at a desired rate then yes a PID can be implemented, but if are not worried about the speed at which the sliding gates actuates then a simple ramp control should do. Having said that If you already have your PID implemented though, you can try increasing your Integral gain to get to your setpoint faster, i would test these in little increments by trial and error and see what you get.
 
Thanks for the suggestion!! We are using a CompactLogix 1769-L32E PLC.
Please tell me if I got this right or not- During shutdown, Move 0 to the setpoint of air PID loop and try changing the Integral gain in small increments to reach the setpoint (zero) faster.

Thanks,

Andy
 
Why don't you override the PID and mov 0 directly to the output. that will close it the quickest. When you hit start again, remove the override and let PID do it's thing.
 
Last edited:
No. First let us be clear on what you are controlling is it the Air or the Flow rate?
You said quote that "I'm using PID loop with a Timer On Delay to slowly open the valve and ramp up to desired flow rates. It works fine."

So based on that im under the assumption that your controlling the flow rate using a PID loop. Either way if when a Stop PB is pressed you want to instantly go to zero. That looks like to me you want your Valve fully closed meaning, not open meaning a 0% output. So you can use a MOV instruction moving a value of 0 only when the Stop PB is on into the PID loops control variable .CV register should be it, and if the Stop is not have have your PID control the same way it already is. Now its up for you to decide if instantly going to zero is fine for the application.

Some things to be aware of. The PID instuction in logix 5000 software has a loop update time. For the timer that your are using to execute the PID instruction make sure the timers preset is set to the same time as the Loop update time. You could have used a periodic task, but in a continuous task that should be fine.

I was suggesting to use ramping logic where you control the ramp up and ramp down of the sliding gate, thats why i didn't think it was wise to have the value go to zero for the sliding gate because it won't be a controlled stop, but it seems to me your are just controlling a control valve for the flow rate.
 
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@dginbuffalo, i think what you meant by an override is by putting the PID into manual mode when the Stop PB is pressed.
So @ddeshi2 what you can do is when the stop PB is true, then put the PID into manual mode i believe it the .MO bit setting it to a value of 1 to put it into manual mode then assign a value of 0 to the .SO bit which is your output %, and if your Stop PB is not true then set the .MO to o to put it into auto mode.

Example:

If Stop_PB THEN
PID_LOOP.MO = 1;
PID_LOOP.SO = 0;
Else
PID_LOOP.MO = 0;
END_IF;
 
Last edited:
Yes, we are using the valve to control flow rates. I didn't understand the term 'override'. Does it mean putting the PID loop in software manual mode(. SWM =1)and moving 0 (or 4,000) to .cv register during shutdown?? When the operator pushes the start button again, the loop will be put in Auto Mode (.SWM = 0) and the PID takes over the control. Am I right?

Thanks,

Andy
 
i don't remember if its the .swm or the .MO that changes modes for the PID, just experiment and see.
 
@dbh6--- I still have override stuck in my head because I use the PIDE function lately.

Your logic for the standard PID looks correct. I believe its the .SWM bit he wants to use.
 

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