Micrologix PID Help

Sterling

Member
Join Date
Sep 2008
Location
Missouri
Posts
3
We have installed a "iron filtration unit" at work. It filters iron out of our well water. I have control of the well's pump speed and have a water tank that we are pumping into. The tank has a level transmitter on it. I have a PID loop set up with the tank level as the process variable. The pump speed is the control variable. I am simply maintaining a setpoint in the water tank.
Here is the kicker! I have to maintain a flow rate of at least 60 gpm and not more than 300 gpm. I can't figure out how to set it up with the flow limits! Oh, the system also has a flow transmitter so I have access to that also. Do anybody have any ideas on how to set this up? Any help would be greatly appreciated!
Thanks, Sterling.
 
This sounds like a question from an engineering exam, so it makes a good Forum discussion (welcome aboard !).

If you have control of the flow into the tank, you can maintain the level of the tank with Proportional control only, as long as you're willing to live with a little bit of offset. You can remove the offset by adding a small amount of Integral gain, which eliminates the offset as time goes on.

I presume that the flow limits (60 to 300 GPM) are on the outflow from the tank, and that the flow transmitter is on the outflow as well. Is the tank gravity-drained, or is there a pump on it ?

It is important to know for programming purposes which kind of MicroLogix controller you are using, and how the analog scaling for the inputs works out.

How are you controlling the well pump speed; do you have discrete preset speeds you can select, or do you have a range of speeds at which you can control the pump ? Does the pump motor have a minimum speed at which it must run for cooling ?

This is a neat little control theory question but you need to provide some more information to help keep Forum members on track.
 
More information

The min. and max. flow rates are for the water filter. I have a Micrologix 1200 with analog cards for pump speed reference, flow rate, and tank level. All of these inputs I have scaled from 0 to 16383 for easy PID interface. My last attempt was to do what Ken Moore has suggested, which is multiple PID loops and low select logic. And like he has already said, the windup is killing me. I'm not sure how to deal with the windup.
Thanks, Sterling.
 
I did a system like this a long time ago.

What I did is look at the current level and subtracted the target level from the current level. This was then scaled to the output to the drive.

Example:
300 (Target) - 0 (Actual) = 300
Scale 0 to 300 to drive, in this case, send 16383 to drive.

300 (Target) - 150 (Actual) = 150
Scale 0 to 300 to drive, in this case, send 8192 to drive.

So the closer it gets to target, the slower the drive runs.
You will have to play with the offsets a little to get the desired minimum flow rate.
 
PID loop is winding up

I like Mark's idea of using an offset except that during the summer months at work they divert some of the water through the plant for cooling. It's nothing elaborate but it will change the flow from the pump that I'm controlling. So I would really like to use a PID loop if I can only figure out how to set it up without winding up.
Thanks, Sterling.
 
Peter Nachtwey said:
The answer is obvious to me except there are conditions that weren't defined. Ken Roach gave a good hint and asked some important questions. I am still waiting for the answers.

Every time this guy responds the paramaters change with an add on.

I would like to see a schematic of the system showing the well and pump the tank (pressurized or ******t??) the filtration system, what kind of filtration method, (with its limiting paramters max min flow pressure drop set point for iron level) where the filtered iron goes, max min flow rates and pressures, etc to system etc etc.

Dan Bentler
 

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