PID/Micrologix1200 Discrete Out to control Valve

People,

I have used the PID function previously in AB applications, but this is the first for the ML1200 Series. I need help in interfacing the PID to a DISCRETE OUTPUT signal, RATHER than the traditional Analog output.....if possible.
Here is the application....

I need to meet a specific setpoint set in the program. (lets say 20 PSI) I have a validated range (say for ex. +/- 5 PSI) which would make the limits 15 and 25 respectively. I am using an analog transducer to read the value into the analog input card, and moving the data to an integer.

In this application, I am drawing a vacuum....
I have a motor/pump combo, and some solenoid valves.

The problem is that I don't have an analog output device (IE motor or porportioning valve) to control the output to the vacuum motor, so the vacuum will always be on. If the vacuum gets too high, I need to keep the analog value in the correct range.
I first thought the best way to do this is to cycle the solenoid valve that is connected to the vacuum tube.....(as the output of the PID).

Has anyone ever used a DO on the output of the PID loop successfully?

Any help would be appreciated.

come on guys...nobody has an answer???

Is there anyone out there that works on weekends????

Not sure , but is this what your looking for

Also you might look at the PWM instruction for more precise
control. I have not used it myself but think it might be closer to
what your tring to do. Someone else here can , I'm sure shed
some lite on it.

Thanks Mickey.

It looks like I am screwed on this one. I was hoping to have a nice clean routine, and make it easy for the other tech's to understand what I have I've done. The checking limits is OK, but I will investigate the PWM further.

Thanks...and I will let you know what I have done.

I haven't done this, but off the top of my head you might try to roll your own PWM. Maybe compare the process to setpoint, the graeter the
error the longer the on pulse. A oscillator logic (two timers) use the
error as a preset. Just a thought

For a simple solution that can easily be understood by your technicians set up a timer with fixed preset, 10 seconds for example (10,000 milliseconds). Have the timer reset itself when the ACC reaches the preset. Take the output of your PID function that you would normally send to the analog output channel and scale it to the timer preset (0 - 10,000). When the timer ACC is less than the scaled PID output, turn on your discrete output. When the timer ACC is greater than the scaled PID output, turn the discrete output off.

Adjust the timer's preset to find the best compromise between cycling the discrete output on and off and excessive deviation from the setpoint.

Thanks Steve and Mickey.

Steve, your response sounds similar to the example that is used in the AB SLC-Instruction set for PID "time proportioning outputs".

See Page 9-24 of the AB SLC500 Reference Manual.

Apparently, this type of system is used for heating and cooling.
Actually, the example that is given looks like it could be used in the ML1200, but the instructions are not exactly the same.
In the ML1200, what is the PID instruction Done bit? Is it still word 13....loop update?

Thanks for the help guys,

OOOPS!

I didn't realize the HotRod.zip was missing from the download area.

kcarlen, go to download\misc there is a file called HotRod.zip. Ron and I worked on this last fall. It has a lot of good stuff in it. Have you searched for 'Micrologix PID'? You will find more good info.
Use the search engine.