PID using two digital outputs ...

Ron Beaufort

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Jul 2002
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here is an excerpt of a question I got today as a Private Message:



PID instruction help question.



Hey Ron, I am a forum lurker and a student currently studying plc's. I was wondering if you had a moment to answer a PID question.



SLC 5/04 processor.



PID setup aside.. let’s assume everything is setup properly from the PID instruction block point of view, and the CV has been setup as N7:75



Here is my question.



I have 2 valves, for controlling a batch process. One valve brings back the process from a 'below set point' state, and the other valve brings back the process from a 'above set point' state. These valves would not be analog. Valves being addressed O:2/1 and O:2/2. I've seen your instructions on controlling a single valve via a “time proportional signal”. How would you incorporate the second valve to bring the process back from an 'above set point' state?



Thank you for any advice you can offer. I truly appreciate it.



since variations of this question come up fairly often, I decided to take a shot at answering it here on the open forum ... I’m sure that other members will be able to add more insight ...



-----------------------------------------------



Greetings ...



my first question is this: are you SURE that you really need to use PID at all? ...



if the answer is “Yes” then do a forum search for “bipolar” and see if any of the ideas you find there will be helpful ... here are just a couple of links to get you started ...



http://www.plctalk.net/qanda/showthread.php?t=7292&highlight=bipolar



http://www.plctalk.net/qanda/showthread.php?t=20274&highlight=bipolar



on the other hand, if I understand your question correctly, you might want to try something similar to the “gearmotor” logic that I use on my Hotrod trainers ... click here for a picture ... then you can click again on the picture itself to enlarge it for more detail ...



the idea is that in “gearmotor mode” the damper on the left is kept wide open and the damper on the right is driven clockwise or counterclockwise to control the amount of air flowing through the tube ...



note that the damper on the left uses PID and analog signals - which is what most of my PID students are interested in ... the damper on the right uses digital signals - sort of what you’re apparently trying to do with your system ... I use the “gearmotor mode” mostly for my own personal experiments - so that I don’t have to worry with the 3-15 psi air supply required for the left damper ... the gearmotor just runs on 120VAC ... one digital output makes it run clockwise to open the damper further ... another digital output makes it run counterclockwise to close the damper ...



the PDF file attached shows just the rungs necessary to control the gearmotor ... take a look at how that’s set up and see if it gives you any ideas for your project ...






the 5 minute trend below shows the type of “good-enough-for-my-purposes” control that I get with the system ... the setpoint is changed from 20% to 40% and back to 20% again as a test ... in the area where the setpoint is not shown, both of the gearmotor outputs were forced OFF after the PV had stabilized at 20% ... this demonstrates that the “jitter” in the PV is due to natural signal “noise” and is not being caused by the control technique being used ...


gearmotor_trend.JPG


hope this helps ...
 
Last edited:
I was the user who submitted this question to ron.

Several posts here on PLCtalk point towards the same technote on AB's site about setting up SLC 500 PID's for bipolar output.

I however cannot seem to access this technote and was wondering if anyone could post the article in full.

Thank you for any help.
 
The AB article number is 21677

If someone with access could send me this article/programming example in full, either posted here, or to an email address I would greatly appreciate it.

Thank you.

(Ron apparently had problems accessing it as well, perhaps someone has a copy of it downloaded already?)
 
Thoughts assuming this is temperature control

As long as the set point is above ******t then only the heating PWM is required. If the set point is below ******t then the cooling PWM is required. Things only get messy when the set point is close to the ******t temperature. Both outputs may be required then.

It is possible to use the cooling PWM to cool the system quickly even if the set point is above the ******t temperature. The same goes for the opposite case where the heating PWM can be use to heat a system quickly even though the set point is below ******t temperature.

The heater and cooler PWM will probably not have the same gains or time constants.
 
Not quite what I'm looking for yet. But thank you for the help thus far.

I think getting a hold of those tech notes would be inredibly helpful.

Or if someone could post a programming example of the split-range time proportioned outputs from the SLC PID.


Thanks everyone.
 
Could someone with an ab techconnect account indicate whether or not these articles are strictly techconnect accessable, or if the articles are down for amendment.

Thank you.

articles 21677 and 21678
 

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