PLC5/15 PID advice

Hi all,
This is my first posting so be gentle.

I work for a gas pipeline company and need some advice for programming PID control on one of our stations where a PLC5/15 is used.

Amongst many other analogues, the station has a "suction pressure analogue" and a "discharge pressure analogue". Both these signals are fed into the PLC via analogue input cards. I need to implement a control algorithm in the PLC where the station will control on discharge pressure (range = 4000kPa to 7000kPa) but also the suction pressure cannot go below 2500kPa.
A discharge pressure setpoint is also in the PLC (in the range 4000kPa to 7000kPa). The control output from the PLC will be via an analogue output module. In most cases the station will control on discharge pressure, however, at times the suction pressure will drop to near 2500kPa and I will need to control at this pressure.

My question is: What type of PID algorithm(s) do I need to program in the PLC to implement this type of control?
I have read about cascade control, but am not sure if this is what I need. The AB manual talks about a single PID control block, but I have not been able to find info on dual control modes.

thanks

I have done something very similar with a water pumping station, where as the suction pressure starts to reach the lower acceptable limit I reduced the required discharge pressure setting. When the suction increased I allowed the required disharge setting to increase back to the operator requested value.
Mine was a crude method, and it does work, but if I had more time then I am sure that you could tie 2 PIDs together to perform this task.
Regards Alan

You might want to look at the low select option. You basically have two PID loops on based on the discharge pressure as the PV, and one with the suction Pressure as the PV, then select the lower pid output for the analog output, you could reverse the selection and make it a high select, depending on the application. Take a look at this knowledgebase example of low select: LINK

Greetings Runamuck, and welcome to the forum ...



just for the record, the way I read your post it could be taken in one of three ways:



(1) you’re asking for help with how to apply PID control to your specific application ...

(2) you’re asking for general help on how to set up the PID “instruction/control/rungs” in an Allen-Bradley PLC-5/15 system ...

(3) or all of the above ...



my distinguished colleagues Alan and Ken seem to be going down path number (1) ... (they’re probably right - I’m usually kind of late catching on) ... but my point is that if you’ve never worked with PID in an Allen-Bradley PLC-5/15 before, then maybe we need to work on that angle too ...



so if path number (2) is also an issue for you, then I’d suggest that you start out by telling us what specific type of analog hardware you’re using (for example: a 1771-IFE analog input module and a 1771-OFE2 analog output module) ... you also need to tell us how those modules are presently scaled (example: 0 to 4095) ... if you can give us the specific addresses for your inputs and outputs (example: N7:54 and N7:75, etc.) then we can help you set up the PID control blocks ... actually the BEST thing to do is to post your existing .RSP file ... but for some reason (usually proprietary or trade-secret issues) most people are reluctant to do that ... we’ll understand, but seeing the file would make this a LOT easier (and a LOT less time-consuming) for everyone concerned ... especially for you ...



finally, just so that we can help you better, it would be a good idea to tell us whether:



(a) you’re well acquainted with PID control (yes, no, sort of) ...

(b) you’re well acquainted with Allen-Bradley’s version of PID control in the PLC-5/15 platform (yes, no, sort of) ...

(c) you’re just getting started with “process control” systems like this ...



and it helps if we know a little bit about your deadline ... and one more thing ... is it safe to assume (gosh I hate that word) that you’re using RSLogix5 to program this system? ... note: the PLC-5/15 is an “oldie-but-goodie” processor ... (Allen-Bradley calls it a “classic” as opposed to the newer “enhanced” models) ... I’m sure that there are some plants out there who are still programming these things with the old “AI” type software ... or with the even older “6200” style software ... etc. ... not that there’s anything wrong with that - but it would help us if we knew what we’re up against from the beginning ...



hopefully you have RSLogix5 because that’s what most of us use ...



we look forward to hearing from you soon ...

Some more advice on PID's

Do a search of this site for both Ron Beaufort and Peter Nachtwey

You will find some very interesting reading on PID's if you are interested.

In particular Ron wrote three threads on the seperate P, I & D....

So what happens when ....

What happens when the discharge pressure goes below 4000 Kpa and the suction pressure goes below 2500 Kpa.

Actually, Alan Case's idea is similar to what I would recommend. There is nothing crude about it, rather I would say intuitive. It is how you would control the system manual.

I would use 1 main PID to control the discharge pressure and modify the discharge pressure SP as Alan suggested. When things are normal the SP should not be affected by the suction pressure. When the suction pressure gets below some point like 2600 Kpa one should reduce the discharge pressure SP using a second PID that subtracts an offset to the discharge SP. Exactly how this is done you can probably figure out yourself. If more details are provided we may be able to come up with an optimal. Off hand I would suggest that you use a suction PID has a SP of about 2600 Kpa or maybe 2550 Kpa. Normally the suction pressure will be above that so the output will be 0 therefore nothing is subtracted from the discharge SP. When the suction pressure goes below 2600 Kpa the suction PID's output will be positive. This output is then subtracted from the discharge SP. Careful scaling, limiting and error checking is required.

PLC5/15 PID advice "Post 2"

Hi Guys,

Thanks for your replies. I have had to work on another project and been away for the last couple of weeks.

Prior to my initial posting, I had not visited the site that I have to work on. The job had been explained to me. I actually visited the site a few days ago and have found that some of the information I told you is incorrect.

I have made a copy of the PLC file that I need to modify (attached). It is written in RSlogix 5.
The processor is a PLC5/20. From my reading this is an enhanced processor which gives me some additional options as far as PID cntl goes. To answer some of your queries - I have performed basic PLC programming, but have never written an entire program from scratch. I have limited knowledge of PID loops.

In relation to Ron's post, I am after option (3) - all the above. I need help in applying and setting up a PID loop.

As the attached PLC code shows, the existing discharge pressure PID loop is implemented in an external Foxboro PID controller. The setpoint is passed to the Foxboro by a BTW command. The station discharge pressure is hardwired into the Foxboro (as the PV) as well as into the PLC5/20.

Prior to your replies, I couldn't think how to implement this control at all; I at least have some ideas now.
With this new information, is the best option still to use a PID command to modify the discharge pressure setpoint - any ideas how I should do this based on the code I have.

Thanks for your help thus far - much appreciated.