MAX PID in Contrologix

[The Plc Kid]

What is the safe max amount of pid loops that i can put into a contrologix processor without overloading it? These will all be steam valve controlled temp loops (Air Zones).

I will be using the top of the line contrologix controller whatever model that may be. There will be other logix ran in the processor.

I pretty much know i will need multiple processors i am just trying to determine the correct amount of loops per processor.

What is a safe number?

 

[rdrast]

That depends entirely on how fast you need to scan them, and how much memory you have.

I routinely run 25 or so 'slow' temperature PID's, in a periodic task with a 90ms period. Each time through, I update two of the PID's, so each one's update time is set to 13*90ms or 1.17 seconds.

In the same PLC, I run 8 or 9 faster PID's, scanned every pass through a 20ms task.

Remember, CLX is fast, and there are ways of optimizing the scans. Start with your required update rate first.

FWIW, two sheets of FBD PIDE's and associated logic (SRAMP, and some logic) runs on average in 1.5ms on an L62.

 

[The Plc Kid]

Being that these are air zones in what is basiclly a oven so to speak the temp can vary +/- 3 degrees and still be okay so i don't think they need to be real fast.

Total on this line they will be about 85 zones like this.

I was thinking of having some processor just for pid temp control in the remote racks and all the non temp control I/O in that rack owned by another processor.

 

[The Plc Kid]

I thought i saw mention somewhere of a L7 series processor debut at automaton fair?

This project will really get uder way jan 2011.

Just working on a rough budget now.

 

[The Plc Kid]

May be a PITA have 6-8 processors on the line though?

What do you think? Is that hard to deal with when troubleshooting?

 

[creativepaper]

I have a L61 processor running the following PIDE loops. All are scanned every pass at their required scan rate.
50ms = 7
250ms = 16
500ms = 7

I used to have 500ms loops running at 250ms, but since the chests are so big, doubling the scan time didn't change the control one bit. I actually don't think that I change the CPU usage either.
CPU usage = 55%
This processor runs the papermill which consists of 13 drives, 30 PID loops, 31 motors.

Being a novice, I wouldn't have thought that my code is fully optimised compared to what the vets could do to it.

Depending upon what else is on the PLC and desired scan time, you could try a single processor

 

[TConnolly]

If these are basic PIDE loops I don't see any reason why you can't do all 85 in less than 100 mS on an L61. If you run them on a one second update then 90% of your CPU time is dedicated to something besides PID.

 

[The Plc Kid]

They are all air zones which have a 4-20 controlled valve to regulate the amount of steam into the coils to heat the zone.

They are heat only no cooling.

 

[shoelesscraig]

I have an L55 running over 60 PIDE loops updating every 150ms, with no problems.

 

[The Plc Kid]

All of them update at 150ms? If so i guess i was worried for no reason then.

Craig are you running much other non pid related logic on that processor also?

I remember on slc if you had too many pid loops you would have some funky things happening sometimes. I guess thats just not the case with contrologix.

 

[The Plc Kid]

I will still most likey go with 2 processors though. I don't want to put all those pid loops on the guard logix processor.

I can put pid loops and all auxilliary drives on another processor.

 

[Honga]

L7s failed first testing I think. So the scheduled release is delayed.

Pretty much what I've seen they're 2x more powerful than the l6's

Probably should contact someone from RA about it, you may even be able to get a demo model.

 

[Honga]

Your other option I suppose is an industrial computer with softlogix.

Or you could OPC with a vb program or something, not sure about the comms speed of that though.

EDIT: Should have read the post.

 

[shoelesscraig]

All of them update at 150ms? If so i guess i was worried for no reason then.

Craig are you running much other non pid related logic on that processor also?

Thousands and thousands of rungs of other stuff...works for us

 

[ghettofreeryder]

I have roughly 100 PID loops in a L63, with thousands of rungs of code (700 analog transmitters with scaling and alarming, 300 discretes, 50 motors, 50 valves, all the control logic), and we have no problem. Average scan time is about 75 ms.