Loop Control PLC Vs. Loop Module

I have a question in regards to doing PID temperature control loops in a CompactLogix L23E PLC vs. using a dedicated loop module to do the temp control. In the past we have always done our temperature control using dedicated loop modules. We communicate to the module from our PLC using Ethernet IP or DeviceNet. The load we are controlling the temperature on is typically a 12” x 18” x 1 ½” resistive heater (a fairly slow loop). The modules have the typical built in Auto tune function, which sets the PID values, thus removing the manual tuning process and room for error. In a nutshell we’ve had good success doing the PID outside of the PLC. I’ve been looking at the PIDE function in the RSLogix 5000 software and I’m not sure if this might be the way to go. Any input on Pro’s and Con’s of PLC vs. Loop Module would be much appreciated.

Thanks

On a slow loop it would be more cost effective and easier to manage in the plc with less hardware and less points of failure IMHO.

All our loops are handled in the plc. When there is a sh*$load of them or they are real tight and real critical we have used a second controller in the same chasis and it was still more cost effective than using individual loop controllers IMHO.

To each his own though.

Thanks for info,

I'm thinking about using the CompactLogix L23E with some point I/O to feed the thermocouples into. Would you us the PIDE or PID instruction in the RSLogix 5000 software? I do like the thought of the auto tune feature in the PIDE instruction. I have not tuned a lot of control loops.

We have always used the PID instructions in the PLC for loop controls. I dont know what it is about the extruder manufacturers and other similar industries that feel like they have to have external single loop controllers. The autotune feature in the logix works pretty good once you get the hang of it. I have heard it can be difficult to tune some very slow temperature loops. Also, if your process variable is unstable, you will not be able to get the autotune to work (ie a pressure signal that is affected by other areas of the process). You may want to do a litte research on the L2x series as I think they left the autotune capability out of those processors. You may have to move up to the 3x series to achieve what you want. Also dont get caught by the limited modules allowed on that series. That can be somewhat circumvented by plopping I/O on your ethernet/IP link.

ddnnis said:

Thanks for info,

I'm thinking about using the CompactLogix L23E with some point I/O to feed the thermocouples into. Would you us the PIDE or PID instruction in the RSLogix 5000 software? I do like the thought of the auto tune feature in the PIDE instruction. I have not tuned a lot of control loops.

Click to expand...

The short answer. It depends.

Using the PIDE is coll because you can autotune but you can't use PIDE in ladder only Function Block.

I have guys here who get all confused and just throw their hands up at anything that is not ladder. So most of mine are in ladder but I use control Station and expertune to do the tuning depending on the loop and the type of processor.

If you are building equipment the end customer may request it either way so if this will be your standard at some point and you have enoughloops and make enough equipment to justify it may be wrth getting some tuning software so you can set it and walk away to do other things

Here is the response when I asked that exact same question 5 weeks ago.

http://www.plctalk.net/qanda/showthread.php?t=64914&highlight=controller

There was no real consensus for which way to go. I still haven't decided if I want to integrate the 10+ heaters into the PLC (I will put in a controllogix) or if I will keep the Partlow and use it on Ethernet.

Bruce,

Thanks for the info. I just looked at your link. Very helpful. Once again thanks.

@ The OP

The PIDE function an be a little funky if your PV is hopping around as some other memebers have said but if your PV is hopping around due to noise or whatever then fix the problem.

Filter the PV a little or use some signal conditioners.

If people want precision temp controls and have noise issues then they should not be using T/C's in the first place they should be using RTD's

I do 99% of our loops in the plc using the PID and PIDE instructions on SLC,PLC 5, and contrologix and have no issues. I have over 2000 loops and counting so use that info as you will. Just my opinion.

Point taken.

Thanks