Temp. Control: PLC PID vs Discrete

I'd appreciate hearing opinions and experiences of going from discrete controllers to PLC control of temperature. We use a lot of Watlow and Eurother controllers here and I'd like to take a stab at eliminating them in favor of PLC's. I think the cost and elimination of parts would be benifitial. Some of the applications use both cooling (water) and heating (heating elements) and others just heating (IR lamps).

This is probably an uphill battle considering the mindset here. I put a small PLC and HMI on an extruder drive and you should have heard the comments. Everything from 'it'll never work' to 'the ET's will never figure that out.'

WildeKurt said:

I'd appreciate hearing opinions and experiences of going from discrete controllers to PLC control of temperature. We use a lot of Watlow and Eurother controllers here and I'd like to take a stab at eliminating them in favor of PLC's. I think the cost and elimination of parts would be benifitial. Some of the applications use both cooling (water) and heating (heating elements) and others just heating (IR lamps).

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When you say, "take a stab", I have to ask, just how committed are you? Speaking from experience, most PLC PID loops that I've dealt with were out-performed by similar processes using stand-alone controllers. That is not to say that PLC control is necessarily inferior, just more complicated. There are so many factors to consider. What type of PLC, and what type of PID algorithm is utilized? What kind of thermocouple interface/card will you use? How fast does the delta time need to be and can your PLC handle it?

These questions have to be answered just to determine if PLC control of your process is possible.

To determine if making the switch is advantageous, other things must be considered. Do you need the extra capability that PLC control offers (i.e. gain scheduling, cascaded loops, dynamic bias control, easy HMI compatibility, trending, etc)? Ask yourself is the potential cost savings from eliminating discrete controllers worth the investment in a PLC, or the investment in engineering time for you and your crew, and training time for the maintenance staff.

You also have to prepare for the possibility that a PLC PID's autotuning feature (if even available) may well be useless. You will probably have to spend many hours observing the process, taking copious notes about PV to CV response behavior, and do a bit of math yourself to get a well-tuned system.

I don't say all this to discourage you from your undertaking, in fact, quite the opposite. You just have to make sure you are prepared for the work. The old adage, "If it ain't broke, don't fix it" applies, unless you (and your boss!) believe you can make things better.

This is probably an uphill battle considering the mindset here. I put a small PLC and HMI on an extruder drive and you should have heard the comments. Everything from 'it'll never work' to 'the ET's will never figure that out.'

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Get used to it . Only one thing matters: Did it work?
If so, just let those comments be fuel for your next project. Just remember: in this business, you're only as good as your last job

Anyway that's my $.02. I wish you luck in your project.

Cheers,
Dustin

I used Automation Direct's DL205 series with the DL260 CPU. System was a quartz tube furnace. Six PID loops. Autotuned, then fine tuned parameters manually. Nice interface in the software. Very tight control in the 800-900 C range.

You could meet in the middle with a Red Lion http://www.redlion.net/Products/ProcessControl/ModularController/ModularControllerModules/CSOUT.html

Dustin,

Those are all concerns I've been pondering. My preference in PLC's would be a CompactLogix system though perhaps one of the MicroLogix systems would work too.

Stand-alone temperature controllers still offer

- local HMI display for PV (temp) and SP, typically right at the location where the heat's generated. For local batch operations that can be convenient.

- menu driven setup. The non-Asian models have largely gotten away from those awful legacy numerical codes for functions and selections.

- options for heat treat functions, like carbon control with its special probes and curves

- option to drive a slidewire feedback actuator motor without a lot of programming (whole threads dedicated to that here)

They have their place.

Dan

WildeKurt said:

Dustin,

Those are all concerns I've been pondering. My preference in PLC's would be a CompactLogix system though perhaps one of the MicroLogix systems would work too.

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I would go with the Compactlogix. The ladder PID instruction in the 5K logix family is superior in my opinion because it has anti-reset windup built in and optional built in bumpless xfer. Plus, if you have the Function Block Editor, then you can use the enchanced PID, or PIDE, instruction. For slow response and long integration processes like temp control, I think the PIDE is much better suited to. It's a different algorithm, and a different philosophy, but there's plenty of good documentation on the r.a. website about how to utilize the instruction.

I used Automation Direct's DL205 series with the DL260 CPU.

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I have used this as well as the dlo6( same processor less cost and smaller foot print) with a c-more hmi for trending. worked just fine on a gas fired 2 burnner oven. However i like to use the 1/4 din controlers just as well I think the compact logix is way over kill!!!!

If the extruder is very sensitive (i.e. vulcanizing temperature band very small or thermal intertia very low) then discrete (relay...or solid state relay) really bites and can become unstable. However, this on/off contro, should still be done with PID and directed to a discrete output /duty cycle(dwell) control. If your system is sensitive and difficult to control, then get an SCR controller with whatever analog input you like. You'll achieve much more precise, predictable, noise-free and stable power control. Most SCR controllers compensate for line voltage fluctuation (theatre light targeted) to maintain a level power output for a given analog setting. This is a huge benefit in such situations and takes a variable out of the very "black-art" of tricky extruder heat control.

I have little real experience with temperature control but I do know that PLC PIDs are crude and are the bare minimum implementation just to say they have a PID. If the stand alone temperature PID controllers have good communication capabilities I would tend to use one of the dedicated temperature controllers. i know that I could make a dedicated temperature that would control much better and be much easier to implement than using a PLC PID.

If you really know what you are doing you can probably use a PLC PID to control temperature more efficiently and cheaper.

The extruders currently use on/off control through discrete controllers which use PID internally. Heating is controlled via SSR's and cooling through solonoid actuated valves. A big advantage with the discrete controllers is they have the ability to auto tune. How much of an advantage does using a PID loop have when ultimately controlling via on/off devices?

If you set the cycle time to a small number (a few seconds, or less) and if there is thermal mass (extruder barrel or die zones), then fast on/off control via SSRs is very good. The auto-tune in most good-quality discrete temperature controllers will perform well.

Modern stand alone controllers can do a better job than a PLC PID loop in most cases. Good reasons to use a PLC:
1) The PLC is controlling aspects of the machine and therefore has info which could be useful to the PID system in order to schedule changes to setpoints, gains, feedforward. The PLC may make the temperature control better by being proactive rather than strictly reactive.
2) Save panel space, reduce costs. For example, 8 loops may fit better and cheaper into a PLC system with a small panel footprint.

Drawbacks:
1) If the PLC goes down, or an analog card fails, you just lost your temp. controller(s) too.
2) Usually the PLC will not do as good a job (see Peter's comments).
3) Joe the Tech may have more difficulty tweaking the tuning in a PLC, than he would performing the same operation in a modern stand alone controller.