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).
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.'
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