PLC Vs. autotuning PID controller

[evol 0]

I am working on a project for an internship. I more or less inherited this project from the previous intern and well... I think that for what is desired he was reinventing the wheel. My employer wants to upgrade the controls on a reflow oven that will give the ability to store multiple recipes, programs or whatever you would like to call them. I was thinking of replacing the existing PID controller with a programable autotuning PID controller. The intern prior to me had them buy a micrologox plc, a panel view 550, various cables, and various expansions modules..... (i.e. spent alot of money in my eyes) any thoughts? I am new to PLC's and the like and I have been doing my homework on it but I wanted to get maybe someones opinion whos experienced in this before I go to my boss and say " I think the company wasted alot of money for nothing on this project and it is going in the wrong direction." I also do not feel comfortable w/ a project that is, from what I see going down the wrong path. Ideas?

[Peter Nachtwey 15]

I think you have a good idea. Now what controller are you going to get and how are you going to connect it to the computer with the recipes? Are there controllers that can store the recipes? I didn't look.

[DanW 41]

It sounds like you have an existing single loop PID controller and that maybe someone else already bought the micrologix stuff? Here's my take on it: I've done temp controls on ovens, furnaces and kilns for 20+ years, and until recently found that most useable stuff for operators (who are not always very bright lights) when multiple recipes of setpoint profiles were involved was a low cost Windows HMI, like SpecView, that would download recipes or on-line setpoint changes into single loop controller over Modbus. Trying to keep track of SP profiles on the controller itself was a nightmare, because of the poor operator interface on the controller. The Windows HMI was needed primarily for the ease of entering SP profiles and maintaining the library of profiles. The same level of complexity for handling SP profiles in a PLC/HMI exists as it does with a PID controller. However, just last week I saw West Instrument Solutions (division of Danaher) new Provu 1/4 DIN single loop controller with an LCD display. It is truly incredible the ease with which this controller/setpoint programmer is configured AND operated. The era of grotesque 7 segment display characters attempting to convey meaning as English alphabet characters is gone. Entire words and phrases appear in fairly well lit LCD display. That makes it operator friendly. The setpoint profile option stores up to 256 segments, but a USB port can load as many config setups as your heart desires. I never thought I'd live long enough to see a functional USB port on a 1/4 DIN controller ! ! ! I will be using these for temp controllers with the setpoint profiler option lots in the future. They even have 1 Mb of memory for tracking PV, SP, or output, which displays on the LCD display as a trend! Like a built-in recorder. So, you ask, how does this play into your situation? Well, if someone has already dished out the dough and bought all the PLC stuff, if I were you, I'd take the opportunity, as an intern, to learn the ins and outs of programming the PLC. It's already bought and paid for. Great learning opportunity. PLCs are a fixture in industrial control and aren't going away. Doing PID puts you several levels ahead of the straight sequence guys. It isn't clear what the business cost of having the oven out of commission is. If the boss is willing to let you spend the next number of weeks dinking around learning PLCs fine. I'd say do it. But if he wants his oven back on line, then the easiest, cleanest, quickest route I could see would be to spend $800 on the West Provu, stick it in the panel hole where the other PID controller is, wire it up in an hour, configure it in an hour, and be running the oven that afternoon. While the oven's running its first profile, you can use the Windows software to crank in a bunch more SP profiles and save 'em on a PC so they'll be there when product DEF or GHI or JKL needs to be run. So tackle the PLC if the owner is willing. If he's not, go get the easiest to use controller/programmer ever made and get the oven running. Dan

[DanW 41]

I realized I made the assumption that 'recipes' means setpoint profiles, with maybe the associated 'event' or two (DO's whose state coincide with specific segments of setpoint profile program). If the recipes involve more than the above, like, belt speed changes or more complicated start logic or whatnot, there can be trade-offs with a 1/4 DIN single looper. I noticed that the West box has a couple dry contact DI's on it, but I did not pay close attention to the specific functionality associated with them. In general, 1/4 DIN loop controllers have very limited, specific functionality associated with discrete input logic, so the thought behind the Micrologix might have been for more sophisticated logic at some point. The point might be to find out what the Micrologix stuff was supposed to do that the single looper doesn't, and it could be to just hold more SP profiles. Dan