hamadi

PID PWM RSLOGIX5000

6 posts in this topic

Hi

I want to order a static relay to regulate the temperature by PID (PWM)
How can I do it by studio5000? I can't find a ready command that gives pulses to control static relay according to the temperature.
Thank you

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I assume we're using the same acronyms and by "PWM" you mean Pulsed Width Modulation. Off the top of my head I don't know of any Allen Bradley output cards or instructions that create PWM. There may be a process card out there that does that but I'm not familiar with it, and a quick glance at the ControlLogix Analog I/O Modules User Manual didn't turn anything up. 

When I was doing IR welding we installed solid state SCR device that were like a very dumbed-down DC drive. A voltage signal directly from a PLC analog output card went to the input of the SCR driver, which then pulsed an output to the IR bulbs to control the temperature of the weld. Is this similar to what you're trying to accomplish? If you're in a closed loop system providing the actual temperature back to the PLC, you can certainly use the PID instruction to get tighter control of that temperature. I just don't think you're going to be able to PWM out directly from the PLC to a field device. 

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Mr. Google returned this video about an AOI that does PID PWM.   - No warranty on the find.

No

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At a previous employer, we had some SLC 5/04 PLCs using straight digital outputs controlling SSRs off of PID controls. Basically, the Process Variable (PV) of the PID control became a timer preset that set how long the digital output turned on as a portion of a full cycle. I don't remember the exact code, but imagine two timers running in parallel. One has a fixed preset of "x" time while the other has a variable preset based on the PID output. The output turns on when the fixed timer starts and turns off when the variable timer turns off. Then the fixed timer resets itself once it times out. The PID output becomes a share of a full cycle. I don't remember what the fixed time was, but I want to say it was like 10 seconds or something. This was a reasonably stable temperature loop, so that seemed to work.

We had others that used an analog output to an analog SSR. Those were simpler to program but used more expensive PLC outputs and more expensive SSRs.

Edited by Joe E.
clarified when the fixed timer resets itself...
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As a drives guy, when I think of PWM I tend to think in terms of both positive and negative pulse polarity (as opposed to only true and false), and those pulses are almost always in gigahertz. When I've needed pulse control for SCRs I've always used an external piece of hardware, as I described above. So... this particular thread is opening a new door for me.

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