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Old February 10th, 2020, 02:25 PM   #1
stu
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Pid information needed rslogix 500

Hi good people
I have a question regarding a program on a machines heater control, I have 3 x heater contactor controlled via a plc
It looks like the contactor is controlled via timers rather then the pid in the program?
Can you explain why pid is not being used full and the timers energize the output to the contactor, could it gave been done with the output of the pid ?
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Old February 10th, 2020, 02:29 PM   #2
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Old February 10th, 2020, 04:41 PM   #3
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Without having the full code to poke about in, I'd say there are two likely possibilities:

1. The PID loop does effectively control the "on time" of the heaters by virtue of adjusting the timer presets, or using N50:14/N50:2 to switch B3:13/0 on and off, or some other similar method. I have used this sort of approach before (although I'd like to think my finished code was somewhat less opaque) when I'm trying to control a temperature with a PID - you effectively have an on/off duty cycle for the heater output, and increasing your PID output increases the on time/decreases the off time.

2. Somebody tried to implement a PID-based temperature control loop, couldn't get it to work, threw in an undocumented fixed-rate on/off duty cycle, got it working by trial and error, and then called it a day.

If I had to take a bet, I'd bet on #2
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Old February 10th, 2020, 04:59 PM   #4
PLCplease
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Yes, it rather looks like they cobbled together a feedback loop using the Less Than instruction to turn on the heater.

Those timer branches are a bit baffling to me, though. It appears that the only thing controlling the heater is the B3:13 bit
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Old February 11th, 2020, 06:40 AM   #5
DaDaDadeo
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The best way to control a digital output is by either Pulse Width Modulation (PWM) or Timed Proportional. I prefer the timed proportional option. There are several topics in this forum addressing this. Basically the PID output percentage is compared to a timer that resets itself. The output percentage is scaled to match the timerís preset value. If the timer preset is 4 seconds (4000ms) and PID output is at 50% (2000), the digital output is on for 2 seconds and off for 2 seconds. If the PID output is 25%, the DO is on for 1 second and off for 3 and so on. To keep an output from cycling off/on too quickly when the percentage is very close to 0 or 100% , add a 1/2 second or more delay on the outputt
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Old February 11th, 2020, 09:02 AM   #6
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If you have a tech connect contract, the knowledgebase has examples of both time proportional and split range time proportional PID examples for RSLogix500. I have used them on extruders and seems to work well.
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