Jeff Jordan

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  • in reply to: Time proportioning output using PID CV rs logix500 #5441
    JKJTechJeff Jordan
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    Alex,

    Fred is right that a simple ON/OFF control for heating could work. This is called thermostatic control. However if the heater is large in respect to the load, meaning that if only 20% of the heater capacity was necessary to maintain temperature, ON/OFF control will probably be too unstable. Do time proportioning control like this. You start with what “update time” your PID loop needs to run.  The update time is part of the PID loop configuration. This means that the PID loop will perform its calculations every update time period. Here’s an example. Lets say a PID loop updates every 10 seconds. The loop output would be scaled to a value between 0 and 1000.  This scaled output then would be moved to the preset value of a timer. If one uses a TOF timer with a .01 timebase, then by enabling the TOF rung with a one shot every time the PID loop updates, then the timer will time out with the correct time proportioning control. That is, if the PID output was 50%, then the scaled output would be 500 which is also the preset of the TOF.  When the TOF time rung is pulsed, then the .TT bit of the timer would be on for 5 seconds and off for 5 seconds, or 50% of the time base. Control the contactor output with the .TT bit. Also be careful with using solid state relays or contactors with heating. These devices can fail in the shorted state thus making a run away heating situation possible. Hopes this helps. If you would like some sample code I could send it to you. I have used this technique for decades on over 500 PID loop applications.

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