BCMaint (Customer) asked a question.

October 3, 2022 at 8:06 PM
PID Control issue

Using Productivity 1000 with P1-4ADL2DAL-1. What we're trying to do is control vacuum with a 4-20ma control valve. My set vacuum is 25inHG. The vacuum sensor isn't installed in the system yet, but if I pull a slight vacuum, the output just bounces between 0 and 4095. I have tried the auto tune by pulling a slight vacuum (10inHG) and holding it forever, nothing changes in the auto tune. It's not even showing the 10inHG on the PV plot. My variables are in min/max=0/30 out min/max=0/4095. The loop is in auto except for autotune. I've never used the 1000 PID before. I assumed the analog output would slowly drop as the PV got closer to the setpoint.

  • 6 answers
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  • BCMaint (Customer)

    Thanks for replying. I think I have it figured out. I was being impatient with the auto tune, so I watched the video series on PID and manually tuned it. It seems to be working now. I'll try the autotune again once the system is put together. Thanks again.

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  • BCMaint (Customer)

    Follow up: I tried autotune again, this time with setpoint of 8. If the PV went above 8, output went to zero. When it dropped below 8, output went to 4095. It finished and gave me gain-0, reset=21, rate=3. It still isn't control the way I had hoped. It's either 0 or 4095, no in between. The way these work on our other systems is the valve stays fully open until it get's within 1-2inHG, then it starts to close until the setpoint is reached. Normally it will go from 100% open to like 20-30% open to maintain the desired vacuum.

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    • ADC Community_02 (Automationdirect.com)

      Can you post your project or screen shots of the PID instruction Major Setup & Advanced tabs? Plus screenshot of data view showing values for each data field?

      • BCMaint (Customer)

        Thanks for replying. I think I have it figured out. I was being impatient with the auto tune, so I watched the video series on PID and manually tuned it. It seems to be working now. I'll try the autotune again once the system is put together. Thanks again.

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  • Todd Dice (Customer)

    Just a thought but I would recommend filtering the analog input (with the "Average" instruction) to a new float and use its value in your PID.

  • Tinker (Customer)

    I don't know how the auto-tune algorithm works, but if the feedback device is not connected to the system can't see how it could give any useful result.

    Ideally if PV doesn't change at all, it would indicate an error and change nothing, second best, if there was no provision for error indication, would be to just change nothing, the bad option would be to change to weird values. Note that due to noise a "constant" input might not be literally constant, and perhaps result in nonsense parameters being calculated (like a gain of 0). If gains were very high even a little noise may cause significant output changes.

     

    In short I would NOT try auto-tuning an incomplete system.

     

    Terms can be used differently and in some implementations a value might be the inverse of what it might be in another implementation, but a gain of zero, is either zero which it seems would do nothing at all (if we ignore reset, but a pure integral controller seems weird), or, infinity (if we allow dividing by zero) , which would be an on/off control or in other words: " It's either 0 or 4095, no in between"

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    • BCMaint (Customer)

      I did have the sensor attached for feedback, but my control of it wasn't very consistent, which could explain my poor autotune results. I'm thinking I'll get a better tune once I run it on the actual system. For now my "rough" PID settings seem to be working fine, Thanks for the feedback

 

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