stgislander (Customer) asked a question.

March 24, 2023 at 8:43 PM
PID bias in Do-more

I'm using the Do-more PID for position control. I am running the PID controller Automatic mode (no transitions to manual).

 

I am wondering what the Bias value does for me. I'm finding the definition in Help a little lacking. I've recorded a plot of a servo control hydraulic cylinder. My default Bias is 50. As I review the spreadsheet, the BIAS follows the PLC Output but not exactly.

 

Does changing the Bias value, say from 50 to 49.5, provide an offset to my Output value?

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stgislander and Garry like this.

  • ADC_PLC_ENG (AutomationDirect)

    Yes it does. Below is an explanatin from Bob O with HOST:

     

    Bias is added to the proportional term to produce the initial state of output and is effectively just an offset...a bias. The proportional term is essentially just "Output = Gain * Error + Bias".

     

    For loops with 'I' terms, the integral affects the loop's output by integrating error into the bias over time. So the user can change the bias, and the loop can as well.

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  • ADC_PLC_ENG (AutomationDirect)

    Yes it does. Below is an explanatin from Bob O with HOST:

     

    Bias is added to the proportional term to produce the initial state of output and is effectively just an offset...a bias. The proportional term is essentially just "Output = Gain * Error + Bias".

     

    For loops with 'I' terms, the integral affects the loop's output by integrating error into the bias over time. So the user can change the bias, and the loop can as well.

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    Selected as Best

    • stgislander (Customer)

      In my PID controller (MyPID) setup I have the following:

       

      Scale from .Output %: Yes

      Scaled Min (0%): 2047

      Scaled Max(100%): -2047

       

      When the PID is running .Bias value is automatically calculated as 50(%).

       

      During testing, a .Bias of 48.8(%) was found to eliminate the customer's concerns about forces on a clevis pin when the actuator is locked. It appears it cancelled out a physical null bias in the servo valve.

       

      Since inputting a "manual" .BIAS value is not part of the PID controller setup, what is the best way to enter the 48.8% .Bias I want and not the automatically calculated 50% value? I assume I can use the Move command to initialize MyPID.Bias to 48.8.

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      • stgislander (Customer)

        Ugh! I'm pretty sure I answered my own question. .Bias is not being automatically calculated. It is being initialized to 50% in an INIT block following my PIDINIT blocks.

         

        I completely passed over it. Too many late work days the past week. 😴

  • stgislander (Customer)

    Thanks for the fast reply. I'm in MD realtime troubleshooting a Do-more PID loop in CA.

     

    I started by running just the P term. With no hydraulic pressure, the SP and PV are both 24.51%. When pressure come on, PV increases to 24.99%. The mechanical engineers in CA do not like that because it increases the load on a linkage pin.

     

    I added a 5 second I term to see if that would drive PV back closer to the SP value. From what I could tell, it didn't anything before they had to turn off hydraulic pressure. I'm putting together some tests that do not require any hydraulic pressure to characterize the PID operation.

     

    I have this nagging feeling that I'm chasing my tail trying to resolve what is a 0.05" error.

     

     

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