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Bug in Control Loop Library, Setpoint off by 1

Engineering 🇬🇧
janderson
2023-08-03
2023-08-08

  • janderson - 2023-08-03

    I may have discovered a bug in the control loop library.

    I am using a PID Controller and when I use an anti-wind-up strategy the response will settle at exactly the setpoint-1.

    When the anti-windup strategy is removed the controller settles at the setpoint correctly.

    PROGRAM temperature_control
VAR
    //PID Controllers

    PID_1_output: LREAL := 0.0;
    PID_1_error: Ctrl.Controller_Error;

    PID_1_integrator : Ctrl.Integrator_ParabolicApproximation;
    PID_1_diff : Ctrl.Differentiator_LinearAverageApproximation;
    b_array : ARRAY [0.. 99] OF LREAL := [100(0.01)];
    filter_1 : Ctrl.Filter_FIR := (palrCoefficientsB := ADR(b_array), udiSizeCoefficientsB := SIZEOF(b_array));
    filter_1_error : Ctrl.Controller_Error;
    PID_1: Ctrl.Controller_PID;

    kp: LREAL := 0.3;
    ki: LREAL := 0.03;
    kd: LREAL := 0.00;
    PID_1_limits_active : BOOL;

    //PWM Generators
    PWM_1_cool : Ctrl.PWM_Creator_FixedCycle;
    PWM_1_heat : Ctrl.PWM_Creator_FixedCycle;

END_VAR

//config
PWM_1_cool.udiPWMCycleTime := 100000; //microseconds
PWM_1_heat.udiPWMCycleTime := 100000; //microseconds

//turn on heatsink fans
GVL.heatsink_fan := TRUE;


// filter temperature signal

filter_1(
    lrValue := GVL.TC1,
    xEnable := GVL.temperature_control_1_enable,
    lrFilteredValue => GVL.TC1_filtered,
    eErrorID => filter_1_error);

//PID Controllers
PID_1(
    //inputs
    lrActualValue := GVL.TC1_filtered,
    lrSetPoint := GVL.temperature_setpoint_1,
    xEnable := GVL.temperature_control_1_enable, //output will go to zero on disable

    //config
    lrOffset := 0,
    lrMinValue := -1,
    lrMaxValue := 1,
    lrKP := kp,
    lrKI := ki,
    lrKD := kd,
    itfIntegrator := PID_1_integrator,
    itfDifferentiator := PID_1_diff,

    //outputs
    lrOutput => PID_1_output,
    eErrorID => PID_1_error,
    xLimitsActive => PID_1_limits_active,
);

IF PID_1_output <= 0 THEN
    PWM_1_cool(
        lrProportionValue := -1*PID_1_output,
        xOutput => GVL.h_bridge_1_cool,
        eErrorID =>,
        xEnable := GVL.temperature_control_1_enable);
    PWM_1_heat(
        lrProportionValue := 0,
        xOutput => GVL.h_bridge_1_heat,
        eErrorID =>,
        xEnable := GVL.temperature_control_1_enable);
ELSIF PID_1_output > 0 THEN
    PWM_1_cool(
        lrProportionValue := 0,
        xOutput => GVL.h_bridge_1_cool,
        eErrorID =>,
        xEnable := GVL.temperature_control_1_enable);
    PWM_1_heat(
        lrProportionValue := PID_1_output,
        xOutput => GVL.h_bridge_1_heat,
        eErrorID =>,
        xEnable := GVL.temperature_control_1_enable);

END_IF

    Does anyone else experience this behavior?

     

  • janderson - 2023-08-08

    Nevermind, i did not recognize that the clamp is placed on the value of the integrator prior to it being scaled by KI. After increasing the max and decreasing the min, it functions normally.

     

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