FB_CTRL_PID_EXT

Transfer function:

The following transfer function can be declared for this block, if the boolean inputs bPInTheFeedbackPath and bDInTheFeedbackPath are set to FALSE, otherwise this transfer function only describes a part of the blocks behaviour:

Functional diagram:

The standard functional diagram of a PID controller in additive form has been expanded by the two active boolean inputs bPInTheFeedbackPath and bDInTheFeedbackPath (which act as "switches"), so that a modified functional diagram can be activated.

Control background: due to the differential component of the control algorithm, large control values are generated at setpoint step-changes, which cause a strain on the control elements and may cause the control system to oscillate. A control algorithm with a differential component that is only applied to the controlled variable ( bDInTheFeedbackPath := TRUE ) can avoid this problem.

The bPInTheFeedbackPath and bDInTheFeedbackPath inputs permit the closed control loop to implement the following transfer functions:

with:

! The default setting for the two inputs bPInTheFeedbackPath and bDInTheFeedbackPath is FALSE. The PID controller then acts as a standard PID controller in additive form.

Switching off the I component in the Outer Window

Integration of the system deviation is halted if the system deviation is greater than the fOuterWindow parameter. In this way it is possible to prevent an extremely large I component from developing if the system deviation is large, since this could lead to a marked overshoot. If it is not wanted, the function can be disabled by setting fOuterWindow := 0.

Linear reduction of the I component in the Inner Window

With this function it is possible to drive the I component linearly down to zero in the range specified by the fInnerWindow parameter. If it is not wanted, the function can be disabled by setting fInnerWindow := 0.

Output dead band

If the parameter fDeadBandOutput is set > 0, the output is set to zero when it is within the range of [ -fDeadBandOutput ... fDeadBandOutput ].

Input dead band

If the parameter fDeadBandInput is set > 0 then the output is held constant for as long as the system deviation remains within the range of [ -fDeadBandInput ... fDeadBandInput ].

Step response:

ARW:

VAR_INPUT

VAR_INPUT
    fSetpointValue  : FLOAT;
    fActualValue    : FLOAT;
    fManSyncValue   : FLOAT;
    bSync           : BOOL;
    eMode           : E_CTRL_MODE;
    bHold           : BOOL;
END_VAR

fSetpointValue : Setpoint of the controlled variable.

fActualValue : Actual value of the controlled variable.

fManSyncValue : Input with which the PID element can be set.

bSync : A rising edge at this input sets the PID element to the value fManSyncValue.

eMode : Input that specifies the operation mode of the function block.

bHold : A TRUE at this input will hold the internal state (and therefore also the output) constant at its current value, independently of the control deviation.

VAR_OUTPUT

VAR_OUTPUT
    fOut        : FLOAT;

    bMaxLimitReached    : BOOL  := FALSE;       (* minimum limitation active ? [TRUE/FALSE] -> ARW *)
    bMinLimitReached    : BOOL  := FALSE;       (* maximum limitation active ? [TRUE/FALSE] -> ARW *)
    bARWActive      : BOOL  := FALSE;       (* ARW active ? [TRUE/FALSE] -> freeze I-part *)

    fCtrlDerivation     : FLOAT;            (* controller command derivation dy/dt *)

    eState      : E_CTRL_STATE;
    bError      : BOOL;             (* error flag *)
    eErrorId        : E_CTRL_ERRORCODES;    (* error code *)

END_VAR

fOut : Output of the PID-element.

bMaxLimitReached : The output is TRUE when the block is at its upper limit.

bMinLimitReached : The output is TRUE when the block is at its lower limit.

bARWactive : A TRUE at this output indicates that the PID element is being restricted.

fCtrlDerivation : A TRUE at this output indicates that the PID element is being restricted.

eState : State of the function block.

eErrorId : Supplies the error number when the bError output is set.

bError : Becomes TRUE, as soon as an error occurs.

VAR_IN_OUT

VAR_IN_OUT
    stParams        : ST_CTRL_PID_EXT_PARAMS;
END_VAR

stParams : Parameter structure of the PID element. This consists of the following elements:

TYPE
ST_CTRL_PID_EXT_PARAMS :
STRUCT
    tCtrlCycleTime     : TIME     := T#0ms;     (* controller
cycle time [TIME] *)
    tTaskCycleTime     : TIME     := T#0ms;     (* task cycle
time [TIME] *)
    fKp        : FLOAT    := 0;     (* proportional
gain *)
    tTn        : TIME     := T#0ms;     (* Tn *)
    tTv        : TIME     := T#0ms;     (* Tv *)
    tTd        : TIME     := T#0ms;     (* Td *)
    fDeadBandInput     : REAL     := 0.0;       (* ctrl
deviation dead band/neutral zone for const output *)
    fDeadBandOutput    : REAL     := 0.0;       (* ctrl output
dead band/neutral zone for zero output *)
    fInnerWindow       : REAL     := 0.0;       (* inner window
for reduced I-part (dE-window) *)
    fOuterWindow       : REAL     := 0.0;       (* outer window
for disabling I-part (dE-window) *)
    fOutMaxLimit       : FLOAT    :=  1E38;     (* maximum
output limit *)
    fOutMinLimit       : FLOAT    := -1E38;     (* minimum
output limit *)
    bPInTheFeedbackPath : BOOL;
    bDInTheFeedbackPath : BOOL;
    bARWOnIPartOnly    : BOOL;
END_STRUCT
END_TYPE

tCtrlCycleTime : Cycle time with which the control loop is processed. This must be greater than or equal to the TaskCycleTime. The function block uses this input value to calculate internally whether the state and the output values have to be updated in the current cycle.

tTaskCycleTime : Cycle time with which the function block is called. If the function block is called in every cycle this corresponds to the task cycle time of the calling task.

fKp : Controller amplification / controller coefficient

tTn : Integral action time. The I-component is deactivated if this is parameterized as T#0s.

tTv : Derivative action time. The D-component is deactivated if this is parameterized as T#0s.

tTd : Damping time

fDeadBandInput : see above description

fDeadBandOutput: see above description

fInnerWindow : see above description

fOuterWindow : see above description

fOutMaxLimit : Upper limit at which integration is halted and to which the output is limited (ARW measure). Reaching this limit is indicated by a TRUE at the bARWActive output.

fOutMinLimit : Lower limit at which integration is halted and to which the output is limited (ARW measure). Reaching this limit is indicated by a TRUE at the bARWActive output.

bPInTheFeedbackPath : Input value of the P element can be selected with this input (see functional diagram).

bDInTheFeedbackPath : Input value of the D element can be selected with this input (see functional diagram).

bARWOnIPartOnly: If this parameter is FALSE (default setting), integration of the I component is halted if the complete controller output reaches the upper or lower limit.
If TRUE, the integration is halted if the I component (the output of the integrator) reaches a limit. (Cf. functional diagram)

Requirements