FB_CTRL_PID_EXT_SPLITRANGE

The function block provides an extended PID transfer element in the functional diagram. With this controller it is possible to switch between two different parameter sets while the regulation is active. The functionalities of the inner and outer windows and of the input and output dead bands are available in addition.

Description

This function block is an extension of FB_CTRL_PID_EXT, which means that the controller can be used to control systems with two controlled devices for which the transfer behavior are different. A system with one actuator for heating and another actuator for cooling would be a typical application. To optimize the regulation of such an arrangement, it is possible to switch between two PID parameter sets. The parameter set changeover is carried out in such a way that a continuous control value is maintained even during the changeover.

The switching algorithm calculates a linear, time-dependent transition between the two parameter sets. The nParameterChangeCycleTicks parameter can be used to specify the number of task cycles over which the continuous change between the two parameter sets takes place.

Transfer function

The following transfer function can be specified if the boolean inputs bPInTheFeedbackPath and bDInTheFeedbackPath are FALSE. Otherwise the transfer function only describes part of the transfer behavior of the function block.

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

Additional functions

Switching off the I-component in the Outer Window

Integration of the control deviation is halted if the control deviation is greater than the fOuterWindow parameter. In this way it is possible to prevent an extremely large I-component from developing if the control 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 > 0 is set, the output is set to zero when it is within the range of [ -fDeadBandOutput ... fDeadBandOutput ].

Input dead band

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

Step response

ARW

VAR_INPUT

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

VAR_OUTPUT

VAR_OUTPUT
    fOutPos                   : FLOAT;
    fOutNeg                   : FLOAT;
    fOut                      : FLOAT;
    bARWActive                : BOOL := FALSE;
    bParameterChangeActive    : BOOL;
    bError                    : BOOL;
    eErrorId                  : E_CTRL_ERRORCODES;
END_VAR

VAR_IN_OUT

VAR_IN_OUT
    stParams    : ST_CTRL_PID_EXT_SPLITRANGE_PARAMS;
END_VAR 

stParams consists of the following elements:

TYPE
ST_CTRL_PID_EXT_SPLITRANGE_PARAMS :
STRUCT
    tCtrlCycleTime              : TIME := T#0ms;
    tTaskCycleTime              : TIME := T#0ms;
    fKp_heating                 : FLOAT := 0;
    tTn_heating                 : TIME := T#0ms;
    tTv_heating                 : TIME := T#0ms;
    tTd_heating                 : TIME := T#0ms; 
    fKp_cooling                 : FLOAT := 0; 
    tTn_cooling                 : TIME := T#0ms; 
    tTv_cooling                 : TIME := T#0ms; 
    tTd_cooling                 : TIME := T#0ms; 
    nParameterChangeCycleTicks  : INT;
    fDeadBandInput              : REAL := 0.0;
    fDeadBandOutput             : REAL := 0.0;
    fInnerWindow                : REAL := 0.0;
    fOuterWindow                : REAL := 0.0; 
    fOutMaxLimit                : FLOAT := 1E38; 
    fOutMinLimit                : FLOAT := -1E38; 
    bPInTheFeedbackPath         : BOOL;
    bDInTheFeedbackPath         : BOOL;
    bARWOnIPartOnly             : BOOL;
END_STRUCT
END_TYPE