FB_CTRL_PID_SPLITRANGE

FB_CTRL_PID_SPLITRANGE 1:

The function block provides an extended PID transfer element in the functional diagram. This controller enables switching between two parameter sets when control is active.

Description

This function block is an extension of FB_CTRL_PID, 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. Parameter set switchover is implemented in such a way that the control value remains continuous even as the parameter sets are changed.

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

FB_CTRL_PID_SPLITRANGE 3:

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:

bPInTheFeedbackPath	bDInTheFeedbackPath	G(s)
false	false
true	false
false	true
true	true

with:

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

FB_CTRL_PID_SPLITRANGE 11:

Step response

FB_CTRL_PID_SPLITRANGE 12:

ARW

FB_CTRL_PID_SPLITRANGE 13:

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

Name	Type	Description
fSetpointValue	FLOAT	Setpoint of the controlled variable
fActualValue	FLOAT	Actual value of the controlled variable
eParameter Record	E_CTRL_ PARAMETER_ RECORD	Index of the active parameter set
fManSyncValue	FLOAT	Input with which the PI element can be set.
bSync	BOOL	A rising edge at this input sets the PI-element to the value "fManSyncValue".
eMode	E_CTRL_MODE	Input that specifies the operation mode of the function block.
bHold	BOOL	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
    fOutPos                   : FLOAT;
    fOutNeg                   : FLOAT;
    fOut                      : FLOAT;
    bARWActive                : BOOL := FALSE;
    bParameterChangeActive    : BOOL;
    eState                    : E_CTRL_STATE;
    bError                    : BOOL;
    eErrorId                  : E_CTRL_ERRORCODES;
END_VAR

Name	Type	Description
fOutPos	FLOAT	Output of the PID-element when the control value is positive. A zero is output otherwise.
fOutNeg	FLOAT	Output of the PID-element when the control value is negative. A zero is output otherwise.
fOut	FLOAT	Output of the PID element
bARWActive	BOOL	A TRUE at this output indicates that the PID-element is being restricted.
bParameter ChangeActive	BOOL	A TRUE at this output indicates that the change from one parameter set to the other is in progress.
eState	E_CTRL_STATE	State of the function block
bError	BOOL	Becomes TRUE, as soon as an error occurs.
eErrorId	E_CTRL_ERRORCODES	Supplies the error number when the output `bError` is set.

VAR_IN_OUT

VAR_IN_OUT
    stParams        : ST_CTRL_PID_SPLITRANGE_PARAMS;
END_VAR

Name	Type	Description
stParams	ST_CTRL_PID_ SPLITRANGE_ PARAMS	Parameter structure of the PID element

stParams consists of the following elements:

TYPE
ST_CTRL_PID_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;
    fOutMaxLimit                  : FLOAT := 1E38;
    fOutMinLimit                  : FLOAT := -1E38;
    bPInTheFeedbackPath           : BOOL;
    bDInTheFeedbackPath           : BOOL;
    bARWOnIPartOnly               : BOOL;
END_STRUCT
END_TYPE

Name	Type	Description
tCtrlCycleTime	TIME	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	TIME	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.
Range eCTRL_PARAMETER_RECORD_HEATING:
fKp_heating	FLOAT	Controller amplification / controller coefficient
tTn_heating	TIME	Integral action time: The I-component is deactivated if this is parameterized as T#0s.
tTv_heating	TIME	Derivative action time: The D-component is deactivated if this is parameterized as T#0s.
tTd_heating	TIME	Damping time
Range eCTRL_PARAMETER_RECORD_COOLING:
fKp_cooling	FLOAT	Controller amplification / controller coefficient
tTn_cooling	TIME	Integral action time: The I-component is deactivated if this is parameterized as T#0s.
tTv_cooling	TIME	Derivative action time: The D-component is deactivated if this is parameterized as T#0s.
tTd_cooling	TIME	Damping time
nParameter ChangeCycle Ticks	INT	The number of task cycles over which the change from one parameter set to the other takes place.
fOutMaxLimit	FLOAT	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	FLOAT	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.
bPInTheFeed backPath	BOOL	Input value of the P-element can be selected with this input (see functional diagram).
bDInTheFeed backPath	BOOL	Input value of the D-element can be selected with this input (see functional diagram).
bARWOnIPartOnly	BOOL	If this parameter is FALSE (the standard setting), the integration of the I-component is halted if the complete controller output reaches the upper or lower limit. If it is TRUE, the integration is halted if the I-component (the output of the integrator) reaches some limit. (Cf. functional diagram.)