FB_CTRL_PID_SPLITRANGE

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 behaviours are different. A system with one actuator for heating and another actuator for cooling would be a typical application. To optimise the regulation of such an arrangement, it is possible to switch between two PID parameter records. Switching between the parameter records is implemented in such a way that the control value remains continuous even as the parameter records are changed.

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

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.

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

fSetpointValue : Set value of the controlled variable.

fActualValue : Actual value of the controlled variable.

eParameterRecord : Index of the active parameter record

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

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

eMode : Input that specifies the block's operating mode.

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

VAR_OUTPUT

VAR_OUTPUT
    fOutPos             : FLOAT;
    fOutNeg             : FLOAT;
    fOut            : FLOAT;

    bARWActive          : BOOL  := FALSE;       (* ARW active ? [TRUE/FALSE] -> freeze I-part *)
    bParameterChangeActive  : BOOL;

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

END_VAR

fOutPos : Output of the PID element when the control value is positive. A zero is output otherwise.

fOutNeg : Output of the PID element when the control value is negative. A zero is output otherwise.

fOut : Output of the PID element.

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

bParameterChangeActive : A TRUE at this output indicates that the change from one parameter record to the other is in progress.

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_SPLITRANGE_PARAMS;
END_VAR

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

TYPE
ST_CTRL_PID_SPLITRANGE_PARAMS :
STRUCT
    tCtrlCycleTime         : TIME     := T#0ms; (*
controller cycle time [TIME] *)
    tTaskCycleTime         : TIME     := T#0ms; (* task
cycle time [TIME] *)
    fKp_heating        : FLOAT    := 0;     (*
proportional gain *)
    tTn_heating        : TIME     := T#0ms; (* Tn
*)
    tTv_heating        : TIME     := T#0ms; (* Tv
*)
    tTd_heating        : TIME     := T#0ms; (* Td
*)
    fKp_cooling        : FLOAT    := 0;     (*
proportional gain *)
    tTn_cooling        : TIME     := T#0ms; (* Tn
*)
    tTv_cooling        : TIME     := T#0ms; (* Tv
*)
    tTd_cooling        : TIME     := T#0ms; (* Td
*)
    nParameterChangeCycleTicks : INT;
    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.

Area eCTRL_PARAMETER_RECORD_HEATING:

fKp_heating : Controller amplification / controller coefficient

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

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

tTd_heating : Damping time

Area eCTRL_PARAMETER_RECORD_COOLING:

fKp_cooling : Controller amplification / controller coefficient

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

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

tTd_cooling : Damping time

nParameterChangeCycleTicks The number of task cycles over which the change from one parameter set to the other takes place.

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 (the default 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)

Requirements