FB_CTRL_PID

FB_CTRL_PID 1:

The function block provides a PID transfer element in the functional diagram.

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 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 whose differential component is only applied to the control value (bDInTheFeedbackPath := TRUE) avoids 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 11:

Step response

FB_CTRL_PID 12:

ARW

FB_CTRL_PID 13:

VAR_INPUT

VAR_INPUT
    fSetpointValue    : FLOAT;
    fActualValue      : FLOAT;
    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
fManSyncValue	FLOAT	Input, to whose value it is possible to set the internal state of the PID-element.
bSync	BOOL	A rising edge at this input sets the PID-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
    fOut          : FLOAT;
    bARWactive    : BOOL;
    eState        : E_CTRL_STATE;
    eErrorId      : E_CTRL_ERRORCODES;
    bError        : BOOL;
END_VAR

Name	Type	Description
fOut	FLOAT	Output of the PID element
bARWactive	BOOL	A TRUE at this output indicates that the PID-element is being restricted.
eState	E_CTRL_STATE	State of the function block
eErrorId	E_CTRL_ERRORCODES	Supplies the error number when the output `bError` is set.
bError	BOOL	Becomes TRUE, as soon as an error occurs.

VAR_IN_OUT

VAR_IN_OUT
    stParams        : ST_CTRL_PID_PARAMS;
END_VAR

Name	Type	Description
stParams	ST_CTRL_PID_PARAMS	Parameter structure of the PID element

stParams consists of the following elements:

TYPE
ST_CTRL_PID_PARAMS :
STRUCT
    tCtrlCycleTime         : TIME := T#0ms;
    tTaskCycleTime         : TIME := T#0ms;
    fKp                    : FLOAT := 0; 
    tTn                    : TIME := T#0ms; 
    tTv                    : TIME := T#0ms; 
    tTd                    : TIME := T#0ms; 
    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.
fKp	FLOAT	Controller amplification / controller coefficient
tTn	TIME	Integral action time; if this is parameterized to T#0s, the I component is deactivated.
tTv	TIME	Derivative action time; if this is parameterized to T#0s, the D component is deactivated.
tTd	TIME	Damping time
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.
bPInTheFeedbackPath	BOOL	Input value of the internal P-element can be selected with this input (see functional diagram). Standard setting: FALSE
bDInTheFeedbackPath	BOOL	Input value of the internal D-element can be selected with this input (see functional diagram). Standard setting: FALSE
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.)