Description
Original instructions
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Function description
The function block displays a cascade-controller, consisting of a P-master controller and a PI-sub controller.
A system deviation ERR is formed by the difference between the reference variable SP and the controlled variable PV.
The master controller generates a sub controller setpoint value SP2 through this system deviation. Due to the difference between SP2 and PV2 the sub controller generates the manipulated variable Y.
NOTE: This function block performs an internal initialization in the first program cycle after a warm start or cold start (e.g. application download or power cycle) of the PLC program.
Due to this, you have to make sure that the function block is invoked in the first program cycle. In case of inkoving the function block in a later program cycle, the internal initialization will not be performed and the ouputs may deliver wrong values.
WARNING
UNEXPECTED OUTPUT BEHAVIOUR
Make sure that the function block is always invoked in the first program cycle.
Failure to follow these instructions can result in death, serious injury, or equipment damage.
EN and ENO can be configured as additional parameters.
Properties
The function block has the following properties:
Transfer function
The transmission function for the controller says:
Controller
Transfer function
Master controller (P-controller)
Sub controller (PI controller)
Proportional action coefficient
The proportional action coefficient is determined as follows:
Representation in FBD
Representation:
Representation in LD
Representation:
Representation in IL
Representation: 
CAL PPI_Instance (SP:=ReferenceVariable,
    PV:=ControlledVariable, PV2:=AuxiliaryControlVariable,
    MODE:=OperatingModes, PARA:=Parameter,
    YMAN:=ManualValue, SP_FIX:=FixedValue, OFF:=Offset,
    Y:=ManipulatedVariable, ERR=>OutputSystemDeviation,
    SP2=>SubControllerSetpointValue,
    STATUS=>StatusOfOutput_Y)
Representation in ST
Representation: 
PPI_Instance (SP:=ReferenceVariable,
    PV:=ControlledVariable, PV2:=AuxiliaryControlVariable,
    MODE:=OperatingModes, PARA:=Parameter,
    YMAN:=ManualValue, SP_FIX:=FixedValue, OFF:=Offset,
    Y:=ManipulatedVariable, ERR=>OutputSystemDeviation,
    SP2=>SubControllerSetpointValue,
    STATUS=>StatusOfOutput_Y) ;
PPI parameter description
Description of input parameters:
Parameter
Data type
Description
SP
REAL
Reference variable for the master controller
PV
REAL
Controlled variable for the master controller
PV2
REAL
Controlled variable for the sub controller (auxiliary control variable)
MODE
Mode_PPI
Operating mode
PARA
Para_PPI
Parameter
YMAN
REAL
Manual value (of output Y)
SP_FIX
REAL
Fixed value (reference variable as manual value for the sub controller)
OFF
REAL
Offset at the output of the P-controller
Description of input / output parameters:
Parameter
Data type
Description
Y
REAL
Manipulated variable
Description of output parameters:
Parameter
Data type
Description
ERR
REAL
System deviation
SP2
REAL
Sub controller setpoint value
STATUS
Stat_MAXMIN
Y output status
Parameter description Mode_PPI
Data structure description
Element
Data type
Description
man
BOOL
"1": Manual mode
halt
BOOL
"1": Halt mode
fix
BOOL
"1": Fixed setpoint control
Parameter description Para_PPI
Data structure description
Element
Data type
Description
gain1
REAL
Proportional action coefficient (gain) for P controller
ti
TIME
PI controller reset time
gain2
REAL
Proportional action coefficient (gain) for PI controller
ymax
REAL
Upper limit
ymin
REAL
Lower limit
Parameter description Stat_MAXMIN
Data structure description
Element
Data type
Description
qmax
BOOL
"1" = Y has reached the upper control limit
qmin
BOOL
"1" = Y has reached the lower control limit
Runtime error
An error message is returned if
NOTE: For a list of all block error codes and values, see CLC_PRO.