Description

Function description

The function block replicates a three-point step controller, and exhibits a PD-like behavior due to a dynamic feedback path.

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.

and can be configured as additional parameters.

Properties

The function block SCON3 has the following properties:

Reset and automatic operating modes
One internal feedback path (delay 1st Order)

Representation in FBD

Representation:

Representation in LD

Representation:

Representation in IL

Representation:

CAL SCON3_Instance (SP:=SetpointInput, PV:=ProcessVariable, 
    PARA:=Parameters, R:=ResetModeFlag, 
    Y_POS=>PosManipulated_ERR_EFF, 
    Y_NEG=>NegManipulated_ERR_EFF, 
    ERR_EFF=>EffectiveError)

Representation in ST

Representation:

SCON3_Instance (SP:=SetpointInput, PV:=ProcessVariable, 
    PARA:=Parameters, R:=ResetModeFlag, 
    Y_POS=>PosManipulated_ERR_EFF, 
    Y_NEG=>NegManipulated_ERR_EFF, 
    ERR_EFF=>EffectiveError) ;

SCON3 parameter description

Description of input parameters:

Parameter	Data type	Description
SP	REAL	Setpoint input
PV	REAL	Actual value input
PARA	Para_SCON3	Parameter
R	BOOL	"1" = Reset mode

Description of output parameters:

Parameter	Data type	Description
Y_POS	BOOL	"1" = positive manipulated variable at output ERR_EFF
Y_NEG	BOOL	"1" = negative manipulated variable at output ERR_EFF
ERR_EFF	REAL	Effective switch value

Parameter description Para_SCON3

Data structure description

Element	Data type	Description
gain	REAL	Proportional action coefficient (gain)
ti	TIME	Integral time
t_proc	TIME	Nominal actuating time of the controlled valve
hys	REAL	Hysteresis from three point switch
db	REAL	Dead zone

Runtime error

If hys > 2 * db, an error message is shown in the diagnostic viewer (Tools → Diagnostic viewer).

NOTE: For a list of all block error codes and values, see CLC_PRO.

Warning

In the following cases, a warning is shown in the diagnostic viewer (Tools → Diagnostic viewer):

gain ≤ 0 : The controller operates without feedback response.
ti = 0: The controller operates without feedback response.
t_proc = 0: The controller operates with a predetermined value of t_proc = 60 s.

NOTE: For a list of all block error codes and values, see CLC_PRO.

REAL

The REAL type is encoded in a 32 bit format.

The possible value ranges are shown in the figure below:

When a result is:

between -1,175494e-38 and 1,175494e-38, it is considered to be a DEN;
less than -3.402824e+38, the symbol -INF (for - infinity) is displayed;
greater than +3.402824e+38, the symbol INF (for + infinity) is displayed;
undefined (square root of a negative number), the symbolNAN is displayed.

NOTE: The IEC 559 standard defines two classes of NAN: the silent NAN (QNAN) and the signaling NAN (SNAN). A QNAN is a NAN with a most significant fraction bit while an SNAN is a NAN without a most significant fraction bit (bit number 22). QNANs can be propagated via most arithmetic operations without throwing an exception. As for SNANs, they generally indicate n invalid operation when they are used as operands in arithmetic operations (see %SW17 and %S18).

NOTE: When a DEN (non-standardized number) is used as an operand, the result is not significant.