Description

Function description

This function block is suitable for simple three-point step-action controls.

Control of the actuator proceeds according to the direction of the actual/setpoint value deviation in relation to the upper and lower threshold value. The control of the threshold value describes a configurable hysteresis.

This controller can also be used for temperature regulation. A traditional controller (such as a PI_B controller), which a function block such as the PWM1 should be switched to is preferable for complex regulation.

and can be configured as additional parameters.

Properties

The control block has the following properties:

Limiting the setpoint between pv_inf and pv_sup
The control input values (actual value, setpoint, and corresponding parameters) are expressed in physical units.

Representation in FBD

Representation:

Representation in LD

Representation:

Representation in IL

Representation:

CAL STEP3_Instance (PV:=ProcessValue, SP:=SetPoint,  
    MAN_AUTO:=OperatingMode, PARA:=Parameters, 
    OUT_NEG=>LogicOutputNegativ, 
    OUT_POS=>LogicOutputPositiv, DEV=>Deviation, 
    MA_O=>CurrentOperatingMode, STATUS=>StatusWord)

Representation in ST

Representation:

STEP3_Instance (PV:=ProcessValue, SP:=SetPoint,  
    MAN_AUTO:=OperatingMode, PARA:=Parameters, 
    OUT_NEG=>LogicOutputNegativ, 
    OUT_POS=>LogicOutputPositiv, DEV=>Deviation, 
    MA_O=>CurrentOperatingMode, STATUS=>StatusWord) ;

Parameter description STEP3

Input parameter description:

Parameter	Data type	Meaning
PV	REAL	Process value
SP	REAL	Setpoint
MAN_AUTO	BOOL	Controller operating mode: "1" : Automatic mode "0" : Halt mode
PARA	Para_STEP3	Parameter

Output parameter description:

Parameter	Data type	Meaning
OUT_NEG	BOOL	Logical output: is set to 1 for negative deviations
OUT_POS	BOOL	Logical output: is set to 1 for positive deviations
DEV	REAL	Deviation (PV-SP)
MA_O	BOOL	Current operating mode of the function block (0: Halt, 1: Automatic)
STATUS	WORD	Status word

Parameter description Para_STEP3

Data structure description

Element	Data type	Meaning
dev_ll	REAL	Lower deviation threshold (≤ 0)
dev_hl	REAL	Upper deviation threshold (≤ 0)
hyst	REAL	Hysteresis
pv_inf	REAL	Lower limit of the process value range
pv_sup	REAL	Upper limit of the process value range

Base	Lower limit	Upper limit
Hexadecimal	16#0	16#FFFF
Octal	8#0	8#177777
Binary	2#0	2#1111111111111111

Data	Representation in one of the bases
0000000011010011	16#D3
1010101010101010	8#125252
0000000011010011	2#11010011

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.