Description

Function description

The function block RATIO carries out ratio control when it is attached to a controller.

The aim of ratio control is to establish a ratio of one process variable PV (controlled variable) to another PV_TRACK (reference variable). The role of the RATIO function block is to calculate the Control setpoint corresponding to the control variable.

and can be configured as additional parameters.

Properties

The function block contains the following properties:

The ratio can be controlled remotely (K) or (RK)
Upper and lower threshold for K or RK
Upper and lower threshold for the calculated Setpoint SP
Calculation of the real ratio: KACT = (PV - bias) / PV_TRACK

Formula

Calculation of the control setpoint

Representation in FBD

Representation:

Representation in LD

Representation:

Representation in IL

Representation:

CAL RATIO_Instance (PV:=ProcessVariable, 
    PV_TRACK:=ControllingProcessVariable, 
    RK:=ExternalRatioCoefficient, 
    K_RK:=CoefficientTypeFlag, K:=InternalRatioCoefficient, 
    PARA:=Parameters, KACT=>RealRatioCoefficient, 
    SP=>CalculatedOutput, STATUS=>StatusWord)

Representation in ST

Representation:

RATIO_Instance (PV:=ProcessVariable, 
    PV_TRACK:=ControllingProcessVariable, 
    RK:=ExternalRatioCoefficient, 
    K_RK:=CoefficientTypeFlag, K:=InternalRatioCoefficient, 
    PARA:=Parameters, KACT=>RealRatioCoefficient, 
    SP=>CalculatedOutput, STATUS=>StatusWord) ;

Parameter description RATIO

Input parameter description:

Parameter	Data type	Meaning
PV	REAL	Process value regulated by the control loop (only used to calculate KACT)
PV_TRACK	REAL	Reference variable of the control loop
RK	REAL	Remote relationship coefficient
K_RK	BOOL	Coefficient type for ratio used "1": remote ratio RK "0": local ratio K
K	REAL	Coefficient for local ratio
PARA	Para_RATIO	Parameter

Output parameter description:

Parameter	Data type	Meaning
KACT	REAL	Coefficient for real ratio
SP	REAL	Calculated output
STATUS	WORD	Status word

Parameter description Para_RATIO

Data structure description

Element	Data type	Meaning
k_min	REAL	Lower threshold with K or RK
k_max	REAL	Upper threshold with K or RK
sp_min	REAL	Lower threshold of the calculated output SP
sp_max	REAL	Upper threshold of the calculated output SP
bias	REAL	Offset coefficient

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.