[CHARFNCT] Characteristics with linearization

The CHAR function block uses a characteristic curve (function) to calculate an output value for each input value.

The CHARFNCT function block uses a characteristic curve (function) to calculate an output value for each input value.

This characteristic curve is defined by entering auxiliary points [Xn],[Yn]. The type of interpolation between the auxiliary points can be selected:

Functioning

CHARFNCT calculates the corresponding function from the parameterized characteristic curve [X1],[Y1]...[Xn],[Yn] and the defined type of interpolation [ChlinMod].

This function is used to calculate the output value [Out] for each input value [In].

Outside the function range defined by the auxiliary points, the output value corresponds to either the first or last auxiliary value.

Types of interpolation

[ChlinMod] = 1 (Linear):

This function carries out linear interpolation (straight-line) between the auxiliary points [Xn],[Yn].

[ChlinMod] = 2 (Smooth):

This function carries out smoothed interpolation (curves) between the auxiliary points [Xn],[Yn].

[ChlinMod] = 3 (Linear inverse):

Inverted function of [ChlinMod] = Linear

The characteristic curve must not be fully monotonous (rising or falling, not constant, not rising, and not falling).

[ChlinMod] = 4 (Smooth inverse):

Inverted function of [ChlinMod] = Smoothed

The characteristic curve must not be fully monotonous (rising or falling, not constant, not rising, and not falling).

[ChlinMod] = 5 (Stepped):

For the input value, point CHARFNCT searches for the next smaller auxiliary point [Xi] and sends its function value [Yi] to the output. When input value [In] is less than [X1], [Y1] is issued. The stepping function is given as a number of [NumVldXY] value pairs [X], [Y] (number of auxiliary points). There is not interpolation between the auxiliary points.

Pins

Input

Description

Data type

Default value
Value range

In

Input

Real

0.0

ChlinMod

Characteristics and linearization mode

Integer

1: Linear

1: Linear - This function carries out linear interpolation (straight-line) between the auxiliary points [Xn],[Yn].
2: Smooth - This function carries out smoothed interpolation (curves) between the auxiliary points [Xn],[Yn].
3: Linear inverse - The characteristic curve must be strictly monotonous (rising or falling, not constant, not rising, and not falling).
4: Smooth inverse - The characteristic curve must be fully monotonous (rising or falling, not constant, not rising, and not falling).
5: Stepped - For input value [In], CHARFNCT searches the next smaller auxiliary point [Xi] and issues its function value [Yi] at the output. When input value [In] is less than [X1], [Y1] is issued. The stepping function is given as a number of [NumVldXY] value pairs [X], [Y] of the auxiliary points. There is not interpolation between the auxiliary points.

NumVldXY

Number of valid sample points X, Y

Integer

10

2…10

X1…X10

Auxiliary point X1…Auxiliary point X10

Required value.

Real

0.0

Y1…Y10

Auxiliary value Y1…Auxiliary value Y10

Required value.

Real

0.0

 

Output

Description

Data type

Default value
Value range

Out

Output

Real

0.0

Application

[ChlinMod] = 1 (Linear) and [ChlinMod] = 2 (Smoothed):

This block typically is used to correct the room setpoint in dependence of the outside air temperature in air conditioning plants (summer/winter compensation).

[ChlinMod] = 3 (Linear inverted) and [ChlinMod] = 4 (Smoothed inverted):

Compensation of non-linear characteristics in air handling plants to improve overall control response represents a typical application.

Troubleshooting

CHARFNCT generates an error message in three cases. In these cases, [Out] = 0 is the output value.

  • If the number of auxiliary points is smaller than 2 and greater than 10.
  • If the auxiliary point X[i] >= X[i+1].
  • If the characteristic curve entered for "inverse linear" or "Smoothed inverted" interpolation is not monotonous (rising or falling, not constant, not rising, and not falling at the same time).
Application example

Linearization of the characteristic curve of the non-linearity between damper position < > volume flow of an air damper:

The manipulated variable of the volume flow controller influences the non-linear actuating device "air damper". Control chain "controller + linearization + air damper" exhibits a near-linear response, as inverting the characteristic curve compensates for non-linearity.