Simulink Reference

Look-Up Table (2-D)

Approximate a two-dimensional function using a selected look-up method

Library

Simulink Look-Up Tables and Fixed-Point Blockset LookUp

Description

The Look-Up Table (2-D) block computes an approximation to some function z=f(x,y) given x, y, z data points.

The Row index input values parameter is a 1-by-m vector of x data points, the Column index input values parameter is a 1-by-n vector of y data points, and the Matrix of output values parameter is an m-by-n matrix of z data points. Both the row and column vectors must be monotonically increasing. These vectors must be strictly monotonically increasing in the following cases:

• The input and output data types are both fixed-point.
• The input and output data types are different.
• The lookup method is not Interpolation-Extrapolation.
• The matrix of output values is complex.
• Minimum, maximum, and overflow logging is on.

The block generates output based on the input values using one of these methods selected from the Look-up method parameter list:

• Interpolation-Extrapolation--This is the default method; it performs linear interpolation and extrapolation of the inputs.
• If the inputs match row and column parameter values, the output is the value at the intersection of the row and column.
• If the inputs do not match row and column parameter values, then the block generates output by linearly interpolating between the appropriate row and column values. If either or both block inputs are less than the first or greater than the last row or column values, the block extrapolates using the first two or last two points.
• Interpolation-Use End Values--This method performs linear interpolation as described above but does not extrapolate outside the end points of x and y. Instead, the end-point values are used.
• Use Input Nearest--This method does not interpolate or extrapolate. Instead, the elements in x and y nearest the current inputs are found. The corresponding element in z is then used as the output.
• Use Input Below--This method does not interpolate or extrapolate. Instead, the elements in x and y nearest and below the current inputs are found. The corresponding element in z is then used as the output. If there are no elements in x or y below the current inputs, then the nearest elements are found.
• Use Input Above--This method does not interpolate or extrapolate. Instead, the elements in x and y nearest and above the current inputs are found. The corresponding element in z is then used as the output. If there are no elements in x or y above the current inputs, then the nearest elements are found.

To avoid parameter saturation errors, the automatic scaling script autofixexp employs a special rule for the Look-Up Table (2-D) block. autofixexp modifies the scaling by using the output look-up values in addition to the logged minimum and maximum simulation values. The output look-up values are converted to the specified output data type. This prevents the data from being saturated to different values.

When the Show additional parameters check box is selected, some of the parameters that become visible are common to many blocks. For a detailed description of these parameters, refer to Block Parameters in the Fixed-Point Blockset documentation.

Parameters and Dialog Box

Row index input values

The row values for the table, entered as a vector. The vector values must increase monotonically.

Column index input values

The column values for the table, entered as a vector. The vector values must increase monotonically.

Matrix of output values

The table of output values. The matrix size must match the dimensions defined by the Row and Column parameters.

Show additional parameters

If selected, additional parameters specific to implementation of the block become visible as shown.

Look-up method

Specify the lookup method. See Description for a discussion of the options for this parameter.

Require all inputs to have same data type

Select to require all inputs to have the same data type.

Output data type mode

You can set the output signal to a built-in data type from this drop-down list, or you can choose the output data type and scaling to be the same as the input. Alternatively, you can choose to inherit the output data type and scaling by backpropagation. Lastly, if you choose Specify via dialog, the Output data type, Output scaling value, and Lock output scaling against changes by the autoscaling tool parameters become visible.

Output data type

Specify any data type, including fixed-point data types. This parameter is only visible if Specify via dialog is selected for the Output data type mode parameter.

Output scaling value

Set the output scaling using radix point-only or [Slope Bias] scaling. This parameter is only visible if Specify via dialog is selected for the Output data type mode parameter.

Lock output scaling against changes by the autoscaling tool

If selected, scaling of outputs is locked. This parameter is only visible if Specify via dialog is selected for the Output data type mode parameter.

Round integer calculations toward

Select the rounding mode for the fixed-point output.

Saturate on integer overflow

If selected, overflows saturate.

Examples

In this example, the block parameters are defined as

• Row:      [1 2]
  Column:   [3 4]
  Table:    [10 20; 30 40]
  

The first figure shows the block outputting a value at the intersection of block inputs that match row and column values. The first input is 1 and the second input is 4. These values select the table value at the intersection of the first row (row parameter value 1) and second column (column parameter value 4).

In the second figure, the first input is 1.7 and the second is 3.4. These values cause the block to interpolate between row and column values, as shown in the table at the left. The value at the intersection (28) is the output value.

Characteristics

Dimensionalized	Yes
Direct Feedthrough	Yes
Sample Time	Inherited from driving blocks
Scalar Expansion	Of one input if the other is a vector
Zero Crossing	No

See Also

Look-Up Table, Look-Up Table (n-D)

Look-Up Table

Look-Up Table (n-D)