Using Simulink

Look-Up Table

Perform piecewise linear mapping of the input.

Library

Functions & Tables

Description

The Look-Up Table block maps an input to an output using linear interpolation of the values defined in the block's parameters.

You define the table by specifying (either as row or column vectors) the Vector of input values and Vector of output values parameters. The block produces an output value by comparing the block input with values in the input vector:

• If it finds a value that matches the block's input, the output is the corresponding element in the output vector.
• If it does not find a value that matches, it performs linear interpolation between the two appropriate elements of the table to determine an output value. If the block input is less than the first or greater than the last input vector element, the block extrapolates using the first two or the last two points.

To map two inputs to an output, use the Look-Up Table (2-D) block. For more information, see Look-Up Table (2-D).

To create a table with step transitions, repeat an input value with different output values. For example, these input and output parameter values create the input/output relationship described by the plot that follows:

Vector of input values:   [-2 -1 -1  0 0 0 1 1 2]
Vector of output values:  [-1 -1 -2 -2 1 2 2 1 1]

This example has three step discontinuities: at u = -1, 0, and +1.

When there are two points at a given input value, the block generates output according to these rules:

• When u is less than zero, the output is the value connected with the point first encountered when moving away from the origin in a negative direction. In this example, when u is -1, y is -2, marked with a solid circle.
• When u is greater than zero, the output is the value connected with the point first encountered when moving away from the origin in a positive direction. In this example, when u is 1, y is 2, marked with a solid circle.
• When u is at the origin and there are two output values specified for zero input, the actual output is their average. In this example, if there were no point at u = 0 and y = 1, the output would be 0, the average of the two points at u = 0. If there are three points at zero, the block generates the output associated with the middle point. In this example, the output at the origin is 1.

The Look-Up Table block icon displays a graph of the input vector versus the output vector. When a parameter is changed on the block's dialog box, the graph is automatically redrawn when you press the Apply or Close button.

Data Type Support

A Look-Up Table block accepts and outputs signals of type double.

Parameters and Dialog Box

Vector of input values

The vector of values containing possible block input values. This vector must be the same size as the output vector. The input vector must be monotonically increasing.

Vector of output values

The vector of values containing block output values. This vector must be the same size as the input vector.

Characteristics

Direct Feedthrough	Yes
Sample Time	Inherited from driving block
Scalar Expansion	No
Dimensionalized	Yes
Zero Crossing	No

Logical Operator

Look-Up Table (2-D)