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Lookup Table Dynamic - Approximate one-dimensional function using dynamic table

Library

Lookup Tables

Description

How This Block Differs from Other Lookup Table Blocks

The Lookup Table Dynamic block computes an approximation to a function y = f(x) using xdat and ydat vectors. The lookup method can use interpolation, extrapolation, or the original values of the input.

Using the Lookup Table Dynamic block, you can change the table data without stopping the simulation. For example, you can incorporate new table data if the physical system you are simulating changes.

Inputs for Breakpoint and Table Data

The xdat vector is the breakpoint data, which must be strictly monotonically increasing. The value of the next element in the vector must be greater than the value of the preceding element after conversion to a fixed-point data type. Due to quantization, the xdat vector can be strictly monotonic for a floating-point data type, but not after conversion to a fixed-point data type.

The ydat vector is the table data, which is an evaluation of the function at the breakpoint values.

You define the lookup table by feeding xdat and ydat as 1-by-n vectors to the block. To reduce ROM usage by the generated code for this block, you can use different data types for the xdat breakpoint data and the ydat table data. However, these restrictions apply:

The xdat breakpoint data and the x input vector must have the same sign, the same bias, and the same fractional slope. Also, the precision and range for the x input vector must be greater than or equal to the precision and range for the xdat breakpoint data.
The ydat table data and the y output vector must have the same sign, the same bias, and the same fractional slope.

Tip Evenly-spaced breakpoints can make Real-Time Workshop generated code division-free. For more information, see fixpt_evenspace_cleanup in the Simulink documentation and Identify questionable fixed-point operations in the Real-Time Workshop documentation.

How the Block Generates Output

The block uses the input values to generate output using the method you select for the Lookup Method parameter:

Lookup Method Action by the Block

Interpolation-Extrapolation

Performs linear interpolation and extrapolation of the inputs.

If the input matches a breakpoint, the output is the corresponding element in the table data.
If the input does not match a breakpoint, the block performs linear interpolation between two elements of the table to determine the output. If the input falls outside the range of breakpoint values, the block extrapolates using the first two or last two points.

Note If you select this lookup method, Real-Time Workshop software cannot generate code for this block.

Interpolation-Use End Values

Performs linear interpolation but does not extrapolate outside the end points of the breakpoint data. Instead, the block uses the end values.

Use Input Nearest

Finds the element in xdat nearest the current input. The corresponding element in ydat is the output.

Use Input Below

Finds the element in xdat nearest and below the current input. The corresponding element in ydat is the output. If there is no element in xdat below the current input, then the block finds the nearest element.

Use Input Above

Finds the element in xdat nearest and above the current input. The corresponding element in ydat is the output. If there is no element in xdat above the current input, then the block finds the nearest element.

Note The Use Input Nearest, Use Input Below, and Use Input Above methods perform the same action when the input x matches a breakpoint value.

Data Type Support

The Lookup Table Dynamic block accepts signals of any numeric data type supported by Simulink software, including fixed-point data types.

Parameters and Dialog Box

The Main pane of the Lookup Table Dynamic block dialog box appears as follows:

Lookup Method: Specify the lookup method. For details, see How the Block Generates Output.

The Signal Attributes pane of the Lookup Table Dynamic block dialog box appears as follows:

Output data type

Specify the output data type. You can set it to:

A rule that inherits a data type, for example, Inherit: Inherit via back propagation
The name of a built-in data type, for example, single
The name of a data type object, for example, a Simulink.NumericType object
An expression that evaluates to a data type, for example, fixdt('double')

Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output data type parameter.

See Specifying Block Output Data Types for more information.

Lock output data type setting against changes by the fixed-point tools

Select to lock the output data type setting of this block against changes by the Fixed-Point Tool and the Fixed-Point Advisor. For more information, see Fixed-Point Tool and Fixed-Point Advisor in the Simulink Fixed Point documentation.

Integer rounding mode

Select the rounding mode for fixed-point operations. For more information, see Rounding in the Simulink Fixed Point User's Guide.

Saturate to max or min when overflows occur

If selected, fixed-point overflows saturate. Otherwise, they wrap.

Examples

For an example of entering breakpoint and table data, see Entering Data Using the Lookup Table Dynamic Block's Inports in the Simulink User's Guide.

For an example that illustrates the lookup methods that this block supports, see Example Output for Lookup Methods in the Simulink User's Guide.

Characteristics

Direct Feedthrough	Yes
Scalar Expansion	No
Zero-Crossing Detection	No