Create a Logarithm Lookup Table

Suppose you want to approximate the common logarithm (base 10) over the input range [1, 10] without performing an expensive computation. You can perform this approximation using a lookup table block as described in the following procedure. (If you want to skip the procedure, you can open the model directly.)

1. Copy the following blocks to a Simulink^® model:

   • One Constant block to input the signal, from the Sources library

   • One n-D Lookup Table block to approximate the common logarithm, from the Lookup Tables library

   • One Display block to display the output, from the Sinks library

2. Assign the table data and breakpoint vector to the n-D Lookup Table block:

   1. In the Number of table dimensions field, enter 1.

   2. In the Table data field, enter [0 .301 .477 .602 .699 .778 .845 .903 .954 1].

   3. In the Breakpoints 1 field, enter [1:10].

   4. Click Apply.

   The dialog box looks something like this:

   

3. Double-click the Constant block to open the parameter dialog box, and change the Constant value parameter to 5. Click OK to apply the changes and close the dialog box.

4. Connect the blocks as follows.

   

5. Start simulation.

   The following behavior applies to the n-D Lookup Table block.

   Value of the Constant Block	Action by the n-D Lookup Table Block	Example of Block Behavior
   		Input Value	Output Value
   Equals a breakpoint	Returns the corresponding output value	5	0.699
   Falls between breakpoints	Linearly interpolates the output value using neighboring breakpoints	7.5	0.874
   Falls outside the range of the breakpoint vector	Linearly extrapolates the output value from a pair of values at the end of the breakpoint vector	10.5	1.023

   For the n-D Lookup Table block, the default settings for Interpolation method and Extrapolation method are both Linear.