Use Preelokup and n-Direct Lookup Table blocks in Simulink without interpolation

Hi,

I want to create a lookup table that based on some inputs, where the indeces of the lookup table are found and the field value is output.

I attach the lookup table I made with its corresponding breakpoints.

I am using two Preelokup blocks to find the indeces of this 2D table. These indices are used then as input to the Direct LookupTable block (see the attached block diagram picture).

My issue is that if I input e.g., 28 in "in1", the index found is correct. However, if I put a value within two breakpoint values (e.g., 30) it uses interpolation, flat or rounding, and I would like to avoid that. Is there a way where if the input value does not match with any of the breakpoints array it simply does not return any index or the Lookup table simply returns no value or a zero?

Thanks in advance

Answer 1

[Gayathri]

Hi @Adrian,

The 1-D, 2-D, and n-D "Lookup Table" blocks evaluate a sampled representation of a function in N variables where the function F can be empirical.Hence, there is no option as of now to return a value of zero, if the input values does not match the breakpoints array mentioned in the "Lookup Table". In such cases it would be better to use a "MATLAB Function" block where you can define the breakpoints and the table data. You can find the code as shown below.

function output = exactMatchLookup(in1, in2)
    breakpointsX=[29 45 65 78 99]; %fill in the breakpoints
    breakpointsY=[2 3 4 5 6]; %fill in the breakpoints
    tableData=[4 5 6 22 10;16 19 20 3 20;10 18 23 5 40;5 9 88 4 50;7 34 67 99 40];
    idxX=0;
    idxY=0;
    for i = 1:length(breakpointsX)
        if breakpointsX(i) == in1
            idxX = i;
            break; % Exit loop once the index is found
        end
    end
    
    % Check if in2 is in breakpointsY and find the index
    for j = 1:length(breakpointsY)
        if breakpointsY(j) == in2
            idxY = j;
            break; % Exit loop once the index is found
        end
    end
    % Check for exact match
    if idxX==0 || idxY==0
        output = 0; % Default value if no exact match
    else
        output = tableData(idxX, idxY); % Lookup table value
    end
end

Then you can design the model as shown in the screenshot below.

For more information on "Lookup Tables", please refer to the documentation link below.

https://www.mathworks.com/help/releases/R2023a/simulink/slref/ndlookuptable.html

Hope you find this information helpful!

[Adrian]

In the end I decided to fill the data with zeroes within those breakpoints without data. But this approach looks more neat, so will definitely give it a try! Thanks a lot