Error: "Expected either a logical, char, int, fi, single, or double. Found an mxArray."

I have a "Matlab Function" block in simulink containing the following code:

function found   = FFTscan( signal, Fs )

PSD=real(fft(signal,2^nextpow2(length(signal)))); %est. power spec density
PSDn=PSD./max(PSD);                            %normalizes values from FFT

framen=[.0007 .0642 1 .394 .0063];             %frame for XCORR
xcn=xcorr(PSDn,framen);               %xcorrelates normalized FFT w/ frame
xcn1 = xcn(((length(xcn)-1)/2):end);  %turns 2sided xcorr into 1sided  

   coder.extrinsic('findpeaks');
  [pks, loc] = findpeaks(xcn1,'MINPEAKHEIGHT',.4,'MINPEAKDISTANCE',30); 

found=loc.*(Fs/length(signal)); %multiply index by Fs/N to get frequency

The purpose is to take a frame of an incoming signal in the time domain, transform it to the freq domain, then cross-correlate it with a frame of a known signal. This function works fine if I run it in the matlab workspace, but if I try to use it in a simulink model it throws the error mentioned in the title.

How can I use the data in "loc" as output for my simulink block?

Thanks,

-Will

[Walter Roberson]

Which line is the problem occurring on? If it is that last line, could you try breaking it up into an assignment to a temporary variable and then on the (new) next line, assign the temporary to "found" ? I am thinking the problem might have to do with the "loc" rather than the "found".

[Will Forfang]

Yes, it's the last line. Do you mean like this?

coder.extrinsic('findpeaks');
[pks, loc] = findpeaks(xcn1,'MINPEAKHEIGHT',.4,'MINPEAKDISTANCE',30); 
temp = loc.*(Fs/length(signal)); %multiply index by Fs/N to get frequency
found= temp;

It still outputs the same error. Here's the full error report:

Expected either a logical, char, int, fi, single, or double. Found an mxArray. MxArrays are returned from calls to the MATLAB interpreter and are not supported inside expressions. They may only be used on the right-hand side of assignments and as arguments to extrinsic functions.

Function 'Subsystem1/Algorithm/MATLAB Function' (#76.609.612), line 16, column 8:
"*loc*"
Launch diagnostic report.

I've also tried it this way, and yielded the same error:

temp = loc; %multiply index by Fs/N to get frequency
found= temp.*(Fs/length(signal));

Thanks for your reply,

-Will

Answer 1

[Walter Roberson]

Please see http://www.mathworks.com/help/simulink/ug/calling-matlab-functions.html#bq1h2z9-47

[Will Forfang]

Thanks, Walter!

[Juan Ramos Zayas]

Hi, I am having the same issue, could you please repost the solution? the link isn't working anymore. Thanks!

[Walter Roberson]

https://web.archive.org/web/20201025151057/https://www.mathworks.com/help/simulink/ug/calling-matlab-functions.html has an archive of the page.

[Juan Ramos Zayas]

Many thanks Walter!