How can I plot the fft of a torque signal from simulink?

I want to plot the fft of the torque waveform that I obtain from Simulink in from 0.5s to 1s. I have the time array and the torque array but the sample step is not homogeneous. I have tried to discretize the torque by using delay and so having the same sample step but it really slows my simulation so I need to use variable step. I would like to know how to obtain in an easy way the harmonic content of the following signal:

r=find(torque.time>= 0.5 & torque.time<= 1); plot(torque.time(r),torque.signals.values(r)); timesim=(torque.time(r)); %1x673 torquesim=(torque.signals.values(r)); %1x673

torquesim= [46.1075152808528 46.1075436322296 46.1075934164804 46.1076432009408 46.1076929856262 46.1077427705538 46.1077925557160 46.1078595784897 46.1079477122389 46.1080358465634 46.1081239814094 46.1082417296371 46.1083594787239 46.1085684779717 46.1091532834247 46.1097381099678 46.1103229575235 46.1112002680397 46.1120776253737 46.1133937486221 46.1147099758724 46.1165069905329 46.1183041954988 46.1210003551979 46.1236969317822 46.1277425631066 46.1317890948029 46.1378605304829 46.1439338684488 46.1677152735873 46.1915208076147 46.2153473409379 46.2421682890254 46.2690180901965 46.2829669879802 46.2924129250715 46.2989683827304 46.3034572114448 46.3079569319853 46.3102403511590 46.3125310989621 46.3138957089359 46.3152663592675 46.3161711729579 46.3170818923648 46.3176897401430 46.3183034850002 46.3187203209313 46.3191432478446 46.3193897264550 46.3196404883689 46.3198380931267 46.3199772383485 46.3201195068677 46.3202659557154 46.3203796174783 46.3204975416532 46.3205833884035 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Find the torque courbe attached. I want to display the fft of that courbe (It has to be something like peaks on 35Hz and 60Hz)

Answer 1

[Sebastian Castro]

Well, the first thing you need to do is make sure that the output signal from Simulink is output at a fixed sampling rate. Furthermore, this rate should be sufficiently high enough that you can capture the 35 and 60 Hz peaks... so, your rate should be AT LEAST 120 Hz (though I'd go for higher).

You can do this by doing either of the following:

1. Discretizing the signal with a Rate Transition block prior to exporting to the workspace.
2. Going to Configuration Parameters > Data Import/Export and changing the Output options to "Produce specified output only" and an evenly sampled time vector of your choice. For example, 0.5:(1/120):1.

Once you have your evenly sampled data, you can follow the example on the fft documentation: http://www.mathworks.com/help/matlab/ref/fft.html

If you have a DSP System Toolbox license, you could alternative plot the FFT on a Spectrum Analyzer block: http://www.mathworks.com/help/dsp/ref/spectrumanalyzer.html

- Sebastian

[Sebastian Castro]

Oh, I just re-read and saw that you didn't want to take all those time-steps because it slows down your simulation.

This may or may not work, but here's a 3rd alternative: Use the interp1 function on your data to interpolate the non-homogeneous data with an homogeneous sampling time of your choice.