필터 지우기
필터 지우기

Generating Frequency domain with FFT function of accelerometer data

조회 수: 2 (최근 30일)
An Van
An Van 2020년 7월 7일
댓글: An Van 2020년 8월 16일
Hello,
I have accelerometer data that are recorded in excel sheet.
I would like to extract the frequency domain features from the data.
In order to do that, I need to convert the data into frequency by using the FFT function.
First of all, I generate the whole frequency of the data with this code:
freX = fft(accX)/N;
freX_2 = abs(freX);
freX_1 = freX_2(1:N/2+1);
freX_1(2:end-1)= 2*freX_1(2:end-1);
f_freX = fs*(0:(N/2))/N;
It works fine.
Now, I need to cut the data into windows with 200 data point for each window.
However, my code is hitting errors and I couldnt debug it.
for i =1:number_of_windows % there are 700 windows
% generating the frequency domain
a = fft(accX((1+(i-1)*s_win):(i*s_win)));
freX_2 = abs(a/N);
freX_1 = freX_2(1:N/2+1);
%error: Index exceeds the number of array elements (200).
% extracting the frequency domain features
meanfreX(i) = meanfreq(freX_1(1+(i-1)*s_win):(i*s_win)); % mean frequency
peakX(i) = findpeaks(freX_1(1+(i-1)*s_win):(i*s_win)); % peak frequency
medfreX(i) = medfreq(freX_1(1+(i-1)*s_win):(i*s_win)); % median frequency
powX(i) = ((abs(freX_1(1+(i-1)*s_win):(i*s_win))).^2)/s_win; %power spectrum
end
My whole code look like these:
clear all;
close all;
% Load the data for treadmill subject
t1= xlsread('data1',1);
% 1st column is accelerometer on X axis
accX = t1(:,1) ;
% set up window length
window = 2;
% set up frequency signal
fs = 100;
% set up size window
s_win = window*fs;
N = length(accX);
number_of_windows = floor(N/s_win);
% Fourier Transform
for i =1:number_of_windows % there are 700 windows
% generating the frequency domain
a = fft(accX((1+(i-1)*s_win):(i*s_win)));
freX_2 = abs(a/N);
freX_1 = freX_2(1:N/2+1);
%error: Index exceeds the number of array elements (200).
% extracting the frequency domain features
meanfreX(i) = meanfreq(freX_1(1+(i-1)*s_win):(i*s_win)); % mean frequency
peakX(i) = findpeaks(freX_1(1+(i-1)*s_win):(i*s_win)); % peak frequency
medfreX(i) = medfreq(freX_1(1+(i-1)*s_win):(i*s_win)); % median frequency
powX(i) = ((abs(freX_1(1+(i-1)*s_win):(i*s_win))).^2)/s_win; %power spectrum
end
Please help me debug the fourier transform.
Thank you.

채택된 답변

Raunak Gupta
Raunak Gupta 2020년 8월 8일
Hi,
In the line
freX_1 = freX_2(1:N/2+1);
freX_2 is built to have 200 elements but when you are calculating freX_1, the indexing is done with N/2 which is essentially the size of full accX and not of freX_2. In the above line if you replace N with s_win, I think the error will be resolved. But again when I see down you have taken freX((1+(I-1)*s_win) :(i*s_win)) which can be correct if freX_1 has size s_win. I think the code in the for loop should only work for a single window. Also you have used findpeaks which will return variable number of frequencies as local maxima. Instead I think you want max frequency so I have replaced it with max(freX_1) for it work.
So, I think the for loop may be written like below for it to work.
for i =1:number_of_windows % there are 700 windows
% generating the frequency domain
a = fft(accX((1+(i-1)*s_win):(i*s_win)));
freX_2 = abs(a/s_win);
freX_1 = freX_2(1:s_win/2+1);
% extracting the frequency domain features
meanfreX(i) = meanfreq(freX_1); % mean frequency
peakX(i) = max(freX_1); % peak frequency
medfreX(i) = medfreq(freX_1); % median frequency
powX(i) = (sum(abs(freX_1).^2))/s_win; %Average power spectrum
end

추가 답변 (0개)

카테고리

Help CenterFile Exchange에서 Fourier Analysis and Filtering에 대해 자세히 알아보기

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by