Matlab Accelerometer Data to Velocity

[MarkTsu]

Hello, this is my first post.

I've searched and seen some similar quesitons in the past, but thus far I'm still stuck, so I'll ask.

I'm trying to convert acceleration results into velocity using
"Omega Arithmetic". The Matlab code I wrote is something like this:

AccelFreqDomain=fft(AccelTimeSignal);
%n is the # of samples
%dt is the sampling rate
freq=(-n/2:n/2)/(n*dt) ;

VelocityFreqDomain=AccelFreqDomain./(2*pi*freq*1i);
VelocityTimeDomain=ifft(VelocityFreqDomain);
plot(VelocityTimeDomain);
hold on
plot(AccelerationTimeDomain);

When I plot the accel. and the velocity on the same graph, the phase shift is correct. When I change the amplitude of the input acceleration, the amplitude of the velocity output changes as expected. The problem is, if I change the input acceleration frequency, the output velocity amplitude remains the same, when I would expect it to change.

Any idea what I'm doing wrong?

Once I get the program responding logically, I'll probalby have some more questions about units.

Thanks for your time,
Mark

 

[MikeyP]

The check to see if you have done it right is to look at the complex part of velocity time domain signal (I see you have a 'real' command in the plot line). If the imaginary part is greater than machine precision then you have done it wrong.
To get a real number in the time domain, the FFT values for the negative frequencies must be the conjugate of their positive values.

M

--
Dr Michael F Platten

 

[MarkTsu]

My program has been working well. Thanks again for all the help.