first convert your raw acceleration units to ft/sec/sec (or whatever
unit you want velocity and displacement) Then use the integrate
function (twice for displacement). You may use FFT for frequency
content but it does not help for velocity or displacement. If you
have already tried this, you may have seen undesirable results, you
will typically have to filter the accel signal in the range of your
mechanical system response. If your system can not vibrate faster
than X, you should filter just above this frequency.
