bandpass filter problem

[AliasAlreadyOnFile]

I originally posted this problem as a reply HERE but haven't recieved any responses so I'm trying a standalone thread. Anyway here's a repeat of my problem...

I had an offset in the raw voltage data from the accelerometer that was causing the plots to fly off to extremely large values after two integrations (conversion from m/s^2 to m). I solved this by subtracting the Offset output from Normalize Waveform VI from the original data stream. This worked really well and I can now get a displacement signal that doesn't fly off sharply away from 0. I do now have one further problem with this signal, though. In each of the attached files are two data sets, one exported from the FFT and the other from the displacement plot. In the 3Hz file the displacement plot shows a large peak just short of 0.1s - corresponding to ~2.5Hz. It turns out this is related to the bandpass filter, which passes frequencies between 3 and 500Hz in my VI. The filter worked really well to attenuate a large peak below 2.5Hz that was originally in the FFT but it introduces another large peak that corresponds with the low frequency cut off for the bandpass filter, ~3Hz in this case.

As expected, this feature moves when the cut off is changed so is directly related to it. The further away from 0 the cut off is the smaller and broader the feature is in the FFT but I want the low freq cut off to be as low as possible. Changing the filter settings - order, type, etc, do nothing. I've attached a second data file with the cut off at 10Hz. The frequency of the main feature in the displacement plot is now 4*1/0.0257 = ~10Hz and the FFT shows a feature around 10Hz. I don't understand what's happening, any suggestions?

This happens only after the double integration to get the displacement from the acceleration signal. Keeping the units in 'g' gives the expected response.

As an aside, I assumed the bandpass filter was a low-pass then a high-pass filter in series, but I get very different results when I use this combination. Any ideas why they could be different, even though the settings are the same?

Thanks!

[johnsold]

I have been looking at the filters question and think I may be able help.

I am not sure what peaks you are seeing in the data you posted. The column labels in the spreadsheet files confused me.  Is the raw acceleration data in there? 

Lynn

[AliasAlreadyOnFile]

There are now responses in the two threads I have about this so just going to use the one linked above now. This one should be closed. Lynn I will respond to you in the other thread.