Requirements for filters external for electronics modules

If one is measuring low-frequency seismic signals from an accelerometer
and the accelerometer is feed to a vibration monitoring module, such as an
Allen-Bradley XM-121 LF, or equivalent, manufactured prior to 2012, should
one use an analog filter external to the module, or is the  module's
low-pass filters, which are selected in the software setup, adequate?

The frequency range of interest is 1.5 to 3.0 Hz, and the modules under
consideration are rated for frequencies above 0.8 Hz. 

The accelerometers are saturating at high frequencies (above 800 Hz), and
I am curious for frequencies well below 800 Hz, if should one use analog
filters between the accelerometers and the module?


Additionally, I used a signal generator to feed frequencies of constant
amplitude into the selected modules.  I configured the modules to
calculate displacement based on acceleration and velocity.  My
observations were that the modules seemed to over-report displacement in
the range of low frequencies of less than 1.5 Hz.  Any comments on this
observation.


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On Fri, 04 Sep 2015 08:41:49 -0500, Frank Wirsing wrote:

> If one is measuring low-frequency seismic signals from an accelerometer
> and the accelerometer is feed to a vibration monitoring module, such as
> an Allen-Bradley XM-121 LF, or equivalent, manufactured prior to 2012,
> should one use an analog filter external to the module, or is the 
> module's low-pass filters, which are selected in the software setup,
> adequate?

I dunno.  How long is a string?

> The frequency range of interest is 1.5 to 3.0 Hz, and the modules under
> consideration are rated for frequencies above 0.8 Hz.
> 
> The accelerometers are saturating at high frequencies (above 800 Hz),
> and I am curious for frequencies well below 800 Hz, if should one use
> analog filters between the accelerometers and the module?

Well, if the _accelerometers_ are saturating in a region that you need 
them to work, then they're destroying data (assuming that you're using 
"saturating" correctly in this context).  No amount of filtering after 
the fact will save you.

> Additionally, I used a signal generator to feed frequencies of constant
> amplitude into the selected modules.  I configured the modules to
> calculate displacement based on acceleration and velocity.  My
> observations were that the modules seemed to over-report displacement in
> the range of low frequencies of less than 1.5 Hz.  Any comments on this
> observation.

Upon what do you base this?  You are keeping in mind that for a constant 
amplitude of acceleration, the displacement goes as the inverse square of 
frequency -- yes?

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com