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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: vibration measurement circuit (vibration gurus please help)
Date: Sun, 27 Oct 2002 03:34:38 GMT
NNTP-Posting-Date: Sat, 26 Oct 2002 20:34:38 PDT
mook johnson wrote:
> I'd like to get a graph of the vibration level (Gs) over a frequency range
> of 10 to 5kHz
> The vibration to be measured would be continuous or slowly changing over
> several minutes.
> Seven bands over that interval would be sufficient resolution of the
> Is it valid to do this using an Analog Devices accelerometer to feed 7
> bandpass filters.
> then measure the peak amplitude of each of the 7 BP outputs?
> I was thinking of using peak detectors to hold the peak experienced in each
> band over a 300mS period then use an A2D to sample the peak in each band.
> I only have an 8051 or MicroChip PIC running at 7.3MHz to do the processing,
> A2D manipulation and serial communication to the host at 9600 bps. 3
> different axes will be sampled over the 1 second period and the host will
> request an update at 1 /second.
> I think FFT is out due to limited processing power and limited local RAM for
> storing the raw waveform.
> Is this a valid approach? Will this be useful data for verifying
> qualification vibration levels for a device to be used in the same
> The desired display on the computer screen would be 3 graphs with G(peak) on
> the Y axis and frequency on the X axis.
The method you mention is valid. I used it back in the early 1970s on a
project to detect formation of microcracks in samples during working of
the samples to cause fatigue. My frequency range was in the megahertz
but the method makes sense. However, with a full second to do
processing, I would consider some sort of DFT method, say an FFT. I
don't know how much RAM you have but if you have enough, the processing
might fit in the time. The embarrassing thing is you want 10Hz because
that means that you need to sample at 10kHz and collect 1024 points for
a simple FFT. If the processor can have external RAM and careful coding
will meet time constraints, use the processor. Otherwise go with the
filters. If you do the digital approach, you will have to have an
For either approach, you will need to use a shaker table for
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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