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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)
References: <6EGu9.firstname.lastname@example.org> <3DBB5EB6.D2A4AECA@webaccess.net> <9kTu9.email@example.com>
Date: Sun, 27 Oct 2002 16:14:00 GMT
NNTP-Posting-Date: Sun, 27 Oct 2002 08:14:00 PST
mook johnson wrote:
> "Chuck Simmons" wrote in message
> > mook johnson wrote:
> > >
> > > I'd like to get a graph of the vibration level (Gs) over a frequency
> > > 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
> > > frequency.
> > >
> > > 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
> > > band over a 300mS period then use an A2D to sample the peak in each
> > >
> > > I only have an 8051 or MicroChip PIC running at 7.3MHz to do the
> > > 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
> > > request an update at 1 /second.
> > >
> > > I think FFT is out due to limited processing power and limited local RAM
> > > 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
> > > environment?
> > >
> > > 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
> > antialiasing filter.
> > For either approach, you will need to use a shaker table for
> > calibration.
> > Chuck
> We only have about 128 bytes of SRAM available with the 8051 and 350 bytes
> with the PIC.
> Not enough room for storage of the waveform.
Analog filters then unless you can find a microP with enough memory or
can do external RAM.
... 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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