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From: "mook johnson"
References: <6EGu9.firstname.lastname@example.org> <3DBB5EB6.D2A4AECA@webaccess.net>
Subject: Re: vibration measurement circuit (vibration gurus please help)
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sun, 27 Oct 2002 15:08:53 GMT
NNTP-Posting-Date: Sun, 27 Oct 2002 09:08:53 CST
Organization: Road Runner - Texas
"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
> > 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
> ... The times have been,
> That, when the brains were out,
> the man would die. ... Macbeth
> Chuck Simmons email@example.com
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.
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