From: "mook johnson"
References: <6EGu9.firstname.lastname@example.org> <6oadnSBXeP61vSCgXTWcrg@netlabs.net>
Subject: Re: vibration measurement circuit (vibration gurus please help)
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 29 Oct 2002 02:35:28 GMT
NNTP-Posting-Date: Mon, 28 Oct 2002 20:35:28 CST
Organization: Road Runner - Texas
Thanks Big John,
This limited IC availability is due to the environment. This thing will be
stuck down a oil well about 3 to 4 miles down. The temperature down there
can be as high as 150 degrees CO. We have to be really selective about
which components we put down there or they won't operate or last for a
significant length of time. We've used the Analog Devices accelerometers at
150 degrees C before and they do work with somewhat degraded performance.
Temperature correction in the software takes care of most of this.
The data will be extracted from the device once per second transmitted to
the surface with our long distance telemetry system at 600 bps. This sensor
is replacing another sensor that has 9 bytes allocated to it in the bit
stream going back the surface. This is why the data needs to be processed
downhole. Otherwise I'd send the sampled waveform up and FFT on the surface
I'll probably have to resort to custom built band pass circuits with known
good Op-amps that work at temperature.
"Big John" wrote in message
> I have some experience with vibration testing from a job I had a couple
> years ago. Your method sounds OK but there are some details you need to
> know about. First, you have to worry about the mounted resonance
> of the accelerometer you are using. To get accurate readings you need to
> far below the resonance frequency at 5kHz. I'm not familiar with the
> devices accelerometer but the manufacturer will provide this data. Also
> mass of the accelerometer must be low enough not to load the structure.
> Smaller accelerometers are less likely to load the structure and have a
> higher mounted resonance frequency, but they are also less sensitive so it
> is important to know what accelerometer you need for your application.
> There are two general ways of specifying filter types for vibration
> - constant bandwidth filters
> - constant percentage bandwidth filters
> Constant bandwidth filters divide the 10 Hz to 5kHz bandwidth into a
> number of "lines" or individual filter bands. FFT works like this. You
> might have 512 or 1024 lines. Each "line" being about 5 or 10 Hz wide.
> Constant percentage bandwidth filters have a bandwidth that increases as
> the frequency increases. Common types are 1/3 Octave or Full Octave
> filters. They are usually specified by their center band frequency.
> for Full Octave the center band frequencies are 16, 31.5, 63, 125, 250,
> 1000, 2000, 4000, 8000 Hz. For 1/3 Octave the center frequencies are 10,
> 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400,
> 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300, 8000
> Hz. Full Octave bands are usually used for airborne sound, 1/3 Octave are
> often used for vibration measurements and pro-audio work.
> So 7 bands is fairly small a number, even at 1/1 Octave band
> You can make your own custom filters but it won't be consistent with
> industry standards. I think analog filtering is the way to go here
> FFT requires a fair amount of processing power and memory, but if you
> to go this route you'll need to add a Hanning window and an anti-aliasing
> filter to make it work. You can make your own active filters using
> but it's easier to buy them. IIRC there are chips available to do this.
> Once the filters are available you can use your microcontroller to read
> voltage at the outputs. It's been a while, but I believe the standard way
> is to measure the RMS voltage output at each band to get the vibration
> Lastly, reading the amplitude on a linear scale can make it difficult to
> see anything. The standard way is to look at a Log scale of AdB
> (acceleration decibels) referenced to 1 micro G. The formula is AdB = 20
> Log(G's measured / 1 micro G). Of course, you normally work with the
> measured voltages and a calibration factor for actual measurements.
> probably need at least 60 dB of measurement range to get usable data, but
> this depends on your application.
> Hope this helps,
> Big John
> mook johnson wrote in message
> >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
> >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 band.
> >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 will
> >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
> >The desired display on the computer screen would be 3 graphs with G(peak)
> >the Y axis and frequency on the X axis.