Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Vibration sensing
Summary: Vibration measurement parameters determine transducers
Keywords: Vibration Peak Sensor
Message-ID: <2884@kitty.UUCP>
Date: 1 Jan 89 04:52:55 GMT
References: <10457@obiwan.mips.COM>
Organization: Recognition Research Corp., Clarence, NY
Lines: 72

In article <10457@obiwan.mips.COM>, freeptos@mips.COM (Dan Freitas) writes:
> Does anyone have any ideas for sensing vibration peaks?  I want to  detect
> the maximum peak (in a given orientation) of a vibration of frequency ~1-2KHz.
> Something like what they use on the dynamic wheel balancers but operating at
> a higher frequency.  In the wheel balancer you place the sensor (mechanical
> pendulum with contacts or some such) on the swing arm of the car.  The wheel
> is spun and the sensor fires a strobe every time the vibration peaks in given
> position around the circle. You then add weight to the opposite side of the 
> wheel and presto, a dynamically balanced wheel.
> 
> I'm not sure a mechanical pendulum would work so well at 2Khz.  I would think
> it might act as the mechanical equivalent of a low-pass filter.

	Vibration measurement and analysis can range from a simple to a
complex undertaking, depending upon the application and what parameter you
want to measure.  In general, vibration mneasurement can deal with any or
all of the following parameters plotted against a frequency domain or
rotational position domain:

1.	Displacement of the measurement surface; most commonly quantitatively
	measured in micrometers peak-to-peak.

2.	Velocity of the measurement surface; most commonly quantitatively
	measured in millimeters/sec peak.

3.	Acceleration of the measurement surface; most commonly quantitatively
	measured in g peak (1 g = 980 cm/sec^2).

4.	Spike energy of the measurement surface; most commonly quantitatively
	measured in g-Spike Energy.

	The frequency domain for vibration measurements is usually expressed
in Hertz or counts/minute (CPM).  Most vibration analysis apparatus covers
the range of 1 Hz to 10,000 Hz (60 to 600,000 CPM).

	Transducers fall into two common categories: (1) electrodynamic and
(2) piezoelectric.

	Electrodynamic transducers are not unlike that of a dynamic
microphone, having a coil and a weighted diaphragm with a known mass and known
displacement range.  Electrodynamic vibration transducers are available with
frequency ranges up to 5 kHz, so your 2 kHz requirement is not a problem.
Electrodynamic transducers will generally measure displacement and velocity;
some designs are not a "true" accelerometer.

	Piezoelectric transducers are generally a true accelerometer, and
use a piezoelectric element connected to an electrometer "charge amplifier".
Many newer piezoelectric transducers have integral charge amplifiers.
Piezoelectric transducers will measure velocity, acceleration and spike
energy.  Piezoelectric transducers will easily measure signals up to 10 Khz,
but have a low frequency cutoff (usually in the 10's to 100's of Hz,
depending upon the device).

	The wheel balancer application which you mentioned most likely uses
an electrodynamic transducer optimized for displacement measurement, with
a displacement signal above a given threshhold beiong used to trigger a
strobe lamp.  This is sort of a "semi-quantitative" application, and
represents a simple use of vibration measurement since there is no
measurement made with respect to a frequency domain.

	Designing a vibration transducer for accurate, repeatable and
reliable quantitative measurement is no trivial task.  However, for some
less exacting applications, a "transducer" as simple as a musical
instrument contact microphone will suffice.

	If you have further details of your application, perhaps I can give
you more specific advice.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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