Vibrometry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 662

33.5 Summary

In this chapter the basic concepts of displacement, velocity, acceleration, resonance, coordinate systems
for measurements, and spectrum analysis are presented and integrated for an understanding of their
application to whole-body and hand-arm occupational vibration. Generic acceleration measurement
systems and methods are discussed, which included piezoelectric and piezoresistive accelerometers, con-
ditioning preamplifiers, data recording systems, and Fourier spectrum computers. The chapter concludes
with a discussion of the various occupational whole-body and hand-arm vibration standards guides cur-

rently used in the U.S. and their application to the evaluation of triaxial acceleration data from the work-
place; because of the complexity of each standard, users are encouraged to obtain copies of standards
which are to be used before obtaining vibration data.

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Defining Terms
Acceleration: The time rate of change of velocity of a moving object.
Accelerometer: A device designed to convert mechanical motion into a corresponding electrical
analog voltage, charge, or current proportional to acceleration.
Conditioning Preamplifier: An electronic solid state amplifier designed to faithfully amplify, both in
amplitude and frequency bandwidth, the minute electrical signals emanating from an accelero-
meter. Some preamplifiers are called “charge amplifiers” and convert the voltage generated
across the face of a crystal (piezoelectric) accelerometer into corresponding charge, thereby allow-
ing long cables to be used for measurements without loss of signal. Other preamplifiers are called
“differential amplifiers,” which act as an amplifying voltmeter when used with “piezoresistive”
type accelerometers.
DAT or Digital Audio Tape: A new type of instrumentation tape system with large dynamic input
range and wide frequency bandwidth, whereby an analog input signal is converted and stored
on a cassette tape in digital format. The original signal so stored can be retrieved either in
digital format or reconverted again into its original analog version.
Displacement: Movement traversed away from a reference position.
Fourier Spectrum Analysis: The analysis of vibration data by mathematically converting time
domain information into its corresponding frequency domain; the underlying assumptions are
that the data are linear and that time domain information can be dissected and represented as
a mathematical series of elemental sines and cosines. Computers which perform this function
are called Fast Fourier Transform (FFT) analyzers or Real Time Analyzers (RTA).
Resonance: The tendency of an object to (1) move in concert with an external vibrating source and
(2) to internally amplify the impinging vibration from that source; resonance is the optimum
energy transfer condition between the source and the receiver.
Vector Coordinate System: A mutually perpendicular set of vectors, originating at the same motion
point, which define the vector motion of that point. Typically, there are three linear and three
rotational vectors which comprise motion at a point.
Vector: A mathematical quantity defined by both its magnitude and direction.
Velocity: The time rate of change of displacement of a moving object. Also called speed.
Vibration: At any one point, vibration is motion defined by six vectors, three mutually perpendicular
linear vectors and three rotational vectors moving around these linear vectors (pitch, yaw, roll).

References
1. Wasserman, D., Badger, D., Doyle, T., and Margolies, L. 1974. Industrial vibration — An overview.
J. Am. Soc. Safety Engrs. 19(6):38-43.
2. Wasserman, D. 1987. Human Aspects of Occupational Vibration, Elsevier Publishers, Amsterdam, The
Netherlands.