Vibrometry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 654

Resonance thus represents the Achilles heel of human response to vibration. As is the auditory system
to sound, human response to vibration is therefore frequency dependent and nonlinear because at res-
onance the impinging vibration finds its easiest pathway to the person; at other vibration frequencies, the
vibration pathway is not as easy and thus it requires more acceleration at a nonresonant frequency to
produce the same level of human response.

33.3 Vibration Measurements Basics 2-4

The major reason for performing occupational vibration measurements is to evaluate the vibration
impinging on persons. Evaluations are performed using the various WBV and HAV standards guides.

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These standards guides are the critical link between the various health and safety effects of WBV or

HAV and the vibration hazard levels experienced by workers. It is important to note that there are
many esoteric types of vibration measurements (i.e., mechanical: impedance, mobility, stiffness, compli-
ance, etc.) and other methods of data analysis, such as modal analysis, but intentionally in this chapter we
briefly describe only performing acceleration measurements as required by the applicable health and
safety standards

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guides in order to use them. Finally, note that since displacement, velocity, and accel-
eration are all mathematically linked, then from a measurement of acceleration, the velocity function can
be derived by electronic integration; repeating the integration next on the velocity function yields the
displacement function. Thus, if desired, an acceleration measurement can yield additional data.
Figure 33.3 shows a basic vibration acceleration measurement setup. Since we must simultaneously but
separately measure in all three X, Y, Z axes acceleration data from a vibrating tool, for example, or from a
driver's truck seat, three separate data channels are needed. Three perpendicularly mounted lightweight
accelerometers are used to measure each axis acceleration, followed by three appropriate preamplifiers to
amplify and electronically condition the tiny millivolt signals coming from each accelerometer. The
outputs of each of these three X, Y, Z preamplifiers are then individually recorded and stored on a multi-
track tape system known as a digital audio tape (DAT) for later Fourier spectrum analysis. It is also desir-
able to have: (1) a microphone

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voice track on the DAT to note the chronology of events being recorded,
and (2) an oscilloscope or similar device monitoring the X, Y, Z acceleration axes for possible signal over-
load conditions leading to distortion of the recorded signal(s) and resulting in erroneous data processing
results. After the three channel or “triaxial” data have been measured, stored, and recorded, then a

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Accelerometers

Multichannel
Monitoring
Oscilloscope

Fourier
Spectrum
Analyzer

Conditioning
Preamplifier

X

ZYX

Selector
Switch

Conditioning
Preamplifier

Chart
Recorder

Y

Multichannel
DAT Tape
Recorder

Conditioning
Preamplifier

Z

X

Y Z Voice Input

Microphone

FIGURE 33.3 Basic triaxial vibration acceleration measuring system.