15A.6 Vibration-Level Measurements

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This Appendix briefly discusses the analysis

concepts associated with performing vibrationlevel

measurements and how one can use the

Vibration Level VIs located on the Vibration Level

palette to perform vibration-level measurements.

15A.6.1 Measuring the Root Mean

Square Level

A basic requirement of vibration measurements is

measuring the level of the signal returned by an

accelerometer. The level of the accelerometer signal

generally is expressed in root-mean-square (RMS)

acceleration ðgrmsÞ:

15A.6.1.1 Single-Shot Buffered

Acquisition

The block diagram in Figure 15A.24 illustrates a VI

designed to perform a single-shot acquisition and

compute the RMS levels.

The sampling frequency is 10 kS/sec. A buffer

containing 1 sec of data is returned by the read VI.

FIGURE 15A.20 Frequency response for single

integration.

FIGURE 15A.21 Frequency response for double

integration.

FIGURE 15A.22 Integration in the time domain and in the frequency domain.

Virtual Instrumentation for Data Acquisition, Analysis, and Presentation 15-89

© 2005 by Taylor & Francis Group, LLC

15A.6.1.2 Continuous Signal Acquisition

One can use the block diagram in Figure 15A.24 with a while loop to continuously acquire signals from an

accelerometer and display the vibration level in a chart. The block diagram in Figure 15A.25 illustrates how

to measure the RMS value once every 100 msec and display the results in a strip chart. In this example, the

RMS value is computed based on the last 100 msec of acquired data.

FIGURE 15A.23 Power spectra of the integrated signal.

FIGURE 15A.24 Single-shot buffered acquisition and RMS level VI.

FIGURE 15A.25 Continuous data acquisition and RMS level VI.

15-90 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

Note: Set the restart averaging control on the SVT RMS Level VI to TRUE. Otherwise, the SVT RMS

Level VI accumulates intermediate results to compute the RMS vibration level over the entire data

acquisition instead of just over the last block of data.

15A.6.2 Performing a Running RMS Level Measurement

The SVT Running RMS Level VI returns the RMS value computed over the last N seconds, which is the

integration time. The block diagram in Figure 15A.26 illustrates an application using the SVT Running

RMS Level VI. The sampling frequency is 10 kS/sec. The read VI reads 1000 samples at a time.

15A.6.3 Computing the Peak Level

Use the SVT Peak Level VI to compute the peak level of a signal. In peak-hold averaging, the largest

measured level value of all previous values is computed and returned until a new value exceeds the

current maximum. The new value becomes the new maximum value and is the value returned until a new

value exceeds it.

15A.6.4 Computing the Crest Factor

The crest factor is the ratio of the peak value over the RMS value of a given signal and indicates the shape

of the waveform. The crest factor is defined by the following equation:

Fc ¼

Vpk

Vrms where

Fc is the crest factor.

Vpk is the peak value of the signal.

Vrms is the RMS value of the signal.

The block diagram in Figure 15A.27 illustrates an application using the SVT Crest Factor VI. Along

with the crest factors, the SVT Crest Factor VI also returns the peak and RMS levels.

FIGURE 15A.26 Running RMS VI.

FIGURE 15A.27 Crest factor VI.

Virtual Instrumentation for Data Acquisition, Analysis, and Presentation 15-91

© 2005 by Taylor & Francis Group, LLC

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