15A.1 Dynamic Signals

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This Appendix introduces how to properly obtain data to analyze with the LabVIEW Sound and

Vibration Toolkit, as well as issues that can affect the quality of the data. One can simulate data with

the generation Virtual Instruments (VIs) located on the Generation palette as well as with other VIs.

The Appendix particularly supplements the material presented in Chapter 10 and in Chapter 15 to

Chapter 18, on vibration instrumentation, testing, data acquisition, and analysis.

15A.1.1 Acquiring and Simulating Dynamic Signals

This section discusses obtaining data and some key issues when acquiring or simulating dynamic

signals to ensure valid measurement results. The three techniques that allow one to obtain data

List of Abbreviations

Symbol Quantity

ADC Analog-to-Digital Converter (A/D)

AI Analog Input

ANSI American National Standards Institute

AO Analog Output

DAC Digital-to-Analog Converter (D/A)

DAQ Data Acquisition

DFT Discrete Fourier Transform

DOF Degree of Freedom

DSA Dynamic Signal Acquisition

DUT Device Under Test

DZT Discrete Zak Transform

EU Engineering Unit

FFT Fast Fourier Transform

FRF Frequency Response Function

IEC International Electrotechnical

Commission

Symbol Quantity

IEPE Integrated Electronic Piezoelectric

Excitation

IMD Intermodulation Distortion

JTFA Joint Time Frequency Analysis

NI National Instruments

PXI PCI eXtensions For Instrumentation

RMS Root Mean Square

RPM Revolutions per Minute

SDOF Single Degree of Freedom

SRS Shock Response Spectrum

STFT Short Time Fourier Transform

SVL Sound and Vibration Library

SVT Sound and Vibration Toolkit

THD Total Harmonic Distortion

VI LabVIEW Virtual Instrument

Virtual Instrumentation for Data Acquisition, Analysis, and Presentation 15-73

© 2005 by Taylor & Francis Group, LLC

are as follows:

* Acquisition of data with a data acquisition (DAQ) device or system

* Reading of data from a file

* Simulation of data with a generation VI (LabVIEW Virtual Instrument) or other source

It is important that you keep certain considerations in mind when you obtain your data. Measurement

and analysis software such as the LabVIEW Sound and Vibration Toolkit does not compensate for

inaccurate data. Therefore, the test equipment and test procedure should be calibrated to ensure accurate

results. Generally, the test equipment should have specifications at least ten times better than those of the

device under test (DUT). Use a verifiable and repeatable test procedure to get accurate results.

Whether one is obtaining the data from a DAQ system, reading the data from a file, or simulating the

data, aliasing and time continuity are common issues, which should be considered in the measurement

analysis.

15A.1.1.1 Aliasing

When a dynamic signal is discretely sampled, aliasing is the phenomenon in which frequency

components greater than the Nyquist frequency are erroneously shifted to lower frequencies (see Chapter

10). The Nyquist frequency is calculated with the following formula:

fNyquist ¼ sample rate =2

When acquiring data with an NI Dynamic Signal Acquisition (DSA) device, aliasing protection is

automatic in any acquisition. The sharp antialiasing filters on DSA devices track the sample rate and filter

out (attenuate) all frequencies above the Nyquist frequency.

When performing frequency measurements with an NI E Series DAQ device, you must take steps to

eliminate aliasing. These antialiasing steps can include the following actions:

* Increasing the sample rate

* Applying an external low-pass filter

* Using an inherently band-limited DUT

FIGURE 15A.1 Simulated data aliasing.

15-74 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

Simulated data also can exhibit aliasing. The signals often are generated according to a time-domain

expression and, therefore, have high-frequency components that are aliased in the discretely sampled

data. Figure 15A.1 shows an example of this aliasing for a simulated square wave.

The only way to protect data from aliasing is to apply appropriate aliasing protection before the data

are generated or acquired. Aliasing occurs when the data are generated or sampled, and it is not possible

to remove aliased components from the data without detailed knowledge of the original signal. In

general, it is not possible to distinguish between true frequency components and aliased frequency

components. Therefore, accurate frequency measurements require adequate alias protection.

15A.1.1.2 Time Continuity

When you acquire data in a continuous acquisition, you can use the t0 parameter in the waveform datatype

to ensure there are no gaps between successive blocks of waveforms returned by sequential calls to

the DAQmx Read VI or AI Read VI. When signals are generated with one of the waveform generation VIs,

in the Generation palette or the Waveform Generation palette, the t0 of the current waveform is one

sample period later than the timestamp of the last sample in the previous waveform. Continuity is

enforced in this way until the generation is reset.

The waveform data type is integral for testing time continuity in the Sound and Vibration Toolkit. If

you read data from a file or simulate a signal using one of the VIs in the Signal Generation palette, wire a

t0 that meets the continuous timestamp condition to the waveform data type connected to the

measurement analysis VIs. This action prevents unexpected resets of the measurement analysis due to

detected discontinuities in the input signal.

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