From Harris' Shock and Vibration Handbook


Acquisition of data that are used in the formulation of a modal model involves many important technical concerns. The primary concern is the digital signal processing, or the converting of analog signals into a corresponding sequence of digital values that accurately describe the time-varying characteristics of the inputs to and responses from a system. Once the data are available in digital form, the most common approach is to transform the data from the time domain to the frequency domain by use of a discrete Fourier transform algorithm. Since this algorithm involves discrete data over a limited time period, there are large potential problems with this approach that must be well understood. (Data acquisition and analysis are discussed in detail in Chap. 27.)


In order to determine modal parameters, the measured input (excitation) and response data must be processed and put into a form that is compatible with the test and modal parameter estimation methods. As a result, digital signal processing of the data is a very important step in structural testing. This is one of the technology areas where a clear understanding of the time-frequency-Laplace domain relationships is important. The conversion of the data from the time domain into the frequency and Laplace domains is important both in the measurement process and subsequently in the parameter estimation process.

Digital signal processing of the measured input and response data is used for the following reasons:

  • Condensation. In general, the amount of measured data tremendously...

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Data acquisition systems and instruments collect, digitize, and process multiple sensor or signal inputs for the purpose of monitoring, analyzing and/or controlling systems and processes.
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IC electronic filters are frequency-selective circuits that consist of devices such resistors, capacitors, inductors, transistors, or operational amplifiers coupled with reactive components.
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Digital-to-analog converter (DAC) chips convert digital signals that represent binary numbers into proportional analog voltages.
Data Acquisition
Data acquisition is the digitizing and processing of multiple sensor or signal inputs for the purpose of monitoring, analyzing and/or controlling systems and processes. Signal conditioning includes the amplification, filtering, converting, and other processes required to make sensor output suitable for rereading by computer boards.

Topics of Interest

MEASUREMENT FORMULATION For current approaches to experimental modal analysis, the frequency response function is the most important, and most common, measurement to be made. When estimating...

MODAL ANALYSIS THEORY While modal analysis theory has not changed over the last century, the application of the theory to experimentally measured data has changed significantly. The advances of...

MODAL PARAMETER ESTIMATION Modal parameter estimation, or modal identification, is a special case of system identification where the a priori model of the system is known to be in the form of modal...

4.1 The Fast Fourier Transform (FFT) Analysis The vibration of a machine is a physical motion. Vibration transducers convert this motion into an electrical signal. The electrical signal is then...

MODAL DATA PRESENTATION/VALIDATION Once the modal parameters are determined, there are several procedures that allow the modal model to be validated. Some of the procedures that are used are...