18.6 Commercial EMA Systems

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Commercially available EMA systems typically consist of an FFT analyzer, a modal analysis processor, a

graphics terminal, and a storage device. Digital plotters, channel selectors, hard copy units, and other

accessories can be interfaced, and the operation of the overall system can be coordinated through a host

computer to enhance its capability. The selection of hardware for a particular application should address

specific objectives as well as hardware capabilities. Software selection is equally important. Proper

selection of an EMA system is difficult unless the underlying theory is understood. In particular, the

determination of transfer functions via FFT analysis; extraction of natural frequencies, modal damping

ratios, and mode shapes from transfer function data; and the construction of mass, stiffness, and

damping matrices from modal data should be considered. We have already presented the underlying

theory. In the present section, we will describe the features of a typical EMA system.

18.6.1 System Configuration

The extraction of modal parameters from dynamic test data is essentially a two-step procedure

consisting of:

1. FFT analysis

2. Modal analysis

In the first step, appropriate frequency transfer functions are computed and stored. These raw transfer

functions form the input data for the subsequent modal analysis, yielding modal parameters (natural

frequencies, damping ratios, and mode shapes) and a linear differential equation model for the dynamic

system (test object).

18.6.1.1 FFT Analysis Options

The basic hardware configuration of a commercial modal analysis system is shown in Figure 18.9. Notice

that the FFT analyzer forms the front end of the system. The excitation signal and the response

measurements can be transmitted on line to the FFT analyzer (through charge amplifiers for piezoelectric

sensors); many signals can be transmitted simultaneously in the multiple-channel case. Alternatively, all

measurements may first be recorded on a multiple-track FM tape and subsequently fed into the analyzer

through a multiplexer. In the first case, it is necessary to take the FFT analyzer to the test site; an FM tape

recorder is needed at the test site in the second case.

Through advances in microelectronics and LSI technology, the FFT analyzer has rapidly evolved into a

powerful yet compact instrument that is often smaller in size than the conventional tape recorder used in

vibration data acquisition; either device can be used in the field with equal convenience. On-site FFT

analysis, however, allows one to identify and reject unacceptable measurements (e.g., low signal levels and

high noise components) during data acquisition, so that alternative data that might be needed for a

complete modal identification can be collected without having to repeat the test at another time. The

main advantage of the FM tape method is that data are available in analog form, free of quantization

error (digital word-size dependent), aliasing distortion (data sampling-rate dependent), and signal

truncation error (data block-size dependent). Sophisticated analog filtering is often necessary, however,

18-24 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

to remove extraneous noise entering from the recording process (e.g., line noise and tape noise), as well

as from the measurement process (e.g., sensor and amplifier noise).

The analog-to-digital converter (ADC) is normally an integral part of the analyzer. The raw transfer

functions, once computed, are stored on a floppy disk or hard disk as the “transfer function file.” This

constitutes the input data file for modal extraction. Some analyzers, instead, compute power spectral

densities with respect to the excitation signal and store these in the data file. From these data, it is possible

instantly to compute coherence functions, transfer functions, and other spectral information using

keyboard commands. Another procedure has been to compute Fourier spectra of all signals and store

them as raw data, from which other spectral functions can be conveniently computed. Most analyzers

have small CRT screens to display spectral results. Low-coherent transfer functions are detected by

analytical or visual monitoring and are automatically discarded.

In principle, the same processor can be used for both FFT analysis and modal analysis. Some

commercial modal analysis systems use a plug-in programmable FFT card in a common processor cage.

Historically, however, the digital FFT analyzer was developed as a stand-alone hardware unit to be used as

a powerful measuring instrument in a wide variety of applications, rather than just as a data processor.

Uses include measurement of resonant frequencies and damping in vibration isolation applications,

measurement of phase lag between two signals, estimation of signal noise levels, identification of the

sources of noise in measured signals, and measurement of correlation in a pair of signals. Because of this

versatility, most modal analysis systems do come with a standard FFT analyzer unit as the front end and a

separate computer for modal analysis.

18.6.1.2 Modal Analysis Components

In addition to the transfer function file, the modal analysis processor needs geometric information about

the test object, typically coordinates of the mass points and directions of the DoF. This information is

stored in a “geometry file.” The results of modal analysis are usually stored in two separate files: the

“parameter file” containing natural frequencies, modal damping ratios, mass matrix, stiffness matrix, and

damping matrix; and the “mode shape file,” containing mode shape vectors that are used for graphics

display and printout. Individual modes can be displayed on the CRT screen of the graphics monitor

either as a static traces or in animated (dynamic) form. The graphics monitor and printer are standard

components of the system. The entire system may be interfaced with other peripheral I/O devices using

an IEEE-488 interface bus or the somewhat slower serial RS-232 interface. For example, the overall

operation can be coordinated, and further processing done, using a host computer. A desktop (personal)

computer may substitute for the modal analysis processor, graphics monitor, and storage devices in the

standard system, resulting in a reasonable reduction of the overall cost. An alternative configuration that

CRT

Screen

Memory

Multi-Channel

FFT

Analyzer

Modal

Analysis

Processor

Data Storage

(Hard Drive,

CD ROM)

Input/Output Devices

(Keyboard, Graphics Monitor,

Printer, etc.)

Sensors and Transducers

from Test Object

(or Tape Player)

FIGURE 18.9 The configuration of a commercial experimental modal analysis system.

Experimental Modal Analysis 18-25

© 2005 by Taylor & Francis Group, LLC

is particularly useful in data transfer and communication from remote test sites uses a voice-grade

telephone line and a modem coupler to link the FFT analyzer to the main processor.

The first step in selecting a modal analysis system for a particular application is to understand

the specific needs of that application. For industrial applications of modal testing, the following

requirements are typically adequate:

1. Acceptance of a wide range of measured signals having a variety of transient and frequency band

characteristics

2. Capability to handle up to 300 DoF of measured data in a single analysis

3. FFT with frequency resolution of at least 400 spectral lines per 512

4. Zoom analysis capability

5. Capability to perform statistical error-band analysis

6. Static display and plot of mode-shape extremes

7. Animated (dynamic) display of mode shapes

8. Color graphics

9. Hidden-line display

10. Color printing with high line resolution

11. Capability to generate an accurate time-domain model (mass, stiffness, and damping matrices)

The capabilities of four representative modal analysis systems are summarized in Table 18.4.

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