15.1 Introduction

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Measurement and associated experimental techniques play a significant role in the practice of vibration.

Academic exposure to vibration instrumentation usually arises in laboratories, in the context of learning,

training, and research. In vibration practice, perhaps the most important task of instrumentation is the

measurement or sensing of vibration. Vibration sensing is useful in the following applications:

1. Design and development of a product

2. Testing (screening) of a finished product for quality assurance

3. Qualification of a good-quality product to determine its suitability for a specific application

15-1

© 2005 by Taylor & Francis Group, LLC

4. Mechanical aging of a product prior to carrying out a test program

5. Exploratory testing of a product to determine its dynamic characteristics such as resonances,

mode shapes, and even a complete dynamic model

6. Vibration monitoring for performance evaluation

7. Control and suppression of vibration

Figure 15.1 indicates a procedure typical of experimental vibration, highlighting the essential

instrumentation. Vibrations are generated in a device, the test object, in response to some excitation. In

some experimental procedures, primarily in vibration testing (see Figure 15.1), the excitation signal has

to be generated in a signal generator in accordance with some requirement (specification), and applied to

the object through an exciter after amplification and conditioning. In some other situations, primarily in

performance monitoring and vibration control, the excitations are generated as an integral part of the

operating environment of the vibrating object and may originate either within the object (e.g., engine

excitations in an automobile) or in the environment with which the object interacts during operation

(e.g., road disturbances on an automobile). Sensors are needed to measure vibrations in the test object. In

particular, a control sensor is used to check whether the specified excitation is applied to the object,

and one or more response sensors may be used to measure the resulting vibrations at key locations of

the object.

The sensor signals have to be properly conditioned, for example by filtering and amplification, and

modified, for example through modulation, demodulation, and analog-to-digital conversion, prior to

recording, analyzing, and display. The purpose of the controller is to guarantee that the excitation is

correctly applied to the test object. If the signal from the control sensor deviates from the required

excitation, the controller modifies the signal to the exciter so as to reduce this deviation. Furthermore, the

controller will stabilize or limit (compress) the vibrations in the object. It follows that instruments in

experimental vibration may be generally classified into the following categories:

1. Signal-generating devices

2. Vibration exciters

3. Sensors and transducers

4. Signal conditioning/modifying devices

5. Signal analysis devices

6. Control devices

7. Vibration recording and display devices

Analog/

Digital

Interface

Digital

Signal

Recorder,

Analyzer,

Display

Filter/

Amplifier

Filter/

Amplifier

Signal

Generator

and Exciter

Controller

Reference (Required)

Signal (Specification)

Power

Amplifier

Mounting

Fixtures

Test

Object

Response

Sensor

Control

Sensor

Exciter

Swivel

Base

FIGURE 15.1 Typical instrumentation in experimental vibration.

15-2 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

Note that one instrument may perform the tasks

of more than one category listed here. Also, more

than one instrument may be needed to carry out

tasks in a single category. In the following sections

we will provide some examples of the types of

vibration instrumentation, giving characteristics,

operating principles, and important practical

considerations. Also, we will describe several

experiments which can be found in a typical

vibration laboratory.

An experimental vibration system generally

consists of four main subsystems:

1. Test object

2. Excitation system

3. Control system

4. Signal acquisition and modification system

These are schematically shown in Figure 15.2. Note that various components shown in Figure 15.1 may

be incorporated into one of these subsystems. In particular, component matching hardware and object

mounting fixtures may be considered interfacing devices that are introduced through the interaction

between the main subsystems, as shown in Figure 15.2. Some important issues of vibration testing and

instrumentation are summarized in Box 15.1.

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