Vibration Measurements

A vibration sensor is a device that converts some property of the vibration of a structure into an electrical signal. Conversely, a vibration generator works on the opposite principle of converting an electrical signal into a mechanical vibration. Both vibration sensors and generators may be termed transducers, since they convert one physical variable into another [4].

Vibration sensors can be made to work using several different principles and to measure surface displacement, velocity, acceleration, and strain. They may be arbitrarily designated as contacting or noncontacting devices. Contacting sensors are often convenient to use since they can measure vibration at a specific location on a structure. They do have the disadvantage, however, that they can change the vibration of the structure by adding mass, stiffness and damping. This is particularly a problem if the structure is lightweight.

There are three quantities of most interest in vibration studies [5]. These are displacement, velocity, and acceleration. A fourth quantity, strain, is also frequently measured. In the early 1900s, most vibration measurements were made using mechanical or optical devices. Such devices are still used satisfactorily for low‐frequency measurements (a few hertz). With the advent of electronics in the 1920s, transducers that converted mechanical into electrical signals were developed. Before about 1960, displacement and particularly velocity‐sensitive transducers were utilized, especially when higher frequency measurements were needed. However, since that time acceleration sensitive transducers (accelerometers) have become preferred. The reason for this is mainly because excellent lightweight accelerometers were developed to measure very high frequency vibrations (5000 Hz or more) in aircraft and spacecraft. Most velocity‐sensitive transducers have an upper limiting frequency of about 1000 Hz, while piezoelectric accelerometers can be made to have an upper limiting frequency of 40 000 Hz or more [5].

For many measurements it is unimportant whether displacement, velocity, or acceleration is measured. For simple harmonic motion, the amplitudes of these three quantities are simply related to each other by multiplying or dividing by the angular frequency ω. Even if the vibration is random in nature, if frequency filtration is used, this principle can still be applied. Alternatively, the conversion from acceleration to velocity and displacement may be made by using electronic integration.