The purpose of this Recommended Practice is to provide guidelines for selecting transducers to measure shock and vibration in laboratory and field testing environments. Some special applications are not covered because of their unique nature and the rapid advancements taking place in their disciplines. These include a variety of biodynamic and biomechanical tests. Even in those applications not specifically addressed, however, these recommendations may be helpful.

There are basically two classes of motion transducers: fixed-reference and mass-spring (relative motion). Non-contact transducers, such as laser interferometric displacement and laser Doppler velocity transducers, are fixed-reference designs. Although they offer some unique properties, these instruments are used to measure shock and vibration only in applications where a fixed reference is available, and where their cost, size, and physical space and geometry requirements are acceptable. Similarly, video and photographic displacement measurement techniques are sometimes useful for motion analysis of large, complex structures or mechanisms, but they are used mostly for verification. These fixed-reference techniques, which have different constraints, are discussed elsewhere. This Recommended Practice will concentrate on the more common mass-spring type transducers, with the sensing element(s) represented by the spring.

The following recommendations apply to dynamic measurements on objects over 10 grams, with frequencies ranging from DC (0 Hz) to over 100 kHz. Only measurements of linear (translational) motion are considered; measurement of angular or rotational motion is addressed as an application at the end of the document.

Recent developments in smart sensor technology have the potential to revolutionize the measurement industry, and play an important role in transducer selection. However, part of the IEEE standard directly related to transducer selection had not been finalized as of publication of this Recommended Practice. We therefore limit our discussion in this document to the basic concept of mixed-mode, pointto- point smart sensor where it is relevant. A complete look at multi-drop smart sensor technology will be included in future revisions.

BACKGROUND AND PURPOSE

Shock and vibration are seldom measured directly. The structural response of an item is normally obtained using measurements of acceleration, velocity, displacement, or dynamic force.

Except for a few unique cases, acceleration is the preferred measurand of motion. However, acceleration is a quantity that cannot be measured directly in practice. As a result, strain or displacement in a massspring motion transducer is used to obtain acceleration information indirectly.

To understand why acceleration is the preferred measured parameter, consider the following.

The nature of mechanical systems is such that appreciable displacements occur only at low frequencies. Displacement measurements, due to their limited bandwidth, are of little value for most structural measurements. Also, displacement sensors, such as linear variable differential transformer (LVDT) displacement transducers, tend to be large, heavy, and limited to frequencies below 500 Hz.

Most practical velocity sensors, such as velocity coils, are based on the electrodynamic principle. This type of transducer is designed to be used at frequencies above its natural frequency, and is not very useful at frequencies above several thousand Hz due to its inherent design characteristics. Most velocity coils therefore are used on industrial rotating machineries where vibration frequencies are relatively low. One uniquely fabricated velocity coil has been designed to measure intense shock up to 100 kHz, but it is not commercially available. In general, a single wide-band measurement of a signal in a shock and vibration environment often requires transducers with much broader bandwidth.

With modern accelerometers, satisfactory acceleration measurements can be made from steady state (DC) to over 10 kHz. Many of these new designs offer more than 100 dB signal-to-noise ratio, giving the accelerometer a definite edge over displacement and velocity transducers. In addition, with a suitable accelerometer, velocity and displacement information can be derived from acceleration data by single- and double-integrating the signals during post-processing.

Dynamic force transducers are conceptually similar to mass-spring accelerometers, with the mass made negligible or as small as possible, and the sensing element(s) again represented by the spring. This Recommended Practice therefore covers basic theories and applications of force transducers following a similar discussion of accelerometers.

In this document, recommendations are made in terms of performance characteristics rather than instrument construction, brand, or model number. Various sensing technologies are discussed in detail to provide a good understanding of design trade-offs. It is assumed that instrument suppliers provide accurate performance specifications, enabling users to select the optimum available instrument. In a few instances, advice or precautions are provided for interpreting manufacturers' specifications. In general, manufacturers and users should use the referenced ANSI and ISA specifications.