Be sure to evaluate system attributes and characteristics of the signals of
interest--such as the vibration type, system type, and signal frequency range--to determine the
most applicable transducer for you.
Relative or Absolute Vibration—Use a displacement transducer, such as a proximity probe, for
measuring relative vibrations. Use an accelerometer for measuring absolute vibrations.
For example, if the vibration you are monitoring is a signal such as the relative
displacement from a shaft or the relative displacement from a bearing clearance, then
you need to use a proximity probe. If the vibration you are monitoring is a signal such
as the casing vibration, or absolute vibration, of a gearbox or motor, then you need to
use an accelerometer.
System Rigidity—A mechanical system like turbo machinery is composed of flexible heavy rotors
and fluid film bearings. Thus the vibration of turbo machinery does not transmit well to
the outer casing. You therefore need to use proximity probes to directly measure shaft
motion. If the system or DUT components are stiff enough to transmit vibrations
effectively, such as with most rolling-element bearings, you can use accelerometers to
measure the vibration. You also can use a combination of proximity probes and
accelerometers to obtain a vibration signal.
Low or High Frequency—The sensitivity of proximity probes, accelerometers, and velocity
transducers varies over different frequency ranges, as shown in the following illustration.
Figure 3. Frequency Effect on Transducer
Vibration Amplitude
Use proximity probes to measure lower frequency vibrations like shaft motion. For
systems with high-frequency elements, such as rolling-element bearings, gearboxes, or
spinning blades, use an accelerometer mounted on the casing or housing. Because of the
limited response range and other limitations of velocity transducers, you usually use
accelerometers instead of velocity transducers in many applications.
Accelerometers, sensors that represent acceleration as voltage, come in two axial types: single axis and tri-axial. Choosing between the two depends on the type of vibration you are measuring.
