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You can use the following three types of vibration transducers to obtain a vibration signal:

• accelerometers
• velocity transducers
• displacement transducers

Each vibration transducer has its own distinguishing characteristics and is suitable for different applications.

Accelerometers

Accelerometers are the most widely used vibration transducers for measuring vibrations on stationary machine elements. An accelerometer is a full-contact transducer mounted directly on a system or device under test (DUT). The benefits of an accelerometer include linearity over a wide frequency range and a large dynamic range. You can use most accelerometers in hazardous environments because of their rugged and reliable construction.

You can use accelerometers in applications involving frequencies from a few hertz to tens of kilohertz. The following illustration shows the typical frequency response characteristics of an accelerometer.

Because most accelerometers have a low dynamic signal response below 10 Hz, you cannot use accelerometers for low frequency measurements. The usable frequency range of an accelerometer is typically 15 Hz to 10 kHz or above.

You can use accelerometers to measure casing or machinery vibrations. You also can use accelerometers to test mechanical parts like gearboxes, bearings, and motors.

Velocity Transducers

Velocity transducers measure the absolute motion of a system or DUT with a response range from 15 Hz to 1.5 kHz. Because of the limited response range and other limitations of velocity transducers, you often use accelerometers instead of velocity transducers in many applications. You can obtain the velocity output from an accelerometer by using the integration function.

Displacement Transducers

You can use displacement transducers to obtain measurements like the relative displacement of a rotating shaft surface. A typical displacement transducer is a shaft-sensing proximity probe. A proximity probe is a non-contacting transducer mounted on a stationary mechanical structure to measure the distance between the probe tip and the shaft surface. A proximity probe has signal response between DC and 1.5 kHz, as well as flat phase response in the operational range. You typically use proximity probes in lower frequency measurements.

You can use a proximity probe to perform machine monitoring and protection measurements for DUTs with fluid film bearings. Because the flexible fluid film bearing and heavy housing usually generate low-frequency, external vibration responses, an accelerometer or a velocity transducer cannot measure the vibration effectively. By measuring the relative displacement of the shaft, a permanently mounted proximity probe can measure the vibration through shaft motion. You also can use proximity probes for functions such as radial or axial position monitoring and rotational speed calculation. A proximity probe is a common type of tachometer.

A proximity probe is susceptible to shaft surface scratches, circular irregularity, shaft bow, and variations in electrical properties. Accurate displacement signal measurements require compensation to remove these signal errors. The NI Sound and Vibration Measurement Suite provides Weighting and Filtering VIs and Reference Data Processing VIs to perform compensation to remove these signal errors.

Selecting a Vibration Transducer

System attributes and characteristics of the signals of interest, such as the vibration type, system type, and signal frequency range, determine the transducer to use to obtain vibration signals.

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.

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.