Transducers: The Sensors of Vibration Analysis

Definition: What is a Transducer?

In the context of vibration analysis, a transducer is a device that converts physical, mechanical motion (vibration) into a proportional electrical signal. This electrical signal can then be processed, measured, and analyzed by a data collector or monitoring system. Transducers are the primary sensors and the very first link in the measurement chain; without a reliable transducer, no meaningful analysis is possible. The choice of transducer is a critical decision that depends on the machine type, its speed, and the specific faults being monitored.

The Three Main Types of Vibration Transducers

There are three principal types of transducers used in industrial machine monitoring, each measuring a different physical parameter of vibration.

1. The Accelerometer (Measures Acceleration)

The accelerometer is by far the most common and versatile vibration transducer. It measures the acceleration of the structure it is mounted on.

• Principle: Most industrial accelerometers are piezoelectric, using a crystal that generates a voltage when stressed by an internal mass.
• Strengths: They have an extremely wide frequency range, are very robust, come in many different designs, and can be used on almost any type of machine. They are particularly excellent for detecting high-frequency events.
• Primary Use: General-purpose machinery monitoring, from low-speed machines to very high-speed turbomachinery. They are the go-to sensor for detecting high-frequency faults like bearing and gear defects.

2. The Velocity Transducer (Measures Velocity)

A velocity transducer is an electrodynamic sensor that directly measures the velocity of vibration.

• Principle: It works like a microphone, with a coil of wire suspended in a magnetic field. As the housing vibrates, the relative motion between the coil and magnet generates a voltage proportional to velocity.
• Strengths: They produce a strong, low-noise signal in the mid-frequency range (approx. 10 Hz to 1,000 Hz), which is where most common faults like unbalance and misalignment occur. They do not require external power.
• Weaknesses: They are more fragile than accelerometers, have a limited frequency range, and are sensitive to magnetic fields and mounting orientation. They have largely been replaced by accelerometers combined with electronic integration to calculate velocity.
• Primary Use: Historically used for general machine monitoring before robust accelerometers became common. Still sometimes used for permanent monitoring in the mid-frequency range.

3. The Proximity Probe (Measures Displacement)

The proximity probe (or eddy current probe) is a non-contact transducer that measures the displacement of a rotating shaft.

• Principle: It uses an electromagnetic field to induce and measure eddy currents on the shaft’s surface, which allows it to measure the gap between the probe tip and the shaft.
• Strengths: It measures the actual motion of the shaft itself, not the machine casing. It is non-contact, and its measurement goes down to 0 Hz (DC), allowing it to measure the shaft’s average position as well as its vibration.
• Primary Use: Essential for protecting and monitoring critical, high-speed turbomachinery with fluid-film bearings, such as turbines and compressors. It is used to analyze phenomena like shaft orbits and centerline position.

Choosing the Right Transducer

The selection of a transducer is a critical step in setting up a monitoring program. The general rule is to choose the transducer that is most sensitive in the frequency range of the expected faults:

• Use proximity probes for shaft motion on fluid-film bearing machines.
• Use accelerometers for everything else, as they are the most versatile. The signal can be integrated to velocity for analyzing mid-frequency faults or used directly as acceleration for high-frequency faults.

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