Understanding the Velometer (Velocity Sensor)

1. Definition: What is a Velometer?

A Velometer, more formally known as a velocity transducer or velocity sensor, is a type of transducer used to measure vibration. Its key characteristic is that it generates an electrical output signal that is directly proportional to the velocity of the vibration.

This is in contrast to an accelerometer, which outputs a signal proportional to acceleration, and a proximity probe, which outputs a signal proportional to displacement. While less common than accelerometers in modern portable data collection, velometers still have important applications, particularly in permanent monitoring of medium-speed machinery.

2. How a Velometer Works

Traditional velometers are electrodynamic sensors. They work on the same principle as a dynamic microphone, based on Faraday’s law of induction:

1. Inside the sensor’s housing is a coil of wire suspended by soft springs.
2. A permanent magnet is fixed to the housing of the sensor.
3. When the sensor is mounted on a vibrating machine, the housing and the magnet move with the machine.
4. Due to its inertia, the suspended coil remains relatively stationary in space.
5. This relative motion between the moving magnet and the stationary coil induces a voltage in the coil.
6. According to Faraday’s law, this induced voltage is directly proportional to the velocity of the relative motion.

Modern “piezoelectric velometers” also exist. These are essentially accelerometers with a built-in electronic integration circuit that converts the acceleration signal into a velocity signal within the sensor itself.

3. Advantages and Disadvantages of Velometers

Advantages

• Direct Velocity Output: They directly measure velocity, which is the most commonly used parameter for assessing the severity of vibration on general-purpose machinery in the 10 Hz to 1,000 Hz range (per ISO 10816). No integration is required by the analyzer, which can simplify the measurement.
• Self-Powered (Passive): Traditional electrodynamic velometers do not require an external power source to operate, which can be an advantage in certain industrial applications.
• Good Low-Frequency Response: They are generally more sensitive at lower frequencies than many general-purpose accelerometers.

Disadvantages

• Limited Frequency Range: They have a more limited usable frequency range compared to accelerometers. They are not suitable for detecting the high-frequency impacts associated with bearing defects or gear defects.
• Moving Parts: The internal spring and coil system can wear out or break over time, especially in high-vibration environments.
• Sensitivity to Orientation: They must be mounted in the orientation for which they were designed (e.g., vertical or horizontal).
• Larger and Heavier: They are typically much larger and heavier than modern accelerometers.
• Sensitivity to Magnetic Fields: Strong external magnetic fields (e.g., from large motors) can interfere with their operation.

4. Velometer vs. Accelerometer

For modern portable data collection and vibration diagnostics, the accelerometer is the sensor of choice. This is because accelerometers have a much wider, flatter frequency response, allowing them to measure both the low-frequency vibrations from unbalance and the very high-frequency vibrations from bearing and gear faults. The vibration analyzer can then easily integrate the acceleration signal to display velocity or displacement as needed.

Velometers are now most often found as permanently installed sensors on medium-speed machines like fans, pumps, and motors, particularly in older installations where they were specified as the original monitoring sensor.

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