Displacement vs. Velocity vs. Acceleration: Selecting the Right Sensor

Sensor selection is dictated by the measurement requirement: Eddy Current Sensors measure displacement (shaft movement), Velocity Probes measure velocity (mid-frequency casing vibration), and Accelerometers measure acceleration (high-frequency casing faults).
Eddy current sensors are non-contact and are mandated for shaft displacement and position monitoring by the API 670 standard.
Velocity is the preferred metric for overall machine health, while acceleration is best for detecting early-stage faults in rolling element bearings and gear mesh problems.

The accuracy and efficacy of a vibration monitoring system rely entirely on selecting the correct sensor for the application. The three main sensor types—Accelerometers, Eddy Current Sensors, and Velocity Probes—each measure a different physical parameter of vibration and are suited for distinct monitoring tasks.

Accelerometer

An accelerometer measures acceleration forces (typically in units of ‘g’), which can be static (like gravity) or dynamic (caused by movement or vibrations).

• Operation: Converts mechanical motion into an electrical signal using a piezoelectric crystal. When the sensor accelerates, an attached mass causes the crystal to deform, generating a proportional electrical signal.
• Applications: Ideal for high-frequency measurements, such as detecting early-stage faults in rolling element bearings, gear mesh, and cavitation.
• Industrial Monitoring: Monitoring vibrations and accelerations in machinery casings. The FM-ACCL-100 Series sensors are standardized at 100 mV/g sensitivity.

Eddy Current Sensor (Proximity Probe)

Eddy current sensors detect displacement, position, or proximity of conductive materials without physical contact. They are non-contact devices that conform to the stringent API 670 Standard for turbomachinery protection.

• Operation: The sensor coil generates an alternating magnetic field. When a conductive target (e.g., a rotating shaft) approaches, eddy currents are induced, creating an opposing magnetic field. The change in the magnetic field corresponds to the distance (displacement) of the target.
• Applications: Mandatory for monitoring the relative movement of the shaft inside the bearing, including shaft vibration (radial), axial thrust, and differential expansion in turbines.
• Key Specifications: Typically have a sensitivity of 200 mV/mil (displacement) and a power supply of Max. -24V DC.

Velocity Probes (Velocimeters)

Velocity probes measure the velocity of vibrations in a structure or machine casing.

• Operation: Based on electromagnetic induction. When the probe (attached to a vibrating surface) moves, the relative motion between a coil and an internal magnet induces a voltage in the coil that is proportional to the vibration velocity.
• Applications: Preferred for monitoring mid-to-low frequency overall vibration severity in machinery casings, bearings, pumps, and motors.
• Key Specifications: The FM-VEL-004B Series has a typical sensitivity of 3.99 mV/mm/sec. They are robust and effective over a broad frequency range.

Vibration Sensor Comparison Chart