Turbine Supervisory Guide - SENSONICS

Catalogue Turbine Supervisory Guide

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Eddy Probe are used for low frequency measurements (typically dc to 1kHz), and the velocity pick-ups cover the mid-frequency band (10Hz to 2kHz). Industrial accelerometers generally have the widest frequency range and certainly the highest (1Hz to 10kHz typically).

In practice, combinations of transducers are employed to supply different types of information simultaneously. For example, an Eddy Probe would be required to measure shaft vibration relative to a bearing housing, while an accelerometer (or velocity pick up) would measure the vibration of the housing itself and the bearing.

Each type of transducer has an area of measurement in which it is particularly useful. In the figure below, the relationship is clearly shown. Note that the velocity curve is constant. Acceleration (the first derivative of velocity) increases with frequency, while displacement (the first integral of velocity) decreases.

At 2000Hz (or 120000rpm) this example shows that measuring displacement would be impractical: the signal would be too low. A velocity pick up would be adequate, since the 0.3in/sec velocity produces a substantial voltage. However, the highest voltage level would be produced by an accelerometer.


TRANSDUCER	ADVANTAGES	DISADVANTAGES
Eddy current probes	Static and dynamic displacement measurements Immune to non-conductive materials such as plastic, wood oil and water Large linear ranges Non-contacting	Requires power supply Calibrated cable length between probe and driver unit
Velocity Transducer	Self-Generating Doesn't require an amplifier Can provide displacement data via integrator Good general purpose vibration transducer	Frequency response limits its lower frequency to 5-10Hz Moving parts make it prone to wear Phase response varies with frequency
Accelerometer	Wide frequency range High operating temperature (external electronics) No moving parts Can provide acceleration, velocity or displacement data	Requires charge amplifier Displacement information normally restricted to greater than 10Hz due to problems of double integration
LVDT	Infinite resolution Robust High temperature rating Adjustable zero and gain controls	Contacting Limited frequency response