Vibration Measurement Instruments.
You can understand the basics of Vibration measurement from following article. Basics of Vibration Measurement
A transducer is a device for converting the mechanical motion of vibration into an electrical signal,commonly called pickup.
Vibration transducer measure motion.
There are three kinds of transducers-
The most common type of displacement transducer is proximity probe.
Operates on the eddy current principle.
It sets up a high-frequency electric field in the gap between the end of the probe and the metal surface that is moving.
Senses the change in the gap.
Measures relative displacement not absolute displacement.
Sensitive to shaft surface defects such as scratches,dents and vibrations in conductivity and permeability.
Senses shaft runout,and it is very difficult to distinguish vibration from runout.
The practical maximum frequency of proximity probes is about 1500 Hz.The minimum frequency is zero.It can also measure static displacement.
A useful application of proximity probes is to measure very slow relative movement like thermal expansion.
It is useful in situations where the vibrating part cannot tolerate the mass of the pickup.