Vibration Monitoring & Analysis- Sensor selection Criterion

Sensor selection criterion:

Sensor selection is one of the most important things in vibration monitoring / Analysis system. Basically it’s the quality of vibration data input that decides the accuracy & relevance of the measurement. No matter how advanced electronics & complex analysis algorithm is used, the sensor being the first element in the line of measurement, has high importance in determining the quality of measurement. The mantra is gold in gold out, garbage in garbage out. Various factors and operating conditions like ambient temperature, magnetic field interferences, g range, frequency range, electromagnetic compatibility etc. decide the ruggedness of sensors in the field. Other machine parameters like type of bearings (Rolling element or journal) & machine speed decide the choice of measurement units & kind of sensor to be put in the field. Once the proper sensor selection has been done, the proper installation & sensor orientation becomes very important.

DISPLACEMENT SENSORS: These are also known as proximity sensors or eddy current sensors & are used to measure relative shaft vibration, shaft position and clearance. These probes are put on sleeve or oil film bearings (Journal bearings). These sensors are best suitable to measure low frequency and low amplitude displacements. If there is not any viability of mounting proximity probes on the machine, the accelerometer with double integrator circuit is also used for the displacement measurement. Normally in turbines & large compressors we find these probes.

VELOCITY SENSORS: Unlike proximity probes, velocity sensors are contact type sensors. Earlier electromagnetic sensors were used but due to mounting constraints & change in sensitivity issue with respect to time, now much rugged & sturdy piezoelectric sensors are used. These are accelerometers which are integrated once to get the velocity output. These sensors are used for low to medium frequency measurements (approx. up to 5000Hz). Majority of the machines lie in this frequency range (low to medium RPM) for vibration monitoring & balancing operations.

ACCELEROMETERS: Accelerometer is made of piezoelectric wafers & produces emf when there is force applied to it. These are the most preferred sensors for measuring vibration & have a very wide frequency range (almost from DC to 20 KHz). They are very useful for high speed machines & rolling element bearings. These are rugged devices & can sustain hash ambient conditions like corrosive environment & extremely high temperature (e.g. gas turbines). The sensor & associated electronics (charge amplifier) can be separated for high temperature applications. They have very good signal to noise ratio.   

 One should ask the questions like application frequency, type of bearing, environmental conditions, machine type, sensor size & mounting constraints before selecting suitable sensor.