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Vibration measurement

 

Vibration measurement


Definition: 

Vibration is defined as a periodic motion of the particles of an elastic body in alternatively opposite direction from the position of equilibrium.




Acceleration, velocity, and displacement are the main parameters in vibration measurement.

Unit of vibration 

 unit of vibration is mm/ sec.




Types of vibration


There are two main types of vibration measurement performed in the industry.

 

Axial Vibration

This is a longitudinal vibration, it is measured in parallel to the shaft of a motor. This type of measurement is done to detect a shaft misalignment.


Radial Vibration

  This vibration occurs as a force applied outward from the shaft. Radial vibration would occur if there is a heavy spot in the motor as it rotates.

If there is a deformed fan blade, as the fan spins, the deformed fan blade would pull outwardly on the shaft of the motor causing radial vibration.


Types of vibration sensor

Following are types of vibration measurement

1. Accelerometer 

2. Velocity sensor

3. Proximity probes (capacitive or eddy current)

4. Laser displacement sensor

1. Accelerometer 

Accelerometers are used to measure the acceleration of a body. 


 Piezoelectric

In this type of accelerometer Acceleration acting on a seismic mass exerts a force on piezoelectric crystals, so these piezoelectric crystals then produce a proportional electric charge, This electric charge is directly proportional to the acceleration, so by amplifying this signal and converting it to a standard signal usually 4 to 20 mA, we can measure acceleration.

 

2. Velocity sensor

 Electromagnetic linear velocity transducer typically used to measure oscillatory velocity. A permanent magnet moving back and forth within a coil winding induces an emf in the winding. This emf is proportional to oscillatory velocity.

 Electromagnetic tachometer generator Used to measure the angular velocity of vibrating objects. Produce an output voltage that is proportional to the angular velocity. 

DC tachometer used a permanent magnet, while AC tachometer operates as a variable coupling transformer.

3. Proximity sensor

 Eddy current sensor probe

 Eddy current is formed when a moving magnetic field interacts with a conductor or vice versa. The relative motion causes a flow of electrons in a conductor.

Reluctive Accelerometer

 Made up of differential transformer type or inductance bridge type.

AC output of these varies in phase as well as amplitude varies with change in acceleration, this change in inductance is converted to a standard signal that is directly proportional to the acceleration.

Capacitive proximity sensor

 The capacitive proximity sensor uses the capacitance change due to the motion of the body which acceleration we want to measure, so thus capacitance change is converted to a standard signal that is directly proportional to the acceleration of the body.

Capacitive 

 In a capacitive type accelerometer, two electrodes are used, one is a fixed electrode and the other one is a moving electrode with fixed and moving bodies, and change in capacitance due to vibration is measured and then converted to a standard signal.

4. Laser displacement sensor

Uses a laser to measure displacement. In this measurement system a  laser source, a laser splitter, a reflector, a Bragg cell, and a photocell are necessary to measure displacement.


Potentiometer

 Relatively cheap and used to measure slow varying acceleration. In these, the displacement of a spring-mass system is mechanically linked to a wiper arm which moves along a potentiometric resistive element. This change in resistance is directly proportional to acceleration, the resistance change is then converted to a standard signal of 4 to 20 mA.


Servo accelerometer

Strain gauge accelerometer

The displacement of the spring mass system is converted into a change in resistance of the Wheatstone bridge.

Common faults in vibration sensors

* No power

* Targets are broken or bent

* Sensor faulty

* Wire broken

* Sensor loose







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