Vibration measurement techniques

interviewquestions

#1

What is vibration?

Vibration is the motion of an object moving back and forth from its point of rest. When a force is applied to a stationary object connected a spring system, the object moves upper limit to lower limit continues the movement even if the force is removed and rest eventually due to damping or friction.

All the rotating machines produce the vibration that are functions of dynamics of the machines. Measuring the vibration of the machines can give information of machine condition on shaft alignment and balance the condition of bearings or gears, and the effect on the machine due to resonance from the housings, piping velocity, acceleration of the machine and other parameters of the machine.

The displacement, velocity and acceleration characteristics of vibration are measured to determine the severity of the vibration and these are often referred to as the ‘amplitude’ of the vibration.

There are different types of transducer used in vibration measuring devices, Microphones are the best example of vibration transducers which converts the sound vibration to an electrical signal, accelerometer used to measure the acceleration of solid structure, Force transducer to measure the force on a solid structure.

Different types of vibration transducers are:

Accelerometer:

An accelerometer is mounted on the object for which the acceleration should measure. The accelerometer is composed of an internal mass and a load cell attached to it with a spring. The system damping is negligible for an accelerometer.

The accelerometer load cell is usually a piezoelectric crystal and thus registers only compressive loads, necessitating a preload spring to keep it in compression. However, the piezoelectric crystal is inherently quite stiff in comparison to the preload spring.

Displacement transducer:

The displacement transducer consists of a moving part and a fixed part. The mobile part is attached to the moving part of the instrument to be tested while fixed acts as a reference. The internal of many rotary types of machinery has a number of quite small annular radians clearance gap between rotor and stator, Therefore one obvious potential consequence of excessive rotor vibration is rotor-stator rubbing contact.

Measuring the gap between transducer tip and the target rotor is done using two principles, the capacitive and inductive principle.

Velocity transducer:

The velocity transducer is self- generating transducer. The velocity transducer uses the principle of electromagnetism. As the test object attached to the transducer vibrates, the coil inside transducer forced the magnetic field by vibratory motion, thus a voltage signal representing the vibratory motion is produced.

This type of transducer can install easily on machines and less cost than other vibration transducers


Position and Motion Measurement