What is vibration
Vibration is the response of an elastic system to a dynamic disturbance. Vibration can be classified into two they are free and forced, free vibration occurs when a system oscillates under the action of forces inherent in the system itself, and when external impressed forces are absent. A system under free vibration will vibrate at one or more of its natural frequency and it is the property of a dynamic system determined by it’s mass and stiffness distribution. The vibration which takes place under the excitation of an external force can be called forced vibration.
How to select vibration sensors
- According to the sensitivity and frequency range
- Defect monitoring
- Sensors must be able to survive extreme temperatures
- Sensors signal range must be able to cover high vibration amplitude and also the highest vibration levels present at the measurement point
- It must be able to use in a hazardous environment
What are the characteristics of vibration
Vibration can be seen in many operating machines, machines condition and mechanical problems can be determined by its vibration characteristic. Vibration can be quantified in terms of displacement, velocity, or acceleration these characteristics of vibration are measured to determine the amount of severity of the vibration. The displacement, velocity or acceleration of a vibration can be referred to as the amplitude of the vibration.
The total distance traveled by a vibration part from one extreme limit of travel to the other extreme limit of travel can be referred to as the peak to peak displacement. There is movement in vibration and the movement will be at some speed, velocity of the motion is a characteristic of the vibration but it is changing constantly so the highest velocity is selected for measurement. Vibration velocity approaches zero at the extreme limits of travel so it must accelerate to pick up speed as it travels towards the other extreme limit.
Eddy current probes are non-contact sensors which are primarily used to measure shaft vibration, shaft/rotor position, and clearance, they are also referred to as displacement probes eddy current sensors are usually applied on machines they have excellent frequency response with no lower frequency limit and they can also be used to provide a trigger input for phase-related measurements.
Velocity sensors can be used for low and medium frequency measurements they are useful for vibration monitoring and balancing operations on rotation machinery when compared to accelerometers velocity sensors have lower sensitivity for high-frequency vibrations. Traditional velocity sensors have mechanical design and velocity signal is generated by using an electromagnetic system but now piezoelectric velocity sensors are widely used because of their improved capabilities and because of their rugged and smaller size design.
Accelerometers operate on the piezoelectric principle, piezoelectric accelerometers have a constant signal over a wide frequency range for a given mechanical acceleration level and are very useful for all types of vibration measurements. Acceleration coordinated to velocity could be used for low-frequency measurements. Basic acceleration sensor has a good signal to noise ratio over a wide dynamic range they can be used to measure low to very high frequencies and are available in a wide variety of general-purpose and application-specific designs, piezoelectric sensors are versatile, reliable and the most popular vibration sensor for machinery monitoring.