Measurement Devices and Sensors

To measure vibration related phenomena it is good to have a basic grasp of the idea behind vibration sensors. Isaac Newton, an early physicist, stated that an object in motion will tend to remain in motion and an object a rest will tend to remain at rest. If we apply this principle by mounting a mass on a spring within a vibrating object, we acquire a vector quantity that describes the relationship between the mass and the object. These dimensions will constantly change relation to the object moving around the mass as well as the induced movement of the mass. The mass is commonly retained by some type of damping mechanism or material to limit the amount of induced movement. Most sensors have a useable frequency range which quantifies the relationship of the damping material to the suspended mass. Above this range is found the resonant frequency of the device. Below the useable frequency range is a non-responsive range, in which the damping device will not allow the mass to move.

A common instrument for vibration measurement is an accelerometer. An accelerometer is a mass mounted to a piezo material. This unique material generates a very small electrical charge under stress, whether compressive or shear. This assembly is then encased within a sealed package, often a cylindrical, stainless steel design. The piezo material is often excited with an electrical bias, a DC voltage.

Another device, often used in turbines and large sleeve bearing applications, is a proximity probe. This device creates a small magnetic field near the tip of the sensor. The position of the shaft causes fluctuations within this magnetic field. These small fluctuations provide an indirect(non-contacting) measurement of shaft movement with the bearings.

Another method is the one used on speakers and microphones for years. When a magnet is placed within a coil, the movement of the magnet induces and electrical charge. Often the magnetic is damped by light oil solution within these sensors.

The sensor selected to acquire vibration signatures must take into account several factors and be specifically designed to give the desired result. This selection process can be intensive, for this reason a variety of pre-packaged equipment is available. If such packages are not available for a specific application, consulting with several manufacturers may provide the necessary equipment. Consultants are also a good source of sensor information. They often operate specialty and prototype packages that eventually become a standard product line for manufacturers.

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Last updated: November 24, 2002