Velocity Sensors:
Abstract:
In the monitoring and analysis of vibrations in industrial machinery and other devices, machine mounted sensors are of critical importance. There are three parameters representing motion that can be detected by vibration monitors, and they include velocity, acceleration and displacement. These parameters are mathematically related and therefore they can be derived from a variety of vibration sensors. Due to this relationship, the choice of sensor, whether acceleration, velocity or displacement is dependent on the signal levels involved and the frequencies of interest. This paper briefly explores and discusses the history, theories, and applications of velocity sensors as well as future advancements in this sensors technology.
Introduction:
In the early 1900’s analysts, relied heavily on velocity sensors and vibration severity charts based on velocity measurements. Even today, majority of vibration information in machinery is recorded and quantified in terms of velocity usually measured in mm/sec in the SI unit system or inches per second (IPS) in the United States. Velocity readings are usually recommended for measurements in the frequency band ranging from 1.7 to 500Hz (Guy).
Some of the earliest vibration sensors were electro-dynamic pickups and in the 1930’s Westinghouse Electric made the first vibration transducers that use speaker coils. When driven by mechanical vibrations, the coil produced a voltage output that was directly proportional to velocity. Later, this technology evolved into the modern electro-dynamic velocity sensors which have been used for many years successfully.
Velocity sensors basically consist of a permanent magnet with a moving coil suspended in its magnetic field. Velocity is given as input and this causes the coil to rotate in the magnetic field consequently generating an electromotive force (e.m.f) in the coil. The induced e.m.f is proportional to the input velocity, and is thus used as a measure of the actual input velocity. The instantaneous voltage generated is given by the equation:V=N((d∅)/dt)
Where, V is the voltage, N is the total number of turns in the coil, and ((d∅)/dt) is the rate of flux change in the coil.
The above equation has been developed based on Faraday’s Law of Induction which is based on the premise that changing magnetic flux (F) is proportional to the length of coil wire, the relative velocity between the coil and the magnet, and the magnetic field strength. The voltage generated is still proportional regardless of the type of velocity in question such as sinusoidal, linear or random. Since damping is electrically obtained, it is assumed that the voltage sensor is highly stable under different temperature conditions (Harris and Piersol).
Applications of velocity sensors:
Velocity sensors have wide applications in measuring low to medium frequencies. They are especially useful for monitoring vibrations and for balancing operations in rotating machines such as paper rolling mills. However, compared to other sensors such as accelerometers, velocity sensors have low sensitivity to vibrations of high frequency and therefore have less susceptibility to amplifier overloads. Overloads can affect the accuracy of low frequency, low amplitude signals.
Future of velocity sensors:
Traditional velocity sensors used electromagnetic systems but for current and future applications, there are more robust piezoelectric velocity sensors which are essentially internally integrated accelerometers (Coleparmer.com). These new sensors have gained popularity due to their improved capabilities such as less susceptibility to magnetic interference, and the ability to measure frequencies accurately down to 1Hz or less (Guy).
Works Cited:
Coleparmer.com, 'Sensor Selection Guide'. N.p., 2014. Web. 17 Oct. 2014.
Guy, Kevin R. 'Monitoring and Analysis with Electronic Data Collectors'. 16Th Vibration Institute National Meeting. 1992. Print.
Harris, Cyril M, and Allan G Piersol. Harris' Shock and Vibration Handbook. New York: McGraw-Hill, 2002. Print.
