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SENSORS
Sizing and applying
Many types of industrial processing equipment use sensors to provide information needed to monitor and control processes. This allows design parameters to be maintained or modified in order to manufacture products at the desired level of quality and throughput. Typical sensors measure variables such as temperature, pressure, flow, force, or position. The type of measurement, as well as the sensor technology, dictates the set of criteria that is important in specifying and sizing the appropriate sensor.
Linear position sensors measure absolute distance along a motion axis. They are available in several technologies, each having its own advantages and disadvantages. This article presents information on the application of magnetostrictive linear position sensors, which are gaining popularity due to their accuracy and reliability. A comparison of magnetostrictive sensors to other linear position sensors is also included.
Theory of operation
A magnetostrictive position sensor measures distance between a position magnet and the head end of the sensing rod. The position magnet does not touch the sensing rod, so there are no parts to wear out.
The sensing rod is mounted along the motion axis to be measured, and the position magnet is attached to the member that will be moving. The head includes an electronics module, which reports position information to a controller (or other receiving device) in the appropriate analog or digital format.
As shown in Figure 1, a magnetostrictive position sensor includes five basic components position magnet, waveguide, pickup, damp, and electronics module. A protective tube usually covers the waveguide.
The position magnet is a permanent magnet, often made in the shape of a ring, which travels along the sensing rod. The waveguide is housed within the sensing rod, and is a small diameter (approximately 0.30 to 0.80 mm) tubing or wire made from a magnetostrictive material.
Magnetostriction is a property of certain materials, including iron, nickel, cobalt, and some of their alloys, in which application of a magnetic field causes strain that results in a change in the size or shape of the material.
The waveguide is so named because a sonic wave travels in it during operation of the sensor. The sonic wave is generated by interaction between the magnetic field from the position magnet and a second magnetic field...