Magnetostrictive Sensors


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A magnetostrictive displacement sensor uses a ferromagnetic element to detect the location of a position magnet that is displaced along its length. The position magnet is attached to a member whose position is to be sensed, and the sensor body remains stationary. The position magnet moves along the measuring area without contacting the sensing element.

Ferromagnetic materials such as iron and nickel display the property called magnetostriction. Application of a magnetic field to these materials causes a strain in the crystal structure, resulting in a change in the size and shape of the material. A material exhibiting positive magnetostriction will expand when magnetized. Conversely, with negative magnetostriction, the material contracts when magnetized.

The ferromagnetic materials used in magnetostrictive displacement sensors are transition metals, such as iron, nickel, and cobalt. In these metals, the 3d electron shell is not completely filled, which allows the formation of a magnetic moment (i.e., the shells closer to the nucleus are complete, and they do not contribute to the magnetic moment).

Magnetostrictive for displacement measure:

magnetostrictiveee

In a displacement sensor, a ferromagnetic wire or tube called the waveguide is used as the sensing
element. The sensor measures the distance between the position magnet and the pickup. To start a measurement, a current pulse I (called the interrogation pulse), is applied to the waveguide. This causes a magnetic field to instantly surround it along its full length.

In a magnetostrictive position sensor, the current is a pulse of approximately 1 to 2 ms duration. A
torsional mechanical wave is launched at the location of the position magnet due to the Wiedemann
effect. Portions of this wave travel both toward and away from the pickup. The wave travelling along the waveguide toward the pickup is detected when it arrives at the pickup. The time measurement between application of the current pulse (launching of the torsion wave at the position magnet) until its detection by the pickup represents the location of the position magnet.

The time measurement can be buffered and used directly as the sensor output, or it can be conditioned inside the sensor to provide various output types, including analog voltage or current, pulse width modulation, CANbus, SSI, HART, Profibus, etc.