With magnetostrictive position sensors, you can attach the position magnet to the part to be measured, "floating" it. Alternately, you can use a sensor with a captive magnet enclosed in a sliding bearing.

The properties of magnetostriction have received much attention lately with their recent application in torque sensors. One of the earliest uses of magnetostriction, however, has been in linear position measurement. Sensors based on this technology are nearly immune to the effects of electrical noise, vibration, and temperature. In addition, because they are non-contact devices, they have a long operating life. They also offer the longest stroke ranges of linear position sensors.

How these sensors use magnetostrictive properties, though, is slightly different from the techniques used in torque sensors. In position sensors, the Villari and Wiedemann effects play important roles in determining position.

The effects of the moment

Like other linear position sensors, magnetostrictive versions have one part that moves with the machine component or device while the other part stays stationary. Instead of physical contact and the resultant wear, though, interaction between the parts is achieved through a magnetic field.

With magnetostrictive position sensors, you can attach the position magnet to the part to be measured, "floating" it. Alternately, you can use a sensor with a captive magnet enclosed in a sliding bearing.

Two magnetic fields detect the position of a movable object. A third changes the result of that detected position into a signal usable by a controller.

While the position magnet tracks the motion of an object, such as a machine tool or cylinder, it supplies an axial magnetic field – one of the two fields used in position detection. The second field comes from a stationary wire made of magnetostrictive material, which is known as the waveguide. That field is generated when the sensor electronics send a current through the wire. The current creates a uniform magnetic field along the wire length.

Magnetostrictive linear position sensors make use of the Wiedemann Effect to determine position. This effect is the mechanical torsion that occurs when an electric current passes along or through a long, thin ferromagnetic material while it is subjected to an axial magnetic field.
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The Wiedemann effect occurs at the point where the axial field intersects the wire's field. The wire undergoes torsional strain due to the interaction of the magnetic fields.

Because the current is applied as a pulse of approximately 1 or 2 μsec duration, the torsional-strain pulse travels in the wire like a sonic wave. It travels at the speed of sound in the waveguide material, approximately 3,000 mps, and is linear, repeatable, and insensitive to temperature, depending on the alloy.

As the sensor electronics apply the current, they also start a timer. When the torsional-strain pulse is created, it travels outward from its point of origin toward both ends of the sensor. At one end, a device called a pickup detects the wave and stops the timer. The elapsed time indicates the distance between the position magnet and the pickup. At the other end, a device called a damp absorbs the strain pulse to prevent any interference by reflections from the end of the waveguide.

The pick up

Applying a magnetic field to magnetostrictive material causes stress which changes its physical properties. Conversely, applying a stress to the material changes its magnetic properties, such as permeability. This "reverse" magnetostriction is the Villari effect, which the sensor's pickup device uses to help change the position data into an electronic signal.

A position magnet applies an axial magnetic field to a point on a magnetostrictive wire. Sensor electronics send a current through the wire, magnetizing it. The intersection of the two magnetic fields causes the wire to undergo torsional stress. This results in a sonic wave that moves away from its origin and toward each end of the waveguide. At one end, the sensor tracks how long it took the wave to reach it. Through another piece of magnetostrictive material, a bias magnet, and a coil, the sensor turns this location data into an output signal for the controller. At the opposite end of the waveguide, a damping element stops waves traveling in this direction to prevent interference by reflections.
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A small piece of magnetostrictive material, the "tape," is welded to the waveguide near one end. The tape passes through a coil and is magnetized by a small permanent magnet called the bias magnet. When a sonic wave propagates down the waveguide and then the tape, the tape's magnetic flux density changes due to the change in permeability (the Villari effect) and produces a voltage output pulse from the coil (the Faraday effect). The electronic circuitry detects the voltage pulse and converts it into the desired output, which can be dc voltage, current, PWM, or start-stop digital pulses. The output can transmit across the CANbus, Profibus, Serial Synchronous Interface, HART, and other communication protocols.

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