All about magnetostriction

A magnetostrictive sensor is an absolute, contact-free and wear-free position measuring system. It is extremely sturdy, making it suitable even for applications where other measuring techniques would fail. This linear measuring system consists of a magnetostrictive wave guide and a movable magnet for determining position. The measuring principle is based on two physical effects: the Wiedemann effect and the Villari effect.

To create the Wiedemann effect, a current impulse is sent through the sensor’s wave guide. This current impulse generates a circular magnetic field, which propagates at the speed of light around the wave guide. If this circular magnetic field makes contact with a magnetic field anywhere along its length, a torsional mechanical-elastic density wave is triggered at the overlap area of the two magnetic fields as a result of magnetostriction. This wave propagates in the position sensor at ultrasonic speed.

The sensor head of the position sensor contains a receptor which detects the arrival of this wave. The magneto-elastic Villari effect is used as the method of detection. The position between the detector coil and the magnet which can be moved lengthways along the sensor is determined by measuring the time difference between the electrical induction current impulse and the voltage pulse generated via the Villari effect in the detector coil (time-of-flight principle).

This time difference can be converted using various well-known methods into analog (4-20mA, 0- 10V) or digital output signals. The time-of-flight signals can however also be evaluated directly by commonly-available interface modules or counter and time-measuring devices.

To adapt a magnetostrictive sensor to a hydraulic cylinder, a position magnet is installed onto the cylinder’s piston, which reflects the current impulse in the waveguide at every point of the stroke cycle.