Magnetostrictive displacement transducer

Magnetostrictive displacement transducers are used to measure the distance between two points. The sensor consists of a fixed base, a waveguide , a moving permanent magnet and a transducer that converts mechanical vibration into an electrical signal. The result of the displacement transducer is the position, which is determined with the help of magnetostriction . The measuring system is largely insensitive to environmental influences such as temperature, shock, shock, vibrations and contamination. One example of application are rolling mills , steel mills and level measurement in tanks.

construction

Schematic structure of the displacement transducer

The measuring element is the waveguide and consists of a magnetostrictive material. This can e.g. B. be a tube made of a nickel-iron alloy with 0.7 mm outside and 0.5 mm inside diameter. A copper conductor is threaded through this tube. An alternative design is z. B. from a nickel wire with 0.8 mm outer diameter.

The measurement process is triggered directly by a short current pulse through the copper inner conductor or the nickel wire. The current generates a circular magnetic field which is bundled in the waveguide due to the soft magnetic properties of the waveguide. At the point of the path to be measured, a movable permanent magnet is used as a position sensor, the magnetic field lines of which run at right angles to the pulse magnetic field and are also bundled in the waveguide. In the area of the waveguide where the two magnetic fields are superimposed, an elastic deformation occurs in the micro area of the crystal structure due to magnetostriction, which generates a mechanical wave that propagates in both directions. The wave traveling to the end of the waveguide is dampened there, while the wave traveling to the signal converter generates an electrical signal by reversing the magnetostrictive effect. Alternative designs do not attenuate the signal in the direction of the waveguide end, but allow it to be reflected there. This has the advantage that the reflected signal can be used for error analysis and temperature compensation.

Measurement

The speed of propagation of the wave in the waveguide is z. B. 2800 m / s and is quite insensitive to environmental influences. Because the speed of the wave in the conductor is known (e.g. through calibration) and the time between sending the current pulse and receiving the magnetostrictive echo is measured, the path can be approximately determined as follows: ${\ displaystyle v}$ ${\ displaystyle t}$

{\ displaystyle s \ approx v \ cdot t.}

As a result, the distance can be determined with an accuracy limited only by the resolution of the time measurement. Typical arrangements achieve resolutions of approx. 1 µm. ${\ displaystyle s}$

Sources of error

A disadvantage of this method is that a. the speed of propagation of the wave in the conductor depends on the temperature of the conductor: ${\ displaystyle T}$

{\ displaystyle v = v (T) \ propto {\ sqrt {T}}.}

If no measures are taken to compensate for this deviation, the measurement becomes less accurate as the deviation of the conductor temperature from the calibration temperature (usually room temperature ) increases.

literature

David S. Nyce: Linear Position Sensors: Theory and Application. John Wiley & Sons, Hoboken (NJ) 2003, ISBN 0-471-23326-9 .