Wiegand effect

Wiegand sensors or pulse-wire sensors included as an essential component Wiegand wires having by parallel soft and hard magnetic regions with a hysteresis curve (usually two ) distinct jump points, a kind of macroscopic Barkhausen effect. The sudden change in the magnetization caused in a nearby coil a voltage pulse whose size and shape does not depend on how fast the external magnetic field changes. The name goes back to John Richard Wiegand, in 1972 dealt with metallic alloys and as a consequence the said wires patented ..

Wiegand sensors are primarily used for motion detection means (rotary) encoders and encoding of identification means of access control systems.

Design and function

The most important part of a Wiegand sensor is the Wiegand wire. It is made of a special alloy with a hard magnetic metal a coat and a soft magnetic metal as the core. The typical diameter of such wires is 0.25 mm.

In these two areas a different magnetization (mantle and core ) can now be observed. Considering the possibilities of the magnetization, there are three cases: one in which the core and shell is equal magnetized, and two with opposite magnetization.

The two areas react differently in the presence of a magnetic field. Thus, the hard magnetic sheath changes its magnetic polarization only in the presence of stronger magnetic fields, when the magnetic fields required for the core material. Now the entire wire by an external magnetic field is surrounded. The field lines of the soft magnetic metal depend on which of the external field.

The external magnetic field exceeds the coercive field, there is an abrupt magnetization of the core. Ummagnetisierungsspannungspuls this is connected to a change of magnetic flux and can be used with the aid of a coil surrounding the Wiegand wire detected.

Both poles of a magnet on the Wiegand sensor passed, so four signals occur, first the core and then the outer skin of the wire is re-magnetized. It is also possible to reset the Wiegand sensor each again by an externally generated field.

Preparation of the Wiegand wire

By cold working and subsequent annealing a thin wire is produced (diameter about 300 microns ). The core of the wire is magnetically soft due to the special manufacturing process, while the outer skin is magnetically hard. The preparation also requires that the magnetic moments are oriented in the axial direction.

Application

Will use the Wiegand sensor among other things as access cards, encoders and as well as distance and proximity sensor.

Access cards

In an access card (door card) a series of short Wiegand wires are embedded. A reader recognizes the pull through the impulses of the Wiegand wires and compares it with the reference pattern previously stored. Other safety features such as a magnetic strip can also be applied and queried. Due to the complicated production of the Wiegand wire Wiegand cards are considered relatively secure against forgery.

Speed ​​measurement ( with flywheel )

A number of Wiegand wires is embedded along the outer circumference of a pole wheel. An externally mounted read head detects the pulses of the Wiegand wires.

Encoders

Here Wiegand sensors are used to realize in magnetic multi-turn absolute value encoders, the acquisition of revolutions in the de-energized state. The shaft of the rotary encoder is rotated, a magnet mounted thereon creates a variable magnetic field on the fixedly mounted Wiegand sensor. The voltage pulses generated thereby are used to store the number of revolutions in a non -volatile memory. This mechanism works even at very slow speeds, it triggers the else from the storage of revolutions used gear / battery.