Wiegand effect

The Wiegand effect is named after its discoverer, John R. Wiegand.

Overview

The Wiegand effect is a pulse-generating phenomenon in a special alloy wire that is processed in such a way as to create two distinct magnetic regions in the same homogeneous piece of wire, referred to as a shell and a core. It occurs when such a specially processed wire (a "Wiegand wire") is moved past a magnetic field. The two distinct magnetic regions react differently to any applied magnetic field: the shell requires a strong magnetic field to reverse its magnetic polarity, whereas the core will revert under weaker field conditions. The polarity of the wire will very rapidly shift and generate strong, short (~10 µs) electrical pulses without any additional external power being supplied. This is known as the "Barkhausen jump" or "Barkhausen effect". The Barkhausen jump can be detected by a coil wrapped around the material, when the small amount of voltage described above is generated.

Wiegand wire

Wiegand wire is produced by cold-working a 0.010 inch (0.254 millimeter) diameter ferromagnetic wire made of Vicalloy, a mixture of cobalt, iron, and vanadium. The cold-working process consists of increasing amounts of twist and de-twist of the wire under applied tension in several steps. The wire is then age-hardened to hold in the tension built up during the cold-working process. This procedure causes the Wiegand wire to have a soft magnetic center, the core, and a work-hardened surface with a higher magnetic coercivity, the shell. When an alternating magnetic field of proper strength is applied to the Wiegand wire, the core's magnetic field will switch polarity and then reverse again, causing a Wiegand pulse to be generated. Fact|date=August 2007

The patented cold-working process that produces the Wiegand wire permanently locks in the ability to exhibit Barkhausen effect discontinuities in the material. To achieve magnetic switching, the wire is put in the presence of alternating longitudinal magnetic fields. The resultant hysteresis loop contains large discontinuous jumps known as Barkhausen discontinuities that occur due to shell and core polarity switching. Fact|date=August 2007

Uses in sensor technology

Sensor based on the proprietary and patented Wiegand effect require only a few simple components: a short length of Wiegand wire, a sensing coil, and alternating fields, which are generally derived from small permanent magnets. Sensors of this type can be used across a wide range of conditions. The Wiegand effect occurs from -80 degrees Celsius to 260 degrees Celsius. Wiegand effect sensors are used in water, gas, and electric meters for electronic indexing. They also have many automotive applications such as antilock braking, speed sensing, and position indicators. They have been used in anemometers and other wind speed applications, machine controls, shaft speed sensing, and numerous rotational counting applications. Wiegand wires are also used in access control systems.

ee also

* Access badge
* ID Card
* Keycard
* Physical Security
* Proximity card
* RFID
* Security
* Security engineering
* Wiegand interface