What is the Wiegand Effect?
Ferromagnetic materials – such as iron, nickel, cobalt, or alloys containing these elements – have a special property: When samples are exposed to an external magnetic field, they become magnetized, creating a magnetic field of their own. Moreover, they remain magnetized when the external field is removed.
Ferromagnetic materials vary in terms of their magnetic ‘hardness’. Magnetically hard materials, such as the materials used for permanent magnets have high coercivity and require strong magnetic fields to become magnetized. They also resist becoming demagnetized when exposed to an external field of opposite polarity. On the other hand, magnetically soft materials, such as mild steel, have low coercivity and easily adopt the magnetic polarity of the external field. These materials are used in applications such as transformer plates and recording heads where the ability to easily change magnetic polarity is an asset.
In 1919, the German physicist Heinrich Barkhausen noticed that when the magnetic polarity of a sample of ferromagnetic material is reversed by a changing external field, the process proceeded as a series of small incremental steps. Barkhausen postulated that the uneven magnetic transformation in his samples was the result of many microscopic magnetic domains in the material initially resisting the change in polarity, then abruptly realigning themselves to the external field when it became strong enough. Barkhausen’s experiments helped to confirm the existence of magnetic domains – microscopic zones in the crystalline structure of a ferromagnetic material where the magnetic moments of atoms all have the same orientation.
John Wiegand, an American inventor, discovered a way of exploiting the Barkhausen effect. Wiegand’s breakthrough was to develop a way of manipulating the properties of Vicalloy wire (vanadium-iron-cobalt alloy) to create a magnetically soft inner core and a harder outer shell. With this arrangement, the magnetically harder shell will initially shield the softer core material, restraining its response to the external field. However, once a critical threshold is reached, the polarity of some domains in the core will change to match the external field. When this occurs, other nearby magnetic domains will feel the combined effect of the external field and the fields of their newly changed neighbors and switch as well. This causes a cascade effect, with the magnetic polarity of the whole body of material changing within a few microseconds. In effect then, the incremental changes in magnetic polarity observed by Barkhausen occur very quickly in a coordinated manner.
The process for manufacturing Wiegand wire involves annealing the wire to provide a uniform base condition, then subjecting it to a combination of twisting and stretching loads to develop a mechanically harder outer shell (strain hardening). This also has the effect of creating a magnetically hard outer layer. Once the material has been selectively hardened, the wire is heat-treated to stabilize its structure. This process – and the machinery that carries it out – were developed by John Wiegand and his associates through a painstaking series of experiments. The process, and the machinery, are still in use today.
Putting the Wiegand Effect to work
The defining feature of the Wiegand effect is that the polarity reversal of a sample of Wiegand wire will always occur abruptly, regardless of how quickly or slowly the external magnetic field changes. Moreover, if a fine coil of copper wire is wrapped around a section of Wiegand wire, a distinct current pulse will be induced in the coil with each polarity reversal. The relationship between moving magnetic fields and induced currents is basic to electromagnetics and is used in many applications, ranging from dynamos to reading heads for disk drives. However, in these ‘conventional’ devices, the strength of the induced current is proportional to the rate of change of the magnetic field. If the change happens too slowly, the power output may be too low to be useful. By contrast, with the Wiegand affect the energy content of the electrical pulse triggered by each reversal is largely consistent. As well, the pulse output has a very high signal-to-noise ratio.
This property opens the door to interesting applications.
Wiegand sensors in fluid meters: Meters for water or natural gas supplies are typically based on an impeller that turns with the fluid. Flow rates in these applications are highly variable, so meter makers need a way of reliably counting revolutions over a wide range or rotational speeds. Enter the Wiegand effect! A Wiegand sensor (a segment of Wiegand wire wrapped in a fine copper coil) is positioned next to a permanent magnet that rotates with the impeller wheel. Each complete rotation of this magnet will trigger two polarity reversal in the Wiegand wire (N-S to S-N, then S-N to N-S). By counting the current pulses generated by the Wiegand sensor, the meter can reliably measure the volume of fluid delivered.
Multi-turn magnetic rotary encoders are another application area for Wiegand sensors. These instruments are used in a wide array of industrial applications to measure the rotation of motors, wheels, cable drums, and other rotating mechanisms. Multiturn encoders provide the control system with both angular displacement within a single rotation and a tally of the total number of rotations that the device has experienced. Here again, a Wiegand sensor is ideal for rotation counting. (Angular position within a single turn is typically measured by Hall-effect sensors that can determine the orientation of the magnetic field to a high level of precision.) Multiturn encoders from POSITAL and other manufacturers use Wiegand sensors both to mark each complete rotation and to harvest enough energy from the electrical pulse to power the counter circuitry. This gives these encoders a distinct advantage: these rotation counters are effectively self-powered and capable of maintaining a reliable rotation count, even if the external electricity supply to the devices is interrupted. This eliminates any need to re-set the machinery from a known starting point in the event of a power failure. No backup batteries are needed!
Energy harvesting and transmission with Wiegand sensors.
The energy harvesting concept has applications far beyond rotary encoders. With no need for mechanical or electrical connections between a rotating or oscillating magnet and a Wiegand sensor, this technology can be used to provide power to isolated electronic devices. This could include monitoring instruments for canned or hermetic pumps or even providing energy for surgically implanted medical devices.
Wiegand sensors in surface-mountable modules
The amount of electrical energy generated by current-generation Wiegand sensors is quite modest – about 190 nanojoules. However, research underway at POSITAL (funded in part by the German ministry for science and technology) aims to increase this significantly by combining several Wiegand wire assemblies in a single unit. Once this has been accomplished, the available energy will be sufficient to power more complex electronic devices – potentially including low-power or passive wireless communications. This could lead to self-contained sensors that take their power entirely from a nearby magnetic field – suitable for the Internet of Things.
Proximity sensing with the Wiegand effect. Wiegand systems can also be used as proximity sensors, where the presence of a large ferromagnetic body distorts the magnetic field around the Wiegand wire enough to trigger a polarity reversal.
Proximity sensing can also work with moving Wiegand wires. An early commercial application of the Wiegand effect was access cards for security systems. Short lengths of Wiegand wire were embedded side-by-side in plastic cards. The spacing between these pieces of wire would be different for each card and would spell out a unique binary code. In use, a card would be swiped through a card reader that contained permanent magnets arranged to trigger polarity reversals in the Wiegand wires. By detecting these polarity changes, the device could read the coded number and decide whether to grant access to the card’s owner. The reader would work effectively regardless of how slowly or quickly the card was swiped. This technology was used successfully for several decades before being supplanted by RFID technology.
The Future
R&D efforts are underway to increase the power output of Wiegand sensors and to improve manufacturing processes through better understanding of the microscopic magnetic domain structure within Wiegand wires. With improved performance and lower costs on the horizon, engineers are exploring innovative ways of using the unique characteristics of this fascinating phenomenon.
Posital
www.posital.com/en/