The “Wiegand effect” was discovered more than 40 years ago and while it has been used successfully in several specialized applications, its full potential for energy harvesting and signal generation has received only limited recognition. With enhancements to the energy output from Wiegand devices and the emergence of a new generation of ultra-efficient electronic chips, the technology is showing significant promise, especially in the exciting area of the Industrial Internet of Things (IIoT).
About the Wiegand Effect
The Wiegand effect is a physical phenomenon discovered in the 1970s by John Wiegand, an American musician and inventor who was interested in the use of magnetic effects in audio equipment. Wiegand found that when a specially prepared piece of ferromagnetic alloy (the Wiegand wire) is subject to a reversing external magnetic field, it will retain its magnetic polarity up to a certain point, then abruptly “flip” to the opposite polarity. This change in magnetic polarity takes place within a few microseconds.
The special magnetic properties of Wiegand wire are formed through a process that involves annealing, cold-working, and heat-treating Vicalloy wire (an alloy of vanadium, iron, and cobalt) to produce adjacent zones of magnetically hard and soft material. Magnetically soft materials change their magnetic polarity easily when exposed to external fields, while magnetically hard materials retain their original polarity until their coercivity limit is exceeded by a strong external field. The combination of hard and soft zones creates the conditions for the near-instantaneous polarity reversals that define the Wiegand effect.
Harnessing the Wiegand Effect
The Wiegand effect has two important properties that make it valuable for practical applications. First, the strength and duration of the polarity reversal remain nearly constant, regardless of how quickly or slowly the external magnetic field changes. Second, there is no mechanical contact between the sensor and the source of the alternating magnetic field. With no moving parts to wear out, service lifetimes can extend into billions of operating cycles.
An early application of the Wiegand effect was for security access cards. Short lengths of Wiegand wire were embedded in the plastic cards in an array that represented a binary number (the presence of a wire segment would represent a 1, while an empty space would mean 0). Each card would receive a unique number; when a card was swiped through a reader, magnets would trigger polarity reversals in the card’s wire segments. These would be detected by the reader to disclose the number encoded in the card.
Another important use for the Wiegand effect has been energy harvesting. Here, the rapid polarity reversals are used to generate current pulses in a coil of fine copper wire wrapped around a segment of Wiegand wire. The strength and duration of each current pulse is independent of the rate of change of the external magnetic field. This differentiates the Wiegand effect from other energy harvesting methods.
While simple dynamos convert rotary motion into electrical energy, their output power depends on the rate of rotation; at low speeds, power levels can be too low to be useful.
For piezoelectric systems, the energy output depends on the degree of resonance between the piezoelectric element and the source motion. With a Wiegand wire system, the amount of electrical energy generated with each reversal of the magnetic field remains consistent, no matter how quickly or slowly the external magnetic field changes. While the amount of electrical energy produced with each polarity reversal is modest — about 200 nano-joules — it is sufficient to activate low-power electronic circuits.
Energy harvesting by the Wiegand effect has been used successfully in more than a million devices including water and gas meters and industrial encoders (rotation measurement instruments). Here, the polarity reversals are triggered by a rotating permanent magnet, resulting in a rotation counter system that is entirely self-powered, with no need for external power sources or batteries.
The Wiegand effect has also been used as a reliable form of signal generation. Here again, the special advantage of the Wiegand effect is that it produces clear, readily detectable magnetic signals with each polarity flip. This makes it ideal for tachometers that must accurately measure a wide range of rotational speeds.
As mentioned earlier, an early application of the Wiegand effect was in access cards and the first company to produce Wiegand wire at commercial scale was HID Global, a security system provider. When RFID technologies replaced Wiegand-based access systems, POSITAL (a manufacturer of industrial position sensors) took over the production equipment and became the main source for high-quality Wiegand wire. In addition to supplying components for its own series of multi-turn industrial encoders, the company provides “micro power plants,” each comprised of a short length of Wiegand wire and a surrounding coil of fine copper wire, all packaged in a surface-mountable plastic housing. These are sold to manufacturers of fluid meters, encoders, and other types of rotating equipment.
As the main supplier of Wiegand technology, POSITAL has undertaken an extensive R&D effort aimed at improving product quality and power output. They have recently developed miniaturized versions of the SMD package and created new magnet configurations for special applications such as a ring-shaped encoder.
A new R&D program, funded by POSITAL and the German ministry of science and technology, aims to increase energy output of Wiegand sensors by a factor of 20 or more. When this goal is reached — and with the continued emergence of ultra-efficient electronic chips and low-power wireless communications technologies — there is potential for entirely energy-self-sufficient, maintenance-free instruments that can function as nodes on the Industrial Internet of Things.
Christian Fell, head of technology development for POSITAL, said, “The vision of the IIoT calls for thousands of smart sensors distributed throughout a digital factory, collecting data for process monitoring and optimization. If these devices can harvest the electricity they need directly from their surroundings, there will be enormous benefits in terms of simplifying network deployment and reducing maintenance costs — including the cost of installing, checking, and disposing of millions of backup batteries.”
But it it’s not only about sensors and the IIoT. In a totally different area, researchers at Japanese universities are exploring the possibility of using Wiegand sensors to recharge batteries in biomedical devices transcutaneously. Here, an external alternating magnetic field would trigger current pulses in a Wiegand sensor implanted in a patient’s body. Because the Wiegand effect operates efficiently over a wide range of frequencies, the magnetic field could be tuned to a frequency that would transmit energy efficiently. By comparison, radio-frequency electromagnetic waves are strongly attenuated by living tissues.
The Wiegand effect is an intriguing option for energy harvesting and signal generation. While power levels are modest when compared to piezoelectric, thermoelectric, or photovoltaic, the Wiegand effect is a well-proven technology with unique properties that make it a viable solution for many imaginative new applications.
This article was written by Tobias Best, Global Product Manager, Wiegand Sensor Technology, at FRABA Pte, Singapore. For more information, visit here .