Magic magnetic field sensors sound the alarm

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In the future, a new sensor cable could be used to protect airports, industrial complexes and people’s yards without a great deal of expense. It registers even the tiniest changes in the Earth’s magnetic field.

Plenty of things go unnoticed by our senses. One of them is the Earth’s magnetic field. Unlike migratory birds and sea turtles, we need technical aids to make use of it. Like the good old compass. It has been helping seamen navigate the seven seas since the 12th century. Sort of a primitive precursor of today’s magnetic field sensors. Then in 1832, mathematician and physicist Carl Friedrich Gauss laid the cornerstone for modern magnetic sensor technology when he developed a method for measuring both the direction and intensity of the Earth’s magnetic field.

Since then, magnetic-field sensors have become quite important because they make entirely new solutions for difficult measuring tasks possible. And not just for research and industry—also for our private lives. For example, they help determine location and position in our smartphones. And with Apps such as Telemeter 11th, they even turn a digital Swiss knife into a metal detector.

Magnetic burglar alarm

Saarland University’s “All-round Warning Alarm” is based on a similar principle. When attached to fencing, a thin magnetic field sensor cable can tell whether the wind, a bird or wire cutters are “interacting” with the wire mesh. Buried in the ground of future traffic-guidance systems, it can tell which direction automobiles are driving. Not even smartphones or zippers can go undetected. That is because everything within a few meters that influences the Earth’s magnetic field is registered by highly sensible magnetic field sensors and transmitted to a smartphone via Bluetooth.

The sensor cable is flexible and can be adapted to a wide variety of requirements, and it consumes very little electricity. It is also practically wear free, and measurements do not dependent on weather conditions. In addition, no data is stored and the sensor system has proved a hard nut for hackers to crack.

The technology is based on the fact that the Earth’s weak magnetic field (approx. 50 microtesla) is always everywhere. And that every ferromagnetic object measurably disturbs this field for magnetic field sensors with sensitivities in the nanotesla range. Corresponding electronics and algorithms then determine metallic properties, size and direction of motion. Every type of “disturbance” has its own magnetic fingerprint.

Magnetic field sensors for every purpose

Magnetic field sensors (Image: Sensitec)
Three magnetoresistance effects with resistance changes of varying intensity: AMR: weak, GMR: stronger, TMR: strong. (Image: Sensitec).

Researchers have been working a “magnetic” recognition systems for a good 15 years. As part of the development process, experiments were conducted with so-called AMR (anisotropic magnetoresistance) and GMR (giant magnetoresistance) sensors. The latter can be found in billions of read heads in hard disk drives, and the physicists who discovered them, Peter Gruenberg from Forschungszentrum Jülich and Albert Fert from Université Paris-Sud, were awarded the Nobel Prize in Physics in 2007. Both work sensors are based on the so-called magnetoresistance effect, which says that ferromagnetic materials change their resistance in a magnetic field.

Burglars in a “tunnel”

The first trials with the third member of the magnetoresistance team, i.e. GMI (giant magnetoimpedence) sensors, are now underway in Saarland. In this case, impedance (alternating current resistance) depends on the intensity of an applied, relatively weak external magnetic field.

But that’s not all: The prototype recently introduced by Saarland University researchers uses a fourth variant, i.e. TMR (tunnel magnetoresistance) sensors, which have only been commercially available for a short time. As the word “tunnel” suggests, these magnetic field sensors make use of quantum mechanics effects. Due to their high change in resistance of 20%, TMRs are extremely interesting for a number of applications. They are provided by Sensitec (Germany), one of the partners in this project, which is sponsored by Germany’s Federal Ministry of Research.

With the exception of the AMR effect, all magnetoresistance effects were discovered after 1988. In other words, this is still a relatively new, rapidly growing research sector with the prospect of extraordinary sensor solutions for various electronics sectors in the years to come. It will be interesting to see what happens!


Magnetic field sensors (Image: Saarland University))

Sensitec in Germany provides magnetoresistance sensors to Saarland University as part of a project funded by Germany's Federal Ministry of Research. (Image: Saarland University).