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Earth’s own magnetic field underpins this unusual tripwire

April 9, 2016 | By Anthony Capkun

Led by Uwe Hartmann (with necktie), the team is building a model to demonstrate its flexible security solution at Hannover Messe. Photo courtesy Saarland, by Oliver Dietze.

April 9, 2016 – Experimental physicists at Saarland University say they have developed a cable security solution that—underpinned by the Earth’s magnetic field—issues warnings to, say, a smartphone, whenever it registers a change in field strength… as when a burglar sneaks onto your property.

The sensor cable system developed by prof. Uwe Hartmann and his team issues a warning signal indicating when and where someone has attempted to cross over the cable. It can be affixed to long stretches of fencing, hung in trees or even buried underground.

“These changes can result from vibrations in a chain-link fence when somebody tries to scale it, or they can be caused by the zipper on the intruder’s jacket,” explained Hartmann. “If the cable has been buried in the ground of a driveway and the intruder passes over the buried cable, the sensors register the deviation from the previous magnetic field values and signal patterns, and transmit this change to the central controller.”

This thin cable was originally developed by Saarland physicists to secure long stretches of airport perimeter fencing. The cable contains a linear array of sensitive magnetic-field sensors that are capable of detecting even the smallest of changes in the ambient magnetic field, they say, at distances of up to several metres from the cable.


The tiny sensors are networked and communicate even the smallest of disturbances to the analyzer unit contained in a microcontroller. The signal arriving from the sensors is processed and automatically checked to filter out any false alarms, such as those triggered by wind affecting the position of the fence.

“The work we have carried out so far lets us distinguish many different kinds of vibrations and changes to the magnetic field, and to identify whether the signal was caused by a human, the wind, a car or an animal,” added Hartmann. “The sensor cable system can, therefore, be used to check whether farm animals are still grazing the pasture or the family dog is still in the back garden—all that is needed is for the animals to have a small piece of metal attached to their collars.”

The research team uses mathematical models to carry out simulations of typical magnetic field disturbances on test fences. The results are then used to program the central analyzer unit, teaching it to identify and automatically attribute the signals received to specific types of disturbance, which enables the system to filter out false alarms.

“Our aim is to further refine the sensor technology,” said physicist Haibin Gao. When the system registers a critical situation, it sends a message via a Bluetooth connection to a smartphone or tablet. “The system identifies the exact location of the disturbance, which is a very useful feature when dealing with large plots of land.”

And the sensor does not draw a lot of power, say researchers, and could be powered by solar generators.

“Fences and, say, gateways could be secured using just a single system, with the cable installed above-ground in the one case, and below-ground in the other. The cable can also be split up into different sections, making installation simpler and more flexible,” said Hartman.

The contactless sensors are essentially wear-free, the team adds, and measurements are independent of weather conditions. Rain or fog do not affect the system.

“The measurements are also unproblematic from the point of view of privacy and data security, as no information is stored. The system doesn’t violate the privacy of neighbours and it doesn’t provide a target for hackers,” Hartmann noted. The sensor system can also be connected to cameras or lighting that are activated whenever a disturbance is detected.

“If these sensor cables are going to be used in the private sector, a low-cost method of manufacture will need to be found,” said Hartmann. The team is looking for commercial and industrial partners with whom they can develop their system into a market-ready product. As such, the cable will be showcased at Hannover Messe April 25-29, where the team will be exhibiting at the Saarland Research & Innovation Stand.

PHOTO: Led by Uwe Hartmann (with necktie), the team is building a model to demonstrate its flexible security solution at Hannover Messe. Photo courtesy Saarland, by Oliver Dietze.

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