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Optical alarm system recognizes attempted break-ins

11 Dec 2017

Fraunhofer partnership develops optical surveillance of glass panes to maximize building security while cutting false alarms.

The window panes of jewelry stores, art galleries and banks are usually protected by alarms and fitted with security glass. However, the pane or part of it has to break before the alarm is triggered. Conventional security glass contains metal threads that tear in the event of mechanical damage, activating the alarm.

If a cutting torch or a drill is used to damage the glass, conventional systems typically react either too late or not at all. Burglars can exploit this weakness and use a drill or a blowtorch instead of a hammer. Researchers at the Fraunhofer Institute for Technological Trend Analysis (INT) and the Fraunhofer Institute for Photonic Microsystems (IPMS) have jointly developed a smart anti-burglary protection system that overcomes this problem.

The new system rapidly and dynamically records thermal and mechanical stresses on the target from external causes. Even a gentle knock against security glass or manipulation of a surface by a flame would be sufficient to trigger the alarm. The external force applied to the pane changes its mechanical characteristics, and the system detects this change.

Light-assisted surveillance

This method of monitoring glass panes is based on a glass break sensor built inside an optical fiber by means of fiber Bragg grating. Specifically this comprises optical interference filters inscribed in optical waveguides. The fiber optics can be fitted in the corner of the windowpane or in other suitable positions.

The sensor with the fiber Bragg grating is an optical sensor, which reflects a specific wavelength that is changed by deviations in temperature and/or elongation stresses. Udo Weinand, engineer at Fraunhofer INT, explained the functioning of this patented system.

“If somebody puts pressure on the glass pane or heats it, the distance between the grating elements changes, which in turn alters the transmitted wavelength. Sensitive optical measuring devices are capable of detecting these changes.

He added, “If the changes are greater than a predefined threshold, signals are transmitted to the alarm system,” said Udo Weinand, engineer at Fraunhofer INT, explaining the functioning of this patented system.

“We can adjust our system in a very fine-tuned, targeted manner. It can react both to slight knocks and to strong ones. As a result, it can be adapted individually to the specific application,” added Dr. Peter Reinig, a scientist at Fraunhofer IPMS.

The innovative break-in protection consists of a Bragg grating, a fiber optic supply cable, an interface to the alarm system, and evaluation electronics, which contain the optical measuring device. In future, the evaluation unit, to which various fiber optics can be connected, is planned to be fitted in the window frames.

In high-security zones, the evaluation unit can be located far away from the security glass. “Measurement with fiber optic sensors is a good solution for these requirements, because it uses light instead of electricity and widely available fiber optics instead of copper wires,” says Weinand.

Cutting false alarms

Another advantage of the Fraunhofer system is that fiber optics are resistant to electromagnetic interference. Electronics can be disturbed by things such as microwave emissions, whose pulses can incapacitate conventional alarm systems or cause an unwanted alarm. In addition, pattern recognition rules out the potential for false alarms triggered by everyday vibrations.

“A football or a bird leave behind a different signature than a hammer or a baseball bat,” said Reinig. The smart alarm system was rigorously tested in various attack scenarios on a wide range of different security glass panes involving hammers, baseball bats, drills, firearms, axes and heat guns to determine when the alarm is reliably triggered.

The effectiveness of the sensor with fiber Bragg grating has been demonstrated in numerous tests, including the “VdS test” by security testing company VdS Schadenverhütung in Cologne, a well-known standard for German industry.

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