Novel alarm system

Window panes of jewelry stores, galleries or banks are alarm-protected and equipped with safety glass. The disadvantage is that the pane or part of the pane must break before the alarm is triggered. This is because conventional safety glass is equipped with metal threads that only break in the event of mechanical damage, thus activating the alarm. If the glass is damaged by a cutting torch or locally with a drill, for example, conventional systems do not react at all or react too late. Burglars take advantage of this circumstance and use a drill or gas burner instead of a hammer. Researchers at the Fraunhofer Institute for Scientific and Technical Trend Analysis (INT) and the Fraunhofer Institute for Photonic Microsystems (IPMS) have jointly developed an intelligent burglary protection system that overcomes this disadvantage: The system detects thermal and mechanical stresses caused by external impact promptly and dynamically. Even a light blow to the security glass or manipulation by a flame is enough to trigger the alarm. The impact of force changes the mechanical property of the pane, which the new system detects. Monitoring of the glass pane is based on a glass breakage sensor implemented by a fiber Bragg grating, optical interference filters inscribed in optical fibers. The optical fiber can be inserted in the corner of the window pane or in other positions.

Light-based monitoring of glass panes
The fiber Bragg grating sensor is an optical sensor that reflects a specific wavelength of light that is changed by temperature and/or strain variations. "If someone applies pressure to the glass pane or it is heated, the distance between the grating elements changes and so does the transmitted wavelength. Sensitive optical measuring devices can detect these changes. If the changes are greater than a previously identified threshold, signals are transmitted to the alarm system," says Udo Weinand, a graduate engineer at Fraunhofer INT, explaining how the patented system works. "We can adjust our system very finely and specifically; it can respond to both light and strong blows. This can be individually adapted depending on the application," adds Peter Reinig, a scientist at Fraunhofer IPMS.

The new type of burglary protection consists of a Bragg grating, an optical fiber feed line, an interface to the alarm system and evaluation electronics containing the optical measuring device. The evaluation unit, to which different optical fibers can be connected, is to be installed in the frame of the window pane in the future. In high-security areas, the evaluation unit can be located at a great distance from the security glass, since the fiber Bragg grating is capable of transporting light in the optical fiber even over several kilometers. "Measurement with optical fiber sensors is a good solution for these requirements because it uses light instead of electricity and commercially available optical fibers instead of copper wires," Weinand said.

Pattern recognition avoids false alarms
Another advantage of the Fraunhofer system is that optical fibers are resistant to electromagnetic interference. Electronics can be disturbed by emitting microwaves, for example. Their pulses can disable conventional alarm systems or generate an unwanted alarm. In addition, pattern recognition rules out false alarms caused by everyday vibrations. "A football or a bird leaves a different signature than a hammer or a baseball bat," Reinig comments. The smart alarm system has been put through its paces in various attack scenarios on different types of security glass with a hammer, baseball bat, drill, firearm, axe and hot blower to determine when the alarm will reliably go off.
The effectiveness of the sensor with fiber Bragg grating has been proven in numerous tests, including the VdS test - a test control conducted by VdS Schadenverhütung in Cologne.

The Fraunhofer researchers' intrusion protection system is now available as a demonstrator. The evaluation electronics in the form of a small box has a size of 14x9x7 cm³ and can be further miniaturized if required. Not only jewelry stores and other burglary-prone objects can be protected with the system, it is also suitable for monitoring bridges, buildings, pipelines, load-bearing structures in the aerospace industry and wind turbines, for example.

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