Modern glass break detectors work due to the presence of metal threads in security glass, which tear when mechanically damaged, causing the alarm to activate. Now, Fraunhofer researchers have come up with the next generation of detectors which are able to detect all the other ways in which burglars can interfere with a pane of glass.
Conventional glass break detection systems require glass panes to break before the alarm is triggered. However, if a cutting torch or a drill is used to damage the glass, reaction times can be delayed. Burglars exploit this weakness, using a drill or blowtorch to bypass the alarm.
The new system developed by the team from the Fraunhofer Institute for Technological Trend Analysis INT and the Fraunhofer Institute for Photonic Microsystems IPMS overcomes this problem, by quickly and dynamically recording thermal and mechanical stresses from external causes. The system is able to register temperature changes in real time as well as vibrations caused by external interference with the glass.
The system works because the external force applied to the pane changes its mechanical characteristics, and the system detects this change. The actual glass break sensor works on the basis of fibre Bragg grating built inside an optical fibre, effectively optical interference filters inscribed in optical waveguides. The fibre optics can be fitted in the corner of the windowpane or in other positions.
The optical sensor with the fibre Bragg grating detects when the wavelength of light is changed due to deviations in temperature or elongation.
“If somebody exercises 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 recording these changes,” explained Udo Weinand, an engineer at Fraunhofer INT.
“If the changes are greater than a predefined threshold, signals are transmitted to the alarm system,” he said.
The system can be adjusted and fine-tuned in a targeted manner, adapting the sensitivity of the sensor to the specific application.
The system, being based on fibre optics, is resistant to electromagnetic interference. It can also recognise patterns to rule out the potential for false alarms triggered by everyday vibrations.
The system consists of a Bragg grating, a fibre optic supply cable, an interface to the alarm system, and evaluation electronics, which contain the optical measuring device. It is possible to locate the evaluation unit far away from the security glass, because the signals from the glass are transmitted via light through the fibre optics, rather than copper wires.
Next steps include further miniaturising the electronics, and deploying the system commercially.
[Image: Test series with the smart security glass in the laboratories of Schott Technical Glass Solutions GmbH © Fraunhofer INT]