Tamper Sensing using Low-Cost Accelerometers

Sammanfattning: Certain security products, in this case sirens, must according to industry specifications generate a tamper warning in case the system is significantly moved from its place of mounting or subject to significant violence such as drilling. Currently, detecting removal from the place of mounting is typically done by mechanical switches placed between the siren and wall. This type of detection can be defeated by using thin blades and moreover complicates the process of mounting. In this M.Sc. thesis carried out in collaboration with the Stockholm-based security equipment manufacturer Indusec AB, we investigate whether it is instead possible to use an inexpensive, but highresolution three-axis micro-electromechanical (MEMS) type accelerometer in conjunction with an equally inexpensive microcontroller to detect such dislodgement. This is done by digital signal processing techniques implemented in the microcontroller. Emphasis is put on finding reliable algorithms, which despite being reliable and fit for the purpose, do not use significant amounts of complex mathematics or memory, so that the detection can run on battery power with acceptable longevity and the algorithms implemented in the 8-bit microcontroller. For the sake of verification and analysis the algorithms were also implemented in MATLAB. The detection methods are primarily derived based on real-world trials and investigations. The reliability of the developed detector algorithms is graphically presented as a functions of relevant detector parameters. The reliability estimates are strongly indicative of that an accelerometer in conjunction with an inexpensive microcontroller can be used as a reliable detector of system dislodging and chassis piercing by electric drills. The risk of misdetection from structural disturbances such as wall knocks or on site machinery seems to be small, with none of the test cases resulting in a false positive or false negative with the detector parameters properly set.