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Monday, October 10, 2011

Researchers develop fail-safe battery-free wireless sensor security system

Energy harvesting from radio signals powers sensors
By Nick Flaherty

Researchers in Germany have developed a fail-safe system for wireless window sensors that is particularly easy to use and needs no wiring or batteries as the sensors harvest the energy they need to run from ambient radio signals.
“Our wireless window contacts draw all their energy from ambient radio signals,” said Dr Gerd vom Bögel, a scientist at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg
Until now, wireless models have been reliant on either batteries or solar cells, but both of these approaches have drawbacks. Batteries need to be changed regularly to keep window contacts operational. Solar-powered systems avoid this problem, but they too are liable to fail: all it takes is for the sunlight to be blocked by something casting an unintentional shadow over the solar cell. Solar systems are also aesthetically less pleasing because they cannot be tucked away in a dark corner of the window. 
The new system can be fitted with little effort and can be positioned very discreetly. Aside from window contacts, each room is equipped with a room controller. This transmitter module not only receives the data from individual window contacts, it also actively provides the sensors with energy via its radio signal. The room controller also has the function of passing the sensor data on to a central base station in the building, from which users can query the status of all windows. Alternatively, the system can be configured to permit remote querying, for instance from a user’s smartphone. The only prerequisite for this is a DSL connection for the base station.
Energy management was the issue which caused the most headaches during development. “Room controllers, too, have to comply with certain limits on the strength of their radio output. This makes it particularly tricky to get enough energy to all the window contacts in bigger rooms,” vom Bögel points out. “But we have made sure all the sensor modules, antennas and components are so finely tuned to each other that the system works reliably even over considerable distances.”
The IMS research scientists have already constructed an initial prototype and are hoping to integrate other types of sensor into the system along the same lines – to regulate room temperature, for example. At the moment, thermostats are generally fitted somewhere just inside the room. If a door is open, the temperature by the door will be lower than in the middle of the room. As a result, the thermostat will then unnecessarily regulate the temperature upwards. The new system would allow a temperature sensor to be placed unobtrusively precisely where a particular temperature is desired – for instance on the display cabinet by the dining room table.
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