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Wireless Sensor Detects Status of Window
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Wireless Sensor Detects Status of Window

German researchers have made advances in the sensor systems that detect if a window is open or closed, by using of all things, a heavily modified washing machine tumble sensor.

The new device, the intelligent window monitor is a wireless device that attaches to the inside of the window frame. It works with a small magnet embedded in the bottom of the opening part of the frame itself. When the window moves relative to the sensor, the magnetic field alters as the magnet changes relative position. It is very simple, but very effective.

The rest of the sensor consists of the standard setup – a wireless network connection, to contact the base station elsewhere in the house, so that the base station can track which windows, if any, are actually open. This is then further networked into the building's Wi-Fi systems, so the status of all windows in the building can be tracked by a PC, or by use of a mobile phone accessing the smart building's system's from outside.

It was developed for home owners mainly, but all manner of commercial applications are suitable as well. It does not matter how many sensors you wish to add to the system,so it will cope with any number of windows (up to just over four million) in any one building. By nature of their design, the sensors are passive; they can tell if a window is open or shut, even if it is only open a tiny crack, but they cannot shut it for you, or open it for that matter.

All that is visible of the system is this sensor plate, the size of an adult thumbnail.

The researchers who developed it are from the Fraunhofer Institute for Integrated Circuits , based in Erlangen and Nuremberg, and have done so in partnership with a commercial firm Seuffer, who make the HallinOne sensor. This was the base component, a 3-D magnetic field sensor which is standard in modern washing machines to determine the exact position of the drum usingthe exact same method.

Seuffer engineer Klaus-Dieter Taschka explained the system.“We’ve adapted our technology for the window application. A fingernail-sized sensor embedded in the inner frame detects sash and handle positions by measuring any changes in the angle and position of a magnet that’s embedded in the bottom of the sash. When you lock the window, for example, the magnet moves to the right.”

“The sensor even detects if a casement window appears to be properly latched, but has actually just been pulled shut. No other system can do this.”

Of course, because the sensor device is on the inside wall, it is quite tamper proof as well. The only part that is feasibly accessible to the outside is the magnet in or on the bottom of the sill. Of course, since the sensor is tracking that anyway, it will pick up immediately if someone tries to remove it – and since the removed magnet is not following the normal movement pattern for that magnet, the base station can be programmed to raise an alarm when that happens.
Another feature of the window sentinel is that it requires no cables or batteries. The sensor draws all the power it needs from its surroundings. It draws heat from the air around it, and even sunlight, to drive its energy requirements.

Thermoelectric generators embedded in the window frame transform heat into power. Solar cells attached to the outer window frame also help power the 3-D sensor. “Our tests showed that this works even in north-facing windows,” says Andreas Buchholz, Head of Research and Development at Seuffer.
Obviously, the system is suitable for everyday use only if all sensors function reliably. To ensure that this is the case, each chip is equipped with a coil that creates a magnetic field as soon as power is applied. If a signal is emitted, then the sensor is intact. “The window monitor is the result of a vigorous exchange of ideas we’ve maintained over the years with Fraunhofer researchers,” says Buchholz.
The window, which includes the sensor, magnet, RF node and solar cell, is currently available as a prototype. By the end of 2012 it is expected to be ready for mass production. Manufacturing will be done by Seuffer, which also developed the electronics and produced the housing.


Wireless window sentinel

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