Sensors: Living off scraps of energy

In the future, your shoes may be able to charge your camera and RFID chips could tell when you leave the room. Images: Energy-saving sensors

Scientists have made machines that can think and drive cars. Now some are working on sensors that can harvest their own energy.

Professor Zhong Lin Wang at the Georgia Institute of Technology has devised a sensor that can harvest mechanical energy and convert it into electricity. Embedded in the boot of a soldier, for instance, the sensor could conceivably gather energy when its wearer walks and use that energy to charge batteries for a radio or flashlight, for example. Similarly, blood flow from the heart could generate energy for an implanted medical device.

At Intel, meanwhile, researchers are looking at ways to let radio-frequency identification tags exploit energy from RFID readers to perform additional tasks. Currently, when a reader is directed at a tag, the tag typically responds by spitting out a serial number. But by inserting a capacitor or other device that can capture energy into the tag, the stored energy could be used to power a temperature sensor or an accelerometer. If someone tried to walk off with a crate, a motion sensor could send a distress signal across the network to security. It could then be periodically recharged with a quick blast.

"You can imagine a moisture sensor. You could embed it into a building and literally never have to get at it again," said Joshua Smith of Intel Research Seattle. "Frozen foods are a big one. Security is another. Another one I heard of is blood. Blood plasma has to be kept at a certain temperature."

These devices are not perpetual motion machines, which are hypothetical machines that produce useful energy in a way that breaks the laws of physics. No such machines currently exist. But from a practical point of view, these devices come close to that ideal because they can survive on energy that otherwise would be unused and they get it on their own. Smith, in fact, calls his sensors a stab at perpetual computing.

Batteries and power consumption have been the skunk in the sensor market for years. Futurists, scientists and others have sketched out visions in which small motes will gather data from faults along the ocean floor or from the center of forest fires. Sensors could also be embedded in walls to help identify intruders or, as some privacy advocates worry, spy on people.

Unfortunately, no one wants to go around replacing batteries in all these things.

Intel's WISP--or Wireless Identification and Sensing Platform--takes advantage of the dynamics of Moore's Law, the observation that the number of transistors on a given chip can be doubled every two years, mostly through shrinking the size of the transistors. Tinier transistors are allowing chip designers to build more intelligence into ever smaller semiconductors.

Images: Sensors

RFID chips may never get to the magical price point of 2 cents apiece, down from a range of 10 cents to more than a half-dollar. However, chipmakers can put more functionality into future chips and sell them for the same price as today's relatively "dumb" tags, Smith said.

The power requirements on low-end microprocessors are also declining because of Moore's Law. Power is a severe problem with high-end microprocessors because designers are pushing the performance envelope. But the trend is going the other way on chips that run at low speeds because the electrons have to travel shorter distances.

"We're getting down to the point where the actual performance of (extremely low-end) computers is getting close to the thermodynamic limits," Smith said. "Microcontrollers have gotten so low with their energy requirements we can now power a general purpose microcontroller off an RFID reader. Compared to all power sources, an RFID reader is a relatively easy case."

One of the first WISPs that Smith developed was a chip that could tell when a lid on a box was opened.

Since then, he has developed an activity/motion sensor as well as a light sensor that can detect different levels of light (not just darkness versus light). In one experiment, a light sensor hung from a suction cup on his window delivered information about light coming through the window over a 12-hour period. However, it did so without external electricity or batteries.

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