Saturday, September 7, 2013

TECH SPECIAL ...Walls listen and talk


Walls listen and talk 

Researchers have embedded ultrathin radios on to walls. The innovation could spawn new devices ranging from an invisible communications to structural monitors for bridges and roads 
 

    Using a modern twist on a technology developed in the 1920s, researchers at Princeton University have embedded ultrathin radios directly on plastic sheets, which can be applied to walls and other structures. The innovation could serve as the basis for new devices ranging from an invisible communications system inside buildings to sophisticated structural monitors.
    “We originally built this for energy management in a smart building,” said Naveen Verma, the principal researcher. “Temperature sensors and occupancy sensors communicate with a central management system using distributed radio arrays that are patterned on wallpaper.”
    The plastic sheets are as thin as wallpaper and can be painted without diminishing their function. They are also flexible and can be applied to irregular surfaces such as bridge decks. And they can be self-powered; solar cells on the plastic sheets supply electricity to the radios.
    Patterning circuits on plastic, a relatively new idea, is challenging because plastic tends to melt at the high temperatures used to create circuitry. In recent years, researchers have developed ways to avoid damaging the plastic. But these required some alterations that lower the performance of electronic components, such as transistors, that are critical to the operation of complex devices likes radio transmitters.
    “Radios have been a real challenge,” Verma said. Radios require a relatively high frequency to operate, and that has been impractical for plastic-based electronics. The transistors developed by two collaborators on this project: Sigurd Wagner and James Sturm uses a new kind of circuit material that reduced that temperature fromabout1,000degreesCelsiusto300degrees.
    To make their low-temperature circuits, Sturm and Wagner used amorphous silicon in transistors rather than crystalline silicon. The amorphous silicon does not require the high temperature of the crystal form, but it also lacks the crystal’s highly ordered inner structure. For electrons, that switch is like changing from a smooth superhighway to a bumpy gravel road.
    “These transistors do not perform nearly as well as the ones that Intel would make on one of its chips,” Sturm said.
    In a modern radio, the frequency depends on the movement of electrons across a transistor: the faster the movement, the higher the frequency. The challenge for the researchers was to find a way to speed up the electrons despite the low-performing amorphous silicon transistors.
    Verma and students Liechao Huang, Warren Rieutort-Louis, Yingzhe Hu and Josue Sanz Robinson hit upon the idea of using a circuit developedin1922bythefatherofFMradio,Edwin Armstrong. Called a super-regenerative circuit, the setup could use other components to increase the radio’s frequency and bypass the relative poor performance of amorphous silicon.
    The super-regenerative circuit bounces electrons between a capacitor and an inductor, causing energy to be stored and discharged from each. The energy changes caused by the rapid bouncing create the radio’s frequency. And because the speed of the bouncing depends on the super-regenerative circuit’s capacitor and inductor – and not the transistors – it can allow the radio to operate at a relatively high frequency even if the transistors are poor quality.
    As a next step, the research team is working with Branko Glisic, a professor of civil engineering, to develop a flexible structural health monitoring system for use on structures. “We will have a prototype in September, but it will probably need several more years of research and development,” Glisic said.
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