Working within the Holst Centre program on Micropower Generation and Storage, researchers have developed a small piezoelectric device capable of harvesting 85 microwatts of electricity from vibrations. Fabricated using MEMS technology, the fully autonomous temperature sensor generates enough power to wirelessly measure and transmit environmental data to a base station every 15 seconds.
The piezoelectric effect (from the Greek Piezo, meaning to press or squeeze) converts mechanical stress or pressure into electric current. It was first demonstrated in 1880 by Pierre and Jacques Curie using various crystals but remained somewhat of a laboratory curiosity until the development of sonar gave it its first practical application.
Probably the most familiar everyday example of the effect in action is the electric cigarette lighter, where depressing a button triggers a spring-loaded hammer to hit a crystal. The resulting electric current flows across a small spark gap and ignites the flowing gas. Other examples include ceramic cartridges on phonographs, pickups on acoustic guitars, ultrasonic transducers, quartz clocks (of course) and auto focus motors in reflex cameras.
The effect can also be used to harvest vibrational energy to power miniature devices like sensor nodes. The harvester built by IMEC (Europe's largest independent research center in nano-electronics and nano-technology) has not only managed to generate a record 85 microwatts of power but the manufacturing process has been undertaken using cost-effective CMOS compatible micro-electro-mechanical systems (MEMS) technology.
MEMS is the microfabrication of mechanical elements, sensors, actuators and other electronics on a common silicon substrate. MEMS augments the decision-making processes of integrated circuits with sensory information about the environment, making complete system on a chip manufacture possible.
Instead of using lead zirconate titanate as the piezoelectric material for the sensor's harvester, IMEC used aluminum nitride, which benefits from better composition control and up to three times faster deposition. By changing the dimensions of the beam on which the silicon mass and the aluminum nitride are suspended, the resonance frequency of the harvester can be modified to meet any value within a 150 to 1200Hz range.
The researchers also found that vacuum bonding glass covers to the top and bottom of the processed wafers significantly increased the power output of the harvester, as opposed to atmospheric packaging. The harvester was attached to a power optimized wireless temperature sensor and subjected to vibrations of 353Hz at 0.64g (indicating a realistic amplitude of the vibrations). The system generated enough power to take environmental readings and send the data to a base station at 15 second intervals.
The researchers presented the findings at the International Electron Devices Meeting in Baltimore last month and hope that industry will be able to use them in the manufacture of fully autonomous harvesters to power such things as tire-pressure monitoring and predictive maintenance of moving or rotating machine parts.
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