EINDHOVEN, Netherlands -- By converting vibrations into usable power, researchers are enabling the battery-free operation of ultra-low-power wireless devices for everything from medical implants to that black-box under the seat of new automobiles. If a sensor is needed but it is inconvenient to supply power--from the inside of a jet engine to the heart's aorta valve, for example--energy harvesters are being designed to convert environmental gradients into usable electrical power.
The latest batch of energy harvesters for vibrations use piezoelectric actuators sized to match the energy required by the application, from centimeter-sized fibers ruggedized to supply milliwatts in harsh environments, all the way down to micron-sized actuators fabricated using micro-electromechanical systems (MEMS) to supply microwatts to wireless sensors.
"We have shown that it is possible to harvest vibrations using a weighted MEMS beam and an on-chip piezoelectric actuator to generate as much as 40 microwatts--enough to power a wireless sensor node," said Jo De Boeck, director of IMEC's Holst Centre, in the Netherlands.
IMEC is a European research center dedicated to anticipating the needs of the electronics industry by three to 10 years, and the Holst Centre houses its most forward-looking outpost. Previously, IMECs' Holst Centre demonstrated a batteryless pulse-oxymeter powered by a thermal scavenger, designed to measure the oxygen level in blood. That project, slated to be commercialized within five years, stimulated development of its MEMS vibration harvester for applications in which thermal gradients were not present.
The IMEC vibration harvester uses a micron-sized cantilever with a carefully calculated mass on its free-end that oscillates in the presence of vibrations. The cantilever's fixed end is embedded in a piezoelectric capacitor formed from two metal electrodes separated by the piezoelectric dielectric called PZT (Lead Zirconate Titanate). As the cantilever oscillates, the tiny mass on its end causes the piezoelectric layer to be alternatively stretched and compressed by about 180 nanometers. Each cycle causes an alternating current (AC) to be generated in the circuit it powers, producing a maximum of 40 microwatts at the cantilever's resonance frequency of 1.8 kHz.
"Our output power is in microwatts, but this is within the range of power needed by wireless-sensor applications," said De Boeck. "We believe our harvesters will begin to be commercialized within five years and will become commonplace by the end of the next decade."
The IMEC researchers are currently working on a new type of vibrating capacitor that outperforms even the most exotic piezoelectric formulations. This capacitor will have permanently charged plates with the cantilever's vibrations changing the distance between them. The result will be an AC output similar to the piezoelectric capacitor, but with the charge carried by this device freed from the constraints of piezoelectric materials.
