Vibrations good for harvesting power

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.

The biggest obstacle to vibration harvesters today is their extreme sensitivity to resonant frequency. A vibration harvester can output as much as 200-times the energy when operating exactly on its resonant frequency, compared with being just a few cycles off resonance. As a result, companies crafting vibration harvesters today are working with their customers to precisely match their application's vibes, as well as manufacturing their own piezoelectric materials that mitigate the effect of resonance.

For instance, in July, Advanced Cerametrics Inc. (Lambertville, N.J.) will announce a vibration harvester using its proprietary piezoelectric fibers that can generate milliwatts of power for remote sensors, wireless transmitters and industrial monitors.

"The key to how it works, is that, even while in sleep mode, it is harvesting vibrations to build up charge on a capacitor," said Farhad Mohammadi, director of research at Advanced Cerametrics. "When the capacitor's voltage gets up to 3.5 (volts), it comes out of sleep mode and turns the application's circuits on - for instance, to wirelessly transmit telemetry data then go back to sleep."

Advanced Cerametrics Inc. was already supplying the Department of Defense with a custom version of its harvester to mine the vibrational energy at the precise resonant frequency of specific pieces of military equipment. But its new line of commercial harvesters is less sensitive to resonant frequency, the company claims, giving it the latitude to serve a variety of applications, from powering industrial wireless sensors to consumer devices, such as pedometers.

To make its piezoelectric fibers less sensitive to resonant frequency, and more ruggedized for a wider variety of applications, Advanced Cerametrics manufactures its own piezoelectric elements, called Bimorphs. Bimorphs are made by spinning 10-micron piezoelectric fibers together with rayon, then embedding them into epoxy. Three Bimorphs power the first harvester model in Advanced Cerametrics new commercial line.

Other companies manufacturing their own piezoelectric materials include Mide Technology Corp, (Medford, Mass.), which earlier this year announced its line of commercial vibration harvesters that are currently being evaluated by several customers, including a railroad that intends to wirelessly transmit data to the operator about the force being applied by brakes on its trains.

"We use a piezoelectric beam with a tip mass, but have encapsulated the fibers in Kapton (a DuPont polymer), making our harvester much more rugged and less sensitive to resonant frequency," said Mide Technology's CEO, Marthinus van Schoor.

As much as 80 percent of an energy harvester's output can be wasted in parasitic losses due to the turn-on voltage requirements for conventional transistors, according to Advanced Linear Devices Inc. To the rescue is the company's invention of a zero-threshold MOSFET, which reduces parasitic losses to as little as five percent.

All normal transistors have a fixed turn-on voltage of 0.7 volts, but ALD's zero-threshold MOSFETs turn on with as little as 10 millivolts, thereby harvesting more energy. The zero-threshold transistor is fabricated by embedding a permanent negative charge on a floating-gate that biases the MOSFET's input to start storing energy immediately, rather than requiring at least 0.7 volts just to turn on and initiate a charge cycle.

"Our zero-threshold MOSFET can work with as little as nanowatts of power, which makes it perfect for harvesting environmental energy," said Bob Chai, ALD founder and CEO.

The zero-threshold circuitry is biased internally at the factory to set the turn-on voltage of each transistor to zero, enabling its circuits to operate at voltage and power levels far below conventional circuitry. ALD claims that its floating-gates will not leak charge for 100 years.