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Posted on December 2, 2008 by  & 

Printing piezo energy harvesters

Over the years there has been a growing interest in the field of low power miniature sensors and wireless sensor networks.
One specific topic that has received little attention is how to supply the required electrical power to such sensors. Conventional power supplies external to such sensors is one way. However, many applications do require such sensors to be completely embedded in the structure with no physical connection to the outside world. Supplying power to such systems is difficult and as a result they need to have their own power supply unit making them self-powered microsystems.
New possibilities offered by micro batteries can make them independent from an external power supply; however the device lifetime still depends on its energy storage capacity.

Vibration energy harvesting

Vibration energy harvesting is receiving a considerable amount of interest as a means for powering wireless sensor nodes. It is the simple notion that mechanical vibration can be transformed into useful electrical power. Researchers are striving to improve the design to enhance the efficiency of electrical power generation from mechanical vibrations and to provide an improved manufacturing method - in particular to provide a low production cost.
Dr Steve Beeby at the University of Southampton, co-ordinator of the "VIBES" project which was concerned with the development of Microsystems for the scavenging of electrical energy from environmental vibrations and movements which was completed last year, published a paper with other researchers last year called "A micro electromagnetic generator for vibration energy harvesting." This paper presents a small (component volume 0.1 cm3, practical volume 0.15 cm3) electromagnetic generator utilizing discrete components and optimized for a low ambient vibration level based upon real application data.
The generator uses four magnets arranged on an etched cantilever with a wound coil located within the moving magnetic field. Magnet size and coil properties were optimized, with the final device producing 46 µW in a resistive load of 4 kΩ from just 0.59 m s-2 acceleration levels at its resonant frequency of 52 Hz. A voltage of 428 mVrms was obtained from the generator with a 2300 turn coil which has proved sufficient for subsequent rectification and voltage step-up circuitry. The generator delivers 30% of the power supplied from the environment to useful electrical power in the load. This generator compares very favourably with other demonstrated examples in the literature, both in terms of normalized power density and efficiency.He has also screen printed piezoelectrics to create energy harvesters supplying 118microWatts with a voltage of 4.1 Volts into a load of 140KOhms. Others have also done this, sol-gel lead zirconate titanate being a popular ink. That is Lead Acetate Trihydrate: Pb(CH3CO2)23H2O, Zirconium n-Propoxide: Zr(C3H7O)4, Titanium iso-Propoxide: Ti(CH3)2CHO4 plus solvents, dilutants and catalysts.

Perpetuum Ltd

In 2004 Beeby co-founded Perpetuum Ltd, a VC-backed spin off company from the University of Southampton in vibration energy-harvesting for wireless sensor networks.
Perpetuum's vibration energy-harvesting microgenerators are based on a highly optimised magnetic circuit coupled to a mechanical resonator. This arrangement successfully transforms the kinetic energy of vibration into electrical current.
Perpetuum's microgenerators produce high levels of power to meet the needs of wireless sensor systems created by the development of low-power sensors, microprocessors and transceivers.
The IEEE 802.15.4 wireless standard defines a short-range, low-power, low data rate wireless interface designed specifically for devices that use limited power. It is the basis of SP100 and wireless HART and ZigBee standards. These developments have been enabled by low power chip sets such as Texas Instruments' Chipcon CC2420 and Nanotron's nanoNET TRX RF Transceiver.
Perpetuum's recently developed wireless sensor node assessment kit (wSNAK), is designed to confirm to OEMs and End Users the advantages of Vibration Energy Harvesting for Wireless Condition Monitoring with a quick, easy and low cost installation.
The wireless sensor nodes will also be available separately for integration in to OEMs own systems and can be adapted for other measurands such as pressure, temperature and flow.
wSNAK gives OEMs the opportunity to evaluate the concept quickly and to provide valuable feedback for the final design. As a result of the short assessment process, they can bring their own new product to market saving typically 12 months of development time and $100,000s in prototype costs.
In the absence of any practical alternative, batteries have been used to power wireless sensor nodes despite end user objections. However, vibration energy-harvesting is now becoming the preferred option as there are none of the reliability, cost of replacement, transportation, safety and disposal issues which are associated with batteries.
The kit consists of four energy harvester-powered wireless sensor nodes which send vibration and temperature data to a laptop-based receiver. Each sensor node runs an industry-standard IEPE accelerometer with integrated temperature sensor on a flying lead. Data is transmitted from each node via the IEEE 802.15.4 transmitter to a receiver situated up to 100m away. Vibration spectra and temperature trends can be displayed on a laptop and basic level alarms may be set similar to the ISO10816-3 standard.
The wSNAK can be installed in just a few minutes. It features an LCD display, so users can instantly observe the power being generated and select an optimum location for the node, while the accelerometer is located precisely where required on the equipment being monitored. It is IP65 rated for protection against the ingress of dust and water allowing OEMs to use it immediately in the field regardless of the weather conditions.

Authored By:

Business Development Director, Research

Posted on: December 2, 2008

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