Wireless sensor networks that learn to adapt to new conditions quickly, and to form new networks on the fly, are being eyed as a way to collect data continuously in a multitude of applications. But among the hurdles technologists must overcome before such networks are widely deployed in real-world applications is how to power the sensors in remote locations.
Many applications require sensor nodes not much larger than a dime, powered by coin-cell batteries. Replacing batteries, even long-lived ones, in remote locations is labor-intensive and cost-prohibitive. As a result, researchers and system designers are looking at the potential of energy harvesting-converting ambient energy from the environment into electrical energy the sensors can use.
Harvesting extracts energy from relatively inexhaustible ambient sources, such as wind, temperature, vibration and other natural phenomena, to provide power to the sensor nodes. A number of smaller companies-including Continuum Control, Ember, EnOcean Applications, Ferro Solutions, Millennial Net, MicroStrain and Microtrend Systems-are pioneering this nascent field, alongside major technology corporations such as Freescale Semiconductor, Rockwell Scientific and Texas Instruments. The payoff will come in enabling such applications as the monitoring of heavy machinery, tire pressure and temperature in vehicles, as well as the detection of potential problems in ships and aircraft.
Development work at both Ferro Solutions (Cambridge, Mass.) and Continuum Control Corp. (Billerica, Mass.) might be a precursor of energy harvesters to come.
Ferro Solutions Energy Harvesters (FSEHs) are independent power sources that generate electricity from vibrations and use it to power wireless transceivers, sensors, micromotors and actuators. FSEHs can provide a continuous, nearly endless supply of electricity from low-level vibrations, the company said-enough power to run sensors, RF transmitters and other electrical devices. Because the harvesters are so sensitive and efficient, they can generate electricity from vibrations that are barely noticeable to the human touch. When not needed immediately, this juice can be stored in a supercapacitor for later use.
Originally, the U.S. Navy funded research into the FSEH to power a new type of wireless health-monitoring system for ships and submarines that would be cost-effective, easy to install and easy to maintain. Ferro Solutions claims to be the first to generate "milliwatts of power from input vibrations of a few tens of milli-g's, with a device the size of a pack of gum." The company says that while competitors generate microwatts from a device of a similar size, the FSEH churns out 10 to 100 times that amount with the same input energy.
The current FSEH model generates 9.3 mW from a cylinder measuring approximately 1.8 inches in diameter x 1.8 inches high. Input vibrations are measured as g's (where 1 g's is equal to the force of the earth's gravity). Tapping on a table creates small vibrations that become recognizable to the human touch at approximately 0.02 g's (20 milli-g's). In an environment with vibrations of 21 Hz and 100 milli-g's, the FSEH has an energy density of 2 mW per cubic inch.
Millennial Net Inc. (Burlington, Mass.) has paired its i-Bean wireless technology with Ferro Solutions' energy-harvesting technology to produce an inductive vibration energy converter that can generate 1.2 to 3.6 mV from a vibration of 28 to 30 Hz with a force of 50 to 100 milli-g's. Millennial's i-Beans include small, ultralow-power, self-organizing wireless devices that enable sensors and other monitoring and control appliances to connect over low-data-rate wireless networks.
Meanwhile, Continuum Control's iPower energy harvesters convert the mechanical energy of motion into electricity to power standalone devices. Continuum harnesses the power of motion from the fusion of a form of piezoelectric material with intelligent, high-efficiency electronic circuits. Its iPower systems consist of two proprietary technologies: what the company calls PiezoFlex composite materials and Self-Powered Electronics.
PiezoFlex composites can be used in a variety of applications, including systems that make it possible for golf clubs or tennis racquets to sense a person's swing and react; automotive panels that sense and control cabin noise; and helicopter blades that adapt for different flight regimens to maximize fuel efficiency, speed and lift. The Self-Powered Electronics extracts, stores and reapplies the power provided by the piezoelectric materials when the latter are subjected to motion or vibration. Using these technologies, Continuum says it can capture up to 10 times the power previously possible in passive approaches for use in wireless sensor nodes.
MicroStrain (Williston, Vt.) applies mechanical strain on piezoelectric materials using a power-management scheme based on charge storage in a capacitor. The wireless circuit is held in the off state until enough charge accumulates to drive it. The power-management circuit detects the voltage from the piezoelectric strip and shuts down the transmitter to allow charge buildup in the capacitor. When a threshold of 9.5 volts is detected on the capacitor, the wireless sensor node is turned on and transmits data. The transmitter sends a 418-MHz frequency-shift-keyed encoded data stream at distances of up to one-third of a mile using 12 milliamps at 3 V.
MicroStrain's network uses addressable sensing nodes that incorporate data-logging capabilities and a bidirectional RF transceiver communications link.
Meanwhile, work is ongoing to develop low-power wireless sensor nets by making each node self-contained and power-stingy. In a paper submitted to the International Symposium on Smart Structures & Materials/NDE for Health Monitoring and Diagnostics 2005, to be held Feb. 26 to March 2 in San Diego, MicroStrain researchers describe work on smart wireless sensing nodes capable of operation at extremely low power. The systems were designed to be compatible with energy-harvesting techniques using piezoelectric materials, solar cells or both. The wireless sensing nodes included a Microchip microprocessor, on-board memory, sensor signal conditioning, 2.4-GHz IEEE 802.15.4 radio transceiver and rechargeable battery. Sensing was accomplished via a 1,000-ohm foil strain gauge . The system slept when not sampling. At 10 Hz, current consumption was 300 microamps at 3 Vdc (900 microW); at 5 Hz, the system generated 400 microW; and at 1 Hz, it generated 90 microW. When the RF stage was not used but data was logged to memory, consumption was cut further.
Piezoelectric-strain energy-harvesting systems delivered approximately 2,000 microW under low-level vibration conditions, the researchers reported. Output power levels were also measured from two miniature solar cells, which provided a wide range of output power (from approximately 100 to 1,400 microW), depending on the light type and distance from the source.
Ultralow power control
Texas Instruments Inc. is collaborating with Ember Corp. on what the companies claim is the world's lowest-power ZigBee wireless networking and microcontroller platform. ZigBee addresses remote monitoring, control and sensor network apps.
Ember has paired its EM2420 802.15.4/ZigBee-compliant semiconductor platform with TI's MSP430F161x series ultralow-power MCU for developers building ZigBee applications that require the lowest possible power. The MSP430 platform MCUs will also support Ember's next-generation EM260 network processor.
The MSP430F161x uses 1.1 microamps in standby with real-time clock operation and has a 300-microamps (1-MHz) active mode. Its total power consumption is 10 times lower than competing devices', said MSP430 marketing manager Juan Alvarez.
TI is working with Microtrend Systems Inc. (South Plainfield, N.J. ) on a development board that uses the MSP430 to control a piezoelectric vibration sensor with standby current of 0.1 microamps and a clock startup time of less than 1 microsecond.
Addressing the consumer electronics market with low-power sensors, Freescale Semiconductor Inc. (Austin, Texas) has developed the three-axis, microelectromechanical-systems-based MMA7260Q sensor with a gravity (g-select) feature that ranges from 1.5 to 6 g's. Built for portable consumer electronics, the device lets designers select the g-force detection level to suit particular applications.
The MMA7260Q chip detects in three dimensions, letting portable devices intelligently respond to changes in position, orientation and movement. The small package size suits the chip for battery-powered electronics. Samsung Electronics designed the Freescale sensor into its YH-J70 and YP-T8 digital audio players.
Freescale is also working on a piezoelectric solution for battery-free tire sensors. Embedded in tires, the unit would transmit pressure data wirelessly to the driver's display. But packaging and interface challenges must be met if this device is to become commercially available by 2007 or 2008, Freescale said.
Startup EnOcean GmbH (Oberhaching, Germany) has a solar-powered wireless sensor module, the STM100, that integrates a controlling microprocessor, RF transceiver and light-energy capture-and-storage circuit, along with three analog and four digital sensor connections-but not the sensors themselves-in a module measuring 20 x 40 x 10 mm. The STM100 has a two-stage solar cell. One provides quickly available startup energy; the other charges an on-board energy reservoir. Designed for use indoors, the device requires only 200 lux to operate. Typical lighting values in a building range from 200 lux in hallways through 500 lux on desktops and up to 1,200 lux in display cases. EnOcean claims that the energy storage is sufficient for the module to operate continuously for up to five days in complete darkness.
Rockwell Scientific (Thousand Oaks, Calif.) has developed an approach that, when implemented into a linear electrical generator, drastically improves its efficiency and enables energy capture from very slight external movements. The core element is a surface-treatment process to reduce the friction between a magnet and a nonmagnetic surface to 40 times lower than the friction between two Teflon surfaces, according to Rockwell.
The primary objective is to demonstrate and fully characterize devices for energy harvesting in a maritime environment. At the end of the program, oceanographic sensors integrated with 1- to 2-W energy-harvesting devices will be deployed for field tests. More-powerful devices are planned for remote power stations and for portable power packs worn by soldiers.
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