Aug. 17, 2004 – Ten wireless sensors stand watch over the Ben Franklin Bridge linking Philadelphia and New Jersey. Developed by MicroStrain Inc. of Williston, Vt., the pager-size devices measure stresses on the bridge whenever commuter trains cross it.
To extend the life of the devices’ batteries, they remain in “sleep” mode until an approaching train rouses them to sentience.
MicroStrain’s bridge-monitoring project illustrates the promise and problem wireless sensors and other devices face: Such self-powered systems have had to contend with the limitations of current battery technologies.
MicroStrain is not alone. Millennial Net Inc., Dust Networks Inc.and other wireless sensor network developers have had to refine hardware and software so that sensing modules will get three, five or 10 years of working life from the coin cell batteries, like those in hearing aids and watches, that power them.
For industrial tasks such as monitoring temperature, humidity and vibrations throughout a semiconductor fab or power plant, wireless sensor nets are designed to improve productivity.
But as Charlie Chi, an analyst with ON World Inc. concluded in a recent market report, a sensor node that has to communicate continuously lasts a few days at best on an AA battery. He noted that frequent replacements add labor and costs, and are impractical for networks in remote or hazardous environments.
Some sensor companies are turning to devices that “harvest” ambient energy such as vibrations for remote or long-term sensing networks. Meanwhile, the makers of medical implants that deliver drugs or stimulate nerves face similar challenges powering micromachines that need to run for years, not days.
Kris Pister, chief technology officer of Dust Networks, said that for most early commercial applications such as building monitoring “sensors won’t need to send or receive data more than once a minute, in some cases once an hour or day.”
Cost and performance outweigh device size, he said, so that coin-cell batteries are an adequate power solution for the near term.
“Shrinking sensor elements and other components with MEMS technology reduces power requirements dramatically,” he said.
Pister said he expects devices to shrink from the size of a pager to a bottle cap to an aspirin over the next five years. As they do, and other microdevices such as advanced radio frequency ID (RFID) tags emerge, market demand will drive the need for equally compact and cheap power packs.
On the RFID front, for example, Power Paper Ltd., based in Israel, has developed a flexible battery material that can be printed on a variety of surfaces.
Lucent Technologies and mPhase Technologies Inc. of Norwalk, Conn., announced a prototype of a nanotech-enabled reserve battery that could be a solution for wireless sensors, RFID tags and other self-powered small devices. Using a layer of “nanograss” embedded with nanoscale pillars, researchers developed an approach to microfluidically control how a battery’s electrode performs.
The nanograss surface is normally hydrophobic, a property that can be exploited to keep the electrolyte separated from the battery’s electrode by liquid. When power is needed, a small voltage pulls the liquid down into the nanograss, so it no longer impedes the discharge.
The power is turned off by removing the voltage. Researchers say the method prevents batteries from slowly discharging, giving them nearly unlimited shelf life. They say the process could scale down to work in an area of as little as a dozen square microns.
High-performance microbatteries exist, but are typically expensive, customized products. Quallion Inc. of Sylmar, Calif, for example, has developed a $400, rechargeable lithium ion model that is slightly larger than a grain of rice.
The company says it should last 10 years or more and can be recharged thousands of times wirelessly via an electrical field. The microbattery has been integrated in a nerve stimulator implant developed by Advanced Bionics Corp.that can treat a variety of neural disorders.
Today, tailoring the architecture of wireless sensor nets to a specific application may offer the most immediate fix. Peter Stein, vice president of business development at Massachusetts-based Sensicast Inc. said its sensor networks for museums feature routers and gateways that plug into existing power lines.
That hybrid approach reduces the power drain on battery-powered wireless sensors. Ardesta LLC, Small Times’ parent company, is an investor in Sensicast.
MicroStrain developed wireless MEMS sensors for monitoring strain and force that “harvest” energy by converting vibrations into current through piezoelectric fibers. Such vibration-powered nodes embedded in the composite skin of a combat helicopter can track structural fatigue, for instance.
Millennial Net of Cambridge, Mass. partnered on similar energy harvesting schemes with Ferro Solutions Inc. and Continuum Control Corp., whose devices also turn vibrations from heating ducts or the motion of a vehicle into power to run Millennial’s i-Bean sensor nodes.
At TPL, Inc. in Albuquerque, N.M., senior scientist Charles Lakeman has been working on a micro electrochemical power supply system that integrates a microbattery with a microsupercapacitor.
Applications for such ultra-small power sources, Lakeman said, include covert intelligence systems, where a small and relatively low-cost device would be necessary.
Indeed, Dust’s Pister said the company’s sensor systems are efficient enough to run off photovoltaic cells, even inside buildings.
Konarka Technologies in Lowell, Mass. is looking to do just that, at least initially for the military. DARPA awarded Konarka, several universities and the Army’s Soldier Systems Center in Natick, Mass. a $6-million contract to develop new materials for hybrid photovoltaic cells.
Potential applications include battery charging on the battlefield and solar-powered sensor networks. Daniel McGahn, Konarka’s chief marketing officer, said the company’s polymer material could be applied to a credit-card-sized device that would work in conjunction with a small battery. Small Times’ parent company, Ardesta, is an investor in Konarka.