Electronic Design

Zombies And Energy Harvesting

Suppose someone were to design a way to power a heart pacemaker by scavenging some of the energy of the beating heart itself, doing away with batteries. Could your heart continue beating after your death in some sort of perpetual-motion scenario? Would you become a zombie?

I had that conversation with Roy Freeland, CEO of a British company called, eerily enough, Perpetuum. Freeland had mentioned the possibility of harvesting the energy of the beating heart at the Darnell Group's first annual nanoPower Forum earlier this month in San Jose, Calif.

Not that Perpetuum is dedicated to Shaun of the Dead pursuits. Instead, the company's microgenerators power systems that monitor vibrations in equipment in places such as refineries and water pumping stations. The pacemaker was just one in a long list of potential applications that Freeland mentioned in his presentation at the conference's plenary session, albeit one that Perpetuum is actually working on.

THE PIEZOELECTRIC PUZZLE
Several vendors of piezoelectric pushbutton switches were at the conference too, and they had various approaches to solving a question that puzzled me— namely, how do you convert the mere press of a finger into a significant burst of energy when piezoelectric materials are brittle and deform so little?

Piezo pushbuttons are a big part of today's energy-harvesting picture. The plenary session started with Boeing's Bradley Mitchell discussing energy-harvesting systems in the 787 jetliner. Some of the technology is thermocouple-based, but a great deal focuses on the Passenger Services System—those buttons on the armrest that turn on the overhead light and change channels on the entertainment system. Today, that's all hardwired, Mitchell said.

Yet in an effort to make things lighter and more reliable, Boeing wants to replace the wiring with a mesh network and power it all with finger-presses. What's more reliable than hardwiring, you ask? Nothing, if the cabin configuration remains unchanged. Airlines change cabins from passenger to cargo use and rearrange seating all the time, though. That implies an interconnect system, and there goes your reliability.

THUNDER AND LIGHTNING
Back to the question of the brittle piezoelectrics. Curiously, one approach is based on a NASA technology called Thunder, which was developed for actuators. Face International has turned the technology on its head for use in switches, calling it "Lightning" to play off the "Thunder" motif.

"A thin ceramic piezoelectric wafer is sandwiched between an aluminum sheet and a steel sheet and held together with an amorphous thermoplastic adhesive. The sandwich is heated in an autoclave, and the adhesive melts," a NASA representative explained.

"When the sandwich cools, the adhesive bonds the parts together into one piezoelectric element," the representative continued. "While they cool, the components of the element contract at different rates, and this differential shrinkage causes the element to warp. The shrinking of the outside metal layers places the inside piezoelectric ceramic under mechanical stress."

"Like a pre-stressed concrete bridge?" I asked.

"Exactly," I was told. So that provides a good quarter-inch of travel and enough energy to flash three NE-2 neon lamps in Face's demo. Mystery solved.

And when I asked Roy Freeland the zombie question, he was a little startled. Then he began to think about the implications. "I'll take it back to my cardiologist," he promised.

Perpetuum Ltd.
www.perpetuum.co.uk

Face International
www.faceinternational.com

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