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A 9 cubic millimeter solar-powered sensor system can harvest energy from its surroundings to operate nearly perpetually.
The system’s processor, solar cells, and battery reside in its tiny frame, which measures 2.5 by 3.5 by 1 millimeters. It is 1,000 times smaller than comparable commercial counterparts, said researchers at the University of Michigan.
The system could enable home-, building- and bridge-monitoring devices as well as new biomedical implants. It could vastly improve the efficiency and cost of current environmental sensor networks designed to detect movement or track air and water quality.
With an industry-standard ARM Cortex-M3 processor, the system contains the lowest-powered commercial-class microcontroller. It uses about 2,000 times less power in sleep mode than its most energy-efficient counterpart on the market today.
Engineers said successful use of an ARM processor the industry’s most popular 32-bit processor architecture is an important step toward commercial adoption of this technology.
“Our system can run nearly perpetually if periodically exposed to reasonable lighting conditions, even indoors,” said David Blaauw, an electrical and computer engineering professor at the University of Michigan. “Its only limiting factor is battery wear-out, but the battery would last many years.”
“The ARM Cortex-M3 processor has been widely adopted throughout the microcontroller industry for its low-power, energy efficient features such as deep sleep mode and Wake-Up Interrupt Controller, which enables the core to be placed in ultra-low leakage mode, returning to fully active mode almost instantaneously,” said Eric Schorn, vice president, marketing, processor division, ARM. “This implementation of the processor exploits all of those features to the maximum to achieve an ultra-low-power operation.”
The sensor spends most of its time in sleep mode, waking briefly every few minutes to take measurements. Its total average power consumption is less than 1 nanowatt. A nanowatt is one-billionth of a watt.
The developers said the key innovation is their method for managing power. The processor only needs about half of a volt to operate, but its low-voltage, thin-film Cymbet battery puts out close to 4 volts. The voltage, which is essentially the pressure of the electric current, must reduce for the system to function most efficiently.
“If we used traditional methods, the voltage conversion process would have consumed many times more power than the processor itself uses,” said Dennis Sylvester, an associate professor in electrical and computer engineering.
One way the UM engineers made the voltage conversion more efficient is by slowing the power management unit’s clock when the processor’s load is light.
“We skip beats if we determine the voltage is sufficiently stable,” Sylvester said.

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