A 9 cubic millimeter solar-powered sensor system developed at the University
of Michigan is the smallest that can harvest energy from its surroundings
to operate nearly perpetually.
The U-M system's processor, solar cells, and battery are all contained in its
tiny frame, which measures 2.5 by 3.5 by 1 millimeters. It is 1,000 times smaller
than comparable commercial counterparts.
The system could enable new biomedical implants as well as home-, building-
and bridge-monitoring devices. 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
The engineers say 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.
Greg Chen, a computer science and engineering doctoral student, will present
the research Feb. 9 at the International Solid-State Circuits Conference in
"Our system can run nearly perpetually if periodically exposed to reasonable
lighting conditions, even indoors," said David Blaauw, an electrical and
computer engineering professor. "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 say 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 be reduced for the system to function
"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 U-M 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,"
The designers are working with doctors on potential medical applications. The
system could enable less-invasive ways to monitor pressure changes in the eyes,
brain, and in tumors in patients with glaucoma, head trauma, or cancer. In the
body, the sensor could conceivably harvest energy from movement or heat, rather
than light, the engineers say.
The inventors are working to commercialize the technology through a company
led by Scott Hanson, a research fellow in the Department of Electrical Engineering
and Computer Science.
The paper is entitled "Millimeter-Scale Nearly Perpetual Sensor System
with Stacked Battery and Solar Cells."
This research is funded by the National Science Foundation, the Defense Advanced
Research Projects Agency, the National Institute of Standards and Technology,
the Focus Center Research Program and ARM.