at Massachusetts Institute of Technology (MIT) have taken a significant step
toward battery-free monitoring systems—which could ultimately be used in
biomedical devices, environmental sensors in remote locations, and gauges in
hard-to-reach spots, among other applications.
work from the laboratory of MIT professor Anantha Chandrakasan has focused on
the development of computer and wireless-communication chips that can operate
at extremely low power levels, and on a variety of devices that can harness
power from natural light, heat, and vibrations in the environment. The latest
development, carried out with doctoral student Saurav Bandyopadhyay, is a chip
that could harness all three of these ambient power sources at once, optimizing
energy-combining circuit is described in a paper being published in the IEEE
Journal of Solid-State Circuits.
harvesting is becoming a reality,” says Chandrakasan, the Keithley Professor of
Electrical Engineering and head of MIT’s Department of Electrical Engineering
and Computer Science. Low-power chips that can collect data and relay it to a
central facility are under development, as are systems to harness power from environmental
sources. But the new design achieves efficient use of multiple power sources in
a single device, a big advantage since many of these sources are intermittent
key here is the circuit that efficiently combines many sources of energy into
one,” Chandrakasan says. The individual devices needed to harness these tiny
sources of energy—such as the difference between body temperature and outside
air, or the motions and vibrations of anything from a person walking to a
bridge vibrating as traffic passes over it—have already been developed, many of
them in Chandrakasan’s laboratory.
the power from these variable sources requires a sophisticated control system,
Bandyopadhyay explains: Typically each energy source requires its own control
circuit to meet its specific requirements. For example, circuits to harvest
thermal differences typically produce only 0.02 to 0.15 V, while low-power
photovoltaic cells can generate 0.2 to 0.7 V and vibration-harvesting systems
can produce up to 5 V. Coordinating these disparate sources of energy in real
time to produce a constant output is a tricky process.
far, most efforts to harness multiple energy sources have simply switched among
them, taking advantage of whichever one is generating the most energy at a
given moment, Bandyopadhyay says, but that can waste the energy being delivered
by the other sources. “Instead of that, we extract power from all the sources,”
he says, by switching rapidly between them. “At one particular instant, energy
is extracted from one source by our chip, but the energy from other sources is
stored in capacitors” and later picked up, so none goes to waste.
challenge for the researchers was to minimize the power consumed by the control
circuit itself, to leave as much as possible for the actual devices it’s
powering—such as sensors to monitor heartbeat, blood sugar, or the stresses on
a bridge or a pipeline. The control circuits optimize the amount of energy
extracted from each source.
system uses an innovative dual-path architecture. Typically, power sources
would be used to charge up a storage device, such as a battery or a
supercapacitor, which would then power an actual sensor or other circuit. But
in this control system, the sensor can either be powered from a storage device
or directly from the source, bypassing the storage system altogether. “That
makes it more efficient,” Bandyopadhyay says. The chip uses a single
time-shared inductor, a crucial component to support the multiple converters
needed in this design, rather than three separate ones.