 www.sensorsmag.com
March
2004
Power Considerations
for Wireless
Sensor Networks
Two ways to power remote, portable devices are
tiny long-life batteries and energy harvesting techniques. The choice
depends on the demands of the particular application.
Tod Riedel, Millennial Net, Inc.
 Figure 1. A self-organizing wireless
sensor node from Millennial Net incorporates all the required
components, except for the sensor, in this form factor.
| | Power conservation is a
critical component of the wireless sensor networks used in hundreds of
commercial, industrial, medical, consumer, and military applications
characterized by low power requirements and low data rates. Long battery
life (typically 5–10 years) is essential in wireless sensor networks
(WSNs) where line power is not available or where the sensor nodes are
mobile. Because the philosophy of sensor networks is “wireless anywhere,”
the size of a sensor node is also critical. Many applications require
nodes not much larger than a dime (see Figure 1). In these cases, even AA
batteries are too bulky and coin cell batteries are the only option. WSNs
are capable of operating on sub-milliampere power, allowing 3 VDC coin
batteries to power a sensor node for up to 10 years and beyond, depending
on the sampling rate.
But in many WSN applications, using and replacing batteries, even
long-lived types, is impractical. Often the sensor nodes are located in
unrecoverable locations. Furthermore, the labor and costs associated with
changing hundreds, if not thousands, of batteries outweighs the ROI that
the sensor network could deliver. In response to these constraints,
systems designers and engineers are looking for alternative ways to
deliver exceptionally long life to sensor network devices. One method that
is gaining traction among designers is the extraction of electrical power
from persistent ambient vibrations.
Advances in vibration-based, environmental harvesting have made it
possible to provide the power required to operate WSNs and make totally
battery-free WSNs feasible. Candidate application areas include machine
condition monitoring, HVAC monitoring through duct- or machine-mounted
sensors, vehicle tire pressure and temperature monitoring, and the remote
colonies of sensors used in aircraft and ships to detect potential
problems.
Because these harvesters are so sensitive and efficient, they can
generate electricity from vibrations that are barely noticeable to the
human touch. Input vibrations are measured in g’s, where 1 g
is equal to the acceleration of gravity. (Tapping on a table creates ~0.02
g, or 20 mg vibrations that are detectable by a hand.) One example is the
Energy Harvester made by Ferro Solutions.
 Figure 2. Continuum Control Corp.’s
harvester package, featuring an integrated transducer and
harvesting electronics including power conditioning up to 3 V,
offers tunable coupling and frequency.
| | In an environment
with vibrations at 28 Hz and 100 mg, this harvester produces a power
output of 9.3 mW from a cylinder measuring ~1.8 in. dia. by ~1.8 in. high.
Doubling the volume of the harvester will double the useful energy. The
power output scales linearly with increased vibration frequency and
exponentially with increased g-force. When the electricity thus generated
is not used immediately, it can be stored in a super capacitor.
Another example is Continuum Control Corp.’s iPower energy harvesters.
These devices, about the size of a pack of gum (see Figure 2), extract
electric energy from mechanical vibrations, motion, or impact, and store
it for use by wireless sensors or other electronic devices. The technology
couples proprietary transducers and circuits to a mechanical system,
creating a solution that maximizes power flow from the mechanical to the
electrical storage and is specifically targeted at converting micromotion
into usable power (see Figure 3).
 Figure 3. Continuum’s technology couples
proprietary transducers to the vibration source and proprietary
circuits, creating a system that maximizes power flow from the
mechanical to the electrical storage. High performance comes from
exploiting knowledge of the states of the structure and circuitry to
control power flow and thus optimize available power and conversion
efficiency.
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Both technologies provide enough energy to power a wireless sensor node
and its attached sensors, and both are viable alternatives to line or
battery power for the appropriate sensor network application.
Environmental energy
harvesting from vibration sources promises nearly unlimited life for
low-power wireless sensor networks. With even conservative projections
putting the number of units to be sold for WSN devices in the vicinity of
hundreds of millions each year, suppliers of WSN technologies must do
their part by making their products capable of supporting power line,
battery, and environmentally powered devices. This cooperative effort will
result in savings for the end users and a new degree of freedom for the
product designers and engineers responsible for implementing these
networks.
The author wishes
to thank Kevin O’Handley of Ferro Solutions (Roslindale, MA; 978-273-4709,
kevin@ferrosi.com) and Nesbitt
Hagood of Continuum
Control Corp. (Billerica, MA; 978-670-4910, nwh@continuumphotonics.com)
for their assistance in preparing this article.
Energy
Harvesting for Wireless Sensor Networks
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Stephanie vL Henkel
Making machines and structures
“smart” allows them to monitor their behavior, report on their
current status, and advise of impending problems before
catastrophic failure claims lives or property. To do this, the
sensors must be able to transmit digital data to a remote
receiver. Hardwiring the transmitters to a source of
electricity is expensive and time consuming. Replacing
batteries, even long-lasting types, can be a never-ending
chore. Energy harvesting is an attractive solution to these
problems.
 MicroStrain’s goal is to combine
energy harvesting with low-power sensors and networks to
create a completely wireless sensor network that can
deployed easily in the field and operate unattended for
long periods.
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Energy can be scavenged from various ambient sources:
solar, wind, thermoelectric, water/wave/tide, vibration, and
strain. The strongest candidate for industrial environments
appears to be strain. Accordingly, MicroStrain is developing
an energy-harvesting scheme (right) based on storing cyclic
strain energy by rectifying piezoelectric fiber output into a
capacitor bank. When the capacitor voltage reaches a preset
threshold, power is transferred to an integrated wireless
sensor node. Alternatively, power can be stored on a
rechargeable thinfilm battery. Software programming allows one
hardware design to operate with many sensor types, including
thermocouples, strain gauges, magnetometers, capacitive and
inductive sensors, and magnetic, temperature, and humidity
sensors.
Advances in single-crystal piezoelectrics have produced
materials with 90% mechanical-to-electrical conversion
coefficients efficiency and 1.4% operating strain
capabilities. The fibers can be directly bonded to a straining
element or structure, and even if some or many of the
individual fibers get broken, the rest of the bundle will
continue to function properly. Moreover, mass production makes
these piezoelectric materials inexpensive.
 MicroStrain’s network uses
addressable sensing nodes incorporating data-logging
capabilities and a bidirectional RF transceiver
communications link.
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The key to making energy harvesting work is to minimize the
power required by the entire system—sensors, conditioner,
processor, data storage, and data transmission (above).
Because the sensors tend to be in sleep mode most of the time,
they draw little current. At intervals controlled by a
randomization timer, the nodes wake up and transmit bursts of
data. MicroStrain’s WWSN ad hoc network architecture allows
thousands of multichannel, uniquely addressed sensing nodes to
communicate to a central, Ethernet-enabled receiver with
extensible markup language data output. Time division
multiple-access is used to control communications.
MicroStrain’s development of energy-harvesting techniques
has received support from National Science Foundation//Vermont
Phase 0 EPSCoR and Navy Phase 1 SBIR, and, currently, from
Navy Phase II programs. n
Contact MicroStrain, Williston, VT; 802-862-6629 or
800-449-3878.
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Tod Riedel is a
cofounder and Vice President, Millennial Net, Inc., Cambridge, MA; 617-225-0100,
x-220, triedel@millennial.net.
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