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An Autonomous 16mm3 Solar-Powered Node for Distributed Wireless Sensor Networks
Abstract
A 16 mm
3
autonomous solar-powered sensor node with bi-
directional optical communication for distributed sensor net-
works has been demonstrated. The device digitizes inte-
grated sensor signals and transmits/receives data over a
free-space optical link. The system consists of three diea
0.25µm CMOS ASIC, a 2.6 mm
2
SOI solar cell array, and a
micromachined four-quadrant corner-cube retroreflector
(CCR), allowing it to be used in a one-to-many network con-
figuration. The CMOS ASIC includes a photosensor, inte-
grated 3 MHz oscillator, 69 pJ/bit optical receiver, and
31 pJ/sample ADC.
Keywords
Smart Dust, CMOS integrated circuits, MEMS, distributed
sensors, low power electronics, sensor networks, microma-
chined sensors, cubic millimeter mote
INTRODUCTION
As sensors have become smaller, cheaper, and increasingly
abundant, there have been commensurate reductions in the
size and cost of computation and wireless communication.
Extrapolating these trends we envision wireless sensor nodes
becoming as small and as numerous as dustdisappearing
into the environment and radically changing the way we
interact with it. These devices will provide more information
from more places in a less intrusive manner than ever before.
The Smart Dust project [1] aims to explore the limits of sys-
tem miniaturization by packing an autonomous sensing,
computing, and communication node into a cubic millimeter
mote
1
that will form the basis of massive distributed sensor
networks, thus demonstrating that a complete system can be
integrated into 1 mm
3
. Some examples of applications that
we are pursuing include defense networks that could be rap-
idly deployed by unmanned aerial vehicles (UAV), tracking
the movements of birds, small animals, and even insects, fin-
gertip accelerometer virtual keyboards, monitoring environ-
mental conditions affecting crops and livestock, inventory
control, and smart office spaces.
Other academic efforts at building small wireless sensor
nodes include the multisensor microcluster [2] at the Univer-
sity of Michigan, Wireless Integrated Network Sensors [3] at
UCLA and the Rockwell Science Center, and PicoRadio [4]
at UC Berkeley. However, these nodes are one or more
orders of magnitude larger, use correspondingly more power,
and utilize RF communication.
The development of Smart Dust will require evolutionary
and revolutionary advances in miniaturization, integration,
and energy management. These advances will be facilitated
by progress in Microelectromechanical Systems (MEMS),
which allows us to build small sensors, optical communica-
tion components, and power supplies; and microelectronics,
which provides increasing amounts of functionality in
smaller areas and with decreasing energy consumption. Fig-
ure 1 shows the conceptual diagram of a Smart Dust mote.
The power system may consist of a battery and/or an energy
harvesting device such as a solar cell with a charge integrat-
ing capacitor for periods of darkness. A variety of sensors,
including light, temperature, vibration, magnetic field,
acoustic, and wind shear, can be integrated on the mote
depending upon the application. An integrated circuit will
provide sensor signal processing, communication, control,
data storage, and energy management. A photodiode will
1. And why beholdest thou the mote that is in thy brother's eye, but con-
siderest not the beam that is in thine own eye? Matthew 7:3 (KJV)
Figure 1. Smart Dust conceptual diagram.
1-2mm
Thick-Film Battery
Solar Cell
Power Capacitor
Analog I/O, DSP, Control
Sensors
Passive Transmitter with
Corner-Cube Retroreflector
Interrogating
Laser Beam
Mirrors
Active Transmitter
with Beam Steering
Laser
Lens
Mirror
Photodetector and Receiver
Incoming Laser
Communication
1-2mm
Thick-Film Battery
Solar Cell
Power Capacitor
Analog I/O, DSP, Control
Sensors
Sensors
Passive Transmitter with
Corner-Cube Retroreflector
Interrogating
Laser Beam
Mirrors
Passive Transmitter with
Corner-Cube Retroreflector
Interrogating
Laser Beam
Mirrors
Interrogating
Laser Beam
Mirrors
Active Transmitter
with Beam Steering
Laser
Lens
Mirror
Active Transmitter
with Beam Steering
Laser
Lens
Mirror
Laser
Lens
Mirror
Photodetector and Receiver
Incoming Laser
Communication
Photodetector and Receiver
Incoming Laser
Communication
Incoming Laser
Communication
An Autonomous 16 mm
3
Solar-Powered Node for Distributed
Wireless Sensor Networks
Brett A. Warneke, Michael D. Scott, Brian S. Leibowitz, Lixia Zhou, Colby L. Bellew,
J. Alex Chediak , Joseph M. Kahn, Bernhard E. Boser, Kristofer S.J. Pister
Berkeley Sensor and Actuator Center
Dept. of Electrical Engineering and Computer Science Dept. of Materials Science & Engineering
University of California at Berkeley
Berkeley, California 94720, USA
{warneke, mdscott, bsl, lzhou, cbellew, jmk, boser, pister}@eecs.berkeley.edu, chediak@uclink.berkeley.edu allow optical data reception, while two transmission
schemes are being explored: passive transmission using a
corner-cube retroreflector (CCR) [5] and active transmis-
sion using a laser diode and steerable mirrors [6].
The diminutive size of the mote makes energy management
a primary design constraint. Current micromachined battery
technology provides 5.6 J/mm
3
[7] while capacitors store up
to 10 mJ/mm
3
. Energy harvesting techniques are attractive
for sensor nodes to allow indefinite lifetimes in the field,
particularly given the small size and abundance of the
motes, but system power consumption must still be mini-
mized to maintain the tiny size of energy havesting devices.
Free-space optical communication provides several advan-
tages over RF communication for small, energy-constrained
wireless nodes. First, optical radiators can be made more
efficient as well as with much higher antenna gain (> 10
6
) at
the millimeter scale. Furthermore, optical transmitters are
more power efficient at low power because of reduced over-
head and since received power only drops as 1/d
2
, compared
with 1/d
4
for RF transmissions subject to multi-path fading.
Preliminary motes [8] have previously demonstrated vari-
ous concepts of Smart Dust. The current work describes the
first fully autonomous mote (Figure 2) utilizing all custom
components to yield the smallest device yet. The mote con-
sists of three diea CMOS ASIC, an SOI solar cell array,
and CCR, with a circumscribed volume less than 16 mm
3
.
This device incorporates a photosensor, an analog to digital
converter (ADC), an optical receiver, a four-quadrant CCR
for optical transmission, solar cells for power, and a simple
finite state machine (FSM) controller (Figure 3). An accel-
erometer has also been fabricated but not yet been demon-
strated in the system. This type of mote would be used in an
application
that
allows
line-of-sight
communication
between one or more base station interrogators and a large
number of sensor nodes, such as environmental, atmo-
spheric, space, or military monitoring.
COMPONENTS
Sensors
Two sensors were designed for this mote to demonstrate
integration of multiple physical sensors into a cubic milli-
meter node. Both sensors are wired through an analog mul-
tiplexer to the ADC and selected by the FSM.
The first sensor is an ambient light sensor integrated entirely
on the CMOS ASIC. This sensor consists of an N-well/sub-
strate photodiode in series with a polysilicon resistor to the
analog voltage supply. The photodiode has an area of
200
µm × 200 µm and a responsivity of approximately
0.1 - 0.3 A/W from visible through near IR illumination.
The resistance is approximately 1 M
, resulting in an over-
all sensitivity of 4 - 12 mV/(W/m
2
).
The second sensor is a simple capacitive accelerometer
(Figure 4) fabricated in an SOI process with electrical
Figure 2.
Mock-up of the 16mm
3
autonomous solar-
powered mote with bi-directional communications and
sensing, composed of a 0.25µm CMOS ASIC, solar power
array, accelerometer (not yet demonstrated in the system)
and CCR, each on a separate die. See Figure 9 for
annotations.
SENSORS
ADC
FSM
RECEIVER
TRANSMITTER
SOLAR POWER
1V
1V
1V
2V
3-8V
PHOTO
8-bits
375 kbps
175 bps
1-2V
OPTICAL IN
OPTICAL OU
Figure
3.
High-level
functional
diagram
of
the
demonstrated mote. The optical downlink and uplink
beams are shown separately for clarity, but in practice may
be the same beam. (Since the downlink beam is modulated
at frequencies well above the uplink signal band, downlink
and uplink can share the same channel.)
Figure 4. Photomicrograph of the gap-closing capacitive
accelerometer with excitation solar cells (four squares on
the left) fabricated in an SOI process. It is 0.9 x 1.3 mm. trench isolation. Small on-chip solar cells are used to pro-
vide the excitation voltage for the sensor. The accelerometer
was designed so that an acceleration of 1 g would produce a
large enough signal to be readily detected by the ADC
several 10s of mV.
To maximize sensitivity, albeit at the expense of bandwidth
and linearity, gap closing capacitive s