The Science Behind Long Lasting Wireless Sensors
By Michael T. Volling, Director of Product Management, Sensys Networks
quired to be met in order to deliver
a viable detection platform. The system needed to be able to communicate with multiple sensors, to have
robust communications, complete
with fail-safe’s, and had to consume
as little power as possible.
The last 20 years have seen an unprecedented explosion in wireless
electronic devices. This advancement continues to grow at a staggering pace. Small wireless devices,
like smart-phones, easily perform
functions that required significant
computer power in the 1980’s and
were thought of as futuristic only a
few decades ago.
The demand to have wireless devices deliver more functionality, while
simultaneously delivering longer
battery life, has created a whole
new field of electrical component
and communication design. This
emerging field is known as ultralow power processing and communication. These devices deliver
a high level of performance, while
operating on virtually no electrical power. When operating, these
ultra low power devices consume
only micro-amps of current, not the
milli-amps of previous generations
of technology.
When combined with commercial
wireless protocol, the new ultra
low power processor, would result
in sensors which would only last
a year or two. Several key characteristics were needed to enable this
technology to deliver on the long
lasting performance required in
the industry. In order to reduce the
power consumption, two key attributes were developed; reducing the
amount of time the sensing electronics are “alive” and minimizing the
power consumption of the wireless
communications.
axis magnetometer, a small microprocessor, and a wireless communications chipset. The magnetometer
is a passive device, which measures
the earth’s magnetic field. The magnetometer measures this field in
very short bursts, turning off its circuitry between each measurement.
The microprocessor also has the
ability to transition from a “sleep
state” which uses virtually no
power, to an “active” state in a very
short time. These two attributes allows the magnetic field to be read,
the information processed and the
vehicle’s detection state prepared
for transmittion, all within a fraction of a millisecond. In striking a
balance between detection accuracy
and power conservation, a detector
sample rate of 128 hertz (times per
second), or just under every 8 milliseconds, was selected. This enables
the detection circuitry to be active
approximately 1% of the time, and
hence “asleep” virtually 99% of the
time, greatly extending the battery
life of the sensor.
Each sensor is comprised of a small
integrated circuit including a three
On the wireless front, several challenging key design criteria were re-
In response to the transportation
industries continued quest for the
ideal vehicle detector, Sensys Networks combined the best of the ultra-low power technology, together
with key wireless protocol elements
to develop the next technology
wave in vehicle detection.
Page 28
With the requirement to have a
central device, or Access Point, communicate with multiple sensors,
a Radio Frequency (RF) channel
sharing scheme needed to be developed and implemented. Leveraging
the work developed in the cellular
communication industry, a time
division multiple access (TDMA)
system enables multiple devises to
share a common RF channel by allowing only a single device to talk
during a pre-defined time interval.
At all other times the radio must
remain silent. Following the need
for relatively quick communication
of the detection to the real world, a
baseline communication interval
of 125 ms was selected. In this interval, every device will have the
opportunity to communicate with
only a small fraction (2 ms) of this
allocated to each sensor.
In order to minimize the amount
of wireless data to be transmitted
in this short time period, the data
stream needs to be minimized to the
smallest amount practical. When
a sensor detects a vehicle, a small
packet is wirelessly transmitted
to a remote access point. In this
packet, the current information on
the vehicle detection is transmitted,
as well as the exact time of the detection. Once the detection information has successfully been received
by the access point, the radio will
no longer communicate detection
information, until a change in detector state is determined. Once the
sensor has determined that a vehicle
is no longer present, the sensor will
again transmit an “undetect” message indicating the vehicle has left,
along with the exact time the vehicle
Continued on page 30
IMSA Journal
The Science Behind Long Lasting Wireless Sensors . . . Continued from page 28
was undetected. This minimizes
the amount of time the radio must
be actively transmitting further
reducing the energy consumption
of the sensor.
With the transmission of detection
information having been reduced to
a state change only, it is critical that
the information be received by the
access point. In order to insure this
data integrity, an acknowledgement
system was developed between the
sensor and the access point. When
the sensor transmits a detector state
change to the Access Point, it must
receive an acknowledgement back
from the Access Point, verifying
the receipt of this information. If an
acknowledgement was not received
by the sensor, it will automatically
re-transmit the information until
it is received by the access point.
Once it is acknowledged by the access point, the sensor will no longer
transmit the information.
As part of the system robustness
and to ensure long battery life during idle times, the sensor is looking
for a synchronization event. The
synchronization event fulfils two
critical functions. It insures that
the sensor and the Access Point are
accurately time synchronized to enable proper communications in the
TDMA architecture as well as maintaining the sensor in an operational
mode. In the event it does not “hear”
from an access point during a predefined interval, it will go to a very
low power state, conserving energy,
while still “listening” for a signal
from an access point. If a sensor is
in sleep mode, it will wake up when
it hears a valid access point and
begin proper operations, including
determining current magnetic field
baseline and begin vehicle detections. In addition, if an access point
does not hear from a sensor within a
pre-determined time, it will go into
a fail safe mode.
With electronics now using as little
power as possible, the next design
hurdle was selecting a battery
Page 30
system to power the sensor under
demanding environmental conditions. After reviewing a variety
of different battery chemistries,
Lithium Thionyl Chloride batteries
were selected.
Lithium Thionyl Chloride batteries
were originally developed in the
1970’s. These batteries offer a number of characteristics making them
the ideal selection for this application. The batteries are field proven
over the last 30 years, so their performance characteristics are very
well known and documented. Due
to the hermetically sealed construction, the batteries lose only 1% of
their energy per year in non-use.
They perform in temperatures from
-55°C to +85°C, easily exceeding the
NEMA temperature range, so they
provide a useable current under all
expected operating conditions.
Lithium Thionyl Chloride batteries
provide the highest energy density
available. Due to the chemistry involved, Lithium Thionyl Chloride
batteries provide 3 times the power
of Li-Ion batteries and 7 times the
power of traditional alkaline batteries. The sensor contains three of
these powerful batteries.
If the sensors were operational at
room temperature consistently, the
batteries would last in excess of 17
years. Taking into account the impact of temperature extremes on the
battery chemistry, Sensys Networks
conservatively rates the battery life
to 10 years.
The most rigorous independent
validation on battery life was performed by AARB in Australia. In
this report, it was methodically
determined the battery life would
easily exceed the 10 years under
adverse temperature conditions.
end of life). Sensys Networks deployed a large deployment in early
December 2006 on Interstate 80 near
the East Bay entrance to the Bay
Bridge. This heavily congested site
easily has over 25,000 vehicles per
day per lane. In the five years since
installation, these sensors have already detected more vehicles than
would be expected in over 10 years
of use at a signalized intersection.
Despite the extreme heavy nature of
the traffic, no voltage variation has
been detected, indicating that the
batteries in every sensor still have
significant life left.
Through the unique combination of
ultra low power electronics, smart
power and communication management, Sensys Networks sensors
deliver long lasting performance.
This will allow traffic professionals to deliver on their objectives to
provide the highest level of performance possible to the motoring
public.
Evaluation of Sensys Networks wireless data station equipment: Stage 1
Laboratory tests by Peter Stewart,
Daniel Stephenson, Michelle Su, Laurie
Cahill, James Luk under contract from
Vic Roads, Australia.
1
One of the unique operating characteristics of lithium cells is that the
output voltage remains constant
until the end of operating life is near
(approximately 9 months prior to
IMSA Journal