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 An early prototype of AmbioMote has been used in sensor powered by bridge vibration. Passing traffic causes deflections of the bridge girder which excite an electromagnetic generator tuned to one of the fundamental frequencies. Energy from the generator is used by AmbioMote.
Energy harvesting as enabling factor for bridge monitoring
Wireless
monitoring of civil engineering structures such as bridges and
overpasses has gained a lot of interest in the recent years. Bridge
collapses happen suddenly and unpredictably and often lead to tragic
loss of human lives. In 2006 Federal Highway Administration
listed 25.8% of nation's 596,842 bridges either as structurally
deficient or functionally obsolete. While many of these bridges will
remain in service for many years, they need monitoring and
rehabilitation.
Presently, bridge monitoring is performed through periodic visual inspections. In the tragic example of I-35W Mississippi River bridge collapse, the bridge passed a visual inspection a year prior to failure. Many researchers are suggesting installing wireless monitoring sensors
that can continuously monitor bridge condition and report any changes
that may lead to failure. Wireless sensors are easy to install and can
be applied to existing highway infrastructure.
The
problem arises in proving power to operate wireless sensors for bridge
monitoring. Since every bridge would have at least several sensors
which could be installed in hard to access locations. Replacing
millions and millions of batteries would become a huge logistic problem
and add to the expense of maintaining bridges. Another important factor
is environmental impact of discarding used batteries.
We
suggest using vibration of bridges caused by passing traffic, wind and
microtremors to power the bridge monitoring sensors. The battery is
completely eliminated from the equation. Hermetically sealed sensors
powered by bridge vibration can remain on the bridge for decades and
provide continuous monitoring.
We use RT11 bridge in Potsdam, NY for a case study.
Figure 1. RT11 bridge in Potsdam (February 2006)
RT11
bridge is a steel girder bridge, one of the most common types of
bridges. Energy can be harvested by locating a generator and sensors on
the bottom or top flange of a girder. The following figure shows the
spot where the self-powered sensor was installed.
Figure 2. Location of sensor installation
Figure 3. Electromagnetic energy harvester
Vibration
of the bridge is harvested by an aircore tubular linear generator which
responds to one of the natural vibration frequencies of the bridge.
Each time a car or a truck pass over the bridge, even in the different
lane from the sensor installation, the whole structure vibrates and
excites the mover in the generator thus producing AC voltages on the
output.
RT11 bridge is located in rural area and has
fairly low traffic. In this sense it presents a more difficult scenario
than a high-traffic bridge in urban areas. Figure 4 illustrates
vibration patterns caused by passing traffic and the output voltage of
the energy harvester. It can be clearly seen the structure experiences
significant vibrations only periodically, while most of the time
harvester output is rather low.
Figure 4. Time history of bridge vibration (acceleration) and open circuit voltage of the generator
A wireless self-powered sensor has the following key tasks:
- Effectively
convert AC voltage from the generator to DC voltage that can be used
for powering of the sensors. Active intelligent conversion may raise
the amount of available energy upto 500%.
- Store the accumulated energy until the level becomes sufficient to perform a useful task.
- Effectively use stored energy to perform a measurement and wireless transmission.
Figure
5 shows a wireless sensor platform that excels at all these tasks. This
is a early prototype of a wireless platform that is now known as AmbioMote a product manufactured and sold by AmbioSystems under a license from Clarkson University
Figure 5. Early prototype of AmbioMote.
Ambiomote
is designed to work with high-impedance high-voltage energy harvester
such as electromagnetic and piezoelectric generators. The devices
handles intelligent energy (AC->DC) conversion, significantly
boosting the efficiency of energy harvesting Figure 6 shows the charge
on a storage capacitor after a single pulse excitation of
electromagnetic generator. Compared to a regular rectifier-capacitor
pair, AmbioMote is capable of capturing 400% greater energy! (E=0.5CV2)
AmbioMote
provides low-leakage multi-stage storage, a high-speed (2Mbps) radio, a
variety of digital and analog sensor interfaces. A single 6-byte radio
transmission uses only 4.5 uJ of energy, a read of a temperature sensor
8 uJ, and so on.
Figure 6. AmbioMote (left) and
rectifier-capacitor harvesting a single 10mm displacement pulse from
the linear generator. on a1000uF storage capacitor.
A
bridge experiment was conducted in June 2007 using the following setup
(Figure 7). The energy harvester was clamped to the bottom flange of
the girder. The output of the energy harvester powered the AmbioMote
prototype. When stored energy became sufficient for an atomic
operation, AmbioMote woke up from sleep mode, measured temperature and
transmitted the measurement via the wireless interface. A receiver
module was connected to a portable computer on a USB port. On each
reception the receiver would display reception time, sensor ID and
temperature value on the computer screen.
Figure 7. Bridge experiment setup.
Bridge energy harvesting video
The
following video illustrates the bridge self-powered sensor in action.
The video consists of two short segments. The first segment shows
wireless transmissions generated by the sensor from excitation
generated by a medium-sized truck. The second illustrates transmissions
generated by a heavy truck.
Please be patient: video size is about 7Mb and may take a while to download.
Alternatively you can download this video .
24-hrs monitoring
The self powered sensor is
capable of monitoring bridge condition for 24 hours. The following graph
shows the number of measurements per hour in one day of September 2007.
As the graph shows, even deep in the night, the energy harvesting
devices generate enough power to perform measurements.
Conclusions
We have created a proof-of the
concept device that uses vibration of a highway bridge to power a
wireless monitoring device. Such sensors can be used to monitor
temperature, ice conditions, traffic flows and health status of highway
bridges without need of maintenance for decades.
Acknowledgements
Support for this project was provided by the Transportation Research Board of the National Academies through a grant to Clarkson University.
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