For detecting and measuring health conditions of bridges, wireless sensor networks are used in these days. However, battery life is critically restricting the application and maintenance cost of sensor network systems. To extend life time, a wireless power transfer system at UHF band is introduced to supply the current wireless sensor network. This power transfer system is based on electric wave at 950 MHz. This power transfer system is redesigned for tiny power transmission, including a combination of a rectenna and a Cockcroft-Walton boost converter, battery board, and a control board. Also, current wireless sensor network is redesigned for power transfer system. The working flow of sensor network is modified to bottom-to-top to save power of sensor modules which are the power bottleneck of this sensor system. As a result, the system is able to support a sensor module continuously with received power of −14 dBmW, when the transmitting antenna is 30 dBmW at 10 meters distance.
In Japan, most landforms are rugged by mountains and rivers. Therefore, bridges played a critical role in daily transports. However, the average age of bridges in Japan is over 20 years, and due to rapid nature disasters, bridge health monitoring techniques have become an important field.
Recently, in the field of bridge health monitoring, wired sensor networks and wireless sensor networks are both used. However, a wired sensor network system usually costs much more than a wireless one. In a previous research on Donghai Bridge, optical fiber and GPS servers are implied to transmit data [
Basic sketch of wireless sensor network bridge health monitoring system.
In this system, sensors are used to acquire bridge vibration. After measurement, sensor modules transfer their data to the data collection device via routers. The data collection device is able to transfer data to a computer via USB cable or to the Internet via a network module.
However, a problem still occurred in this system. Under a current measurement plan, the sensor network activates for 15 seconds per day to measure bridge vibration, and battery is able to supply this system for 10 years. Also, due to the leakage of battery, increasing battery capacity will not extend system life as expected.
Since battery exchange for bridge monitoring system costs many resources, it is expected to find a way for unlimited power. To solve this problem and extend the battery life, some methods are raised such as solar energy or wind energy. However, since the sensors are attached beneath the deck or on the piers, solar panels would not be able to receive enough sunshine for the system. Also, wind power generators will suffer from the moist environment under the bridge, and the maintenance of wind energy generator is technically similar to changing batteries. Therefore, to provide energy generation with a nearly infinite life, or at least the same as a bridge’s life, wireless energy transfer is brought to this system, with expected transmission distance over 10 meters.
Different from other wireless power transfer systems, aiming at large power transfer, this system focuses tiny power transfer under 10 mW. One of the reasons is that, depending on radio law in Japan, output of an antenna at 950 MHz is limited up to 1 W. Therefore, at the receiving side, power decreases to less than 1 mW, considering the antenna gain and the free-space attenuation. On the other hand, electric radiation at 1 W level has not been proved safe for human health yet. Therefore, this research chose tiny power transfer for recharging wireless sensor modules.
In this research, we mainly focused on the receiving side, including the rectenna and boost converter, a low-power controlling board, and modification of the current wireless sensor module. The architecture of this system is shown in Figure
Architecture of the wireless power transfer system.
A rectenna is an antenna with rectifying circuit, which is used to convert microwaves to DC power [
For traditional microstrip antenna, size is the first priority. However, in this wireless power transfer system, the antenna gain and efficiency have more priority than size. To increase the antenna gain, a stacked architecture design is applied to increase the receiving bandwidth and the antenna gain [
The receiving antenna with rectifying circuit.
Moreover, the rectifying circuit uses a low pass filter (LPF) to prevent reradiation of harmonic effect and a quarter-wavelength transmission line for impedance matching [
Evaluation result of the antenna is shown in Figure
Performance evaluation of the receiving antenna.
As a result from the experiment, the rectenna circuit is able to provide 52
Design and implement of boost converter.
The combination of rectenna and boost converter is evaluated by actual wireless power transfer antenna. The experiment’s result of output voltage and current is shown in Figure
Evaluation of the boost converter.
Therefore, the output power after the boost converter became 47
For wireless power transfer system, it is required for frequently charging and discharging. In this system, we choose micro energy cells made by THINERGY. The MEC101 energy cell works at 4.1 V, with 1.0 mAh and 40 mA. The size of this energy cell is 25.4 mm × 25.4 mm × 0.17 mm [
Photo of battery system, control board, and a sensor module. The upper board is the battery system and the right-bottom board is the control unit.
For an automatic controlling system, we used R8c/27 microcontroller produced by Renesas to control this whole system.
Obviously, the control board is also fully powered by the secondary battery unit, which means that it is 100% powered wirelessly. Hence, power consumption of this control board became the first priority. Another reason to use this microchip-based control board is that the sensor module costs over 50
Depending on actual experiment, the transfer system can transfer about 47
Firstly, switch the microchip into wait mode. In oscillator mode or outer clock mode, power consumption of the microchip is not less than 650
Moreover, to keep waiting mode as long as possible, interrupt of the microchip is also modified for the longest time interval. In this case, it is set to 63.5 s.
At last, after other procedures of switching off other functions in the micro-chip and controlling board, average power consumption of the controlling board is decreased to 11
Photo of battery system, controlling board, and a sensor module is shown in Figure
The total capacity of the battery unit is 3 mAh working at 4 V. Therefore, the total energy is 12,000
Figure
Working procedure of previous sensor system and current sensor system. (a) Previous system with top-to-bottom flow, (b) current system with bottom-to-top flow.
In Figure
While processing the collected data, the previous system will save all the collected data into its own buffer and transmit it to the collector after the whole measuring is finished. In this system, since the data is transmitted one by another, all the sensor modules will cost more time (10 seconds by each module, 3 seconds by each axis) and power for waiting in the transmission queue. In the current system, data is transmitted during measuring, which means after measuring, transmission will be terminated immediately, in order to save power. Therefore, in a system with 3 sensor modules, sensor module’s average activation time is 35 seconds in previous system, while current system requires only 15 seconds.
Since some experiments of the rectenna and the boost converter are introduced in Section
Firstly, we tested the discharging curve of the battery system, using our sensor modules. Result shows that the discharging curve is basically a straight line. And after 150 s of discharging, the voltage of the battery system will fall to 3.9 V from 4.1 V.
Moreover, several experiments are carried out for testing this system. Firstly, output performances in different environments of this system are tested. Test result is shown in Figure
Output performance of the wireless power transfer.
To reduce the effect of electrical wave reflection, first experiment is done on a building roof, with no higher buildings around 100 m. Experiment result is shown in Figure
In real application, bridges are made of steel or reinforced concrete, which strongly prevent UHF microwave. For confirmation, we did another experiment, while there are iron fence and reinforced concrete, which locates less than 2 m from the antenna. The wireless transmission result is shown in Figure
The result shows that when sending and receiving antennas are set in-sight, power transfer will be influenced but not obviously, even if we attached both antennas on the iron fence. However, the power transfer is falling suddenly due to the reflection of the concrete, but the boost converter is still able to boost voltage at about 10 m distance in concrete environment.
During the discharging experiment, the following facts are known. First, while the secondary battery is fully charged, the voltage will be 4.1 V. Second, while overdischarging, permanent damage will be done to the battery system. Therefore, one battery unit is able to drive a sensor module for about 150 seconds. Third, after 150 s of discharging, the voltage of the battery system will fall to 3.9 V from 4.1 V. Fourth, the discharging curve can be treated as a straight line.
From these four facts, two conclusions can be inferred. Conclusion 1: for the battery, voltage changing of 0.01 V equals 5% capacity. Conclusion 2: 15 s of measurement equals 10% battery capacity.
Depending on these two conclusions, we did another 72 hours experiment to verify this system. The basic point is whether this system can run continuously and periodically. Theoretically, this system can be driven only by wireless transferred power.
In the next experiment, we cleaned a room and then adjusted distance between the sending and receiving antennas to simulate the power transfer strength over 10 m distance. After setting up devices, we measured the voltage of the battery system every 2 hours for 3 days. The experiment result is shown in Figure
Result of a 72-hour wireless power transfer experiment.
Additionally, 3-axis acceleration data measured by sensor modules is shown in Figure
The 3-axis acceleration data of 10 s measured by a sensor module, with spectrum analysis result.
In this paper, we have realized a wireless power transfer system at 950 MHz using a secondary battery power unit for wireless sensor network.
For realizing this system, a rectenna-boost converter-battery system is proposed to drive sensor modules with working voltage at 3.3 V. The output of the antenna is about −8 dBm. Although there is a boost converter after the rectenna, the combination of the rectenna and boost converter is able to provide power of 47
There is still some problem to be solved. First, generally bridges are made of steel or concrete, whose materials prevent UHF electromagnetic wave. To solve this problem, we can set the power sending antenna and power receiving rectenna to in-sight areas. Depending on our experiments, while antennas are put in-sight, power attenuation is not obvious.
However, this solution brings another problem. This system is proved to be effective at 10 m distance. While setting antennas, distance between two antennas will be over 10 meters. To expand energy transmission range and efficiency, several methods such as increasing sending power and modifying sending antenna are considered. During an academic exchange between our lab and mechanical engineering laboratories of India Institute of Technology Kanpur, a method of flexible antenna is discussed. In that research, antenna is made of flexible materials and is able to change its shape to enhance transmission directivities.
For future work, it is necessary to extend power transfer distance and reduce power consumption of sensor modules. However, radio law in Japan will change in one or two years. After the modification of radio law, power transfer frequency will be changed from 950 MHz to 920 MHz. Receiving antenna will be adjusted at that time.