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Investigation of Applicability of Electromagnetic Energy Harvesting System to Inclined Stay Cable Under Wind Load Hyung-Jo Jung, Jeongsu Park, and In-Ho Kim Department of Civil and Environmental Engineering, KAIST, Daejeon 305-701, Republic of Korea An electromagnetic energy harvesting system that involves the use of wind-induced vibrations of an inclined stay cable is proposed for powering a wireless sensor node installed on a cable, which is important for monitoring the integrity of a cable-stayed bridge. The proposed system is developed by introducing the rotational mechanism of a rigid bar having a moving mass pivoted on a hinged point with a power spring, so that it can be successfully operated on an inclined cable. The performance of the proposed energy harvesting system is validated using its prototype by performing a shaking table test and a field test on an in-service cable-stayed bridge. In addition, strategies for improving the performance of the proposed system are discussed in detail, and the applicability of the system is investigated on the basis of these performance improvement strategies. Index Terms—Energy harvesting, electromagnetic induction, inclined stay cable, rotational mechanism, field test.
I. INTRODUCTION
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NERGY harvesting is one of the most active research topics in the structural health monitoring (SHM) community, because it has the potential to facilitate the stand-alone functionality of an SHM system or some parts of the system such as wireless sensors (i.e., it can enable the operation of these systems without external power or battery replacement). Several resources for scavenging electrical energy from the environment exist, such as sunlight, temperature gradient, wind, and vibration. Among several energy harvesting systems, a vibrationbased energy harvesting system has attracted considerable attention as a stable and continuous power supply system for wireless sensor nodes, owing to the fact that a plethora of vibration sources exist around a bridge site mainly due to wind and traffic loadings [1]. The vibration-based energy harvesting system converts vibrational energy in the form of mechanical movement observed in the application environment to electrical energy. In other words, this system can use the ambient vibrations of cables—which are crucial structural elements of a cable-stayed bridge—and provide electricity to an adjacent wireless sensor node that measures the structural response of the bridge. In 2011, Jung et al. [1] developed an electromagnetic energy harvesting system that involves the use of wind-induced vibrations of a stay cable and investigated its feasibility for powering a wireless sensor node on the cable, through numerical simulations as well as experimental tests. They claimed that their system could generate sufficient electricity for the operation of a wireless sensor node attached on the cable under moderate wind conditions. However, they did not perform a field test under real conditions. Moreover, they did not consider the inclination of the cable and the large static deflection of a spring element; therefore, their system could not be directly implemented in a real stay cable from a practical point of view. For real applications, several issues including the inclination of a cable and the static deflection of a spring should be appropriately addressed. Manuscript received March 03, 2012; revised May 07, 2012; accepted May 24, 2012. Date of current version October 19, 2012. Corresponding author: H.-J. Jung (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2012.2202889
In this study, the applicability of a new electromagnetic energy harvesting system, which is more suitable for providing sufficient electricity to a wireless sensor node in an inclined stay cable, is investigated. To this end, an energy harvesting system consisting of a rigid bar having moving magnets pivoted on a hinged point with a power spring and a fixed solenoid coil has been developed. To verify the performance of the proposed system, a shaking table test and a preliminary field test on an in-service cable-stayed bridge are performed using its prototype. Further, performance improvement strategies are carefully established, and the applicability of the system is investigated on the basis of these performance improvement strategies. II. PROPOSED ENERGY HARVESTING SYSTEM INCLINED STAY CABLE
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Not long after Faraday’s fundamental breakthrough in electromagnetic induction [2], electromagnetic mechanisms have been used for generating electricity since the 1930s. It should be noted that electromagnetic power conversion results from the relative motion of an electrical conductor in a magnetic field. Further, the relative motion between a coil and the magnetic field causes a current to flow in the coil. The electromotive force (or the induced voltage in the coil) is determined by Faraday’s law of electromagnetic induction [2], and the corresponding maximum open circuit voltage across the coil, , can be expressed as (1) where denotes the number of turns in the coil; , the strength of the magnetic field; , the length of the coil; and , the distance that the coil moves through the magnetic field. The proposed system consists of two moving permanent magnets attached on one end of a rigid bar, which is pivoted on a hinged point with a power spring, and a solenoid coil fixed on the base plate as shown in Fig. 1. In the proposed system, rotational oscillation of the magnets is attributed to the base excitation and is responsible for the relative motion between the coil and the magnets. Hence, the proposed system can generate electricity according to Faraday’s law of electromagnetic induction.
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Fig. 1. Schematic representation of the proposed energy harvesting system. Fig. 3. Prototype of the proposed system. TABLE I MATERIAL AND GEOMETRIC PROPERTIES OF THE PROTOTYPE
Fig. 2. Advantages of the proposed energy harvesting system. (a) No effect on the inclination of a cable, (b) Compensation of static displacement.
The natural frequency of the proposed system can be expressed as (2) where denotes the stiffness of a power spring; , the mass of the magnet; , the gravitational acceleration; , the inclination angle of the system; and , the length of the moment arm. In order to maximize its efficiency, the natural frequency of the system (i.e., ) should be tuned with the dominant frequency of the cable vibration. Since the performance of this resonant-type energy harvesting system is highly dependent on its damping level, a ball-bearing system is introduced to minimize the frictional damping in a pivoting joint. The proposed system has two main advantages over a conventional spring-mass-type electromagnetic energy harvester. First, unlike in the case of the conventional energy harvester, the performance of the proposed system is not affected by the inclination of a cable (see Fig. 2(a)). Further, as shown in Fig. 2(b), a large static deflection of a spring in the conventional system may cause several problems such as an increase in size of the harvester and the nonlinear behavior of the spring. By using the resetting capability of a power spring, on the other hand, we can easily solve these problems with regard to the proposed device (see Fig. 2(b)). Therefore, the proposed energy harvesting system can be effectively applied to an inclined stay cable in a cable-stayed bridge. The design strategy of the proposed system can be briefly described as follows: first, the target tuning frequency is se-
lected on the basis of the dynamic characteristics of a stay cable; next, the size and mass of the magnet and coil parts are determined; and then, the stiffness of a power spring is calculated from the equation of motion of the system and an appropriate power spring is prepared; finally, a fine tuning process is conducted through lab and field tests. III. EXPERIMENTAL VALIDATION To experimentally evaluate the performance of the proposed energy harvesting system, its prototype is first designed and manufactured, and then, a series of experimental tests are performed using a shaking table system at the bridge site. A. Prototype The prototype of the proposed energy harvesting system is shown in Fig. 3. As shown in this figure, the permanent magnets in a moving arm (i.e., a rigid bar) can be moved; hence, the tuning frequency can be easily adjusted according to the dominant frequency of the cable. The material and geometric properties of the prototype are listed in Table I. The total mass of the system including the base plate is 1.58 kg. B. Experimental Setup In this study, two experimental tests are performed. First, a laboratory test using a shaking table system is performed to examine the dynamic behavior of the device and its performance under harmonic loading and random loading conditions. Fig. 4(a) shows the experimental setup of the shaking table test. To describe the vibration of an inclined cable, the inclination jig (slope: 25 ) and the vertical motion are considered. After the shaking table tests are performed, the performance of the proposed system is validated at the bridge site. As shown in Fig. 4(b), the prototype of the proposed device is placed on an inclined cable in an in-service cable-stayed bridge in Korea. Its performance is tested under an ambient vibration condition, owing to wind and/or traffic loading.
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Fig. 6. Results of the field test. (a) Acceleration response of cable under wind load, (b) Output voltage generated from the device.
Fig. 4. Experimental test setup. (a) Shaking table test case, (b) Field test case.
Fig. 5. Output voltages generated from the proposed device. (a) Harmonic loading case, (b) Random loading case.
C. Performance Validation Fig. 5 shows the results obtained from the shaking table test. These results are the output voltages generated from the device under harmonic loading with different exciting frequencies ranging from 1 Hz to 9 Hz and random loading based on the measurement data of the cable. As shown in Fig. 5(a), the peak values are obtained at a tuning frequency of 2.32 Hz, and the result obtained for an input acceleration of 20 mg is considerably greater than that obtained for an acceleration of 10 mg. Fig. 5(b) shows the output voltage produced from the device under the random loading condition based on the measurement data of
the cable, with a root-mean-square (RMS) value of 10.0 mg. In the figure, the experimental result is compared with the numerical simulation result obtained from a single-degree-of-freedom model with mass of 0.231 kg and stiffness of 0.189 . It is demonstrated that there is a good agreement between them. Fig. 6 shows the results of the field test. Fig. 6(a) and (b) show the time histories of the acceleration response of the cable and the corresponding output voltage generated from the device, respectively. It is observed that the overall shapes of the two responses are very similar. The RMS value of the acceleration of the cable is 17.7 mg, and the corresponding RMS value of the output voltage is 0.063 V. It also can be seen that the simulation result is similar to the experimental result. According to the maximum power transfer theory [3], the RMS power of approximately 0.09 mW is calculated, which is considerably less than that expected. This is because the tuning frequency (2.32 Hz) is not the dominant frequency of the cable vibration (4.80 Hz in this case) and the configuration of the proposed device is not optimized. Hence, for real applications, strategies for improving the performance of the proposed system should be addressed comprehensively. IV. PERFORMANCE IMPROVEMENT STRATEGIES Through a comprehensive analysis and discussion of the previous test results, we have found that the performance of the proposed system can be significantly improved if the following two strategies are considered: (1) optimization of the device configuration and (2) accurate tuning with the dominant frequency of a cable. First, the optimized configuration of the proposed system can bring about a significant increase in the electrical energy produced from the device. In other words, if a new configuration is adopted, as shown in Fig. 7(a), the magnetic field density is increased more than three times as compared to the original configuration, according to the finite element analysis (see Fig. 7(b)).
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node is investigated by considering the power consumption of the sensor node with the twice-a-day measurement (i.e., 41.48 mWh) [4]. Assuming a charging efficiency of 90% [5], we have found that it takes 1.06 h to generate the electricity required for operating a wireless sensor node for one day. Therefore, the device is fully applicable for powering a wireless sensor node if the performance improvement strategies are well implemented. V. CONCLUSION
Fig. 7. Effect of optimization of device configuration. (a) New configuration of the proposed device, (b) Magnetic flux density vs. distance.
Fig. 8. Effect of accurate tuning with the dominant frequency of the cable. TABLE II GENERATED VOLTAGES AND POWERS FROM PRESENT SYSTEM AND MODIFIED SYSTEM BASED ON IMPROVEMENT STRATEGIES
In this study, a new electromagnetic energy harvesting system was developed, and its applicability was investigated by performing shaking table and field tests. The proposed system is based on a combination of a rigid bar having moving magnets pivoted on a hinged point with a power spring and a solenoid coil fixed on a base plate. Further, this system is more suitable to be implemented in an inclined stay cable of a cable-stayed bridge. To validate the performance of the proposed system, a shaking table test and a field test on an in-service cable-stayed bridge were performed. According to the test results, the proposed system placed on an inclined stay cable worked well. However, the generated power from the proposed system was too low to operate a wireless sensor node. To improve the performance of the proposed system, two effective strategies were established (i.e., optimization of the device configuration and accurate tuning with the dominant frequency of the cable). The output voltage and power generated from the device were estimated by considering the performance improvement strategies. The expected RMS value of the power harvested from the device was 43.5 mW under an ambient vibration condition, which was sufficient to operate a wireless sensor node. However, this needs to be validated through a field test. ACKNOWLEDGMENT
Next, the dominant frequency of the cable of interest should be carefully determined via an extensive field test. Then, a relatively large output voltage can be generated by accurately tuning the natural frequency of the device with the dominant frequency of the cable. Fig. 8 shows the effect of accurate tuning with the dominant frequency (4.80 Hz) on the velocity of the cable, which is directly proportional to the output voltage (see (1)). As shown in the figure, the velocity in the accurate tuning case is considerably greater than that in the original case. The RMS value in the accurate tuning case is increased by more than 330%. Table II compares the output voltage and power generated from the modified system, which are estimated on the basis of the two performance improvement strategies described above, with those harvested from the original system. As shown in this table, the performance of the modified system is considerably better than that of the original system. The applicability of the proposed system to the power supply for a wireless sensor
This research was supported by a grant (07high Tech A01) from High-tech Urban Development Program funded by Ministry of Land, Transportation and Maritime Affairs of Korea and the National Nuclear R&D Program through the National Research Foundation (NRF) funded by the Ministry of Education, Science and Technology (NRF-2010-0018753). REFERENCES [1] H. J. Jung, I. H. Kim, and S. J. Jang, “An energy harvesting system using wind-induced vibration of a stay cable for powering a wireless sensor node,” Smart Mater. Struct., vol. 20, 2011. [2] S. Priya and D. J. Inman, Energy Harvesting Technologies. New York: Springer, 2009. [3] H. W. Jackson, Introduction to Electronic Circuits. Englewood Cliffs, NJ: Prentice-Hall, 1959. [4] S. Jang, H. Jo, S. Cho, K. Mechitov, J. A. Rice, S. H. Sim, H. J. Jung, C. B. Yun, B. F. Spencer, Jr., and G. Agha, “Structural health monitoring of a cable stayed bridge using smart sensor technology: Deployment and evaluation,” Smart Struct. Syst., vol. 6, pp. 439–459, 2010. [5] “R&D Report on the Ubiquitous Micro Power Generation System,” Korea Institute of Science and Technology, 2008.
