Seungkyung Kye

Bridge stay cables that are constantly exposed to fluctuating wind environments are subject to constant or transient vibration, which hinders the serviceability and causes maintenance problems for the cables and dampers. A regenerative hybrid electrodynamic cable damper can semi-permanently reduce the vibration and generate electricity. This study proposes a regenerative hybrid electrodynamic cable damper that can be used as an alternative to existing dampers while reducing maintenance costs. Shaking-table experiments were performed on the proposed damper prototype to measure the dynamic hysteresis according to the excitation conditions and changes in damping characteristics and power generation according to external resistance. The hysteresis, damping force, dissipated energy, and damping coefficient changed non-linearly according to the excitation conditions and external resistance. Further, the maximum damping force, induced current, and power produced increased, compared with those of a coil-type electromagnetic damper. The experimental results confirmed that the proposed damper can be used in autonomous vibration control and monitoring system for stay cables because it possesses a versatile semi-active control characteristic that is achieved using a variable resistor and a self-generation function.

A magnetorheological elastomer (MRE) is one of smart materials which can control the stiffness according to the magnetic field strength. Various types of MRE-based isolators have been studied. However, there was a limit to performance validation owing to the material and structural limitations. In this study, an optimal mixing ratio of the MRE was considered, and a novel electromagnetic system was proposed. A multi-layered electromagnetic system was proposed to constantly maintain a closed magnetic circuit by acting on the deformation of the MRE. As a result, numerical and experimental comparisons were performed with the conventional MRE-based isolator, and its superiority was validated. Moreover, the dynamic characteristics of the proposed MRE-based isolator under a vertical load were investigated. The results show that the proposed MRE-based isolator outperforms the conventional MRE-based isolator in improving the magnetorheological (MR) effects and the dynamic characteristics of the proposed MRE-based isolator under the vertical load were identified.

Crack width estimation through remote imaging and digital image processing are the leading diagnostic technologies used for infrastructure. However, owing to limitations in the accuracy of these techniques, alternative methods to detect target crack widths larger than the diagnostic standard are being developed. In this study, the coordinates of fiducial markers were extracted using the Harris corner detector, and fine crack widths of approximately 0.3 mm or less were estimated using the ground sample distance, varied numbers of adjusted pixels, and varied focal lengths. The results of these examinations were compared with the measured values. As a result, it was confirmed that the number of pixels rather than the ground sample distance was the dominant factor affecting the accuracy of the estimated value, and the accuracy could be improved by adjusting the number of pixels, increasing the focal length, and ensuring an effective object distance. The results showed the feasibility of estimating fine crack widths with a high accuracy through the fiducial-marker-based method under the aforementioned conditions. In the future, if the number of crack samples is increased and more detailed focal length and object distance data are supported, this method may provide estimations with a higher accuracy.

SPIE Digital Library Proceedings

Electromagnetic dampers are emerging as alternatives to conventional dampers applied to stay cables of bridges because they can reduce maintenance costs and allow vibration monitoring owing to their permanent driving characteristics and self-generation function. In this study, the main equations (including those for the induced electromotive force of the active coils and the total damping force of the damper) were derived through magnetic circuit analysis using the main parameters of the electromagnetic damper model. Characteristic tests were performed on electromagnetic damper prototypes to analyze the hysteretic dynamics and derive characteristics according to their structure and excitation conditions. On the basis of the results, we proposed a regenerative hybrid electrodynamic damper with an oxygen-free copper tube and teeth structure. Its physical and electromagnetic behaviors were examined through an electromagnetic analysis of the finite element model of the proposed damper. The results confirmed that attenuation occurred via strengthened magnetic flows, and the estimated power production is suitable for energy harvesting applications. Therefore, we confirmed the feasibility of constructing a system that can simultaneously perform cable attenuation and vibration monitoring using the proposed damper.

The vibration of cables in a cable-supported bridge affects its serviceability and safety. Therefore, cable dampers are essential for vibration control, monitoring, and the further maintenance of such bridges. In this study, the vibration control performance of an electrodynamic damper applied to a cable used in a footbridge was experimentally verified considering the major design variables of the damper. The damping performance was analyzed by varying the damping ratio according to the excitation condition and external circuit resistance. The acceleration and displacement at each measurement point and the frequency-domain response decreased. Considering the dominant response conditions of the cables in the bridge, an effective additional attenuation was observed. In addition, the harvesting power considering the measurement frequency and power loss was sufficient to operate a wireless measuring sensor by examining the energy harvesting performance from the cable measurement data of an actual bridge in service. Finally, a stepwise operation strategy for a cable vibration monitoring system was suggested and examined by analyzing the meteorological observation data and the power output according to the wind environment. The results demonstrate the feasibility of using an electrodynamic damper to build an integrated monitoring system capable of simultaneous cable vibration reduction and energy harvesting.

Bridge cables should always be equipped with vibration mitigation measures and monitoring techniques. The proposed electromagnetic damper was developed to reduce vibration of the cable and utilize induction current as the power source of the wireless sensor. Major parameters for the design of the damper were derived. Then, the cable experiment was carried out under the conditions of free vibration and force vibration. For free vibration conditions, the change of damping ratio according to the acceleration amplitude evaluation point was analyzed through Hilbert transform. As a result, the damping performance of the passive electronic damper applying the external resistance under the free vibration condition was improved to 2.18% of the maximum damping ratio and 1.88% of the average damping ratio. Under the sinusoidal forced vibration conditions, it was found that the acceleration and frequency domain response at each measurement point of the cable decreased by 30% to 50% or more, and the RMS displacement response decreased by 45% to 49% under the excitation with 1st to 3rd natural frequencies. In addition, it has been confirmed that effective damping performance is exhibited in the 2nd and 3rd natural frequency, which are the main response conditions of the cable. Hybrid simulation was carried out to evaluate the energy harvesting performance of the electromagnetic damper. As a result, the output power was 174.6mWh at the mean wind speed of 5.4 m/s. Even if the sensor and the battery loss were considered, enough power was generated to operate the wireless sensor.