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CONTENTS
Volume 22, Number 2, August 2018
 

Abstract
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Key Words
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Address
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Abstract
Increasing interest in prognostics and health management has heightened the need for wireless sensor networks (WSN) with efficient power sources. Piezoelectric energy harvesters using Pb(Zr,Ti)O3 (PZT) are one of the candidate power sources for WSNs as they efficiently convert mechanical vibration energy into electrical energy. These types of devices are resonated at a specific frequency, which has a significant impact on the amount of energy harvested, by external vibration. Hence, precise prediction of mechanical deformation including modal analysis of piezoelectric devices is crucial for estimating the energy generated under specific conditions. In this study, an experimental vibrational system capable of controlling a wide range of frequencies and accelerations was designed to generate mechanical vibration for piezoelectric energy harvesters. In conjunction with MATLAB, the system automatically finds the resonance frequency of harvesters. A small accelerometer and non-contact laser displacement sensor are employed to investigate the mechanical deformation of harvesters. Mechanical deformation under various frequencies and accelerations were investigated and analyzed based on data from two types of sensors. The results verify that the proposed system can be employed to carry out vibration experiments for piezoelectric harvesters and measurement of their mechanical deformation.

Key Words
piezoelectric; beam shape; energy harvester; PZT; mechanical deformation

Address
Changho Kim, Youngsu Ko, Taemin Kim, Youngho Kim1 and Namsu Kim: Department of Mechanical Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul, Republic of Korea
Chan-Sei Yoo and Seung Ho Han: Electronic Convergence Materials & Device Research Center, Korea Electronics Technology Institute, 25 Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, Republic of Korea
BeomJin Choi and YongHo Jang: Technical Research Center, SENBOL Inc., 437-5 Nonhyeon-dong, Namdong-gu, Incheon, Republic of Korea



Abstract
A variable electromotive-force generator (VEG), which is a modified generator with an adjustable overlap between the rotor and the stator, is proposed to expand the operational range of a regular generator through a simple and robust active control strategy. It has a broad range of applications in hybrid vehicles, wind turbines, water turbines, and similar technologies. A mathematical model of the VEG is developed, and a novel prototype is designed and fabricated. The performance of the VEG with an active control system, which adjusts the overlap ratio based on the desired output power at different rotor speeds for a specific application, is theoretically and experimentally studied. The results show that reducing the overlap between the rotor and the stator of the generator results in reduced torque loss of the generator and an increased rotational speed of the generator rotor. A VEG can improve the fuel efficiency of hybrid vehicles; it can also expand operational ranges of wind turbines and water turbines and harness more power.

Key Words
modeling; design; variable electromotive-force generator with an adjustable overlap between the rotor and the stator; hybrid vehicle; wind turbine; active control system

Address
W.D. Zhu, X.F. Wang and P. Kendrick: Department of Mechanical Engineering, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
N. Goudarzi: Mechanical Engineering Technology Program, Department of Engineering Technology and Construction Management, University of North Carolina at Charlotte, Charlotte, NC 28223, USA


Abstract
Maintenance activities are regarded as a key part of the repairable deteriorating system because they maintain the equipment in good condition. In practice, many maintenance policies are used in engineering fields to reduce unexpected failures and slow down the deterioration of the system. However, in traditional maintenance policies, maintenance activities have often been assumed to be performed at the same time interval, which may result in higher operational costs and more system failures. Thus, this study presents two non-periodic preventive maintenance (PM) policies for repairable deteriorating systems, employing the failure rate of the system as a conditional variable. In the proposed PM models, the failure rate of the system was restored via the failure rate reduction factors after imperfect PM activities. Operational costs were also considered, which increased along with the operating time of the system and the frequency of PM activities to reflect the deterioration process of the system. A numerical example was provided to illustrate the proposed PM policy. The results showed that PM activities performed at a low failure rate threshold slowed down the degradation of the system and thus extended the system lifetime. Moreover, when the operational cost was considered in the proposed maintenance scheme, the system replacement was more cost-effective than frequent PM activities in the severely degraded system.

Key Words
preventive maintenance; failure rate threshold; failure rate reduction factor; operational cost; minimal repair

Address
Juhyun Lee and Jihyun Park: Department of Industrial and Management Engineering, Hanyang University, Seoul,15588, Republic of Korea
Suneung Ahn: Department of Industrial and Management Engineering, Hanyang University ERICA Campus, Ansan, 04763, Republic of Korea

Abstract
In the last decade, there has been an exponential increase of scientific interest in smart additive manufacturing (AM) technology. Among the different AM techniques, one of the most commonly applied processes is digital light processing (DLP). DLP uses a digital projector screen to flash an ultraviolet light which cures photopolymer resins. The resin is cured to form a solid to produce parts with precise high dimensional accuracy. During the curing process, there are several process parameters that need to be optimized. Among these, the exposure time affects the quality of the 3D printed specimen such as mechanical strength and dimensional accuracy. This study examines optimal exposure times and their impact on printed part. It was found that there is optimal exposure time for printed part to have appropriate mechanical strength and accurate dimensions. The gel fraction and TGA test results confirmed that the improvement of mechanical properties with the increasing UV exposure time was due to the increase of crosslinked network formation with UV exposure time in acrylic resins. In addition, gel fraction and thermogravimetric analysis were employed to microscopically investigate how this process parameter impacts mechanical performance.

Key Words
additive manufacturing; DLP 3-D Printing; mechanical properties; dimensional accuracy, UV curing, acrylic resin

Address
Younghun Lee, Xing Guan Zhao, Dongoh Lee, Taemin Kim, Hoeryong Jung and Namsu Kim: Department of Mechanical Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea
Sungho Lee: SoC Platform Research Center, Korea Electronics Technology Institute, 25 Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do 13509, Republic of Korea


Abstract
Digital input and output modules are widely used to connect digital sensors and actuators to automation systems. Digital I/O modules provide flexible connectivity extension to numerous sensors and actuators and protect systems from high voltages and currents by isolation. Components in digital I/O modules are inevitably affected by operating and environmental conditions, such as high voltage, high current, high temperature, and temperature cycling. Because digital I/O modules transfer signals or isolate the systems from unexpected voltage and current transients, their failures may result in signal transmission failures and damages to sensitive circuitry leading to system malfunction and system shutdown. In this study, the lifetime of optocouplers, one of the critical components in digital I/O modules, was predicted using Bayesian tracking approaches. Accelerated degradation tests were conducted for collecting the critical performance parameter of optocouplers, current transfer ratio (CTR), during their lifetime. Bayesian tracking approaches, including extended Kalman filter and particle filter, were applied to predict the failure. The performance of each prognostic algorithm was then compared using accuracy and robustness-based performance metrics.

Key Words
digital input and output modules; optocouplers; lifetime prediction; particle filter; extended Kalman filter Bayesian tracking approaches

Address
Insun Shin: Department of System Design and Control Engineering, Ulsan National Institute of Science and Technology,Ulsan, 44919, Republic of Korea
Daeil Kwon: School of Mechanical, Aerospace and Nuclear Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919, Republic of Korea