Improved dimension reduction method (DRM) in uncertainty analysis using kriging interpolation

Uncertainty or reliability analysis is to investigate the stochastic behavior of response variables due to the randomness of input parameters, and evaluate the probabilistic values of the responses against the failure, which is known as reliability. While the major research for decades has been made on the most probable point (MPP) search methods, the dimension reduction method (DRM) has recently emerged as a new alternative in this field due to its sensitivity-free nature and efficiency. In the recent implementation of the DRM, however, the method was found to have some drawbacks which counteract its efficiency. It can be inaccurate for strong nonlinear response and is numerically instable when calculating integration points. In this study, the response function is approximated by the Kriging interpolation technique, which is known to be more accurate for nonlinear functions. The integration is carried out with this meta-model to prevent the numerical instability while improving the accuracy. The Kriging based DRM is applied and compared with the other methods in a number of mathematical examples. Effectiveness and accuracy of this method are discussed in comparison with the other existing methods. This paper was recommended for publication in revised form by Associate Editor Tae Hee Lee Junho Won received B.S. and M.S. degree in Mechanical and Aerospace Engineering from Korea Aerospace University in 2004 and 2006, respectively. He is currently a doctoral course at the departments of Mechanical and Aerospace Engineering, Korea Aerospace University in Gyeonggi, Korea. His research interests are in the area of reliability analysis, multidisciplinary design optimization, and fatigue analysis. Changhyun Choi received B.S. and M.S. degree in Mechanical and Aerospace Engineering from Korea Aerospace University in 2006 and 2008, respectively. He is currently researcher at the SFA Engineering Corp. in kyungnam, Korea. His research interests are in the area of computer control system, high reliable product technology, and factory automation system. Jooho Choi received a B.S. degree in Mechanical Engineering from Hanyang University in 1981. He then went on to receive his M.S. and Ph.D. degrees from KAIST in 1983 and 1987, respectively. Dr. Choi is currently a Professor at the School of Mechanical and Aerospace Engineering, Korea Aerospace University in Gyeonggi, Korea. He is currently serving as an Editor of the Journal of Mechanical Science &Technology. Dr. Choi’s research interests are in the area of reliability analysis, multidisciplinary design optimization, and design optimization using automation system integrated with CAD/CAE.     

The structure and photoluminescence properties of Bi2O3-core/SnO2-shell nanowires

Bi2O3-core/SnO2-shell nanowires have been prepared by using a two-step process: thermal evaporation of Bi2O3 powders and sputtering of SnO2. The crystalline nature of the Bi2O3-core/SnO2-shell nanowires has been revealed by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). TEM analysis and X-ray diffraction (XRD) results indicate that the Bi2O3-core/SnO2-shell nanowires consist of pure tetragonal alpha-Bi2O3-phase momocrystalline cores and tetragonal SnO2-phase polycrystalline shells. The photoluminescence (PL) measurements show that Bi2O3 nanowires have a broad emission band centered at around 560 nm in the yellow-green region. On the other hand, the Bi2O3-core/SnO2-shell coaxial nanowires with the sputtering times of 4 and 8 min have a blue emission band centered at around 450 nm. In contrast, those with a sputtering time of 10 min have a broad emission band centered at approximately 550 nm again. The origin of this yellow-green emission from the core/shell nanowires, however, quite differs from that from Bi2O3 nanowires, i.e., it is not from the Bi2O3 cores but from the SnO2 shells.     

Gasholder level control based on time-series analysis and process heuristics

A novel method to control gasholder levels in an iron and steel company with accurate prediction of the future trend is presented. Although various gasholders are used to recycle by-product gases generated during the ironmaking, coke-burning, and steel-making processes, the capacities of these gasholders are insufficient to handle large amounts of gases. To overcome this problem, tight control of the gasholder level should be maintained by predicting their anticipated changes. However, the current prediction logic cannot show satisfactory results due to the lack of characterization of the relevant processes. In the proposed method, time-series modeling and heuristics from industrial operators are used to correctly reflect the process characteristics and deal with unexpected process delays. By applying the proposed method to an off-line data set, a significant reduction in the discrepancy between predicted values and actual values was observed. The method is expected to be adopted in the prediction system of POSCO.     

A 0.2 V Micro‐Electromechanical Switch Enabled by a Phase Transition

Micro‐electromechanical (MEM) switches, with advantages such as quasi‐zero leakage current, emerge as attractive candidates for overcoming the physical limits of complementary metal‐oxide semiconductor (CMOS) devices. To practically integrate MEM switches into CMOS circuits, two major challenges must be addressed: sub 1 V operating voltage to match the voltage levels in current circuit systems and being able to deliver at least millions of operating cycles. However, existing sub 1 V mechanical switches are mostly subject to significant body bias and/or limited lifetimes, thus failing to meet both limitations simultaneously. Here 0.2 V MEM switching devices with ?10⁶ safe operating cycles in ambient air are reported, which achieve the lowest operating voltage in mechanical switches without body bias reported to date. The ultralow operating voltage is mainly enabled by the abrupt phase transition of nanolayered vanadium dioxide (VO₂) slightly above room temperature. The phase‐transition MEM switches open possibilities for sub 1 V hybrid integrated devices/circuits/systems, as well as ultralow power consumption sensors for Internet of Things applications.     

A safe robot arm with safe joints and gravity compensator

This study presents a robot arm equipped with safe joints and multi DOFs gravity compensators. The safe joint, also referred to as “Spring-clutch”, is simple passive mechanism that consist of a spring, a cam, and a joint torque sensor. When the torque applied is lower than a pre-set threshold, the Spring-clutch serves as a rigid joint between the input and output. When the applied torque exceeds the threshold, the Spring-clutch is released and is free to rotate like a revolute joint, which significantly reduces the collision force to avoid damage to the robot, as well as to humans. In addition, a compact joint torque sensor is installed in the Spring-clutch to measure the torque at the joint. Also, the analysis of energy and torque shows that the proposed mechanism can function as a gravity compensator capable of static balancing. Since joint torques vary in accordance with the pose of a manipulator (i.e., rotation angles), a Spring-clutch with a constant threshold torque cannot always guarantee the maximum collision torque in some poses of a manipulator. To overcome this limitation, a gravity compensator is adopted to eliminate the gravitational torque. In this research a bevel gravity compensator is applied which can perform static balancing completely. This paper describes the design principles and fabrication of the safety mechanisms and the robot arm.     

Solving a Location Problem of a Stackelberg Firm Competing with Cournot-Nash Firms

We study a discrete facility location problem on a network, where the locating firm acts as the leader and other competitors as the followers in a Stackelberg-Cournot-Nash game. To maximize expected profits the locating firm must solve a mixed-integer problem with equilibrium constraints. Finding an optimal solution is hard for large problems, and full-enumeration approaches have been proposed in the literature for similar problem instances. We present a heuristic solution procedure based on simulated annealing. Computational results are reported.     

Modulating Photoluminescence of Monolayer Molybdenum Disulfide by Metal–Insulator Phase Transition in Active Substrates

The atomic thickness and flatness allow properties of 2D semiconductors to be modulated with influence from the substrate. Reversible modulation of these properties requires an “active,” reconfigurable substrate, i.e., a substrate with switchable functionalities that interacts strongly with the 2D overlayer. In this work, the photoluminescence (PL) of monolayer molybdenum disulfide (MoS₂) is modulated by interfacing it with a phase transition material, vanadium dioxide (VO₂). The MoS₂ PL intensity is enhanced by a factor of up to three when the underlying VO₂ undergoes the thermally driven phase transition from the insulating to metallic phase. A nonvolatile, reversible way to rewrite the PL pattern is also demonstrated. The enhancement effect is attributed to constructive optical interference when the VO₂ turns metallic. This modulation method requires no chemical or mechanical processes, potentially finding applications in new switches and sensors.