• Journal of Communications and Networks
• /
• v.15 no.2
• /
• pp.122-131
• /
• 2013

We address the problem of effective vehicular routing in hostile scenarios where malicious nodes intend to jeopardize the delivery of messages. Compromised vehicles can severely affect the performance of the network by a number of attacks, such as selectively dropping messages, manipulating them on the fly, and the likes. One of the best performing solutions that has been used in static wireless sensor networks to deal with these attacks is based on the concept of watchdog nodes (also known as guard nodes) that collaborate to continue the forwarding of data packets in case a malicious behavior in a neighbor node is detected. In this work, we consider the beacon-less routing algorithm for vehicular environments routing protocol, which has been previously shown to perform very well in vehicular networks, and analyze whether a similar solution would be feasible for vehicular environments. Our simulation results in an urban scenario show that watchdog nodes are able to avoid up to a 50% of packet drops across different network densities and for different number of attackers, without introducing a significant increase in terms of control overhead. However, the overall performance of the routing protocol is still far from optimal. Thus, in the case of vehicular networks, watchdog nodes alone are not able to completely alleviate these security threats.

• Journal of the Korean Society for Precision Engineering
• /
• v.30 no.10
• /
• pp.1031-1039
• /
• 2013

This paper presents gain optimization of a 6-DOF underwater robotic platform with 4 rotatable thrusters. To stabilize the 6-DOF motion of the underwater robotic platform, a back-stepping controller is designed with 6 proportional gains and 6 derivative gains. The 12 gains of the backstepping controller are optimized to decrease settling time in step response in 6-DOF motion independently. Stability criterion and overshoots are used as a constraint of the optimization problem. Trust-region algorithm and hybrid Taguchi-Random order Coordinate search algorithm are used to determine the optimal parameters, and the results by two methods are analyzed. Additionally, the resulting controller shows improved performance under disturbances.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2005.10a
• /
• pp.370-374
• /
• 2005

This study deals with the method of post-processing in the automatic tool path generation for 5-axis NC machining. The 5-axis NC machining cannot only cope with the manufacturing of complicated shapes, but also offers numerous advantages such as reasonable tool employment, great reduction of set-up process and so on. Thus 5-axis NC machining has been used fur aircraft parts, mold and die as well as for complicated shapes such as impeller, propeller and rotor. However, most of the present CAM systems for 5-axis NC machining have limited functions in terms of tool collision, machine limits and post-processing. Especially 5-axis machine configurations are various according to the method which the rotational axes are adapted with the table and spindle. For that reason, in many cases the optimal numerical control (NC) data cannot be obtained or considerable time is consumed. To solve this problem, we applied a general post-processor fur 5-axis NC machining. The validity of this post-processor should be experimentally confirmed by successfully milling to a helix shaped workpiece.

• Journal of Drive and Control
• /
• v.14 no.4
• /
• pp.45-50
• /
• 2017

The gas spring is a hydropneumatic adjusting element, consisting of a pressure tube, a piston rod, a piston and a connection fitting. The gas spring is filled with compressed nitrogen within the cylinder. The filling pressure acts on both sides of the piston and because of area difference it produces an extension force. Therefore, a gas spring is similar in function compare to mechanical coil spring. Conversely, optimization is a process of finding the best set of parameters to reach a goal while not violating certain constraints. The AMESim software provides NLPQL (Nonlinear Programming by Quadratic Lagrangian) and GA (genetic algorithm) for optimization. The NLPQL method builds a quadratic approximation to the Lagrange function and linear approximations to all output constraints at each iteration, starting with the identity matrix for the Hessian of the Lagrangian, and gradually updating it using the BFGS method. On each iteration, a quadratic programming problem is solved to find an improved design until the final convergence to the optimum design. In this study, we conducted optimization design of the gas spring reaction force with NLPQL.

• Proceedings of the SAREK Conference
• /
• 2006.06a
• /
• pp.596-601
• /
• 2006

Because of insufficiency of energy resources and pollution of environment, it is necessary to develop alternative energy sources. Nuclear fission energy is used widely for source of electric Power but being restricted due to radioactivity problem. Nuclear fission is highlighted as the new generation of nuclear energy and researched worldwide because of low risk of radiation effect. The representatives of fusion research is China's EAST, KSTAR of Korea and ITER of world. Korea Superconducting Tokamak Advanced Research(KSTAR) project is on progress for the completion in August, 2007. In this study, the research of utility system for KSTAR be carried out. The utility system of KSTAR is consist of water cooling & heating system, $N_2$ gas system, DI water system, service water system and instrument air & auto control system. The progress of KSTAR utility system is under commissioning state after construction completion. The optimal operation scenario will be verified during commissioning and adopted to the KSTAR operation.

• Journal of IKEEE
• /
• v.24 no.4
• /
• pp.950-953
• /
• 2020

A switching differentiator that can estimate the 1st-order time-derivative of a time-varying signal was proposed, and it is extended later to the higher-order switching differentiator(HOSD) that can observe higher-order time-derivatives of a time-varying signal in previous works. By using HOSD, higher-order time-derivatives can be estimated without peaking or chattering, and it has an asymptotic tracking performance. However, there exist many design constants to be determined in HOSD. In this paper, a method of reducing the number of design constants is proposed to solve the problem. Simulations reveal the effectiveness of the proposed method.

• The Journal of Korea Robotics Society
• /
• v.17 no.4
• /
• pp.407-416
• /
• 2022

Multiple AMRs have been proved to be effective in improving warehouse productivity by eliminating workers' wasteful walking time. Although Multi-agent Path Finding (MAPF)-based solution is an optimal approach for this task, its deployment in practice is challenging mainly due to its imperfect plan-execution capabilities and insufficient computing resources for high-density environments. In this paper, we present a MAPF-based fleet management system architecture that robustly manages multiple robots by re-computing their paths whenever it is necessary. To achieve this, we defined four events that trigger our MAPF solver framework to generate new paths. These paths are then delivered to each AMR through ROS2 message topic. We also optimized a graph structure that effectively captures spatial information of the warehouse. By using this graph structure we can reduce computational burden while keeping its rescheduling functionality. With proposed MAPF-based fleet management system, we can control AMRs without collision or deadlock. We applied our fleet management system to the real logistics warehouse with 10 AMRs and observed that it works without a problem. We also present the usage statistic of adopting AMRs with proposed fleet management system to the warehouse. We show that it is useful over 25% of daily working time.

• Journal of the Korean Society for Aviation and Aeronautics
• /
• v.31 no.2
• /
• pp.66-71
• /
• 2023

In the case of the Navy, if some of the co-pilots are included in the long-term promotion process due to the limited number of co-pilots, operational flight and administrative tasks will be added to the co-pilots not included in the rest of the Pilot in commander process. Therefore, to solve this problem, the co-pilot who has passed the PQS step-by-step process minimizes the personnel gap in the flight operation unit through a system that evaluates whether it is possible to perform its duties as a co-pilot through actual flight after entering the school. The advantage of the PQS course is that you can control flight plans on your own and minimize gaps in flight and ground work while carrying out the curriculum, but you can't focus on education or improve your skills due to irregular training flight cycles. Therefore, in this study, after collecting opinions on effective flight cycles through a survey of pilots of P-3C, the Navy's fixed-wing aircraft representative, we will analyze the association of aircraft volume performance by flight cycle to derive the optimal flight cycle of the P-3C pilot course.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• v.18 no.7
• /
• pp.2027-2046
• /
• 2024

Reconfigurable Intelligent Surface (RIS) is an innovative technique to precisely control the phase of incident signals with the help of low-cost passive reflective elements. It shows excellent potential in the sixth generation of mobile communication systems, which not only extends wireless coverage but also boosts channel capacity. Considering that multipath propagation and a high number of antennas are involved in RIS in assisted mega multiple-input multiple-output (MIMO) systems, it suffers from severe channel fading and multipath effects, which in turn lead to signal instability and degradation of transmission performance. To overcome this obstacle, this essay suggests an improved gradient optimization algorithm to dynamically and optimally adjust the phase of the reflective elements to counteract channel fading and multipath effects as a strategy. In order to overcome the optimization problem of falling into local minima, this paper proposes an adaptive learning rate algorithm based on Adagrad improvement, which searches for the global optimal solution more efficiently and improves the robustness of the optimization algorithm. The suggested technique helps to enhance the estimate of channel efficiency of RIS-assisted large MIMO systems, according to simulation results.

• Research in Community and Public Health Nursing
• /
• v.6 no.2
• /
• pp.319-334
• /
• 1995

To investigate the risk factors of low back pain, an epidemiological study was carried out among male workers aged 20-55 employed in an automobile industry in Korea during the time period from February 1993 to October 1995. Workers participated to this study were divided into low back pain group(LBP) and control group, according to the self-reports by written questionnaires. General characteristics, medical history, work related factors, fatigue, and MMPI were compared between two groups. To clarify the relationship between job related low back pain and radiologic features of lumbar spine, radiographic study was carried out. The resultant data were processed for $x^2-test$, t-test, and stepwise logistic regression to confirm the adjusted odds ratios. The results were as follows: 1. History of back disease, lifting and carrying work, excessive physical fatigue, and weakend back strength of individual workers were directly associated with low back pain. Odd ratios of these 4 risk factors of low back pain were 5.07, 3.34, 1.49, and 1.22 respectively. 2. The frequency of low back pain history was significantly higher in LBP group. 3. Back muscle strength of lumbar spine of LBP group were significantly lower than control group. 4. The workers in LBP group revealed high fatigue symptoms. 5. In MMPI test LBP group showed higher scales in hypochondriasis, depression, hysteria, psychopathic deviate, paranoia, psychasthenia, schizophrenia, and hypomania. 6. LBP group were more frequently involved in lifting and carrying, working in awkward position, bending, twisting and using lower extremities. 7. LBP group were exposed more to vibration during working. 8. In the Analysis of radiographs of lumbar spine, Jacob's line not crossing fourth lumhar disc space, transitional vertebrae and lumbar displacement more than 4.4mm in standing lateral view were more frequently observed in LBP group than control group. Through these results, it is concluded that identification of previous history of back problem, change of work or working environment for workers with previous back problem and measures to relieve both physical and psychological fatigue of the workers are required for optimal management of work-related back problems among workers. In the present study, several results were different from the previous reports: Jacob's line not corssing fourth lumbar disc space, lumbarization, and vertebral slipping (spondylolisthesis) more than 4.4mm are related to backache. Meticulous studies are required to elucidate the difference.