• Journal of Drive and Control
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• v.14 no.4
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• pp.9-16
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• 2017

In this study, the force tracking performance of the single-rod and double-rod type EHAs (Electro-Hydrostatic Actuators) was compared by computer simulation and experiments. The force-controlled EHAs exhibit non-linear behavior that are significantly dependent on operation conditions. The investigation focused on localizing the parameters that provide significant rise to the non-linearity. For this, the single-rod and double-rod type EHAs were mathematically expressed to derive their linear models. In parallel, they were modeled by a commercial simulation program including non-linear properties based on experimental results. It was shown that the dependency of the bulk modulus of oil with entrapped air on working pressure dominated the non-linearity in force control performance in case of the double-rod type EHA. The force control of the single-rod type EHA was influenced by much more elements. Besides the asymmetrical piston geometry and the non-linear bulk modulus of oil, its pilot-operated check valves made it dependent not only on the magnitude of reference input but also on its direction.

• Journal of Powder Materials
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• v.30 no.2
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• pp.140-145
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• 2023

Although the Ti-6Al-4V alloy has been used in the aircraft industry owing to its excellent mechanical properties and low density, the low formability of the alloy hinders broadening its applications. Recently, laser-powder bed fusion (L-PBF) has become a novel process for overcoming the limitations of the alloy (i.e., low formability), owing to the high degree of design freedom for the geometry of products having outstanding performance used in high-tech applications. In this study, to investigate the effect of bulk shape on the microstructure and mechanical properties of L-PBFed Ti-6Al-4V alloys, two types of samples are fabricated using L-PBF: thick and thin samples. The thick sample exhibits lower strength and higher ductility than the thin sample owing to the larger grain size and lower residual dislocation density of the thick sample because of the heat input during the L-PBF process.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.18 no.6
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• pp.889-894
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• 1993

Asychronous Transfer Mode (ATM) provides the means to transport different types of bursty traffic such as voice, video, and bulk data. To handle more efficiently the traffic sources and to increase the bandwidth utilization as much as possible, flexible statistical multiplexing schemes must be adopted for the ATM networks. This paper presents an efficient computational procedure to calculate the queue state distribution in a finite buffer queueing system with a number of independent input sources, and the cell loss probability is exactly calculated with the use of this recursion computation method. The cell loss probability is related to a ATM multiplexer with a homogeneous ON-OFF source is also investigated through numerical examples.

• Journal of Korea Soil Environment Society
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• v.3 no.2
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• pp.61-70
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• 1998

This research has been made for influence of forest environmental changes, such as tree-clearcutting affecting to soil chemical and physical properties, on water storage capacity at forest fire land in Keumsan, Chungnam. The analyzed factors were bulk density, porosity, field moisture saturated hydraulic conductivity air permeability and organic matter content, Field moisture saturated hydraulic conductivity and air permeability at uncutting sites were higher than those at clearcutting sites, especially the most differences were appeared at lower slope. After 2 years passed since forest fire, the most changeable parts of soil characteristics were 5-l5cm depth below soil surface. Total Porosity, coarse pore and fine pore at uncutting sites were higher than those at clearcutting sites. Also, as soil depth increased, total porosity and coarse pore were decreased. Bulk density at uncutting sites was lower than that at clearcutting sites, and was decreased as soil depth increased. The order of the change trend in field moisture saturated hydraulic conductivity, air permeability and porosity was slope lower>middle>upper. Organic matter content at uncutting sites were higher than those at clearcutting sites, and decreased as soil depth increased. As soil depth increased, bulk density had the positive correlation, in other hand, porosity, coarse pore, field moisture saturated hydraulic conductivity, air permeability and organic matter content had the negative correlation. It was concluded that forest environmental changes by forest fire degrade soil physical and chemical properties.

• Journal of the Korean Society for Nondestructive Testing
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• v.29 no.4
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• pp.344-350
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• 2009

The goal of this study is to introduce the theoretical background of acoustic nonlinearity in surface wave and to verify its characteristics by experiments. It has been known by theory that the nonlinear parameter of surface wave is proportional to the ratio of $2^{nd}$ harmonic amplitude and the power of primary component in the propagated surface wave, as like as in bulk waves. In this paper, in order to verify this characteristics we constructed a measurement system using contact angle beam transducers and measured the nonlinear parameter of surface wave in an Aluminum 6061 alloy block specimen while changing the distance of wave propagation and the input amplitude. We also considered the effect of frequency-dependent attenuation to the measurement of nonlinear parameter. Results showed good agreement with the theoretical expectation that the nonlinear parameter should be independent on the input amplitude and linearly dependent on the input amplitude and the $2^{nd}$ harmonic amplitude is linearly dependant on the propagation distance.

• 제어로봇시스템학회:학술대회논문집
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• 2001.10a
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• pp.169.1-169
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• 2001

In general, for high performance pressure control system, hydraulic system with electo hydraulic servo valve controls flow rate, it contains many nonlinear term like square-root and change of bulk modulus by flow rate. But, DDV(Direct Drive Valve) contains pressure control loop itself, then it can eliminate nonlinearity and achieve linearity for hydraulic system. In this paper, parameter identification method which uses input and ouput data is applied to obtain DDV's mathematical model and parameter assuming that dynamic characteristic of DDV is first order system. Then, the state feedback controller was designed to implement the force control of hydraulic system , and the control performance was evaluated.

• Transactions of The Korea Fluid Power Systems Society
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• v.1 no.3
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• pp.14-19
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• 2004

The Direct Drive Valve used in this study contains a pressure-feedback-loop in itself, then it can eliminate nonlinearity such as the square-root-term in flow rate calculation and the change of bulk modulus of hydraulic oil. In this study, assuming that the dynamic characteristic of the DDV is modelled as a first order lag system, an parameter identification method using the input data and the output data is applied to obtain DDV's mathematical model. Then, a state feedback controller was designed to implement the force control of hydraulic system, and the control performance was evaluated.

• Journal of the Korean Statistical Society
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• v.31 no.4
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• pp.491-507
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• 2002

We analyze a single server bulk input queue with optional server vacations under a single vacation policy and balking phenomenon. The service times of the customers as well as the vacation times of the server have been assumed to be arbitrary (general). We further assume that not all arriving batches join the system during server's vacation periods. The supplementary variable technique is employed to obtain time-dependent probability generating functions of the queue size as well as the system size in terms of their Laplace transforms. For the steady state, we obtain probability generating functions of the queue size as well as the system size, the expected number of customers and the expected waiting time of the customers in the queue as well as the system, all in explicit and closed forms. Some special cases are discussed and some known results have been derived.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2010.04a
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• pp.268-271
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• 2010

In this paper, forward and reverse analysis is introduced in order to estimate the elastic-plastic properties from a power-law hardening bulk specimen materials with one simple spherical indentation impression test. In order to verify the reliability of the reverse analysis, we have simulated about a large range of materials that essentially cover all engineering materials, using ABAQUS(6.91) program. Then, we could obtained the fitting functions and plastic parameters from the numerical analysis results. Next, through the procedure of reverse analysis we can obtain the yield stress and power-law exponent. Finally, obtain good agreement between the result from reverse analysis and initial input data from experiment.

• Journal of the Korean Institute of Navigation
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• v.25 no.3
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• pp.225-236
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• 2001

In the recent years, major ship registers have demanded improved safety on the hull stress of large bulk carriers which are on navigation or cargo handling in harbour. Under these circumstances, a system that monitors hull stress and ship condition is being more and more important. If efficient and appropriate navigational information is given, safety of navigation would be greatly improved. The major ship registers of the globe are investing a great effort on the development of a system that monitors the hull stress of ship. Using this system, information of hull stress and ship motion is given to the users and also the data is stored on the external data storage system simultaneously. Through this study, a software that monitors hull stress was developed. Not only can randomized input-data of the standard hardwares be applied to the system, but also this system can be operated on and applied to real hardware systems.