Papers by Author: Ja Choon Koo

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Authors: Seong In Moon, Ja Choon Koo, Jae Boong Choi, Young Jin Kim, Sung Sik Choi, Jeoung Gwen Lee
Authors: Yun Jae Kim, Ja Choon Koo, Seong In Moon, Young Jin Kim
Authors: Hui Je Cho, Dae Sung Bae, Jin Hwan Choi, Ja Choon Koo
Abstract: Dynamic analysis of many mechanical systems is often involved with contacts among bodies. This paper presents an efficient and general-purpose contact search algorithm for multibody dynamics in the context of the compliance contact force model. While many conventional collision detection algorithms are based on the absolute coordinate system, this paper proposes to use the relative coordinate system in detecting a contact. A boundary box of a defense surface geometry is divided into many blocks. A contact reference frame is defined on the defense body of a contact pair. Since all geometric variables necessary to detect a contact are measured relative to the contact reference frame attached to the defense body, the variables belonging to the defense body are constant, which significantly reduces computation time associated with the contact search. Therefore, the contact reference frame plays a key role in developing an efficient contact search algorithm. Contour of the defense body is approximated by many piecewise triangular patches, while contour of the hitting body is represented by hitting nodes along its boundary. Bounding boxes inside which contain each body of a contact pair are defined at a preprocessing stage to eliminate an exhaustive contact inspection when two bodies are in a distance. If two bounding boxes are turned out to be in a contact during the pre-search, each node on the hitting boundary is inspected to find out to which block the node belongs in the post-search. Since each block dividing the boundary of the defense body has a list of patches, each node on the hitting boundary is tested for a contact only with the patches in the block that the node belongs. Actual contact calculation is then carried out to find the contact penetration used in calculating the compliant contact force. Numerical example is performed to demonstrate the validity of the proposed method.
Authors: Ja Choon Koo, H.S. Kim, Jae Boong Choi, Young Jin Kim
Abstract: Application of Hamilton’s theorem is limited to rigid body dynamics problems in spite of its benefit that always yield a set of first order differential equations as a model. From the fundamental formulation procedure, introduction of Hamilton’s principle to continuum problems differs from the traditional continuum modeling methodology that relies upon partial differential field equation. For the analysis of impact problems where highly nonlinear coupled models are norm, massively distributed computation schemes are usually employed and they significantly reduce computational cost and improve accuracy. With the parallel resources in mind, the present work applies Hamiltonian modeling approach to a shock propagation problem in continuous media. The formulated model which is in first order ordinary differential equations is efficiently calculated on a Beowulf based Linux parallel machines.
Authors: Ja Choon Koo, Hyouk Ryeol Choi, Min Young Jung, Kwang Mok Jung, Jae Do Nam, Young Kwan Lee
Abstract: Smart polymer based actuators have demonstrated various benefits over the traditional electromagnetic or piezoelectric-material actuators. One of the most significant contributions of the polymers is its soft actuation mechanisms. Hence morphological freedom for actuator construction benefits production of either small scale complex mechanisms or human-like applications. Although many actuation paradigms of polymer actuators are presented in various publications, no significant contributions are made for investigation of modeling and control methods of the material. In the present work, a smart polymer based actuator is constructed. It is then modeled and analyzed for feasible control scheme selection.
Authors: Hyouk Ryeol Choi, Kwang Mok Jung, Min Young Jung, Ja Choon Koo, Jae Do Nam, Young Kwan Lee
Abstract: As ElectroActive Polymers (EAPs) attract keen attentions from various engineering fields, they have been proven more beneficial over the traditional electromagnetic transducers. In the present paper, a new polymeric material that could be adopted for a dielectric elastomer actuator is introduced. The proposed synthetic rubber produces larger deformation at higher energy efficiency compared to previously known dielectric elastomers. A method for the material synthesis and a set of comparative testing of the material to the existing material are to be mentioned in the present work. In addition, benefits of actuators made with the proposed material are discussed.
Authors: Ja Choon Koo, Kwang Mok Jung, Min Young Jung, Hyouk Ryeol Choi, Jae Do Nam, Young Kwan Lee
Abstract: Many publications have demonstrated advantages of smart polymer actuators over the traditional electromagnetic transducers. One of the most significant contributions of the polymers might be their soft actuation mechanism. Hence unlike the traditional actuators, there is morphological freedom for actuator construction that benefits production of either small scale complex mechanisms or human-like applications. Although many different actuation paradigms of polymer actuators presented in previous publications, no significant contributions are made for the actual industrial applications. A noble idea for acquiring controllable actuation is antagonistic drive mechanism of dielectric elastomer. The mechanism provides fairly accurate controllable motion and relatively large actuation forces. A strong dependency to pre-strain of the polymer is however one of the major constraints of the actuator driving mechanism. A detailed characterization of pre-strain effects should be done for the successful construction of the actuators. Hence an experimental and theoretical consideration about mutual effects of pre-strain and actuator performance is to be presented in the present work.
Authors: Hyouk Ryeol Choi, Kwang Mok Jung, Ja Choon Koo, Jae Do Nam, Young Kwan Lee, Mi Suk Cho
Abstract: ElectroActive Polymers (EAPs) are emerging as new actuating means replacing the existing technologies such as piezoelectric, electrostatic, SMA etc. The dielectric elastomer actuator is regarded as the one of the most practically applicable actuators in the near future among the EAPs. In this paper, we introduce a new material capable of being employed as the dielectric elastomer actuator. The proposed material, which is a kind of the synthetic rubber, produces larger deformation as well as higher enegy efficiency, since it has a much higher dielectric constant compared to the previous ones. Beginning with the method of material synthesis, we give the description of its basic material properties by comparing with those of the existing materials for the dielectric elastomers. Also, the advantages of the proposed material as the actuating means are discussed with the several results of the experiments.
Authors: Byung Sun Kim, Dock Jin Lee, Ja Choon Koo, Jae Boong Choi, Young Jin Kim, Jong Nam Lee, Young Bee Chu
Abstract: As the TFT-LCDs are getting more attention for the next generation display device, specifications of the mechanical functionalities of the device is to be more tighter as well as the electrical user specifications. Due to its brittle characteristics of TFT-LCD panels, maintaining mechanical integrity under an impact loading situation is the one of the key design concerns. Furthermore, as the TFT-LCDs are popularly adopted for various mobile equipments such as cellular phones and digital cameras, shock failure of the display should be prohibitive for the design engineers. A major incident being monitored during the shock loading is of course the local material failure of the TFT-LCD panel that might happen at its maximum deformation. The present work delivers a systematic approach for the shockproof design of mobile TFT-LCD. A specially designed shock test setup evaluated by a set of rigorous FEM analyses is shown and comments for the shockproof method is to be also delineated.
Authors: Dong Ho Oh, Nam Hoon Lee, Ja Choon Koo, Hyeon Ki Choi, Yeon Sun Choi
Abstract: As the fluid dynamic bearing spindles are to be actively adopted to various small form factor mobile applications, mechanical specifications for the motors have been aggressively changed to pursue the fierce information technology sector market trend. One of the major technological challenges for the spindles to be successfully employed in the applications is the reduction of power consumption since the most of the mobile applications operate with a limited power source at relatively lower voltage. Recognizing implication of the power consumption that of course affects stiffness of the spindle, few of options for mechanical designers are available but either lowering rotational speed or adopting thinner lubricant. In the present work, a novel design solution for alleviating side effect of the lower stiffness spindle is introduced and verified.
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