Papers by Author: 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.

Abstract: Flexible and thin conductive films of poly(3,4-ethylenedioxythiophene) (PEDOT) on PET substrate were obtained by an in- situ vapor-phase polymerization (VPP) method using ferric toluene sulfonate as an oxidant. The addition of epoxy acrylate resin used as a binder to provide adhesive strength between PEDOT and PET also afforded the possibility of the surface patterning through UV exposure. The electrical and optical properties of the conductive PEDOT films were characterized by UV-Vis spectroscopy and conductivity measurement. Surface resistance below 150./sq. was achieved for 100 nm thick films with UV-vis-spectrum transparency exceeding 80%. The combination of these properties makes the films highly suitable for numerous device applications.

Abstract: A novel core-shell material composed of closely packed gold shells on poly (divinylbenzene) (PDVB) cores was fabricated via the reduction of a gold complex. PDVB beads (2-5 +m) were synthesized by precipitation polymerization. The surface of the PDVB beads was modified by three different methods, viz. sulfonation, chloromethylation, and thiolation. The modification of the surface of the PDVB beads was designed to allow the facile attachment of the gold layer onto the PDVB cores. The gold seeding layer was initially formed on the modified PDVB cores by the chemical reduction of a gold-phenanthroline complex. The subsequent growing reactions of NH2OH and HAuCl4 increased the gold coverage to more than 90%. The structure of the PDVB/Au core-shell material was characterized by SEM, XPS, and FT-IR.

Abstract: This paper presents a new design of an artificial muscle actuator called tube-spring actuator (TSA) that is fabricated with dielectric elastomer. The new actuator construction includes two steps: the first is that a cylindrical actuator is manufactured with dielectric elastomer with multi-layer lamination process. Then a compressed spring is inserted to inside the tube as the second step of the process. The inner spring is used to maximize the axial deformation while constraining the radial contraction. This design enables effective linear actuation with largest amount strain. The monitored strain of active length is up to 15 percents. The manufactured actuator is applied to a robot hand as an example of the actual application.

Abstract: We assessed the effects of muscle forces on ankle joint kinetics during postural balance control of human boy. Nine male subjects (mean age of 25.8 yrs) participated in the experiment. An ankle joint model assumed ball and socket joint was used, which was capable of three dimensional rotations. A six-camera VICON system was used for motion analysis. Waist pulling system and force platform were adopted for forward sway and GRF (ground reaction force) measurement. We used linear optimization programs to calculate the variation of muscle forces and angular displacements of shank and foot segments. With the experimental data and linear programs, we could calculate joint reaction forces, and bone-on-bone forces. The results presented in this study give us the insights to understand the roles of lower limb muscles during postural balance control and ankle injury mechanism.

Abstract: Melt processable plasticized cellulose diacetate (CDA) was prepared using triacetin (TA) as a plasticizer and its mechanical properties were characterized. The processability of the plasticized CDA was further enhanced by using a small amount of epoxidized soybean oil as a secondary plasticizer. The glass transition temperature of the plasticized CDA was observed at 50°C lower than that of the virgin CDA and the incorporation of 5 % of ESO also resulted in an additional 20°C decrease in the Tg value. In order to obtain practical processing conditions, a plasticizer content of more than 20 wt % should be used.

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.

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.

Abstract: Recently, mechanical shock failures of a flat display unit such as TFT-LCD device have been an important concern of designers. In order to achieve the mechanical shock requirement, it is necessary to perform the detailed FE analyses which could be very expensive either by the lengthy computation or by the complicated geometry modeling. The objective of this study is to propose a simplified analysis methodology to simulate impact behavior of thin glass plates. The static problem equivalent to the impact one is found from the concept of solid mechanics to estimate the maximum deflection and stress under impact loading. To show the plausibility of the proposed approach, it is applied to the idealized problem which is a two dimensional beam subjected to impact loading. Based on explicit FE analyses using the LS-DYNA FE program, it was shown that the impact problem can be solved by the equivalent static analysis which is much easier to solve in practice. Therefore, the proposed approach provides significant advantages in design optimization of a TFT-LCD device against shock failure, and enables the designer to avoid ad hoc modeling of the transient dynamics so that product design cycle could be shortened.

Abstract: This article presents a modeling method for air flow analysis of a hard disk drive. Air flow excitation causes disk vibration that aggravates TMR budget of the design of modern high performance hard drives. And it is the most expensive budget consumer so that controlling of the flutter becomes the primary design issue of the data storage industry. In the presented work, air flow excitation forces are characterized by LES modeling and the results are verified with experiments. A squeeze-film-type disk damper is employed in the experiments and it is applied for a hardware design improvement for disk flutter reduction. LES and RANS are compared and alternately used in a calculation in order to minimize computational efforts.