Papers by Author: Hyeon Ki Choi

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Authors: Hyeon Chang Choi, Hyeon Ki Choi
Abstract: The relationship between fatigue crack growth behavior and cyclic crack tip opening displacement is studied. An elastic-plastic finite element analysis (FEA) is performed to examine the growth behavior of fatigue crack, where the contact elements are used in the mesh of the crack tip area. We investigate the relationship between the reversed plastic zone size and the changes of the cyclic crack tip opening displacement along the crack growth. The cyclic crack tip opening displacement is related to fatigue crack opening behavior.
1051
Authors: Hyeon Ki Choi, Si Yeol Kim, Won Hak Cho
Abstract: We investigated the relationship between kinematic and kinetic characteristics of foot joints resisting ground reaction force (GRF). Passive elastic characteristics of joint were obtained from the experiment using three cameras and one force plate. The relationship between joint angle and moment was mathematically modeled by using least square method. The calculated ranges of motion were 7o for TM joint, 4o for TT joint and 20o for MP joint. With the model that relates joint angle and plantar pressure, we could get the kinematic data of the joints which are not available from conventional motion analysis. The model can be used not only for biomechanical analysis which simulates gait but also for the clinical evaluations.
621
Authors: Hyeon Ki Choi, Si Yeol Kim, Hyeon Chang Choi
Abstract: In this study we presented kinematic and kinetic data of foot joints by using passive elastic characteristics of joints during gait. During the calculation process we used approximate equations and partial plantar pressures. The maximum angular displacements of each tarsometatarsal joint ranged from 4o to 7o and the maximum moments were from 200N⋅cm to 1500N⋅cm. It was relatively wide distribution. Foot kinematic data calculated from the approximated equations, which were represented by the correlation between moment and angular displacement, and the data from motion analysis were very similar. We found that the movements of foot joints were mainly decided by the passive characteristics of the joints when ground reaction force acts. The kinematic and kinetic analysis using approximated equations which is presented in this study can be usefully applied to describing the movements of foot joints during gait.
2231
Authors: Hyeon Ki Choi, Si Yeol Kim
Abstract: A computer-graphics based biomechanical model was constructed to investigate the kinematics of foot joints during the stance-phase of walking. In the model, all joints were assumed to act as monocentric, single degree of freedom hinge joints. To obtain the inputs to the model, the motion of foot segments was captured during the gait by a four-camera video system. The model fitted in an individual subject was simulated with these motion data. The ranges of motion of the first tarsometatarsal joint and the first metatarsophanlangeal joint were 8  ∼13  and -13  ∼ 48  respectively. The kinematic data of joints were similar to those of the previous studies. Our method based on the graphical computer model is considered useful for kinematic analysis of small joints including foot joints. Also, the results of this study will provide important information to the biomechanical studies which deal with human gait.
1115
Authors: Hyeon Ki Choi, Jae Hoon Jeong, Sung Ho Hwang, Hyeon Chang Choi, Won Hak Cho
Abstract: We recognized EMG signal patterns of lower limb muscles by using neural networks and performed feature evaluation during the recovery of postural balance of human body. Surface electrodes were attached to lower limb and EMG signals were collected during the balance recovery process from a perturbation without permitting compensatory stepping. A waist pulling system was used to apply transient perturbations in five horizontal directions. The EMG signals of fifty repetitions of five motions were analyzed for ten subjects. Twenty features were extracted from EMG signals of one event. Feature evaluation was also performed by using DB (Davies-Bouldin) index. By using neural networks, EMG signals were classified into five categories, such as forward perturbation, backward perturbation, lateral perturbation and two oblique perturbations. As results, motions were recognized with mean success rates of 75 percent. With the neural networks classifier of this study, the EMG patterns of lower limb muscles during the recovery of postural balance can be classified with high accuracy of recognition.
867
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.
1595
Authors: Ki Won Han, Wan Sik Ryu, Jae Wook Jeon, Hyeon Ki Choi, Hyun Soo Kim, Sung Ho Hwang
Abstract: Drivers are becoming more fatigued and uncomfortable as traffic densities increase, and so, can show slower reaction time. They then face the danger of traffic accidents due to their inability to cope with frequent shifting. To reduce this risk, some drivers prefer automatic transmissions to manual transmissions. However, automatic transmission requires both higher fuel consumptions and costs. For this reason, attention to automated manual transmission that can provide high efficiency, low cost and easy manufacturability has been increasing. In addition, the function and performance of the electronic control unit of automobiles has improved continually and rapidly with the growing electronics technology. The ECU is a representative embedded system in automobiles, which has to satisfy high performance and reliability under the constraints of size and cost. In this paper, the embedded system platform for automobiles is developed on the basis of MPC565, and a test rig is developed to perform the basic function test for automatic clutch actuation. The developed embedded system and clutch control algorithm are validated by the experimental results performed on the test rig.
1427
Authors: Hyeon Ki Choi, Won Hak Cho
Abstract: This study addressed the effect of balance control problems on the high-heeled women. The specific purposes of this study are to quantify the displacements and velocities of center-of-pressure (COP) of a body during waist pulling perturbation and to compare the differences between the bare-feet and the high-heeled. Another purpose of the study is to identify the effects of a high-heeled posture on electromyography (EMG) activities and muscle fatigue. We used a waist pulling system which has three different magnitudes to sway the subjects. The COP displacement of a high-heeled posture was about twice as much as that of bare-feet posture. Also the COP velocity of a high-heeled posture became about twice as much as that of bare-feet posture. Muscle fatigue could be identified by the shift of the median frequency (MF) of the EMG power spectrum toward lower frequencies. Median frequency of the EMG power spectrum from tibialis anterior was reduced more rapidly during high-heeled situation than during bare-feet situation. COP kinematics and muscle fatigue analysis in postural balance researches are considered to provide useful information in understanding the balance control mechanism of women’s high-heeled posture.
1119
Authors: Hyeon Ki Choi, Min Jwa Seo, Ja Choon Koo, Hyeon Chang Choi, Won Hak Cho
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.
871
Authors: Hyeon Chang Choi, Hyeon Ki Choi, Jun Hyub Park
Abstract: The cyclic crack tip opening displacement is well related to fatigue crack opening behavior. In this paper, we investigate the effect of the maximum stress intensity factor, Kmax, when predicting fatigue crack opening behavior using the cyclic crack tip opening displacement obtained from FEA. The commercial finite element code, ANSYS, for fatigue crack closure analysis in this study is used. We derive the prediction formula of crack opening behavior when using the cyclic crack tip displacement obtained from the FEA. The numerical prediction shows the good results regardless of stress ratios. It is confirmed that the crack opening behavior depends upon the maximum stress intensity factor Kmax.
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