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595 results on '"Lee, Ho-Jae"'

1. CusADi: A GPU Parallelization Framework for Symbolic Expressions and Optimal Control

Author

Jeon, Se Hwan, Hong, Seungwoo, Lee, Ho Jae, Khazoom, Charles, and Kim, Sangbae

Subjects
Computer Science - Robotics, Computer Science - Distributed, Parallel, and Cluster Computing
Abstract

The parallelism afforded by GPUs presents significant advantages in training controllers through reinforcement learning (RL). However, integrating model-based optimization into this process remains challenging due to the complexity of formulating and solving optimization problems across thousands of instances. In this work, we present CusADi, an extension of the CasADi symbolic framework to support the parallelization of arbitrary closed-form expressions on GPUs with CUDA. We also formulate a closed-form approximation for solving general optimal control problems, enabling large-scale parallelization and evaluation of MPC controllers. Our results show a ten-fold speedup relative to similar MPC implementation on the CPU, and we demonstrate the use of CusADi for various applications, including parallel simulation, parameter sweeps, and policy training., Comment: RAL 2024 submission

Published
2024

2. Integrating Model-Based Footstep Planning with Model-Free Reinforcement Learning for Dynamic Legged Locomotion

Author

Lee, Ho Jae, Hong, Seungwoo, and Kim, Sangbae

Subjects
Computer Science - Robotics, Electrical Engineering and Systems Science - Systems and Control
Abstract

In this work, we introduce a control framework that combines model-based footstep planning with Reinforcement Learning (RL), leveraging desired footstep patterns derived from the Linear Inverted Pendulum (LIP) dynamics. Utilizing the LIP model, our method forward predicts robot states and determines the desired foot placement given the velocity commands. We then train an RL policy to track the foot placements without following the full reference motions derived from the LIP model. This partial guidance from the physics model allows the RL policy to integrate the predictive capabilities of the physics-informed dynamics and the adaptability characteristics of the RL controller without overfitting the policy to the template model. Our approach is validated on the MIT Humanoid, demonstrating that our policy can achieve stable yet dynamic locomotion for walking and turning. We further validate the adaptability and generalizability of our policy by extending the locomotion task to unseen, uneven terrain. During the hardware deployment, we have achieved forward walking speeds of up to 1.5 m/s on a treadmill and have successfully performed dynamic locomotion maneuvers such as 90-degree and 180-degree turns., Comment: 8 pages

Published
2024

43. Does stress shielding after radial head arthroplasty affect functional outcomes?

Author

Lee, Ho-Jae, Kim, Byung-Kook, Jung, Suk-Han, Lee, Younghun, and Dan, Jinmyoung

Subjects
*PHYSIOLOGICAL stress, *RADIAL head & neck fractures, *PATIENT aftercare, *RANGE of motion of joints, *FUNCTIONAL status, *RADIAL bone, *ELBOW joint, *RETROSPECTIVE studies, *VISUAL analog scale, *LIGAMENT injuries, *ARTIFICIAL joints, *RISK assessment, *TOTAL elbow replacement, *SEVERITY of illness index, *TREATMENT effectiveness, *QUESTIONNAIRES, *PROSTHESIS design & construction, *LONGITUDINAL method, RISK of prosthesis complications
Abstract

Background: Various complications related to the prosthesis, such as implant loosening and stress shielding phenomenon, could develop after prosthetic replacement of the radial head. Stress shielding is known to occur around rigidly fixed implants. The purpose of this study was to evaluate the clinical influence and causative factors of the stress shielding phenomenon after radial head arthroplasty (RHA). Methods: Clinical records and radiographs of 56 patients with unreconstructable radial head fractures who received radial head replacement between 2009 and 2019 were reviewed. Exclusion criteria were infection, loosening, and follow-up of less than 24 months. After exclusion, 35 patients were enrolled. Patients were divided into two groups: an anatomical press-fit group (Anatomical Radial Head System; Acumed, Hillsboro, OR, USA) and a round bipolar cemented group (RHS; Tornier, Montbonnot Saint-Martin, France). Stress shielding around the prosthesis was assessed in the serial radiological examination. Clinical results were assessed using Mayo elbow performance score (MEPS), Quick Disabilities of the Arm, Shoulder, and Hand (q-DASH) score, range of motion (flexion–extension arc and pronation–supination arc), and visual analog scale score (VAS). Correlations between stress shielding phenomenon and demographic data and functional results were analyzed. Results: At an average follow-up of 43.06 (± 14.6) months, 14 (40%) out of 35 fixed stems demonstrated stress shielding. Our results showed that the rate of stress shielding was significantly higher in cases with a bilateral ligament injury and in the anatomical press-fit group (p = 0.028 and p = 0.0091, respectively). However, stress shielding around prostheses did not affect the clinical results (p > 0.05). Conclusion: The stress shielding phenomenon around radial head prosthesis may vary according to prosthetic design and severity of ligament injuries. Stress shielding does not affect the mid-term outcomes in the treatment of acute fractures of the radial head. Level of evidence III: Retrospective Cohort Comparison; Treatment Study. [ABSTRACT FROM AUTHOR]

Published
2023
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49. Design of T–S Fuzzy Classifier via Linear Matrix Inequality Approach

Author

Kim, Moon Hwan, Park, Jin Bae, Joo, Young Hoon, Lee, Ho Jae, Hutchison, David, editor, Kanade, Takeo, editor, Kittler, Josef, editor, Kleinberg, Jon M., editor, Mattern, Friedemann, editor, Mitchell, John C., editor, Naor, Moni, editor, Nierstrasz, Oscar, editor, Pandu Rangan, C., editor, Steffen, Bernhard, editor, Sudan, Madhu, editor, Terzopoulos, Demetri, editor, Tygar, Dough, editor, Vardi, Moshe Y., editor, Weikum, Gerhard, editor, Carbonell, Jaime G., editor, Siekmann, Jörg, editor, Wang, Lipo, editor, and Jin, Yaochu, editor

Published
2005
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