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Junho Choi

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14 papers

2 author rows

IROS Conference 2025 Conference Paper

SaWa-ML: Structure-Aware Pose Correction and Weight Adaptation-Based Robust Multi-Robot Localization

Junho Choi
Kihwan Ryoo
Jeewon Kim
Taeyun Kim
Eungchang Mason Lee
Myeongwoo Jeong
Kevin Christiansen Marsim
Hyungtae Lim

Multi-robot localization is a crucial task for implementing multi-robot systems. Numerous researchers have proposed optimization-based multi-robot localization methods that use camera, IMU, and UWB sensors. Nevertheless, characteristics of individual robot odometry estimates and distance measurements between robots used in the optimization are not sufficiently considered. In addition, previous researches were heavily influenced by the odometry accuracy that is estimated from individual robots. Consequently, long-term drift error caused by error accumulation is potentially inevitable. In this paper, we propose a novel visual-inertial-range-based multi-robot localization method, named SaWa-ML, which enables geometric structure-aware pose correction and weight adaptation-based robust multi-robot localization. Our contributions are twofold: (i) we leverage UWB sensor data, whose range error does not accumulate over time, to first estimate the relative positions between robots and then correct the positions of each robot, thus reducing long-term drift errors, (ii) we design adaptive weights for robot pose correction by considering the characteristics of the sensor data and visual-inertial odometry estimates. The proposed method has been validated in real-world experiments, showing a substantial performance increase compared with state-of-the-art algorithms.

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AAAI Conference 2025 Conference Paper

TSDF-Based Efficient Motion-Compensated Temporal Interpolation for 3D Dynamic Sequences

Soowoong Kim
Minseong Kwon
Junho Choi
Gun Bang
Seungjoon Yang

This paper introduces a method for efficiently interpolating 3D dynamic sequences using truncated signed distance function (TSDF) volumes. The method calculates bi-directional motions between TSDF volumes of two frames and refines them to reconstruct intermediate frames. Unlike point cloud-based methods, which can suffer from varying and irregular point densities, the uniform and dense grid structure of TSDF offers a consistent framework for estimating the true motion of objects within a scene. In our experiments, the TSDF-based method offers more precise and reliable smooth motion prediction compared to the often error-prone surface depiction in point clouds. Experimental results demonstrate improved accuracy and reduced computational complexity, making it suitable for real-time applications.

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ICRA Conference 2024 Conference Paper

A transtibial prosthesis using a parallel spring mechanism

Donggyu Jung
Shinsuk Park
Junho Choi

Prosthetic legs have been used to restore function in the lower limbs lost due to amputation. Early designs including prosthetic legs with a passive joint or without any joint as well as the Energy Storing and Releasing (ESR) feet have shown deficiency in push-off torque, which results in asymmetric gait pattern, slower walking speed, and higher cost of transportation. Although powered prosthetic legs address the aforementioned problems, they suffer from lower energy efficiency, higher volume and weight. In this paper, a powered transtibial prosthesis using a Parallel Elastic Actuator (PEA) is proposed in order to generate the joint torque needed for walking with a lower-powered actuator for lighter and more compact design. A non-linear spring mechanism is proposed to generate the spring torque as needed. The implemented prosthetic leg is evaluated with three intact subjects. The experimental results shows that smaller torque is required for the motor with the spring mechanism. Therefore, less electrical power is consumed when the spring mechanism is used, which implies a lower-powered actuator is sufficient to generate the joint torque needed for walking.

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IROS Conference 2021 Conference Paper

REAL: Rapid Exploration with Active Loop-Closing toward Large-Scale 3D Mapping using UAVs

Eungchang Mason Lee
Junho Choi
Hyungtae Lim
Hyun Myung

Exploring an unknown environment without colliding with obstacles is one of the essentials of autonomous vehicles to perform diverse missions such as structural inspections, rescues, deliveries, and so forth. Therefore, unmanned aerial vehicles (UAVS), which are fast, agile, and have high degrees of freedom, have been widely used. However, previous approaches have two limitations: a) First, they may not be appropriate for exploring large-scale environments because they mainly depend on random sampling-based path planning that causes unnecessary movements. b) Second, they assume the pose estimation is accurate enough, which is the most critical factor in obtaining an accurate map. In this paper, to explore and map unknown large-scale environments rapidly and accurately, we propose a novel exploration method that combines the pre-calculated Peacock Trajectory with graph-based global exploration and active loop-closing. Because the two-step trajectory that considers the kinodynamics of UAVs is used, obstacle avoidance is guaranteed in the receding-horizon manner. In addition, local exploration that considers the frontier and global exploration based on the graph maximizes the speed of exploration by minimizing unnecessary revisiting. In addition, by actively closing the loop based on the likelihood, pose estimation performance is improved. The proposed method’s performance is verified by exploring 3D simulation environments in comparison with the state-of-the-art methods. Finally, the proposed approach is validated in a real-world experiment.

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IROS Conference 2020 Conference Paper

BRM Localization: UAV Localization in GNSS-Denied Environments Based on Matching of Numerical Map and UAV Images

Junho Choi
Hyun Myung

Localization is one of the most important technologies needed to use Unmanned Aerial Vehicles (UAVs) in actual fields. Currently, most UAVs use GNSS to estimate their position. Recently, there have been attacks that target the weaknesses of UAVs that use GNSS, such as interrupting GNSS signal to crash the UAVs or sending fake GNSS signals to hijack the UAVs. To avoid this kind of situation, this paper proposes an algorithm that deals with the localization problem of the UAV in GNSS-denied environments. We propose a localization method, named as BRM (Building Ratio Map based) localization, for a UAV by matching an existing numerical map with UAV images. The building area is extracted from the UAV images. The ratio of buildings that occupy in the corresponding image frame is calculated and matched with the building information on the numerical map. The position estimation is started in the range of several km 2 area, so that the position estimation can be performed without knowing the exact initial coordinate. Only freely available maps are used for training data set and matching the ground truth. Finally, we get real UAV images, IMU data, and GNSS data from UAV flight to show that the proposed method can achieve better performance than the conventional methods.

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ICRA Conference 2017 Conference Paper

A robotic orthosis with a cable-differential mechanism

Jaehwan Park
Seunghan Park
Chan Ho Park
Seungmin Jung
Chankyu Kim
Jong Hyeon Park
Junho Choi

Robotic orthoses have potential to assist people having difficulty in walking due to their neurological disorders. However, it is important to design lighter orthoses since additional weight and inertia from the device cause discomfort to the wearer and even instability during walking in some extreme cases. To address this problem, a robotic orthosis with a cable differential mechanism is proposed in this paper. It is designed to assist stroke patients with hemiplegia. It has 2 active degrees of freedom at the hip and knee joints and one passive degree of freedom at the ankle. A cable differential mechanism is used to transmit the torques generated by the actuators, which are located near the pelvis of the wearer to reduce the inertial effect. The cable differential mechanism allows for the actuators to share the load with each other, which results in decrease of the maximum required torques by the actuators. Therefore, smaller actuators are possible to be used for further reduction of the weight. The proposed robotic orthosis with a cable differential mechanism, which is called “COWALK-Mobile 2, ” is implemented and evaluated.

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IROS Conference 2015 Conference Paper

A methodology to control walking speed of robotic gait rehabilitation system using feasibility-guaranteed trajectories

Chan-Yul Jung
Junho Choi
Shinsuk Park
Seung-Jong Kim

This paper presents a novel methodology to control walking speed of an exoskeleton for gait rehabilitation of stroke patients using feasibility-guaranteed trajectories. The controller uses interaction forces to estimate the desired walking speed. Instead of allowing each joint to move around a nominal trajectory, which could lead to infeasible gait patterns, the control algorithm proposed in this paper chooses joint trajectories for desired walking speed, which generates feasible gait patterns. With the interaction forces measured during walking, the walking speed intended by the patient is estimated. Then, based on the estimated walking speed, a reference trajectory stored in a database, which is checked if kinematic constraints required for walking are met, is chosen. Since checking feasibility is performed off-line before the training sessions, it is possible to ensure stability of walking without causing any computational time on-line.

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ICRA Conference 2014 Conference Paper

A Mechanically Adjustable Stiffness Actuator(MASA) of a robot for knee rehabilitation

Jaewook Oh
Soo-Jun Lee
Myo-Taeg Lim
Junho Choi

This paper presents a Mechanically Adjustable Stiffness Actuator(MASA) for knee rehabilitation of stroke patients. The MASA is designed for safer and more effective physical human-robot interaction with patients in rehabilitation. The MASA consists of cantilever springs, a double-tripod parallel mechanism, and a torque limiter. Using the double-tripod parallel mechanism and two identical actuators, the effective length and the resting position of the cantilever springs are controlled independently. Changes of the effective length of the cantilever springs result in variation of the stiffness of the MASA. One end of each cantilever springs is attached to an axis via the torque limiter. When an external torque beyond the preset threshold is applied from and to the axis, the torque limiter is released so the axis rotates freely regardless of the position of the actuators. The MASA is used for a knee rehabilitation robot. Due to the springs and the torque limiter, physical safety of the patients is guaranteed in case of unexpected involuntary muscle activities (i. e. spasticity) during a therapy session. With changing stiffness of the MASA, the amount of assistance by the robot is possible to be adjusted.

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IROS Conference 2014 Conference Paper

Design and control of an exoskeleton system for gait rehabilitation capable of natural pelvic movement

Chan-Yul Jung
Junho Choi
Shinsuk Park
Jong Min Lee
ChangHwan Kim
Seung-Jong Kim

This paper introduces a novel exoskeleton system for gait rehabilitation, which allows natural pelvic movements. The developed lower extremity exoskeleton system, COWALK, has 14 degrees of freedom including four degrees of freedom for pelvic motion. From 113 healthy human subjects, 3D motion capture data were collected to determine mechanical design parameters and to generate gait patterns for the COWALK system. In order to reduce the affect of the weight of the robot, a gravity compensator was installed to support the weight of the robot. The performance of the COWALK system was validated by experiments. The experimental results from joint trajectories and pelvis movements show that the developed exoskeleton system can produce natural gait movements by augmenting the degrees of freedom for pelvic motion.

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ICRA Conference 2012 Conference Paper

External force estimation using joint torque sensors for a robot manipulator

Le Dinh Phong
Junho Choi
Sungchul Kang

This paper proposes an algorithm to estimate external forces exerted on the end-effector of a robot manipulator using information from joint torque sensors (JTS). The algorithm is combination of Time Delay Estimation (TDE) and input estimation technique where the external force is considered as an unknown input to the robot manipulator. Based on TDE's idea, the estimator which does not require an accurate dynamics model of the robot manipulator is developed. The simultaneous input and state estimation is used to reject not only nonlinear uncertainties of the robot dynamics but also the noise of measurements. The performance of the proposed estimation algorithm is evaluated through simulation of a two degree-of-freedom manipulator and it demonstrates the stability in estimating the external forces. The estimation results show that this approach allows inexpensive sensors as joint torque sensors to be used instead of expensive ones as F/T sensors in robot application.

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ICRA Conference 2009 Conference Paper

A variable stiffness joint using leaf springs for robot manipulators

Junho Choi
Seonghun Hong 0001
Woosub Lee
Sungchul Kang

Safety of a manipulator designed to be used at home requires different approach than industrial robots, where safety is achieved mainly by decreasing the interaction with humans. Robots for applications at home, however, require frequent interaction with humans. Introducing compliant component gives the answer to the safety issue at the cost of performance degradation. In order to reduce the performance degradation, manipulators equipped with variable stiffness have been studied by many researchers. This paper presents a variable stiffness joint(VSJ) designed for a robot manipulator. The stiffness is generated by leaf springs and two actuators are used to control the position and stiffness of the joint. Changing the effective length of the spring results in change in stiffness. The position of the joint is controlled via rotating two actuators at the same speed in the same direction. The stiffness is controlled when the two actuators rotate in the different speed. Experiments are conducted to show that the position and stiffness are controlled independent with each other and having less stiffness at the joint helps in making unexpected collision with object safer.

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IROS Conference 2009 Conference Paper

Spring-Clutch: A safe torque limiter based on a spring and CAM mechanism with the ability to reinitialize its position

Woosub Lee
Junho Choi
Sungchul Kang

Service robots are anticipated to be used in unstructured areas such as homes, hospitals, and public areas in the near future. However, safety issues need to be addressed before this can occur. In particular, robot manipulators that handle objects by physical contact run the risk of colliding with people or objects. Thus, it is important to prevent collisions that could injure people and damage robot manipulators. In this study, a safe joint mechanism is developed to ensure the safe use of a manipulator. This mechanism, termed ‘Spring-Clutch, ’ is a simple passive mechanism that consists of a coil spring and a CAM mechanism. When a torque is applied that is less than a threshold value, Spring-Clutch functions as a rigid joint between the input and the output. However, when an applied torque exceeds the threshold, angular displacement occurs between the input and output to reduce the collision force. If the applied torque is removed, Spring-Clutch immediately returns to its nominal position without the need for additional operations. This paper describes the design principles and performance of Spring-Clutch, and discusses the possibility of its practical use as a joint mechanism for safe manipulation.

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ICRA Conference 2008 Conference Paper

Design of a robot joint with variable stiffness

Junho Choi
Sunchul Park
Woosub Lee
Sungchul Kang

A robot joint with a variable stiffness unit is presented. The variable stiffness unit (VSU) is composed of a motor, two rings that consist of arc-shaped magnets separated by spacers, and a linear guide to change the cross-sectional area of the two rings. Angular displacement between two rings causes the magnets to generate torque, which acts as a nonlinear spring. The stiffness of the joint is varied via changing the overlapping area of the magnets. The VSJ exhibits nearly zero stiffness, which enables robot manipulator to be harmless to humans at a wide range of operating speed. Connected to a joint motor in series, the stiffness by the VSU and the position of the joint are controlled independently by two motors. The torque generated by the magnets is analyzed. Using dynamics of the joint, feedback linearization method is adopted to control the VSJ. In addition to feedback linearization, an integral controller is augmented in order to reduce the effect of model uncertainty and disturbances.

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ICRA Conference 2007 Conference Paper

Frequency Domain Stability Observer and Active Damping Control for Stable Haptic Interaction

Dongseok Ryu
Jae-Bok Song
Junho Choi
Sungchul Kang
Mun Sang Kim

Stable haptic interaction has been studied extensively by an energy-based approach. However, the energy in the haptic system is not directly measurable, but estimated from some measured quantities such as force and velocity; therefore, the estimated energy is occasionally inaccurate. To resolve this problem, a new observer, working in the frequency domain, is proposed in this research. The observer quantifies the degree of instability of a haptic system, and a proposed controller generates variable damping in proportion to this quantitative instability. Especially, for a double layered virtual wall, the proposed methods were much faster in detecting haptic instability than other schemes, and successfully reduced unstable behavior.

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