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Dongjae Lee

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12 papers
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12

ICRA Conference 2025 Conference Paper

Ephemerality Meets Lidar-Based Lifelong Mapping

  • Hyeonjae Gil
  • Dongjae Lee
  • Giseop Kim
  • Ayoung Kim

Lifelong mapping is crucial for the long-term deployment of robots in dynamic environments. In this paper, we present ELite, an ephemerality-aided LiDAR-based lifelong mapping framework which can seamlessly align multiple session data, remove dynamic objects, and update maps in an end-toend fashion. Map elements are typically classified as static or dynamic, but cases like parked cars indicate the need for more detailed categories than binary. Central to our approach is the probabilistic modeling of the world into two-stage ephemerality, which represent the transiency of points in the map within two different time scales. By leveraging the spatiotemporal context encoded in ephemeralities, ELite can accurately infer transient map elements, maintain a reliable up-to-date static map, and improve robustness in aligning the new data in a more finegrained manner. Extensive real-world experiments on long-term datasets demonstrate the robustness and effectiveness of our system. The source code is publicly available for the robotics community: https://github.com/dongjae0107/ELite.

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NeurIPS Conference 2025 Conference Paper

Frequency-Aware Token Reduction for Efficient Vision Transformer

  • DongJae Lee
  • Jiwan Hur
  • Jaehyun Choi
  • Jaemyung Yu
  • Junmo Kim

Vision Transformers have demonstrated exceptional performance across various computer vision tasks, yet their quadratic computational complexity concerning token length remains a significant challenge. To address this, token reduction methods have been widely explored. However, existing approaches often overlook the frequency characteristics of self-attention, such as rank collapsing and over-smoothing phenomenon. In this paper, we propose a frequency-aware token reduction strategy that improves computational efficiency while preserving performance by mitigating rank collapsing. Our method partitions tokens into high-frequency tokens and low-frequency tokens. high-frequency tokens are selectively preserved, while low-frequency tokens are aggregated into a compact direct current token to retain essential low-frequency components. Through extensive experiments and analysis, we demonstrate that our approach significantly improves accuracy while reducing computational overhead and mitigating rank collapsing and over smoothing. Furthermore, we analyze the previous methods, shedding light on their implicit frequency characteristics and limitations. The code is available in https: //github. com/jhtwosun/frequency-aware-token-pruning.

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ICRA Conference 2025 Conference Paper

Safety-Critical Control for Aerial Physical Interaction in Uncertain Environment

  • Jeonghyun Byun
  • Yeonjoon Kim
  • Dongjae Lee
  • H. Jin Kim

Aerial manipulation for safe physical interaction with their environments is gaining significant momentum in robotics research. In this paper, we present a disturbance-observer-based safety-critical control for a fully actuated aerial manipulator interacting with both static and dynamic structures. Our approach centers on a safety filter that dynamically adjusts the desired trajectory of the vehicle's pose, accounting for the aerial manipulator's dynamics, the disturbance observer's structure, and motor thrust limits. We provide rigorous proof that the proposed safety filter ensures the forward invariance of the safety set—representing motor thrust limits—even in the presence of disturbance estimation errors. To demonstrate the superiority of our method over existing control strategies for aerial physical interaction, we perform comparative experiments involving complex tasks, such as pushing against a static structure and pulling a plug firmly attached to an electric socket. Furthermore, to highlight its repeatability in scenarios with sudden dynamic changes, we perform repeated tests of pushing a movable cart and extracting a plug from a socket. These experiments confirm that our method not only outperforms existing methods but also excels in handling tasks with rapid dynamic variations.

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ICRA Conference 2024 Conference Paper

Autonomous aerial perching and unperching using omnidirectional tiltrotor and switching controller

  • Dongjae Lee
  • Sunwoo Hwang
  • Jeonghyun Byun
  • Seung Jae Lee 0002
  • H. Jin Kim

Aerial unperching of multirotors has received little attention as opposed to perching that has been investigated to elongate operation time. This study presents a new aerial robot capable of both perching and unperching autonomously on/from a ferromagnetic surface during flight, and a switching controller to avoid rotor saturation and mitigate overshoot during transition between free-flight and perching. To enable stable perching and unperching maneuvers on/from a vertical surface, a lightweight (≈ 1 kg), fully actuated tiltrotor that can hover at 90 ◦ pitch angle is first developed. We design a perching/unperching module composed of a single servomotor and a magnet, which is then mounted on the tiltrotor. A switching controller including exclusive control modes for transitions between free-flight and perching is proposed. Lastly, we propose a simple yet effective strategy to ensure robust perching in the presence of measurement and control errors and avoid collisions with the perching site immediately after unperching. We validate the proposed framework in experiments where the tiltrotor successfully performs perching and unperching on/from a vertical surface during flight. We further show effectiveness of the proposed transition mode in the switching controller by ablation studies where large overshoot and even collision with a perching site occur. To the best of the authors’ knowledge, this work presents the first autonomous aerial unperching framework using a fully actuated tiltrotor.

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ICRA Conference 2023 Conference Paper

Globally Defined Dynamic Modelling and Geometric Tracking Controller Design for Aerial Manipulator

  • Byeongjun Kim
  • Dongjae Lee
  • Jeonghyun Byun
  • H. Jin Kim

This study presents a globally defined dynamics for a conventional multirotor equipped with a single $n\mathbf{-DOF}$ manipulator using modified Lagrangian dynamics. This enables the reformulation of entire dynamics directly on $\text{SO}(3)$ without exploiting any local coordinates, and thus problems such as the singularity of Euler angles can be avoided. Since skew-symmetric property of Coriolis matrix $C$ and inertia matrix facilitates stability analysis, we propose a method to compute $C$ which guarantees the skew-symmetric property by considering $C$ as a summation of two sub-matrices. Then, a geometric tracking controller is designed based on decoupled dynamics applying passive decomposition. The proposed controller guarantees almost global region of attraction. We validate our method via consecutive aerial flipping experiments.

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IROS Conference 2023 Conference Paper

Minimally Actuated Tiltrotor for Perching and Normal Force Exertion

  • Dongjae Lee
  • Sunwoo Hwang
  • Changhyeon Kim
  • Seung Jae Lee 0002
  • H. Jin Kim

This study presents a new hardware design and control of a minimally actuated 5 control degrees of freedom (CDoF) quadrotor-based tiltrotor. The proposed tiltrotor possesses several characteristics distinct from those found in existing works, including: 1) minimal number of actuators for 5 CDoF, 2) large margin to generate interaction force during aerial physical interaction (APhI), and 3) no mechanical obstruction in thrust direction rotation. Thanks to these properties, the proposed tiltrotor is suitable for perching-enabled APhI since it can hover parallel to an arbitrarily oriented surface and can freely adjust its thrust direction. To fully control the 5-CDoF of the designed tiltrotor, we construct an asymptotically stabilizing controller with stability analysis. The proposed tiltrotor design and controller are validated in experiments where the first two experiments of x, y position tracking and pitch tracking show controllability of the added CDoF compared to a conventional quadrotor. Finally, the last experiment of perching and cart pushing demonstrates the proposed tiltrotor's applicability to perching-enabled APhI.

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IROS Conference 2021 Conference Paper

Real-Time Motion Planning of a Hydraulic Excavator using Trajectory Optimization and Model Predictive Control

  • Dongjae Lee
  • Inkyu Jang
  • Jeonghyun Byun
  • Hoseong Seo
  • H. Jin Kim

Automation of excavation tasks requires real-time trajectory planning satisfying various constraints. To guarantee both constraint feasibility and real-time trajectory re-plannability, we present an integrated framework for real-time optimization-based trajectory planning of a hydraulic excavator. The proposed framework is composed of two main modules: a global planner and a real-time local planner. The global planner computes the entire global trajectory considering excavation volume and energy minimization while the local counterpart tracks the global trajectory in a receding horizon manner, satisfying dynamic feasibility, physical constraints, and disturbance-awareness. We validate the proposed planning algorithm in a simulation environment where two types of operations are conducted in the presence of emulated disturbance from hydraulic friction and soil-bucket interaction: shallow and deep excavation. The optimized global trajectories are obtained in an order of a second, which is tracked by the local planner at faster than 30 Hz. To the best of our knowledge, this work presents the first real-time motion planning framework that satisfies constraints of a hydraulic excavator, such as force/torque, power, cylinder displacement, and flow rate limits.

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IROS Conference 2021 Conference Paper

Robust and Recursively Feasible Real-Time Trajectory Planning in Unknown Environments

  • Inkyu Jang
  • Dongjae Lee
  • Seungjae Lee 0001
  • H. Jin Kim

Motion planners for mobile robots in unknown environments face the challenge of simultaneously maintaining both robustness against unmodeled uncertainties and persistent feasibility of the trajectory-finding problem. That is, while dealing with uncertainties, a motion planner must update its trajectory, adapting to the newly revealed environment in real-time; failing to do so may involve unsafe circumstances. Many existing planning algorithms guarantee these by maintaining the clearance needed to perform an emergency brake, which is itself a robust and persistently feasible maneuver. However, such maneuvers are not applicable for systems in which braking is impossible or risky, such as fixed-wing aircraft. To that end, we propose a real-time robust planner that recursively guarantees persistent feasibility without any need of braking. The planner ensures robustness against bounded uncertainties and persistent feasibility by constructing a loop of sequentially composed funnels, starting from the receding horizon local trajectory’s forward reachable set. We implement the proposed algorithm for a robotic car tracking a speed-fixed reference trajectory. The experiment results show that the proposed algorithm can be run at faster than 16 Hz, while successfully keeping the system away from entering any dead end, to maintain safety and feasibility.

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IROS Conference 2021 Conference Paper

Stability and Robustness Analysis of Plug-Pulling using an Aerial Manipulator

  • Jeonghyun Byun
  • Dongjae Lee
  • Hoseong Seo
  • Inkyu Jang
  • Jeongjun Choi
  • H. Jin Kim

In this paper, an autonomous aerial manipulation task of pulling a plug out of an electric socket is conducted, where maintaining the stability and robustness is challenging due to sudden disappearance of a large interaction force. The abrupt change in the dynamical model before and after the separation of the plug can cause destabilization or mission failure. To accomplish aerial plug-pulling, we employ the concept of hybrid automata to divide the task into three operative modes, i. e, wire-pulling, stabilizing, and free-flight. Also, a strategy for trajectory generation and a design of disturbance-observer-based controllers for each operative mode are presented. Furthermore, the theory of hybrid automata is used to prove the stability and robustness during the mode transition. We validate the proposed trajectory generation and control method by an actual wire-pulling experiment with a multirotor-based aerial manipulator.

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ICRA Conference 2020 Conference Paper

Aerial Manipulation using Model Predictive Control for Opening a Hinged Door

  • Dongjae Lee
  • Hoseong Seo
  • Dabin Kim
  • H. Jin Kim

Existing studies for environment interaction with an aerial robot have been focused on interaction with static surroundings. However, to fully explore the concept of an aerial manipulation, interaction with moving structures should also be considered. In this paper, a multirotor-based aerial manipulator opening a daily-life moving structure, a hinged door, is presented. In order to address the constrained motion of the structure and to avoid collisions during operation, model predictive control (MPC) is applied to the derived coupled system dynamics between the aerial manipulator and the door involving state constraints. By implementing a constrained version of differential dynamic programming (DDP), MPC can generate position setpoints to the disturbance observer (DOB)-based robust controller in real-time, which is validated by our experimental results.

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ICRA Conference 2020 Conference Paper

Trajectory Planning with Safety Guaranty for a Multirotor based on the Forward and Backward Reachability Analysis

  • Hoseong Seo
  • Clark Youngdong Son
  • Dongjae Lee
  • H. Jin Kim

Planning a trajectory with guaranteed safety is a core part for a risk-free flight of a multirotor. If a trajectory planner only aims to ensure safety, it may generate trajectories which overly bypass risky regions and prevent the system from achieving specific missions. This work presents a robust trajectory planning algorithm which simultaneously guarantees the safety and reachability to the target state in the presence of unknown disturbances. We first characterize how the forward and backward reachable sets (FRSs and BRSs) are constructed by using Hamilton-Jacobi reachability analysis. Based on the analysis, we present analytic expressions for the reachable sets and then propose minimal ellipsoids which closely approximate the reachable sets. In the planning process, we optimize the reference trajectory to connect the FRSs and BRSs, while avoiding obstacles. By combining the FRSs and BRSs, we can guarantee that any state inside of the initial set reaches the target set. We validate the proposed algorithm through a simulation of traversing a narrow gap.

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ICRA Conference 2019 Conference Paper

Cargo Transportation Strategy using T 3 -Multirotor UAV

  • Seung Jae Lee 0002
  • Dongjae Lee
  • H. Jin Kim

In this paper, we introduce a cargo transportation method with a new type of multi-rotor UAV platform known as T 3 -multirotor, to achieve stable and constant flight performance regardless of the type of cargo attached to the fuselage. The T 3 -multirotor, which consists of the `Thrust Generating Part' and the `Fuselage Part', can directly control the relative attitude between the two parts using the novel servomechanism. By utilizing the servomechanism with the proposed relative attitude control strategy, the T 3 -multirotor with cargo attached to the fuselage part can behave as a multi-rotor with only the moment of inertia of the thrust generating part during entire transportation. This allows the T 3 -multirotor to achieve the reliable performance in the event of any cargo being attached to the fuselage, achieving stable platform motion control. Detailed hardware description and dynamic analysis of T 3 -Multirotor is performed in this paper, and the validity of the proposed control strategy is also analyzed. The feasibility of the proposed control strategy is verified through experimental results with analysis.

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