Floating-base multi-link robots can change their shape during flight, making them well-suited for applications in confined environments such as autonomous inspection and search and rescue. However, trajectory planning for such systems remains an open challenge because the problem lies in a high-dimensional, constraint-rich space where collision avoidance must be addressed together with kinematic limits and dynamic feasibility. This work introduces a hierarchical trajectory planning framework that integrates global guidance with configuration-aware local optimization. First, we exploit the dual nature of these robots—the root link as a rigid body for guidance and the articulated joints for flexibility—to generate global anchor states that decompose the planning problem into tractable segments. Second, we design a local trajectory planner that optimizes each segment in parallel with differentiable objectives and constraints, systematically enforcing kinematic feasibility and maintaining dynamic feasibility by avoiding control singularities. Third, we implement a complete system that directly processes point-cloud data, eliminating the need for handcrafted obstacle models. Extensive simulations and real-world experiments confirm that this framework enables an articulated aerial robot to exploit its morphology for maneuvering that rigid robots cannot achieve. To the best of our knowledge, this is the first planning framework for floating-base multi-link robots that has been demonstrated on a real robot to generate continuous, collision-free, and dynamically feasible trajectories directly from raw point-cloud inputs, without relying on handcrafted obstacle models.

In recent years, multimodal locomotion capabilities have enabled robots to maneuver in both terrestrial and aerial domains. However, most of these robots are designed only for locomotion, and few possess the manipulation capabilities required for practical tasks. By adding a manipulator, ground robots can perform manipulation, and some drones with robotic arms have demonstrated aerial manipulation. Nonetheless, such multirotors cannot be directly used for manipulation on the ground, and this configuration itself is unsuitable for air-ground hybrid locomotion. This is because their thruster-centralized structure makes it difficult to achieve both sufficient degrees of freedom (DoF) for manipulation and stable motion with contact and transformation. Therefore, in this work, we develop a new multilink multirotor with thrusters on each link and capable of contact with the environments. This robot can perform terrestrial rolling locomotion, aerial flight locomotion, and manipulation in multiple environments using joint actuation. First, we introduce a minimal configuration design of the proposed robot. We also describe a kinematic model and propose a design for each component based on this model. Second, we propose a real-time control method based on nonlinear optimization that considers contact and joint motion, which can be applied to various multirotors. Third, we propose motion strategies that include contact constraints specific to air-ground hybrid multilink multirotors, and analyze the limitations of manipulation capabilities based on multi-contact model. Finally, we demonstrate a variety of motions in both domains using the implemented prototype. To the best of our knowledge, this is the first demonstration of air-ground hybrid locomotion and manipulation by a multilink multirotor.

Aerial robots, especially multirotor type, have been utilized in various scenarios such as inspection, surveillance, and logistics. The most critical issue for multirotor type is the limited flight time due to the large power consumption to hover against gravity. Inspired by nature, various research areas focus on the perching and grasping ability by deploying a gripper on the multirotor to grasp arboreal environments to save energy; however, most of the mechanical design for gripper restricts the approach path, significantly limiting the performance of perching and grasping. In addition, it is also challenging to design a light gripper that also offers sufficiently large grip force to hang itself. Therefore, in this work, we develop a single-actuator hand for aerial robot that enables adaptive grasping of various objects, and thus can perch from various approach directions. First, we present the design of the lightweight three-fingered hand with a pair of special two-dimensional differential plates that enables adaptive grasping with a single actuator. In addition, we develop a unique control method for the over-actuated aerial robot equipped with this hand to perform both adaptive pendulum-like perching and detachment. Finally, we demonstrate the feasibility of the prototype hand via load bearing and object grasping experiments, along with in-flight perching experiments.

This paper introduces a novel method for controlling multirotor aerial robots connected by passive flexible elements. Despite the growing popularity of multirotor aerial robots, their real-world applications remain limited due to difficulties adapting to complex environments. Soft robotics, due to its inherent flexibility, offers a potential solution, although research on integrating flexible elements into aerial robots is still in the early stages. In this study, we propose control methods for a system where multiple aerial robots are interconnected with passive flexible elements. These robotic systems enhance adaptability, enabling tasks like object manipulation. We model the flexible parts using the piecewise constant strain (PCS) model, which allows for model-based closed-loop control and stabilizes various configurations of the system. Through simulations and experiments, we validated that the proposed method achieves both stable flight and flexible deformation. Notably, we succeeded in maintaining stable flight, which traditional methods could not achieve, and demonstrated both positional controllability and the ability of the flexible parts to bend dynamically during flight.

Delivery by aerial robots is an emerging topic in many scenarios, such as logistics, construction industry, and disaster response. Compared to the standard styles that deploy cage or sling, grasping style by gripper can handle objects in various shapes. A multi-limbed structure with distributed vectorable rotors called SPIDAR shows a higher potential to grasp large object in a three-dimensional manner. Therefore, in this paper, we focus on the advanced usage of the vectored thrust forces to achieve aerial grasping by this robot. First, a vectored thrust control to avoid the aerointerference on the underwind segments (e.g., grasped object) during flight is proposed. Then, an optimization-based planning method that utilizes redundant vectored thrust forces for firm grasping is developed. Finally, we demonstrate the feasibility of the proposed flight control and grasp planning by performing challenging grasping and transporting motion with a spherical object of which the diameter is 0.6 m. To the best of our knowledge, this work is the first to achieve multi-finger-like grasping to carry a large object in midair.

Omnidirectional aerial robots offer full 6-DoF independent control over position and orientation, making them popular for aerial manipulation. Although advancements in robotic autonomy, human operation remains essential in complex aerial environments. Existing teleoperation approaches for multirotors fail to fully leverage the additional DoFs provided by omnidirectional rotation. Additionally, the dexterity of human fingers should be exploited for more engaged interaction. In this work, we propose an aerial teleoperation system that brings the rotational flexibility of human hands into the unbounded aerial workspace. Our system includes two motion-tracking marker sets—one on the shoulder and one on the hand—along with a data glove to capture hand gestures. Using these inputs, we design four interaction modes for different tasks, including Spherical Mode and Cartesian Mode for long-range moving, Operation Mode for precise manipulation, as well as Locking Mode for temporary pauses, where the hand gestures are utilized for seamless mode switching. We evaluate our system on a vertically mounted valve-turning task in the real world, demonstrating how each mode contributes to effective aerial manipulation. This interaction framework bridges human dexterity with aerial robotics, paving the way for enhanced aerial teleoperation in unstructured environments.

Flapping-wing drones have attracted significant attention due to their biomimetic flight. They are considered more human-friendly due to their characteristics such as low noise and flexible wings, making them suitable for human-drone interactions. However, few studies have explored the practical interaction between humans and flapping-wing drones. On establishing a physical interaction system with flapping-wing drones, we can acquire inspirations from falconers who guide birds of prey to land on their arms. This interaction interprets the human body as a dynamic landing platform, which can be utilized in various scenarios such as crowded or spatially constrained environments. Thus, in this study, we propose a falconry-like interaction system in which a flapping-wing drone performs a palm landing motion on a human hand. To achieve a safe approach toward humans, we design a motion planning method that considers both physical and psychological factors of the human safety such as the distance from the user, the altitude, the approach direction, and the drone’s velocity. We use a commercial flapping platform with the implemented motion planning and conduct experiments to evaluate the palm landing performance and safety. The results demonstrate that our approach enables safe and smooth hand landing interactions. To the best of our knowledge, it is the first time to achieve a contact-based interaction between flapping-wing drones and humans.

Manipulation performance improvement is crucial for aerial robots. For aerial manipulators, the baselink position and attitude errors directly affect the precision at the end effector. To address this stability problem, fixed-body approaches such as perching on the environment using the rotor suction force are useful. Additionally, conventional arm-equipped multirotors, called rotor-concentrated manipulators, find it difficult to generate a large wrench at the end effector due to joint torque limitations. Using distributed rotors to each link, the thrust can support each link weight, decreasing the arm joints' torque. Based on this approach, rotor-distributed manipulators (RDMs) can increase feasible wrench and reachability of the end effector. This article introduces a minimal configuration of an RDM that can perch on surfaces, especially ceilings, using a part of their body. First, we design a minimal rotor-distributed arm considering the flight and end-effector performance. Second, a flight controller is proposed for this minimal RDM along with a perching controller adaptable for various types of aerial robots. Third, we propose a motion planning method based on inverse kinematics, considering specific constraints to the proposed RDMs, such as perching force. Finally, we evaluate flight and perching motions and confirm that the proposed manipulator can significantly improve the manipulation performance.

In recent years, an increasing number of research works have been focusing on the manipulation by aerial robots. Previous works using aerial robots with robotic arms have two problems: underactuation and external disturbances. We propose the fully-actuated control method and motion strategy using contact with the environment to solve these problems, along with the mechanical approach required. First, each propeller's 1 degree-of-freedom (DoF) thrust vectoring units are applied to enable fully-actuated flight control. In order to obtain the desired thrust and vectoring angle inputs for aerial manipulation satisfying hardware limits, we developed a fully-actuated control method using non-linear optimization. Second, we propose a manipulation motion strategy that treats the multilink robot body as a fixed manipulator by making contact with the environment. The contact mechanism attached to the link end is developed to maintain contact and resist external disturbances. In a real machine experiment, the robot successfully opened the door while in contact with the wall, demonstrating the feasibility of the proposed methods.

Precise aerial manipulation is important for multirotor robots. For multirotors equipped with arms, the root pose error due to the floating body affects the precision at the end effector. Fixed-root approaches, such as perching on surfaces using the rotor suction force, are useful to address this problem. Furthermore, it is difficult for arm-equipped multirotors to generate large wrenches at the end effector owing to joint torque limitations. For multilink aerial robots with rotors distributed to each link, the thrust of rotors can produce large torques. Therefore, such multirotor robots can generate comparatively large wrenches at the end effector. In this paper, we introduce a rotor-distributed multilink robot that can perch on surfaces. First, we designed a root footplate and arm module for a multilink aerial robot. During perching, the joint between these two links can be passive to prevent peeling. Second, we propose a quadratic programming (QP) based controller to calculate the desired thrust for perching motion, considering the static friction and zero moment point (ZMP) conditions on the footplate. Finally, we conducted root-body perching motion tests. The manipulations of the multilink aerial robot during perching become more accurate than those during flight because the root position adheres to the environment.

Quadrupedal robots are skillful at locomotion tasks while lacking manipulation skills, not to mention dexterous manipulation abilities. Inspired by the animal behavior and the duality between multi-legged locomotion and multi-fingered manipulation, we showcase a circus ball challenge on a quadrupedal robot, ANYmal. We employ a model-free reinforcement learning approach to train a deep policy that enables the robot to balance and manipulate a light-weight ball robustly using its limbs without any contact measurement sensor. The policy is trained in the simulation, in which we randomize many physical properties with additive noise and inject random disturbance force during manipulation, and achieves zero-shot deployment on the real robot without any adjustment. In the hardware experiments, dynamic performance is achieved with a maximum rotation speed of 15 °/s, and robust recovery is showcased under external poking. To our best knowledge, it is the first work that demonstrates the dexterous dynamic manipulation on a real quadrupedal robot.

Carrying payloads in air is a major mission for multirotor aerial robot. However, the presence of payloads on multirotor aerial robot has a risk of degrading the performance of the flight controller. This concern becomes obvious especially when carrying objects not securely attached to the body or performing aerial manipulation. Therefore, controller with the ability to adapt itself to the effects of payloads on flight stability is needed. This paper proposes a novel nonlinear multiple-input and multiple-output (MIMO) model reference adaptive control (MRAC) system for attitude control of multirotor aerial robots which can dynamically compensate change in the position of center of gravity and inertia caused by payloads. Stability and robustness of the controller are experimentally confirmed in quadrotor and transformable multirotor, and experiments modeling practical applications are conducted for each aerial robot system, proving the utility of the controller.

Stable flight control under ceilings is difficult for multirotor Unmanned Aerial Vehicles (UAVs). The wake interaction between rotors and upper walls, called the "ceiling effect", causes an increase of rotor thrust. As a result of the thrust increase, multi-rotors are drawn upward abruptly and collide with ceilings. In previous work, several thrust models of the ceiling effect have been proposed for stable flight under ceilings, assuming that the airflow around rotors is in steady states. However, the airflow around rotors in vertical flight is not in steady states and each thrust model in previous work is skillfully determined based on large amounts of precise experimental data. In this paper, we introduce an aerodynamics-based thrust model and a stable control method under ceilings. This model is derived from the momentum theory and the relationship between vertical climbing/descending rates of rotors and an induced velocity. To confirm our proposed model, we collect thrust data at various vertical rates in flight. In addition, we use only onboard sensors to estimate selfstate for structural inspections. Consequently, we reveal that the proposed model is consistent with the experimental results. Based on an aerodynamic model, we need not collect large amounts of precise experimental data to realize stable flight. Furthermore, the vertical flight tests under ceilings demonstrate that our in-unsteady-state-model-based controller outperforms the conventional steady-state ones.

Regrasping is one of the most common and important manipulation skills used in our daily life. However, aerial regrasping has not been seriously investigated yet, since most of the aerial manipulator lacks dexterous manipulation abilities except for the basic pick-and-place. In this paper, we focus on pivoting a long box, which is one of the most classical problems among regrasping researches, using a transformable multilink aerial robot. First, we improve our previous controller by compensating for the external wrench. Second, we optimize the joints configuration of our transformable multilink drone for stable grasping form under the constraints of thrust force and joints effort. Third, we sequentially optimize the grasping force in the pivoting process. The optimization goal is to generate continous grasping force whilst maximizing the friction force in case of the downwash, which would influence the grasped object and is difficult to model. Fourth, we develop the impedance controller in joint space and admittance controller in task space. As far as we know, it is the first research to achieve extrinsic contact-aware regrasping task on aerial robots.

External wrench estimation is very helpful for aerial exploration and manipulation tasks. During the exploration, there might be unseen obstacles to cause dangerous collisions. The estimation of the external force and torque is also beneficial in aerial manipulation tasks. In this paper, we present a framework of estimating the external wrench for the aerial multilink robot based on the onboard inertial measurement unit (IMU) sensors, joints state and robot dynamic models. Compared to the conventional multirotor robot, the center of mass (CoM) is always changing when the robot transforms. The sensor could not be attached to CoM to observe the acceleration data. Consequently, we present a novel method by applying a distributed IMU system to estimate the CoM linear and angular accelerations for the external wrench estimation. With the help of the robot model, the position of the contact point could be estimated, which is useful in exploring tasks to safely interact with the physical world. We design the contact-aided navigation strategy and computationally efficient motion primitives library to help our robot react to the unexpected collision. We experimentally validate our framework with a two-dimensional multilink aerial robot to show the results of external wrench estimator and its further applications2.2Experiment video: https://youtu.be/R-WDReLnWWI

We present a novel transformable multilinked aerial robot which consists of link modules with 1 DoF thrust vectoring mechanism. Commonly used UAV is underactuated due to its simplicity and high flight duration, but can not control the position and orientation independently. To overcome this problem, fully actuated multirotor aerial robots have been developed. In our previous work we developed fully actuated multilinked aerial robot which can transform in the air. However, the transformation range was limited because of a singularity problem. In this paper we propose a new design of link module with a tilted rotor and 1 DoF thrust vectoring joint which enables to avoid singularity forms and keep the flight stable during transformation. We describe modeling and control for the fully actuated multilinked multirotor. Then we propose a transformation planning method utilizing the 1 DoF thrust vectoring angle with consideration of guaranteed minimum force/torque. Finally we perform an aerial transformation experiment with a real platform to demonstrate the feasibility of our proposed design and methods.

Transformable aerial robots are favorable in aerial manipulation tasks for their flexible ability to change configuration during the flight. By assuming robot keeping in the mild motion, the previous researches sacrifice aerial agility to simplify the complex non-linear system into a single rigid body with a linear controller. In this paper, we present a framework towards agile swing motion for the transformable multi-links aerial robot. We introduce a computational-efficient non-linear model predictive controller and joints motion primitive frame-work to achieve agile transforming motions and validate with a novel robot named HYRURS-X. Finally, we implement our framework under a table tennis task to validate the online and agile performance.Supplementary MaterialThis paper is accompanied by a experiment video: http://www.jsk.t.u-tokyo.ac.jp/%7eshifan/paper/iros19/video.mp4.

In this paper, we present the achievement of aerial grasping by shape adaptive transformation to the object shape, using a novel transformable aerial robot called HALO: Horizontal Plane Transformable Aerial Robot with Closed-loop Multilinks Structure. Aerial manipulation is an active research area and using multiple aerial robots is an effective solution for the large size object. However the cooperation is considered that there are some difficulties such as the synchronized flight control and collision with each other. Then, we focus on the transformable aerial robot with two-dimensional multilinks proposed in our previous works, which can transform to the suitable form for the target object and grasp it. However the transformable aerial robot with the serial-link structure could not achieve stable flight in terms of horizontal position and yaw control due to the low rigidity and large inertia in the case of more than 4 links. Thus, first we construct a novel type of multilinks with closed-loop structure to avoid the deformation and a new link module with a tilted propeller for fully-actuated control. Second, we describe transformation method with closed-loop multilinks. Third, we present the optimization planning method for the multilinks form to be adaptive to the two-dimensional shape of the target object. Finally, we present experimental results to demonstrate the feasibility of closed-loop aerial transformation and aerial grasping for the large size object.

In this paper, we introduce the achievement of the flight motion to pass through small opening by the multilinked and transformable aerial robot. Previous works about such motion are based on under-actuated multirotors, indicating that aggressive maneuvering is necessary condition. This involves two crucial problems: i) enough free space for deceleration is necessary, otherwise the robot would collide with unknown obstacle after exiting opening; ii) the multirotor can not traverse the openings that are smaller than the robot body. The proposed transformable aerial robot in our work can solve these problems, since the multilinked model can not only guarantee the near-hover condition during the whole motion sequence, but also slowly traverse relative small openings by changing its form like a snake. We first propose an improved dynamics derivation and flight control method for this multilinked aerial robot based on our previous work. Then, we present the path planning method which takes the flight stability in the near-hover condition into account. Finally we demonstrate the experimental results of the motion to pass through a horizontal and small opening which also involves the borders (the floor and the ceiling).

In this paper, we introduce the achievement of the aerial manipulation by using the whole body of a transformable aerial robot, instead of attaching an additional manipulator. The aerial robot in our work is composed by two-dimensional multilinks which enable a stable aerial transformation and can be employed as an entire gripper. We propose a planning method to find the optimized grasping form for the multilinks while they are on the air, which is based on the original planar enveloping algorithm, along with the optimization of the internal force and joint torque for the force-closure. We then propose the aerial approach and grasp motion strategy, which is devoted to the determination of the form and position of the aerial robot to approach and grasp effectively the object from the air. Finally we present the experimental results of the aerial manipulation which involves grasping, carrying and dropping different types of object. These results validate the performance of aerial grasping based on our proposed whole-body grasp planning and motion control method.

In this paper, we show the achievement of a transformable aerial robot with internal communication system for multiple objects transportation. As it is not easy to make the flight endurance of an aerial robot longer, we study the problem to transport multiple objects at the same time to improve the efficiency of transportation. However, for conventional aerial robots, multiple objects transportation is difficult because the CoG position changes when the number of grasped objects changes, resulting in the instability of the flight. Therefore, to solve this problem, we focus on the multirotor with two-dimensional multilinks proposed in our previous work, which possesses the ability to modify the CoG position actively and can keep the flight stable. First, we introduce the hardware platform including the structure of link module and internal communication system to achieve the extensibility in terms of the link number. We then propose a method to find the optimal form for the multilinks based on the flight stability. Finally, we present experimental results which include aerial transformation and multiple objects transportation.

In recent years, applications of visual tracking algorithms has seen a substantial growth with deployments in intelligent robots such as drones for human tracking. The algorithms for such tasks has to be efficient in terms of computational cost while been robust, accurate and fast. Object tracking algorithms based on handcrafted heuristics and constraints are widely used in uav applications. The handcrafted heuristics are mostly implemented for task-oriented applications which limits the extensions in uav's capability beyond the predefined functions. This paper considers the challenges of tracking and landing an autonomous uav on a speed high moving target, and presents a visual tracking algorithm that integrates correlation filters with deep comparison network for real-time tracking with state-of-the-art accuracy. The method first tracks the target upto translation using an online learnt model via local search technique. The changes in scale is estimated by a deep comparison network (DCN) instead of the commonly used pyramidal approach. In a single network evaluation, DCN can estimate the changes in scale as well as compensate the drifting of the tracker by refining the object region estimated by the correlation filters. The network is end-to-end trained which attempts to learn a powerful matching function for object localization using a known template. Generally, the integrated framework can be viewed as coarse-to-fine level motion estimation. Moreover, the framework can redetect the lost target without a need for a separate detector.

To enable robotic sensing for tasks with requirements on weight, size, and cost, we develop an ultra-tiny line laser range sensor based on the Time-of-Flight (TOF) principle. With delicate circuit design and optical attachments, we create a sensor as small as 35[mm] × 27[mm] × 30[mm] and as light as 20[g]. The line sensor samples 272 pixels (256 effective pixels) uniformly distributed within the measurement field of view customizable using different laser lenses. The optimal measurement range of the sensor is 0.05[m] ~ 2[m]. Higher sampling rates can be achieved with a shorter range. The sensor can also extend its range to 3[m] with reduced accuracy. We model the overall errors of the sensor and formulate calibration methods, achieving repeatable accuracy and measurement bias both within 2[cm] with our tested ambient lighting conditions and measurement ranges. The sensor is applicable to range sensing tasks including humanoid hand-eye measurement, UAV safe landing, tiny robot range sensing, and object detection.

We have developed a robot with a new control mechanism in order to collect information on flying robots in multiple fields. We aimed for a function that could rotate the tilt angle continuously and without limit and a function for flying maintaining any desired tilt angle with a structure that could efficiently use the thrust generated by the propellers. We devised a mechanism that connected two bicopter modules, each of which combines two of the four propellers into one set and named this mechanism the “Bi2Copter”. This mechanism provided movements including landing, take-off, and flying with any desired tilt angle. This ability of this mechanism to fly walls with continuously changing surface angles and full 360° spherical coverage makes possible applications in investigation, measurement, etc. This report covers the design concepts of this flying robot, the structure design, basic control and operations verification.

This paper presents a multi-field universal vehicle that is able to work at land, sea and air. The vehicle consists of a quad-copter with variable-pitch propellers that enable the vehicle to stand on the ground at a given tilt angle, roll on the ground like a wheel, and float and move on the water, in addition to flying like a conventional quad-copter. This article clarifies the behavioral objectives, structural design, basic control mechanism of the ring-shaped robot, and examples of 3D measurements.