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.

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.

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.

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.