Gecko-inspired adhesives have the advantage of being able to grasp and release flat surfaces in a vacuum using their microwedge structures. This makes them an especially attractive solution for perching on and grasping flat objects in space for free-flying robots. To grasp and anchor onto these flat surfaces, the gripper must ensure contact between the gecko adhesives and the surface before applying the appropriate forces to activate their adhesion. However, in the case of a free-flying robot in microgravity, physical contact with the surface induces reaction forces, causing the robot to quickly bounce away from the surface. To solve this issue, we propose a simple passive mechanism and a control method of a robotic arm on a free-flying robot with a gecko adhesive gripper. The gripper utilizes a single-motor controlled tendon-driven mechanism mounted at the end of a robotic arm equipped with controllable stiffness joints and a linear spring-damper system. A free-flying robot on an air-bearing platform can successfully perch on a flat surface with a velocity of up to 72.5mm/s and with an approach angle misalignment of up to 33.0 degrees.

The International Space Station (ISS) crew has to complete various tasks in a limited time. The Int-Ball project is one of the activities aimed at the efficient utilization of crew time. Int-Ball2 is an autonomous mobile robot that conducts imaging operations in place of crew members by remote control from the ground. This paper first introduces the design of the guidance, navigation and control (GNC) and propulsion systems of Int-Ball2, offering autonomous flight inside the ISS. This robot has unique features compared to other free-flying robots in space, including its small size for use in crowded environments and its high thrust despite its size. In addition, the navigation system uses a visual SLAM algorithm, which does not rely on external markers. The results of orbital performance verification tests for 6-DOF translational and rotational motion are also presented in this paper. Our analysis indicated that the proposed GNC algorithm provided sufficient performance to conduct the required photography. Finally, the results of ground tests simulating the microgravity environment are compared with the results of orbital experiments to evaluate the differences in the robot’s mobility performance in these tests. The results confirmed that the ground verification method was valid for achieving stable on-orbit mobility.

In January 2024, a Japanese lunar lander successfully touched down on the moon as part of the Smart Lander for Investigating Moon (SLIM) mission. Accompanying the SLIM on its journey were two small rovers named lunar excursion vehicles (LEV-1 and 2). LEV-2, a transformable nano rover, was particularly designed to capture images of the SLIM on the lunar surface. During the mission, LEV-2 successfully maneuvered and captured images of the lander. Understanding the mobility characteristics of LEV-2 was crucial for the success of this task. This study analyzed the mobility characteristics of LEV-2 through numerical simulation and traveling tests. In the numerical simulation, a dynamics model of LEV-2 was first developed. Subsequently, the motion behaviors of LEV2 were analyzed by utilizing the discrete element method. Traveling tests were then conducted using the LEV-2 engineering model on various terrains covered with a lunar regolith simulant. The results of both the numerical simulations and experiments revealed consistent trends across different moving modes of LEV-2. Furthermore, the simulation results indicated no significant deterioration in the mobility performance of LEV-2 on the moon when compared to that on the Earth. Based on the characteristics predicted from the simulation and the quantitative data obtained from the experiments, it was concluded that LEV-2 is supposed to travel at least 15◦slopes on the moon.

Small free-flying robots to assist astronauts and perform experiments need a propulsion system to move freely in microgravity. Hardware-in-the-loop (HIL) simulators can simultaneously verify guidance, navigation, and control (GNC) systems, including flight hardware and software, in three dimensions. However, it is difficult to incorporate a small free-flying robot into the HIL simulator because of the low propulsive force and gravity compensation associated with its attitude changes. This paper proposes a HIL simulator with a propulsion subsystem mounted on a statically fixed force/torque sensor and a GNC subsystem mounted on a dynamically movable robotic arm. This simulator allows us to verify the GNC algorithms comprehensively using actual navigation sensors and propulsive actuators in an emulated flight environment. The actual capabilities of this simulator were successfully demonstrated in motion verifications of a free-flying robot, the Int-Ball2.

Gecko-inspired adhesives have several desirable characteristics in robotic grasping: controllable activation and deactivation of adhesion, ability to grasp and release with minimal disturbance, and grasping without the need of form closure. Previously proposed grippers with this technology either require a complex activation mechanism or multiple activation steps. In this paper, we present an underactuated gecko-inspired adhesive gripper that can grasp a wide range of curved surfaces using a single actuator through a simple tendon-driven mechanism that attaches and adheres in one step. We derive a theoretical model of the adhesive contact area and resulting gripper grasp force, which is verified experimentally. The actual performance of the proposed mechanism is demonstrated by successfully grasping several surfaces with different curvature diameters.

This paper presents a contact-event-triggered filter using only a force-torque sensor with impedance control for non-cooperative, rotating, heavy object capture. Contact events are modeled for prediction, and detected to trigger the particle filter's updating process. By combining these features, a computationally efficient, contact-event-triggered filter is proposed. For our purpose of capture using impedance control, expected contact events, collisions and sliding are defined for prediction and detection. This novel method is implemented in an air bearing robotic system, and has demonstrated its superiority with the highest success rate (100%) for sliding contact mode cases, whereas the previous method could only yield a success rate of 87.9%. The computation resource is demonstrated to be limited, with a computation time of 4.2 milliseconds on average and 8.3 milliseconds at worst.

This paper proposes real-time collision-based contact mode estimation with only a force-torque sensor for capturing a moving rigid body. The contact modes are defined for determining when to generate the signal to close the robotic hand for establishing object closure. In our particle filter approach, collision-triggered filter is used to determine the contact mode with the least amount of computation. Brach's collision model is used for our collision model-based approach for a rigid body because it is computationally light-weighted and enables the sampling of three collision properties for the particle filter. The validity of our method is experimentally demonstrated by achieving the highest success rate using the reasonable computation resources required (average of 3.9 milliseconds and worst of 6.1 milliseconds with our setup), and verifying each computation resource (or number of particles) based on the size of motion estimation error in the pre-capture phase.

This paper presents the path planning and coordinated control of a space robot with a manipulator for capturing a rotating large object. As the grasping point on a rotating large object is translationally moving fast, an appropriate strategy and coordinated motion control of the spacecraft base and robotic arm must be employed for approaching and tracking such a grasping point. In this paper, we propose a robust control scheme including the online path planning and compliance control for grasping such a target. The path planning is derived in a simple form that allows the desired end-effector trajectory to be easily modified in real-time using the newly updated states without complex numerical calculation. In addition, the compliance control allows the end-effector to track the planned trajectory or the moving grasping point, while using contact force feedback to reduce the end-effector position error from the grasping point when capturing the target. This end-effector motion is implemented by coordinated control on the spacecraft base and robotic arm, which can suitably alter their distribution of motion according to scenes using a weighted pseudoinverse matrix. Experiments are conducted to demonstrate the validity of the proposed path planning and compliance control.

This paper discusses robust capture of a free-floating target using a robotic arm. The position error resulting from sensor errors and time delay can cause undesired contact and unstable control. In this paper, we propose a caging-based rigid gripper and impedance control, which enables the robot to capture the target robustly without precise motion tracking and large force interaction. The performance of the proposed method is verified experimentally using an air-floating system that emulates planar microgravity motion.

This paper addresses a dynamic model and a control method of a space robot with a rigid manipulator and a flexible appendage. The control method has been developed for performing multiple tasks: end-point motion control and vibration suppression control of a flexible appendage. A simple dynamic model that considers coupling between the manipulator and the flexible appendage is proposed for the control method. The tasks are performed simultaneously on the basis of their order of priorities using a redundant manipulator. Additionally, because vibration suppression requires feedback of the state of the flexible appendage, a state estimator of the appendage using a force/torque sensor is developed. Finally, the proposed model, control method, and state estimator were verified experimentally using an air-floating system.

This paper discusses a vibration suppression control method for a space robot with a rigid manipulator and flexible appendage. A suitable dynamic model that considers the coupling between the manipulator and flexible appendage was developed for the controller to accomplish the vibration suppression control of the flexible appendage. The flexible appendage was modeled using a virtual joint model, and the control method was developed on the basis of this model. Although this type of control requires feedback of the flexible appendage state, its direct measurement is generally difficult. Thus, an estimator of the flexible appendage state was constructed using a force/torque sensor attached between the base and flexible appendage. The control method was experimentally verified using an air-floating system.