Capture and removal of space debris is challenging in robotic on-orbit servicing (OOS) activities. A large portion of space debris does not possess any graspable features, which makes conventional grippers inapplicable. To handle such non-graspable objects, a space robotic capture system is presented. A dual-arm space robot simulator that has the advantages of miniaturization and scalability is designed for ground tests. Inspired by robotic caging, we propose a novel capture method that uses a series of hollow-shaped end-effector pairs to cage the antipodal pairs of non-graspable objects. To apply the caging-pair method steadily, space robots need exerting a squeezing action on objects, which can be characterized by the motion and force manipulation of two robotic arms in the assigned directions. Based on the velocity and force manipulability transmission ratios, a caging compatibility index is proposed to describe the capturing ability in this manner. Via the optimization of the desired caging compatibility index, an effective algorithm is proposed to plan near-optimal joint configurations for pre-grasping cages. Finally, both simulation studies and experimental tests are conducted to evaluate the performance of the proposed capture method.

Continuous-bodied “trunk and tentacle” robots have increased self-adaptability and obstacle avoidance capabilities, compared with traditional, discrete-jointed, robots with large rigid links. In particular, continuous-bodied robots have obvious advantages in grasping objects across a wide range of external dimensions. Not only can they grasp objects using end effectors like traditional robots, but their bodies can also be regarded as a gripping device, and large objects with respect to the robot’s scale can be captured by the entire structure of the robots themselves. Existing trunk-like robots have distributed multidrive actuation and are often manufactured using soft materials, which leads to a complex actuator system that also limits their potential applications in dangerous and extreme environments. This paper introduces a new type of elephant’s trunk robot with very few driving constraints. The robot consists of a series of novel underactuated linkage units. With a single-motor drive, the robot can achieve stable grasping of objects of different shapes and sizes. The proposed robot simplifies the requirements of the sensing and control systems during the operation process and has the advantage of accomplishing the capture task without determining the exact shape and position of the target object. It is especially suitable for operations such as non-cooperative target capture in extremely dangerous environments, including those in outer space. Based on theoretical analysis and model design, a trunk robot prototype was developed, and a comprehensive experimental study of the bending/extension and grasping operation functions was conducted to verify the validity of the proposed robot design.

Dynamic coupling is an inherent behavior of the space robot system, which seriously affects the motion accuracy of the system. This article focuses on how to guarantee the base attitude stability of semifloating space robots, from a control perspective—namely, the base attitude decoupling control. The quaternion representation of the system dynamics is derived to avoid the representation singularity of the Euler angles. In consideration of the complicated system dynamics and the limited computing ability of the on-board computer, an efficient decoupling controller is designed based on the time-delay estimation (TDE) and the supertwisting control (STC). Herein, for the proposed TDE-based STC (TDE-STC) scheme, TDE is used to decouple and linearize the nonlinear dynamics, and STC is a second-order sliding mode control algorithm which can compensate for the TDE error and drive the state variables to converge to the equilibrium point robustly in finite time. The model-free decoupling principle of TDE is clearly illustrated by comparison with the classical computed torque control. The global asymptotical stability analysis of the closed-loop system is proven using the Lyapunov theory and the linear matrix inequality theorem. Finally, several comparative simulation studies on a 3-D space robot system with consideration of joint friction, disturbances, and model uncertainties are conducted to verify the effectiveness of the proposed TDE-STC scheme. The corresponding results show that the TDE-STC scheme can achieve a high-accuracy attitude decoupling performance with both the model-free merits and the chatter-free merits.

Variable Stiffness Actuator (VSA) is the core mechanism to achieve physical human–robot interaction, which is an inevitable development trend in robotic. The existing variable stiffness actuators are basically single degree-of-freedom (DOF) rotating joints, which are achieving multi-DOF motion by cascades and resulting in complex robot body structures. In this paper, an integrated 2-DOF actuator with variable stiffness is proposed, which could be used for bionic wrist joints or shoulder joints. The 2-DOF motion is coupling in one universal joint, which is different from the way of single DOF actuators cascade. Based on the 2-DOF orthogonal motion generated by the spherical wrist parallel mechanism, the stiffness could be adjusted by varying the effective length of the springs, which is uniformly distributed in the variable stiffness unit. The variable stiffness principle, the model design, and theoretical analysis of the VSA are discussed in this work. The independence of adjusting the equilibrium position and stiffness of the actuator is validated by experiments. The results show that the measured actuator characteristics are sufficiently matched the theoretical values. In the future, VSA could be used in biped robot or robotic arm, ensuring the safety of human–robot interaction.

Passive variable stiffness joints have unique advantages over active variable stiffness joints and are currently eliciting increased attention. Existing passive variable stiffness joints rely mainly on sensors and special control algorithms, resulting in a bandwidth-limited response speed of the joint. We propose a new passive power-source-free stiffness-self-adjustable mechanism that can be used as the elbow joint of a robot arm. The new mechanism does not require special stiffness regulating motors or sensors and can realize large-range self-adaptive adjustment of stiffness in a purely mechanical manner. The variable stiffness mechanism can automatically adjust joint stiffness in accordance with the magnitude of the payload, and this adjustment is a successful imitation of the stiffness adjustment characteristics of the human elbow. The response speed is high because sensors and control algorithms are not needed. The variable stiffness principle is explained, and the design of the variable stiffness mechanism is analyzed. A prototype is fabricated, and the associated hardware is set up to validate the analytical stiffness model and design experimentally.

Pneumatic soft robotics is known for its high adaptability and strong load capacity, and the pneumatic control system is regarded as the neural system of pneumatic soft robots to transmit signals and control pressure. The control accuracy and responsiveness of the system are critical to the performance of soft robots. In this paper, we proposed a low-cost pneumatic system configuration with a pressure control strategy, which was capable of programmable pressure states with just 2-way 2-state solenoid valves. A pneumatic system prototype with 12 independent air branches was developed. Experimental results showed that the pressure control strategy was able to achieve the control accuracy within 1 kPa, and adjust the speed of inflation and deflation. Further, the three robot applications verified its capability to meet diverse pressure demands of soft robots. This innovative system makes contributions to actuation and control within the field of soft robotics as a versatile pneumatic platform.