The cable-driven redundant manipulator (CDRM) has a light slender and highly dexterous body, which is especially suitable for fine manipulation in confined spaces or unstructured environments. However, for dexterous manipulation and physical interaction with its surrounding environment, the force/torque sensor is indispensable, which will significantly increase the mass, dimension, and cost of the whole CDRM system. In this article, a force-sensing algorithm and compliance control framework for CDRM without a six-axis force/torque sensor are proposed. First, we design a modular cable-driven manipulator with two-level internal sensors, i.e., joint encoders and cable tension sensors. The multispace kinetic model is derived to establish the mapping between motor, cable, joint, and end-effector states. At the same time, we build a recursive dynamics model that takes the cables’ tension, cable–hole friction, links’ gravity, and end-effector forces into account. Then, the indirect force-sensing algorithm of the end-effector is proposed by combining the kinematic and dynamic equations and internal sensor information. Furthermore, a compliance controller based on indirect force sensing is designed. Finally, typical experiments are carried out based on the CDRM. Experiment results indicate that the force-sensing accuracy exceeds 95%, whereas the compliance controller demonstrates outstanding compliant behavior in human–robot interaction tasks.

Cable-driven serial manipulators have significant potential for applications in confined spaces. However, due to the unidirectional driving force and hysteresis characteristics of cables, nonsynchronous motion between motors and tracking errors on each axis can cause cables to become too loose or too tight, leading to cable slack or breakage. To address this issue, we propose a hybrid tension-position control (HTPC) method to achieve high position accuracy while maintaining cable tension within a safe range. First, we establish multiple-space kinematic and static models to reflect the motion and force mapping between motors, cables, joints, and the end-effector. Second, cable terminal tensions are predicted by combining artificial neural networks with friction models and measuring cable tension at the motor side. Based on risk assessment, the desired tensions are determined according to force balance constraints to ensure safety. Third, we present a two-level closed-loop control method composed of resolved motion position control and hybrid tension-position control. Each driving group consists of three driving cables and three motors. The cable with the highest risk is controlled in hybrid tension-position mode to maintain its tension, while the other two cables are controlled in position mode to achieve the desired joint positions. Finally, we conducted several hardware experiments to validate the proposed method. The results show that the HTPC can reduce motion errors by more than half compared to traditional methods while simultaneously avoiding cable slack and breakage.

The cable-driven redundant manipulator (CDRM) possesses remarkable flexibility and holds substantial potential for application in constrained environments. To ensure both the smooth movement of the end-effector during delicate operations and the safety of interactions with the surrounding environment, real-time sensing of forces acting on both the end and links is imperative. Current in situ sensor-based methods face limitations in their applicability to CDRMs due to size and load capacity constraints. Moreover, these methods fall short in measuring contact force and its location along the entire arm. In this article, we introduce an ex situ sensing approach for capturing the six-dimensional (6-D) force at the end and the contact force on the linkages of a CDRM. First, a multispace recursive dynamic model of the CDRM is established using the Newton–Euler method. This model establishes mapping relationships among cable tensions, joint torques, and operational forces at the end-effector. Then, a simplified dynamic model for the recursive subsystem is derived based on joint motion transmission relationships and recursive equations. This model decouples the dynamic equations and provides a versatile force-sensing model. It enables the realization of 6-D force/torque sensing at the end-effector, as well as the determination of the magnitude and location of external forces acting on the links. Finally, compliant controllers are designed based on different external force-sensing methods to cater to diverse operational requirements. Experimental validation of the proposed methods is conducted on a CDRM prototype. The results demonstrate that the accuracy of end-effector force sensing exceeds 95%, torque sensing surpasses 90%, and the positioning error of the link's contact force sensing is less than 20 mm. Furthermore, the compliance controllers exhibit excellent smoothness in tasks involving human–robot interaction.

Cable-driven segmented manipulators (CDSM) have high dexterity and large bending characteristics with less motors, which have great potential for long-range manipulation in structured environment. However, as joints' number and segments' length increase, the synchronous performance will largely decrease due to friction and deformation. This letter presents a composite linkage mechanism for CDSM to improve the synchronous accuracy and increase the transmission distance. The design concept is first introduced based on the analysis of the traditional design. Then, composite linkage mechanism is proposed to overcome the shortcomings of single linkage mechanism. It is composed of short linkage mechanisms and long linkage mechanisms, where the former and the latter are respectively used to constrain adjacent and non-adjacent joints. Subsequently, four types of linkage mechanisms are designed in details. Furthermore, the construction rules of linkage mechanisms are also established, and a typical design example is provided. Finally, a prototype is developed and typical experiments are conducted. The experiments results show that compared with traditional design, the developed composite linkage mechanism can significantly improve the synchronous accuracy and increase the effective transmission distance

The discrete-serpentine heterogeneous multi-arm space robot (DSHMASR) has more advantages than single discrete space robots or single serpentine space robots in complex tasks of on-orbit servicing. However, the mechanical structure complexity of the DSHMASR poses challenges for modeling and motion planning. In this paper, a coupled kinematic model and a coordinated trajectory planning method for the DSHMASR were proposed to address these issues. Firstly, an uncontrolled satellite and the DSHMASR were modeled based on the momentum conservation law. The generalized Jacobian matrix J_g of the space robotic system was derived. Secondly, the manipulation capability of the DSHMASR was analyzed based on the null-space of J_g. Furthermore, the cooperative capturing-monitoring trajectory planning method for DSHMASR was presented through the manipulability optimization. The expected trajectory of each arm’s tip can be obtained by pose deviations and velocity deviations between the tip and the target point. Additionally, the optimized joint velocities of each arm were calculated by combining differential kinematics and manipulability optimization. Therefore, the manipulability of DSHMASR in the direction of the capture operation was enhanced simultaneously as it approached the target satellite. Finally, the proposed algorithm was demonstrated by establishing the Adams–Simulink co-simulation model. Comparisons with traditional approaches further confirm the outperformance of the proposed method in terms of manipulation capability.

Compared with a single discrete or serpentine arm, the discrete-serpentine heterogeneous dual-arm space robot (DSHDASR) combines the advantages of both kinds of arms, enabling it to perform complex on-orbit missions. The structural complexity of DSHDASR and cluttered environments pose a significant challenge in modeling and collision-free motion planning. To tackle the issue, this paper proposes a coordinated collision-free trajectory planning method for DSHDASR based on equivalent kinematics. Firstly, the mechanism and universal kinematics of DSHDASR are analyzed. Then, the equivalent kinematics model is established based on the spatial arc method. The whole kinematics chain of DSHDASR is described by the parameters of equivalent curves composed of space arcs. Furthermore, taking the target satellite transposition as an example, a coordinated collision-free trajectory planning is presented for DSHDASR based on the equivalent kinematics. The trajectory planning problem is formulated as the minimization of the objective function, which consists of kinematics constraint equations and obstacle avoidance constraint equations. The parameters of equivalent curves are obtained by optimizing the objective function, and the joint angles of DSHDASR can be determined using the above parameters. Finally, the mission of the target satellite transposition is simulated, and the results demonstrate the proposed method.