Enhanced Load and Accuracy for a New Cable-Driven Redundant Manipulator With Linkage Quaternion Joint via Variable Stiffness Optimization

Cable-driven redundant manipulators feature a slender structure, large workspace, and flexible motion, making them suitable for operation in confined spaces. However, their positioning accuracy and load capacity are limited, thus restricting their overall performance. This article focuses on a novel linkage quaternion joint and proposes a variable stiffness optimization method to improve positional accuracy, enhance load capacity, and ensure safer interaction through null-space motion. First, a simplified analytical stiffness model is developed to reduce computational complexity and improve efficiency. Next, a null-space variable stiffness optimization method is introduced, with multidirectional composite flexibility as the evaluation metric. Iterative vectors are designed to enable bidirectional continuous control of the end-effector stiffness. Finally, experiments are conducted to validate the performance. The results demonstrate that the proposed variable stiffness optimization method significantly improves positioning accuracy, load capacity, and compliance, thereby broadening the applicability of cable-driven redundant manipulators.