Abstract
A novel two degree-of-freedom (DOF) ball-joint-like hydraulic spherical motion mechanism (SMM) for use in robotic applications is proposed to achieve smooth spherical motion in all directions. Unlike traditional systems that use serial or parallel mechanisms for generating multi-DOF rotations, the proposed SMM is capable of producing continuous 2-DOF rotational motions in a single joint without intermediate transmission mechanisms. The proposed SMM has a compact structure, low inertia, and high stiffness. First, the architecture and operating principle of the proposed SMM is introduced. Then, the kinematic model is established using Euler transformation, following which factors (such as workspace and dexterity) that have an impact on motion performance are evaluated. As the foundation of dynamics analysis and controller design, the Lagrange's equations of the second kind are used to establish the dynamic model. To achieve high tracking accuracy, the radial basis function neural network-based sliding mode controller is applied to the mechanism. The simulation results indicate that the designed controller not only improves trajectory tracking capability but also enhances robustness against external disturbance and system uncertainty. Finally, experiments are performed on a prototype SMM to validate the performance of the proposed SMM and evaluate the control method.