Research On Singularity Analysis And Double Working Mode Control Strategy Of Spherical Wrist

The interaction between robot and environment is not taken into account when a robot is working in structured environment such as factories. With the opening of robot working environment, contact between robot and operation object leads to the situation of interference and collision. The study of adaptability and security of interaction between robot and environment has become a trend. As an important component of robot, the wrist adjusts attitude of robot end-effector by tiny movement, its performance is directly related to the accuracy and flexibility of the robot. Aiming at complex structure, low integration, motion coupling, and poor capacity of compliance and resisting disturbance, a three degree of freedom spherical wrist with high integration and motion decoupling as well as its double working mode control strategy are proposed.In the mechanical structure, the decoupling structure of double hemispheres and double universal joints is employed to guarantee compactness and flexibility of the wrist. To analyze the transitive relation and motion decoupling principle of the wrist, the forward and inverse kinematics and jacobian matrix are derived. The motion transitive relation between the working space and the joint space is established. Dynamic equations and equivalent static conversion relationship are derived to lay a basis on the control system design of the wrist.The global manipulability based on Jacobian matrix is used to evaluate the wrist capability of global transformation for force and motion. Structural parameter of the wrist is optimized, and the optimal inclined angle is obtained. Singularity of the wrist is analyzed by manipulability. According to the characteristics of different work modes, processing methods of singularity free trajectory planning in active mode and adaptive singular parameters in passive mode are proposed to avoid interior singular point effectively.Double mode control includes active mode of trajectory tracking control based on nonlinear Terminal sliding-mode surface and passive mode of compliance control based on friction feedforward compensation. In active mode, trajectory tracking control is fast and accurate, control system is insensitive to uncertain disturbances, higher attitude accuracy is realized. In passive mode, the wrist can move freely without generating any internal force by feedforward compensation based on friction model. The interaction force between the end effector and the environment is measured by six axis force / torque sensor to form a closed loop control. Universal compliance control of the wrist is realized.Finally, the Simulink environment simulation platform and the spherical wrist motion trajectory control and position detection platform are built. The simulation and experiment results prove the validity of spherical wrist double mode control strategy. Spherical wrist modular design method and control theory including light and dexterous structure, trajectory tracking control with precise positioning and compliance control based on friction compensation are established, then adaptability and security of the robot working in unstructured environment are improved.