Research On Trajectory Tracking Control And Compliance Control Based On Spherical Wrist

With the development of modern industry, robots have replaced human to complete the tasks gradually in the production and daily life. Some environments such as aerospace, deep sea and the nuclear industry with dangerous and narrow working space are not suitable for human operating personally, so it requires high-performance robots to do the job instead of human. The characteristics of the robot wrist are compact, light weight and big power-weight ratio, and the wrist is an important component in the hard environment. The robot wrist changes the pose of end actuator by minor adjusting locally, and the wrist will directly affect the position accuracy and flexibility of the robot.This paper presents a highly integration and decouple spherical wrist. Using offset installation in upper and lower hemispheres and using internal and external universal joints in transmission chain, the wrist could achieve three degree of freedoms that is pitch, yaw and rotation. Build kinematics mathematical model by analyzing the motion of spherical wrist and explain the decouple mechanism of the wrist; Jacobian matrix has been also obtained, and analyze the singular position in the working space by Jacobian matrix. Establish the dynamic mathematical model for further motion control.For realizing the wrist motion control, it has been designed two trajectory plan mode: circular interpolation mode and linear interpolation mode. For the problem of wrist’s trajectory tracking, it has been designed a kind of global fast Terminal synovial controller, and the stability of the controller has been proofed by Lyapunov function. The controller makes use of RBF neural network algorithm to assess the uncertain border of the control system, so it effectively shortens the convergence time and eliminates the problem of "tremors" existing in the control system. It has been verified the effectiveness of the algorithm by the simulation analysis in MATLAB.For the problem of wrist’s compliance control, it has been designed a compliant controller which is based on friction compensation. Genetic algorithm has been written to identify friction model parameters and establish wrist’s friction model as feedforward compensation term. Through coordinate transformation and Jacobian force transformation, the six-axis force which is measure from the wrist actuator converts to the motor output torque ultimately. It will realize the purpose that the force between outside and the actuator is zero, i.e., achieve "zero power" trace. It has been verified the effectiveness of the algorithm by the simulation analysis in MATLAB.The spherical wrist proposed in this paper makes a smooth movement and has a good trajectory tracking performance. The compliance control mode also greatly enriched the application of the wrist. This spherical wrist will have a great value in the field of aerospace, rehabilitation robot and so on.