DYNAMIC BEHAVIOR OF 3-DOF PARALLEL MANIPULATORS WITH R-P-S JOINT STRUCTURE NEAR SINGULARITIES

摘要

This article illustrates the dynamics behavior and the functional problems of a 3-DOF parallel manipulator with a Revolute-Prismatic-Spherical (R-P-S) joint structure in the neighborhood of singular configurations. The robot is used as a positioning device, i.e. coordinates of the centre of the moving platform define the robot motion. Three different cases of robot motion along the given path with predefined velocity profile are considered: (ⅰ) the manipulator approaches the singular configuration, but does not cross it; (ⅱ) the manipulator crosses the singularity; (ⅲ) the manipulator avoids the singularities. This variation is achieved by changing a robot design parameter. The three cases are observed with the usage of a robot dynamics simulation model developed based on the principle of virtual work, and it was revealed that in the first case the forces have peaks. In the second one the forces are increasing very fast and go to infinity, which causes functional problems in the robot system, i.e. makes the motion uncontrollable. The third case is the most favorable for the robot operation. It is shown that in the case when the manipulator is used as a positioning device the singular configurations can be avoided without changing of position of the end-effector by selecting a different solution of the inverse kinematics problem for a given position of the end-effector (x,y,z). The results have practical importance for the design of the robot and its motion control.