Spherical robot wrist control with compensation of compliance errors.

A parallel spherical manipulator can provide three degrees of freedom of pure rotation around the virtual center of its platform. Parallel manipulators, where all actuators are fixed to the base of the manipulator, provide better load-carrying capacity and better dynamic properties. This advantage makes it possible to use a parallel spherical manipulator as an additional tool for rotating the end-effector in robots with an incomplete set of degrees of freedom or compensate rotation/position error of the master robot. This solution can be used in precise machining operations and other problems where external forces have great influence or other sources of position errors are significant.Under the influence of external forces robot's end-effector could have an offset in the positioning. In this work, we propose to use a specially designed parallel robot that mounted on the master robot flange between the master robot and its tool. In order to compensate position errors following steps is performed: 1) Estimation of master robot stiffness model; 2) Estimation of spherical parallel robot stiffness model; 3) Estimation of external forces; 4) Estimation of compliance errors for both robots; 5) Trajectory modification for spherical parallel robot; 6) Trajectory execution.In this way, the overall accuracy of the system could be greatly improved, without any modification to the master robot program.