Smart Changes of the End-Effector Orientation for the Automatic Handling of Singular Configurations

Trajectories in the operational space could easily cause feasibility issues when they incur in kinematic singularities. In proximity of singular configurations, as known, joint velocities and accelerations could indeed assume unfeasible values, thus worsening the tracking performances of controllers: in case of strong violations of the assigned bounds, manipulators could even be stopped in order to prevent control losses. Problems are especially evident when trajectories are online defined for nonrepetitive tasks. This paper proposes an algorithm for the real-time handling of wrist singularities for nonredundant manipulators. The proposed method makes it possible to execute otherwise unfeasible trajectories by simultaneously preserving an accurate path tracking, by guaranteeing the fulfillment of the assigned longitudinal time law and by fulfilling a given set of bounds on the joint velocities and accelerations. This result is achieved by acting on the orientation of the end-effector: only minor changes are admitted in order to minimize their impact on the process quality. The proposed method has been verified by means of simulations and real experiments. Comparisons have been made with the performances of an analogous commercial system.