Effect of Torque-Velocity Relationship on Manipulability for Robot Manipulators

This paper presents novel manipulability analysis for robotic manipulators, taking the effect of generating joint torques on generable joint velocities and vice versa into consideration. The conventional manipulability is analysis in velocity domain and cannot concern force effect such as gravity of payload and external forces exerted on the endeffector. Gravitational force has been regarded that it just changes the origin of the manipulability ellipsoid expressing the set of generable tip velocities, and its evaluation (its volume) does not change. However, if robot grasps a heavy object, the robot cannot move with the same speed as the case of grasping a light object, because the power of the robot is limited. It indicates that the robot performance evaluation by conventional manipulability has serious problem that the force effect cannot be included. The power of the robot is determined by the operation range of every actuator, which tells us the relationship between generating torque/velocity and addable velocity/torque. Then, this paper presents novel manipulability analysis which can take the force effect into consideration, based on the torque-velocity relationship. This analysis shows that manipulability is influenced by payload, gravitational force, and external forces.