Criteria based evaluation of stopping trajectories in serial manipulators

Abstract

In the past few years, there has been a large push towards adapting traditional industrial manipulators to other, more consumer-centric applications [1]. These include not only house and elderly care, but also towards medical applications that manipulators may be especially suited for, such as rehabilitation of patients who have suffered neurological trauma [2]. Impeding this push are the strict safety requirements necessary to certify a manipulator for use. These requirements include low speed operation and preventing humans from entering the manipulator workspace [3]. These restrictions effectively prevent a manipulator from being used in many of these applications. Previous work done in manipulator safety research has focused on improving the system’s knowledge of its environment and controlling the manipulator’s motion to keep away from potential hazards. These methods are extremely important in terms of avoiding potential collisions but provide little insight into the situation that occurs once a hazard occurs and the manipulator is forced to react. In order to improve upon the ability to evaluate a manipulator’s overall safety, this report establishes a framework to evaluate the capacity of a manipulator to safely “halt” itself. Two sets of criteria are presented in this report. The first set seeks to quantify both the potential of the manipulator to avoid a collision during the stopping motion and the potential severity of the collision. The second set of criteria quantifies the effect of the stopping motion at the actuator level, allowing the operator to identify potential hardware faults and the capacity to which the actuators are performing. A framework for mapping the manipulator’s actuator parameters for the gear reduction ratio and the motor torque to the potential safety criteria performance is formulated to allow the manipulator designer to match task requirements to the manipulator design. Finally, an examination of the effects on operating parameters such as manipulator configuration, end-effector load, and operating speed is presented with a 6DOF industrial manipulator. This analysis showed that the operating speed of the manipulator is the most important determinant of the safety performance, with the distance traveled by the manipulator increasing by a factor of 15 for all configurations when the speed is increased only by a factor of four. Recommendations for the application of these criteria are presented to the reader as well.