An investigation of a passively controlled haptic interface

Abstract

Haptic interfaces enhance cooperation between humans and robotic manipulators by providing force and tactile feedback to the human user during the execution of arbitrary tasks. The use of active actuators in haptic displays presents a certain amount of risk since they· are capable of providing unacceptable levels of energy to the systems upon which they operate. An alternative to providing numerous safeguards is to remove the sources of risk altogether. This research investigates the feasibility of trajectory control using passive devices, that is, devices that cannot add energy to the system. Passive actuators are capable only of removing energy from the system or transferring energy within the system. It is proposed that the utility of passive devices is greatly enhanced by the use of redundant actuators. In a passive system, once motion is provided to the system, presumably by a human user, passive devices may be able to modify this motion to achieve a desired resultant trajectory. A mechanically passive, 2-Degree-of Freedom (D.O.F.) manipulator has been designed and built. It is equipped with four passive actuators: two electromagnetic brakes and two electromagnetic clutches. This paper gives a review of the literature on passive robotics and describes the experimental test bed used in this research. Several control algorithms are investigated, resulting in the formulation of a passive control law. Several issues specific to controlling passive and/or redundantly actuated mechanisms are presented, and an algorithm for transforming general (active) controller commands into an appropriate set of passive actuator commands is presented. This algorithm capitalizes on the overlapping capabilities of the actuators and the enhanced control capabilities they provide for a passive device.