Haptic teleoperation of mobile manipulator systems using virtual fixtures.

Abstract

In order to make the task of controlling Mobile-Manipulator Systems (MMS) simpler, a novel command strategy that uses a single joystick is presented to replace the existing paradigm of using multiple joysticks. To improve efficiency and accuracy, virtual fixtures were implemented with the use of a haptic joystick. Instead of modeling the MMS as a single unit with three redundant degrees-of-freedom (DOF), the operator controls either the manipulator or the mobile base, with the command strategy choosing which one to move. The novel command strategy uses three modes of operation to automatically switch control between the manipulator and base. The three modes of operation are called near-target manipulation mode, off-target manipulation mode, and transportation mode. The system enters near-target manipulation mode only when close to a target of interest, and allows the operator to control the manipulator using velocity control. When the operator attempts to move the manipulator out of its workspace limits, the system temporarily enters transportation mode. When the operator moves the manipulator in a direction towards the manipulator’s workspace the system returns to near-target manipulation mode. In off-target manipulation mode, when the operator moves the manipulator to its workspace limits, the system retracts the arm near to the centre of its workspace to enter and remain in transportation mode. While in transportation mode the operator controls the base using velocity control. Two types of virtual fixtures are used, repulsive virtual fixtures and forbidden region virtual fixtures. Repulsive virtual fixtures are present in the form of six virtual walls forming a cube at the manipulator’s workspace limits. When the operator approaches a virtual wall, a repulsive force is felt pushing the operator’s hand away from the workspace limits. The forbidden region virtual fixtures prevent the operator from driving into obstacles by disregarding motion commands that would result in a collision. The command strategy was implemented on the Omnibot MMS and test results show that it was successful in improving simplicity, accuracy, and efficiency when teleoperating a MMS.