A sliding mode control approach to reduce the influence of manipulator disturbances on an underwater vehicle's position

Abstract

Manipulator movement produces force/moment disturbances which act on the underwater vehicle and alter its position, and consequently the manipulator end-effector position. The effect of the manipulator motion, assuming perfect manipulator joint angle tracking, on the vehicle's position and consequently the manipulator end-effector position is investigated assuming no vehicle control. Control of the vehicle's yaw angle, in this particular manipulator-vehicle configuration, has been determined to be the single most important factor in reducing the endeffector error variation. Due to the nonlinear dynamics of the vehicle, Slotine's sliding mode approach has been used. This technique allows the expressions developed for the manipulator disturbances to be incorporated in the control law. This is shown to be beneficial in the regulation of the vehicle's yaw angle, and offers improved performance compared with a sliding mode controller that does not incorporate the manipulator disturbances. This technique also demonstrates superior performance and insensitivity to parameter variations compared with a fixed-gain controller.1. I~tr~~~c~~~rĨ t is recognised that there will be increasing activity in the exploration and exploitation of the ocean's resources in the coming decades. Remotely Operated Vehicles (ROVS) equipped with robotic manipulators have an important role to play in a number of shallow and deep water missions for marine science, oil and gas, survey, exploration, exploitation and military applications (Busby and Vadus, 1990). Current underwater robotic systems typically comprise of one or more manipulators mounted on the front of an underwater vehicle, a tethered ROV, equipped with an underwater camera system. In addition an attachment system, a second manipulator or a dedicated hydraulically powered arm mechanism with suction feet, is required to hold the vehicle reasonably static relative to the wc~rkpi~ce due to manipulator disturbances and sea-currents. The use of the second manipulator to prevent vehicle motion is wasteful as coordinated tasks involving both manipulators cannot be performed.The manipulators are operated in a master-slave configuration by an operator on the surface vessel. The movement of the smaller master arm is replicated by the larger slave arm and they forrn an approximately spatially correspondent system. The operator has a number of handicaps that contribute to the difficulty of performing the task at hand (Lane et al, 1991). This is illustrated in Fig were the various delays, All to At5 (human not computational), in the teleoperated masterslave configuration contribute to the reduction of task effectiveness.The operator must use the vehicle thrusters to compensate for sea current disturbances and vehicle motions induced by manipulator movements, and these are the cause of delays Ati and At2. These delays limit the accuracy that can be achieved by the manipulator's end-effector when following a prescribed trajectory. A vehicle control system with the ability to reduce the manipulator...