Design and Implementation of Time Efficient Trajectories for Autonomous Underwater Vehicles

This paper discusses control strategies adapted for practical implementation and efficient motion of autonomous underwater vehicles (AUV’s). For AUV’s we would like efficiency in both the measured time and the energy consumption, the mission dictating the weight to put on each of these cost. As a first approach to this problem, we focus in this paper on time minimization. Based on the structure of the time optimal trajectories and of the pure motions, we develop an algorithm to design time efficient trajectories corresponding to piecewise constant thrust arcs with few actuator switchings. We do that by solving a new optimization problem where the unknows are the time period between two actuator switchings as well as the values of the constant thrust arcs. We apply a direct method to compute numerically the solutions. With our algorithm, we gain considerable computational time. Moreover, with as least as three actuator switchings the duration of our trajectories are within 10% of the time optimal ones. Since our control strategies have a simple structure they can be implemented on a test-bed vehicle. For the experiments displayed in this paper we use a spherical underwater vehicle which moves with almost no preference of direction or orientation for movement, giving us a very controllable and versatile vehicle.