Robust Adaptive Control of Underwater Vehicles: a Comparative Study

Robust adaptive control of underwater vehicles in 6 DOF is analyzed in the context of measurement noise. The performance of the adaptive control laws of Sadegh and Horowitz (1990) and Slotine and Benedetto (1990) are compared. Both these schemes require that all states are measured, that is the velocities and positions in surge, sway, heave, roll, pitch and yaw. However, for underwater vehicles it is diicult to measure the linear velocities whereas angular velocity measurements can be obtained by using a 3-axes angular rate sensor. This problem is adressed by designing a nonlinear observer for linear velocity state estimation. The proposed observer requires that the position and the attitude are measured, e.g. by using a hydroacoustic positioning system for linear positions, two gyros for roll and pitch and a compass for yaw. In addition angular rate measurements will be assumed available from a 3-axes rate sensor or a state estimator. It is also assumed that the measurement rate is limited to 2 Hz for all the sensors. Simulation studies with a 3 DOF AUV model are used to demonstrate the convergence and robustness of the adaptive control laws and the velocity state observer. 1. INTRODUCTION The implementation of advanced adaptive control laws for underwater vehicles in 6 degrees of freedom (DOF) is mainly limited by the performance of the underwater navigation and sensor systems. The surge, sway and heave position can be measured with fairly accuracy by applying a hydroa-coustic short or long base-line system whereas attitude usually is measured by 2 gyros (or incli-nometers) and a compass. The bandwidth limitation of the control system is mainly due to the hy-droacoustic positioning system which usually operates in the interval of 0.1{1.0 Hz.