AUV play an important role in the exploration and utilization of the ocean. Underactuated autonomous underwater vehicles (AUVs) are widely used in these missions due to the associated low manufacturing costs, low power consumption, and high reliability. Among many control problems of underactuated AUVs, the research of trajectory tracking and formation control methods has become a hot topic in recent years. In order to obtain superior control performance, research on trajectory tracking and formation control of underactuated AUVs based on intelligent control methods should be further advanced and deepened. In this context, we present a survey of intelligent trajectory tracking and formation control of underactuated AUVs, which is beneficial to researchers in this field. To facilitate a comprehensive understanding of the subject matter, we review some preliminary knowledge in trajectory tracking and formation control of underactuated AUVs. We also highlight research problems and challenges, including external disturbances, system characteristics, and system faults. We review recent advances in intelligent trajectory tracking and formation control of underactuated AUVs and analyze and discuss their characteristics. Moreover, we put forward some prospects of advanced control techniques, especially artificial intelligence-based techniques for underactuated AUVs.

This article proposes a fuzzy optimal fault-tolerant control scheme with prescribed performance for three-dimensional trajectory tracking control of underactuated autonomous underwater vehicles (AUVs) in complex ocean environments, especially in the presence of unknown actuator faults and dynamic uncertainties. The scheme has the following features. 1) The underactuation problem is overcome by defining new system outputs. 2) An error transformation method that removes restrictions on initial error conditions is adopted to achieve global prescribed performance control. 3) A fuzzy adaptive disturbance observer based on a generalized fuzzy hyperbolic model (GFHM) is proposed to estimate compound disturbances consisting of actuator faults, dynamic uncertainty parts and disturbances. 4) Based on adaptive dynamic programming techniques, a single critic structure fuzzy optimal controller is developed using a GFHM to obtain an approximate optimal control law. In particular, actuator faults, dynamic uncertainties, and disturbances are uniformly handled by structuring an improved cost function and solving optimal control of the nominal system. Through analysis, the proposed scheme can ensure that all signals of the AUV closed-loop system are uniformly ultimately bounded and the original tracking errors can remain within prescribed boundaries. Pure software and hardware-in-the-loop simulations demonstrate the effectiveness and advantages of the proposed scheme.