This article discusses a challenge of the hydrodynamic coefficient identification in autonomous underwater vehicles (AUVs). The considered coefficients correspond to a nonlinear mathematical model of an AUV maneuvering in the horizontal plane. The dynamics of underwater vehicles is intrinsically nonlinear. Hence, to reproduce different types of maneuvers, such as straight line, zigzag, or turning in circles, a nonlinear mathematical model is important. The proposed identification approach combines an analytical and semiempirical estimation method (ASE), based on the hydrodynamics and geometric characteristics of submarine vehicles, with a parameter estimator based on the extended Kalman filter. The identification starts with ASE estimations, and then the estimator based on the extended Kalman filter adjusts the hydrodynamic coefficient estimates based on the experimental data. The experimental data were obtained with the sensors of the Pirajuba AUV during sea trials. This identification method is a cyclical estimation process, where the less reliable parameters are adjusted to the experimental data while the most reliable parameters are kept fixed. Therefore, in the next identification iteration, the originally fixed model parameters are adjusted to the experimental data, keeping the already identified values. Finally, the identified model is used to simulate the vehicle's movement, and the movement variables are compared to the experimental data, thus validating the identified model.

The structure of an autonomous underwater vehicle (AUV), usually composed of a cylindrical shell, may be exposed to high hydrostatic pressures where buckling collapse occurs before yield stress failure. In conventional submarines, welded stiffeners increase the buckling resistance, however, in small AUVs, they reduce the inner space and cause residual stresses. This work presents an innovative concept for the structural design of an AUV, proposing the use of sliding stiffeners that are part of the structure used to accommodate the electronics inside it. Since the sliding stiffeners are not welded to the shell, there are no residual stresses due to welding, the AUV fabrication process is simplified, enabling a reduction of the manufacturing cost, and the inner space is available to accommodate the equipment needed for the AUV mission. Moreover, they provide a higher buckling resistance when compared to that of an unstiffened cylindrical shell. A comparative analysis of the critical buckling loads for different shell designs was carried out considering the following: (i) the unstiffened shell, (ii) the shell with ring stiffeners, and (iii) the shell with sliding stiffeners. Results evidenced that major advantages were obtained by using the latter alternative against buckling.

The use of autonomous underwater vehicles (AUV) has increased in a wide range of sectors, including the oil and gas industry, military, and marine research. The AUV capabilities to operate without a direct human operator and untethered to a support vessel are features that have aroused interest in the marine environment. The localization of AUV is significantly affected by the initial alignment and the calibration of the navigation sensors. In this sense, this paper proposes a thorough observability analysis applied to the latter problem. The observability analysis is carried out considering three types of sensor fusion integration and a set of maneuvers, and the results are validated through numerical simulations. As main contribution of this paper, it is shown how the addition of position errors in the observation vector can decouple some gyro and accelerometer biases from the latitude and altitude errors, particularly in the stationary observability analysis. The influence of oscillations in the diving plane and typical AUV maneuvers are analyzed, showing their relative impacts on the degree of observability of the inertial measurement unit (IMU)/Doppler velocity log (DVL) misalignment and DVL scale factor error. Finally, the state’s estimation accuracy is also analyzed, showing the limitation of the degree of observability as an assessment tool for the estimability of the states.