As an important marine detection platform, two-part towed system is easier to be controlled and could reduce the disturbance from the towing ship. The constant depth and stable attitude of the secondary towed vehicle is a basic precondition of the accuracy of detection data at actual work condition. The control of depth and attitude of the secondary towed vehicle with automatic regulation mechanism is studied in this paper. Firstly, a spring-damping model of the secondary towed cable is established, and the mathematical model of the whole two-part towed system is established based on the previous model. Considering the strong nonlinearity and time-varying characteristics of the system, a self-adaptive fuzzy PID controller for the secondary towed vehicle is designed in order to control the depth and attitude state under different working conditions. Finally a series of simulations is performed. From simulation results, it could be seen that change of sea states has significant influence on depth and attitude state of the secondary towed vehicle. When the vehicle is working without controller, the fluctuation range of depth and pitch is very large. After the self-adaptive fuzzy PID controller is applied, the range is much smaller because of the self-regulation, and could satisfy actual requirement. The validity of mathematical model of towed cable and the feasibility of the adopted controller are verified
The deep-sea towed system has a wide range of applications in marine environment detection, seabed mapping and optical or electric cable detection. Generally the deep-sea towed system is divided into one-part towed system [Yamaguchi 2000, Yokobiki 2000, Buckham 2003] and two-part towed system. Since the two-part towed system could effectively reduce the interference of the mother ship motion on the towed body and has a certain heaving compensation function, more and more scholars focus on the research of the two-part towed system. Wu [Wu 2000, 2001, 2001] established a mathematical model of the two-part towed system by using the finite difference method. Based on this model, simulation and experimental verification of the motion state of the secondary towed body were carried out. Lalu [Lalu 2007, 2013] established a mathematical model of towed cable using lumped mass method, and combined with CFD method, the influence of hydrodynamic coefficients on the motion state of secondary towed body is analyzed. However, none of the above scholars have studied the active control of the secondary towed body. Schuch [Schuch 2004, 2005] and Linklater [Linklater 2005] established the "spring-damping" model of the towed system and studied the active control of the secondary towed, but depth keeping did not draw their special attention. However when the secondary towed vehicle is equipped with high-precision detection equipment such as sonar, magnetic detector, altimeter and so on, depth keeping and attitude stabilization have an important impact on the accuracy of the detection, or even directly related to the success or failure of the detection task. Research on the depth keeping and attitude stability of the secondary towed body has important scientific research value and engineering significance.
