Research On The Underwater Towed Heave Compensation Hydraulic System And Control Strategies

An underwater towed system generally consists of towed body, cable, retraction and towing device. It is used extensively for ocean exploration. Examples include oceanography, ocean resource development, ocean salvage, underwater target detection and so on. When it is working in the ocean, heave motion of the towed ship on the rough sea leads to change of the towed body depth and the cable tension. Obviously, the situation will become more severe with higher sea state. Violent motion of the towed ship may not only disturb the ability of sensors installed in the towed body to perform a task, but also present a great threat to the safety of the towed body and the cable. In order to ensure that the underwater towed system is able to work in the bad sea condition, it must have heave compensation function. At present there is almost no study on this key technology of the underwater towed system in China, therefore the research on towed heave compensation is very significant.Under the background of the above-mentioned, a heavy underwater towed system is selected as the object of study and compensating the influence of heave motion of the towed ship to the towed body depth is the research objective. Through theoretical analysis, modeling, simulation and experiments, the fundamental feature of the controlled object of the towed heave compensation system is understood and the hydraulic system and control strategies which are suitable for the towed heave compensation system are studied. All these works provide some design basis for developing the underwater towed system possessing heave compensation function. They have important academic value and a bright future for engineering application.The main contents of each chapter are summarized as following.In chapter 1, a general view of the underwater towed system is given at the beginning. Then the work principle and classification of the towed heave compensation system are introduced. Present study state of the towed heave compensation system as well as its related fields is overviewed. The research significance and main study contents of this dissertation are put forward at the end.In chapter 2, launch and recovery hydraulic system in which the crane is driven by proportional directional valve and electromagnetic directional valve to carry out horizontal and variable-amplitude motion respectively, and the winch is driven to implement retrieval and release of the towed body by open volumetric speed control is designed. Constant tension synchronous control strategy based on feed forward compensation is proposed. The results of land and sea experiments demonstrate that the designed hydraulic system and proposed synchronous control strategy both have good performance and achieve a satisfied effect.In chapter 3, a passive towed heave compensation hydraulic system, an active towed heave compensation hydraulic system driven by single motor and based on meter-in, meter-out switch control principle, and a semi-active towed heave compensation hydraulic system driven by double motors and having passive and active compensation units are designed. The mathematical models of these hydraulic systems are built with simplified model of the towed body and the cable. In addition, the experiment equipment of the active towed heave compensation hydraulic system is established. The mathematical model analysis shows that the passive system has limited capability, and the active system and the semi-active system are both flexible in design and has higher compensation performance. But compared with the active system, the semi-active version has lower power assumption, higher safety and reliability. Experiments of the active system show that it has good speed regulation performance and quick response.In chapter 4, dynamic model, boundary conditions and initial conditions of the towed heave compensation system are established in normal plane. The equations of the towed cable motion are based on Ablow and Schechter model and the simplified version of six degree-freedom ordinary differential equations are used in motion analysis of the towed body. By perturbation theory, dynamic model of the towed heave compensation system is solved approximately about equilibrium positions. The partial differential equations of the dynamic model are discreted by finite difference method and the transient values of each node in the cable are computed by Newton Iteration. At the end, the input and output parametric model of the towed heave compensation system is analyzed by using numerical calculation results. The analysis of the dynamic model shows that short length of the cable in the towed heave compensation results in that there is none sympathetic vibration between tension in the cable and heave motion of the towed ship, and tension change in the cable is small. The analysis of the input and output parametric model of the compensation system shows that the interrelationship of depth of the towed body, heave motion of the towed ship and retraction of the winch is a time-variable nonlinear function with speed change of the towed ship.In chapter 5, the general control scheme for the towed heave compensation system which is composed of an external depth heave compensation control subsystem and an inner position servo-control subsystem is proposed. The inner subsystem rapidly tracks the reference displacement without steady-state error under time-variable tension torque disturbance, while the external subsystem determines reference displacement of the winch to compensate depth change of the towed body. The inner controller designed by internal model control principle and the external controller designed by the nonparametric model adaptive control approach based on a dynamic linearization of tight format are introduced. Finally, the inner subsystem, the external subsystem and the towed heave compensation control system are respectively simulated. Simulation shows that the inner controller has strong robustness and good ability to resist disturbance, its demands for modeling precision are low, its parameter setting is convenient, and the external controller is simple in design and has high accuracy in case that the control object has time-variable nonlinear feature, and the towed heave compensation control system is able to effectively inhibit influence of heave motion of the towed ship on depth of the towed body at every speed of the towed ship.All achievements of the dissertation are summarized and the further research work is put forward in chapter 6.