Trajectory Tracking Control And Application Of The Electro-Hydraulic 6DOF Parallel Ship Motion Simulator

Ship motion simulators have been used in various applications, due to their advantages such as superior controllability, non-destructivity, economical efficiency, high reliability, etc. Ship motion simulators are often driven by electro-hydraulic six degree of freedom (6DOF) parallel mechanisms, due to the high precision, high capacity and so on. High trajectory tracking accuracy is an important criterion for the use of 6DOF parallel mechanisms. However, the control of electro-hydraulic 6DOF parallel mechanism is challenging as its dynamics is highly nonlinear. Furthermore, the control of hydraulic actuators is more challenging than that of their electrical counterparts, due to the phenomena such as nonlinear servo valve flow-pressure characteristics, fluid compressibility, and valve overlap. On the other hand, the ship motion simulators often subject to uncertainty disturbances from the external environment. This thesis addresses the study of electro-hydraulic 6DOF parallel mechanism. With the help of theoretical analysis, simulation and experimental research, the trajectory tracking control of the 6DOF parallel mechanism with uncertain disturbance is investigated systemically. Then, the application to the simulation of ship equipment in a dynamic environment is studied.This doctoral dissertation consists of eight chapters. The main contents are as follows:In Chapter 1, the support and background of the research is introduced. On the basis of referring to domestic and international associated documents, the overview of ship motion simulators is summarized. Then, the present research situation and trends of 6DOF parallel mechanism and control of 6DOF parallel mechanism are reviewed, and several issues to be resolved are proposed. Finally, the research object and main research content of the subject are illustrated.In Chapter 2, the mathematic model of 6DOF parallel mechanism is derived. Then, based on joint force sensors, a structure parameter calibration method of 6DOF parallel mechanism is proposed. Simulation and experimental results show that this method significantly improves the accuracy of 6DOF parallel mechanism. This chapter provides a basis for high precise control of 6DOF parallel mechanism.In Chapter 3, according to analysis the uncertain disturbances of 6DOF parallel mechanism, the method which uses force sensors to measure load forces is proposed. Through using force sensors, the 6DOF parallel mechanism has been decoupled into six independent legs. Then, the mathematical model of hydraulic system is given, and the adaptive robust controller is derived. Simulation and experimental results show that the proposed controller gives a good performance for the specified tracking task in the presence of uncertain load disturbances.In Chapter 4, observer-based cascade control of 6DOF parallel hydraulic mechanism in joint space coordinate is proposed. Cascade control, nonlinear disturbance observer technique and sliding mode control are integrated to design the controller for 6DOF parallel hydraulic mechanism. In order to reduce costs, disturbance observer, instead of force sensor, is used to obtain disturbance information. Hydraulic actuator dynamics models are incorporated in the controller design by using cascade control algorithm. This algorithm is applied to separate the hydraulic dynamics from the mechanical part, which can mask the hydraulic dynamics with an inner loop. The inner loop controller is designed based on the feedback linearization, and the outer loop is derived by using sliding mode control. Simulation and experimental results confirm the effectiveness of the method.In Chapter 5, observer-based synchronous tracking control of 6DOF parallel mechanism in joint coordinate is studied. The position synchronization error is developed by considering motion synchronization between each actuator joint and its adjacent one based on the synchronous goal. Then the controller is designed with feedback of both position error and synchronization error. The proposed controller is proven to guarantee asymptotic to convergence to zero of both the position errors and synchronization errors. This method solves the problem that joint space control scheme cannot guarantee 6DOF parallel mechanism to work in a synchronous manner. Simulation and experimental results confirm that this method guarantees the tacking accuracy in joint space and can effectively improve system interoperability at the same time. In Chapter 6, the task space control of 6DOF parallel mechanism is developed. The direct kinematics of 6DOF parallel mechanism is used as feedback signal to design controller. Considering the drawbacks of conventional sliding mode control and adaptive control, a lumped disturbance which synthesizes both parametric uncertainties, uncertain external disturbance and un-modeled dynamics is first defined. An adaptive disturbance observer is then constructed to estimate and compensate the lumped disturbance. The backstepping design method is adopted to derive the controller. Simulation and experimental results confirm the effectiveness of the method.In Chapter 7, the method which uses ship motion simulator to reproduce the power spectrum with frequency domain loop and time domain loop is proposed. This method is divided into time domain inner loop and frequency domain outer loop. The aforementioned electro-hydraulic servo control method can be used in the time domain loop. The target power spectrum signal is converted into time domain drive signal in the frequency domain loop. This method does not require additional acceleration sensors, and can be used in ordinary shaking table.In Chapter 8, the main research work, conclusions and innovation points of this thesis are summarized. Then, future development is predicted in order to provide references for the further research on this project.