Research On Modeling And Control Method Of Valve-controlled Asymmetrical Cylinder System

The valve-controlled asymmetrical cylinder system, which has the advantages of small working space, simple structure and large bearing capacity, is widely used. However, due to its structural asymmetry, it brings a great difficulty to system modeling and control, and it has become the most crucial factor to further improve the system performance. Therefore, for the need to meet certain requirements of high performance occasions, it is very necessary to carry out the studies of new modeling and control methods on the valve-controlled asymmetrical cylinder system according to the key problems in the current research, so this has important theoretical research significance and better value in engineering application.According to the current definition of the load pressure, which does not fully consider the asymmetry of the asymmetric cylinder and non-minimum hydraulic spring stiffness under the initial position, this definition will result in problems with a large modeling error and stability margin deficiency. In this dissertation, based on the weighted average method of equivalent bearing area and the hydraulic spring rigidity theory, a linear model of critical center valve-controlled system is established. Then, different models of the asymmetrical cylinder controlled by valve in two directions are unified, and this model can also be used for the symmetrical cylinder system controlled by valve. Moreover, the corresponding design criterions of the system are proposed. In addition, on this basis, a linear model of closed center valve-controlled system based on the harmonic linearization method is established. Then the effects of the closed center dead-zone on the performance of the system are carried out.In the system modeling, according to the general calculation methods on the amount of the leakage and the general linearization method on the model of the system, which result in the problems of a large modeling error. Focusing on this problem, a novel method to calculate the amount of leakage is used at the first time under considering shear flow, and a nonlinear model with a higher precision is established based on this method. Furthermore, by using the exact linearization method for the nonlinear model and introducing the state feedback to make the linear model stabilize, the problems in the actual use which are difficult to solve by the exact linearization are solved.The general control method is difficult to effectively solve the contradict problems between the control performance and robustness. Thus, a new control method of internal model variable structure is developed according to the integration of the internal model and the variable structure control strategy. Then, after accurate linearization based on the linear model and nonlinear model, the internal model variable structure control is studied respectively. Furthermore, the control performance and robustness of the system are improved. The main studies and results are summarized as follows:(1) The study of the linear modeling. The results show that, the control precision and the dynamic performance based on the linear model are better than these performances based on the general model. The zero dead zone has certain effects on the performances of the system, including self amplitude value, self vibration frequency, the peak time under the step response and the control precision under static and dynamic states. In addition, the effectiveness of the modeling and the linearing zero dead zone method are also verified by simulation and experimental research, which provides a reference model for the applications under the general situations.(2) Nonlinear modeling and its exact linearization. The results show that, the established nonlinear model and the exact linearization model both have a higher precision. Moreover, the effectiveness of the new method to calculate the amount of leakage and the exact linearization method are verified by simulation and experiments, which provides a reference model for the applications under high performance occasions.(3) The internal model variable structure control method and the corresponding experimental research. The results show that, the system of the internal model variable structure control based on the linear model has a better tracking performance under the steady state and dynamic performance than these performances based on the PID control method. Furthermore, it also has a good robustness. For the system of the internal model variable structure control, based on an accurate linearization after the nonlinear model, has a better stationary control precision and more stable dynamic transition process than these performances based on the linear model. Finally, the effectiveness of the proposed control methods is also verified by experiments, which provides a reference and experience for the control methods for the high performance of the system.