| It is necessary for the hydraulic press, especially the large-tonnage hydraulic press, to have a variety of operating modes to meet the production requirements of different types of products. As the core part of hydraulic press, the working performance of electro-hydraulic system not only dertermines the working characteristics of hydraulic press, but also significantly affects the quality of the products to be suppressed. Therefore, the electro-hydraulic system of hydraulic press must have the ability of various operating modes coexist to make the hydraulic press have a variety of operating modes. However, few scholars have deeply studied the electro-hyarulic system of hydraulic press, especially the multi-mode electro-hyarulic system of large-tonnage hydraulic press in the past. This thesis takes the multi-mode electro-hydraulic system of large-tonnage hydraulic press as the specific research object, with a large froging hydraulic press as the research background. Systematic researches will be conducted to study the key technologies of the multi-mode electro-hydraulic system of large-tonnage hydraulic press, which include the core component, the external load compensation strategy, the nonlinear control strategy,and so on.The outline of this thesis is as follows:In chapter 1, the characteristics and classification, the research status at home and abroad, and the development trend of hydraulic press are summarized. It is then pointed out that the coexistence of multiple operating modes will be inevitably considered in the future design of large-tonnage hydraulic press. The four key technologies of the multi-mode electro-hydraulic system of hydraulic press are introduced, which include the electro-hydraulic proportional cartridge valve technology, the external load modeling technology, the disturbance observer-based external load compensation technology and the control technology of electro-hydraulic system. The problems of existing technologies applied to the multi-mode electro-hydraulic system of hydraulic press are also pointed. Finally, the origin and significance of this research are expounded, and the research contents are introduced.Chapter 2 conducts systematic research aiming at the single-feedback proportional cartridge valve (SFPCV). The working principle of SFPCV is firstly introduced, then the main components of SFPCV are designed according to the relevant performance indexes, and the nonlinear mathematical model of SFPCV is also established. According to the nonlinear mathematical model of SFPCV, the MATLAB/AMESim co-simulation model of SFPCV is established, whose correctness is proved by comparison between the simulation data and the prototype experiment.Based on the co-simulation model, the influence of the relevant structural parameters on the performances of SFPCV is further studied,which provides the basis for the optimization of SFPCV. According to the simulation results together with the experiments, the main structural parameters and control strategy of SFPCV are optimized. The experiment results illustrate that the dynamic and static performances of the developed SFPCV has reached the design targets; the developed SFPCV can replace imported, and be applied to the multi-mode electro-hydraulic system of large-tonnage hydraulic press.In chapter 3, the working principle of the multi-mode electro-hydraulic system of a large forging hydraulic press is firstly introduced. Experiment platforms are then established for the slow feeding system and accumulator assisted fast forging system of the hydraulic press. In order to facilitate the nonlinear controller design,mathematical models of the experiment platforms are established, whose correctness are verified by the open-loop simulation and experiment comparison. Control algorithm of the loading cylinder is further studied considering that the loading cylinder is applied to load the driving cylinder in the following studies. The experiment results show that the fuzzy PID control algorithm can significantly improve the dynamic response of the loading process under the premise that the steady loading accuracy is similar with the traditional PID control.Chapter 4 conducts systematic research for the high-perfomance motion control of slow feeding system under unknown load. Firstly,two different disturbance observers (DOBs) are proposed to estimate and compensate the external load of the hydraulic press in the slow feeding stage. The first DOB is proposed based on the piecewise linearization characteristic of the external load, its parameter adaptation is driven by the observer error and motion tracking error together. In this thesis, it is called "extended piecewise disturbance observer"(EPDO). The second DOB is proposed based on the characteristic that a fuzzy system can approximate an unknown nonlinear system with any precision, its parameter adaptation is also driven by the observer error and motion tracking error together. In this thesis,it is called "extended fuzzy disturbance observer"(EFDO). Aiming at the inherent strong nonlinearities and model uncertainties in the electro-hydraulic system, and considering the multiple-input single-output characteristic of the individual metering slow feeding system, a nonlinear cascade controller is proposed as the motion controller of the slow feeding system. The motion tracking loop of the nonlinear cascade controller is designed based on the sliding mode control with the desired displacement, velocity,acceleration and jerk as inputs and the desired driving force as output. The pressure tracking loop of the nonlinear cascade controller is designed based on the backstepping technique, so that the actual driving force of the system can precisely track the desired driving force derived from the motion tracking loop. The "minimum equivalent load criterion"(MELC) is proposed to solve the static working point selection problem of the individual metering slow feeding system. The MELC regards the force provided by the rod chamber of driving cylinder as a part of the equivalent external load. With the increase of the estimated external load, the MELC will gradually reduce the desired pressure of the rod chamber of driving cylinder under the premise that the minimum equivalent external load is bigger than the gravity of the slider. By doing this, the maximum driving ability of hydraulic press is digged out,and ensures that the hydraulic press will not appear instability phenomenon in any state. Simulation and experiment results demonstrate that the nonlinear cascade controller, together with the EPDO or EFDO, can make the hydraulic press precisely track the desired displacement curve during the slow feeding stage; with the EPDO or EFDO, the estimated external load can precisely track the actual external load;furthermore, with the proposed MELC, the controller can automatically change the desired pressure of the rod chamber of driving cylinder, which can dig out the driving ability as much as possible to improve the displacement tracking performance.Chapter 5 conducts systematic research for the high-frequency, high-precision and stable displacement control of the accumulator assisted fast forging system.Parameter matching design is firstly conducted for the core components from the basic structure of the accumulator assisted fast forging system. Considering the multiple-input single-output characteristic of the accumulator assisted fast forging system, and the matched or mismatched model uncertainties associated with the accumulator assisted fast forging system, an optimal energy distributed adaptive robust control (OED-ARC) is proposed as the motion controller. In the design of OED-ARC controller,the "lumped flow" is firstly regarded as the equivalent input to make the input degree of freedom and output degree of freedom equal; according to the desired motion trajectory,the desired value of the "lumped flow" is then derived using the backstepping technique; control law of each control valve is finally derived from the "lumped flow" using the "optimal energy distributed criterion". In addition,in the design of OED-ARC controller,a "nonlinear damping regulator" is adopted based on the simplified mathematical model of the accumulator to compensate for the nonlinear parameter uncertainy associated with the accumulator model. Simulation and experiment results illustrate that the proposed OED-ARC controller can make the accumulator assisted fast forging system achieve determined dynamic response and static tracking accuracy even if the system is subjected to various matched or mismatched model uncertainties; what's more, the proposed "nonlinear damping regulator" can not only compensate for the nonlinear parameter uncertainty of the accumulator, but also fine-tune the motion control performance to increase the degree of freedom for adjustment.Chapter 6 summarizes the research work of this thesis, describes the main innovations of the research, and prospects the future work. |
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