| Hydraulic system is an important part of excavator, and its performance is highly related to the operability and energy-saving of machinery. In the conventional load-sensing (LS) systems, the requirements of systems flow and pressure are met by closed-loop pressure control with the feedback of maximum load pressure. However, in the electrohydraulic flow matching control (EFMC) systems, when the electro-proportional valve opens, the electro-proportional pump supplies the required flow by open-loop flow control, and the pressure difference between the system pressure and maximum load pressure is not needed to be preset. Therefore, the flexibility and controllability of EFMC systems are improved. The control methods for flow matching between the electro-proportional pumps and electro-proportional valves are great challenges to mobile machinery. In this thesis, the characteristics and control methods of EFMC systems have been investigated and applied to a2-ton prototype excavator, as compared with the conventional LS system.In this thesis, a new method of pressure and flow compound control was proposed. Using the method proposed, the EFMC systems based upon pre-compensation multi-way valves had the advantages of closed-loop pressure control in electrohydraulic LS systems and the advantages of open-loop flow control in general EFMC systems. Therefore, the closed-loop pressure control and open-loop flow control could be chosen under different working conditions. When the flow of the EFMC system was excessive, closed-loop pressure control was used in the system, or alternatively open-loop flow control was adopted, hence the problems of energy loss and overflow could be solved. Moreover, the pressure shock of the system was effectively surpressed when the actuators instantaneously stopped or quickly switched. When compared with the conventional LS system in excavating test mode, the energy consumption of the EFMC system was reduced by10%and the efficiency was increased by10%in the EFMC system. An experimental prototype of a2-ton excavtor equipped with the EFMC system was developed, and a corresponding virtual prototype model was built based on AMEsim and Adams. Moreover, a new method for flow control was proposed by adding a bypass unloading valve in the EFMC system. According to the system pressure and maximum load pressure, the unloading valve could be opened to discharge excessive flow, hence the problems of overflow and energy loss could be effectively solved. Compared with the conventional LS system, the pressure margin of the EFMC system was reduced by0.6-0.7MPa and the step response time was shortened about0.5s in the EFMC system with the unloading valve, as well as the stability of the system was enhanced. Furthermore, a new control method for improving the energy efficiency has been proposed for the EFMC systems based on LUDV (Load Independent Flow Distribution) multi-way valves (LUDV-EFMC). Adopting the new method, when the flow demand of the actuator was highest, the corresponding control valve was fully opened, and the opening of the other valves were proportional to operator signals. The experimental results showed that, compared with the LUDV LS system in excavating test mode, the pressure margin of LUDV-EFMC was reduced by0.8-1MPa and energy consumption was decreased by12%in the LUDV-EFMC system. All research results could be used as references for the further investigation and application.The thesis is outlined as follows:In chapter1, the principles and characteristics of typical hydraulic systems in existing excvavtors were summarized, and a new kind of hydraulic system-the EFMC system was introduced. The research background and current research progress of EFMC systems were reviewed all over the world. Then, the main research objective, significance and topic of the study in the thesis were presented and discussed.In chapter2, the principles and characteristics of the pre-compensation EFMC (PC-EFMC) system were analyzed. The experimental prototype of the excavator was developed, and the virtual prototyping co-simulation model was built. The response, energy efficiency and stability of the PC-EFMC system were all investigated when compared with the conventional LS system. As key problems, the flow saturation and overhead flow matching control were solved. The controller with the capability of anti-saturation was designed, and the method of the pressure and flow compound control was proposed.In Chapter3, the principles and characteristics of the PC-EFMC system with an unloading valve as a bypass were analyzed in detail. The influences of different flow matching relationships on the operation characteristics and the energy loss of the excvavtor were studied. The system characteristics were experimentally investigated under the condition of flow saturation. The controller with the capability of anti-saturation was developed. The performance of response, energy efficiency and stability all were analyzed comparatively between the LS system and the PC-EFMC system with an unloading valve.In chapter4, the factors and impact of flow error in EFMC systems were analyzed. The influence of load pressure on the flow characteristics of the electro-proportional pump was experimentally studied, and an open-loop compensation method was presented to solve the effect based on pressure characteristics. With the experimental datasheet, the system flow was compensated by linear interpolation. To meet the requirements of high-precision system and working condition, a control method of indirect flow close-loop based on the cylinder velocity was proposed. Then, the velocity characteristics of single and compound action were experimentally investigated with different loads and reference velocities.In chapter5, the EFMC system based on LUDV multi-way valves (LUDV EFMC) was analyzed on the principle and characteristics. The dynamic, static flow and pressure characteristics of the LUDV EFMC system were studied in detail, as compared with the PC-EFMC system. Based on Adams and AMEsim, the simulation model of hydraulic-mechanical was built. A new control method for improving the energy efficiency has been proposed. Compared with the LUDV LS system, the response characteristics, energy efficiency and stability of LUDV EFMC system of the excavator were analyzed under the conditions of different actions and loads.In chapter6, all the research contents are summarized, and some new views are put forward in the future. |
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