Generation And Application Of Ultra-high Pressure Hydraulic Impulse Waveform

Hydraulic systems are widely used in automotive industry, construction machinery, agricultural machinery, and defense industry because of their large power-mass ratio, quick response speed, good control performance, overload protection, and so on. In the work process of hydraulic systems, hydraulic impulse may arise at the sudden switch of hydraulic valves, resulting in sudden change of the hydraulic oil speed or direction. Because of the inertia and compressibility of fluid, hydraulic pressure may rise in an instant and exceed system pressure by several times, which would cause great harm to hydraulic systems. Therefore, to improve the reliability of hydraulic components and systems, it is of vital importance to do impulse test for hydraulic components, the key of which is to generate the desired hydraulic impulse waveform. In recent years, with the development of hydraulic systems towards high pressure and large flow rate, more serious requirement is put forward for hydraulic impulse waveform generation systems. In addition, for the fact that hydraulic impulse waveform generation systems involve ultra-high-pressure and large-flow-rate electro-hydraulic servo control, the research on hydraulic impulse waveform generation systems not only is useful for hydraulic impulse testing for hydraulic components, but also facilitates the development of related theory, technology and equipment.Ultra-high pressure hydraulic impulse waveform is studied in this paper, with the method of theoretical analysis, computer simulation, mathematical calculation and experimental study, aiming at reducing the power consumption, improving system response speed and accuracy. An ultra-high pressure hydraulic impulse waveform generation system with pressure difference feedback from the control chambers of second-stage valve is designed, the characteristic of which is that the system damping ratio can be adjusted at will by changing the feedback gain. Accordingly, the system overshoot of step response can be adjusted between 0 and 50%, providing a new way to produce water hammer waveform under the same square-wave input. Furthermore, the research also provides a theoretical foundation for hydraulic water hammer impulse testing. The ultra-high pressure hydraulic impulse waveform generation system is designed to be energy-saving, adopting an accumulator to recover energy in the return stroke and release energy in the next boost stroke, the result of which is that the system power consumption is reduced by 15%, with system efficiency improved from 63% to 76% and a faster response speed. In consideration of the large variation range of system load, a fuzzy repetitive controller is designed, replacing the low-pass-filter of conventional repetitive controller by fuzzy controller. The designed fuzzy repetitive controller can stabilize the system by reducing high-frequency component without damaging low-frequency component, so higher control accuracy can be obtained. Experimental results indicate that compared with traditional PID control systems, the fuzzy repetitive control system can reduce periodic comprehensive error by 99% and 87% for sine-wave input and water-hammer input, while the relative error reduced by 54% and 40%.The main content of each chapter is summarized as following:In chapter 1, the significance of hydraulic impulse test is stated, and the development of hydraulic impulse test standards and current research home and abroad are summarized. The methods to generate standard hydraulic impulse waveform are introduced. Related technical progress including ultra-high pressure technology, hydraulic system energy-saving technology, and control strategies are studied.In chapter 2, an ultra-high pressure hydraulic impulse waveform generation system is designed, the mathematical model and AMESim simulation model of which are established. A large-flow-rate hydraulic-control-valve utilizing composite throttling window is designed to meet the requirement of hydraulic impulse waveform generation system, with the characteristic of varying flow gain at different valve opening. By changing the pressure difference feedback gain from the control chambers of the hydraulic-control-valve, various damping ratios of the system can be obtained, resulting in the adjustment of system overshoot at will, which is a new way to produce water hammer waveform under the same square-wave input.In chapter 3, an energy-saving hydraulic impulse waveform generation system is designed, adopting an accumulator to recover energy in the return stroke. The power, energy consumption and efficiency of the energy-saving system are analyzed, calculated, and compared with those of the system without accumulator. Results show that the adoption of accumulator can reduce energy consumption and improve system efficiency. Furthermore, the system with accumulator possesses higher response speed when input by step, square wave and water hammer wave.In chapter 4, by analyzing the mathematical model of hydraulic impulse waveform generation system, it is concluded that the open loop gain of the system varies with the system load, so traditional control strategies have difficulty meeting the requirement of stability and accuracy at the same time. Fuzzy repetitive control method is put forward and applied to hydraulic impulse waveform generation system. Results show that the error decreases with the cycle time increasing, proving the accuracy and stability of the fuzzy repetitive control systemIn chapter 5, the designed energy-saving ultra-high pressure hydraulic impulse waveform generation system is applied to hydraulic hose impulse testing. Experiments are carried out about the overshoot of the hydraulic system with pressure difference feedback from control chambers of hydraulic-control-valve, and prove that different waveforms can be obtained by adjusting feedback gain. Experiments are also done on the energy-saving system with accumulator, and the good control performance of the energy-saving system is validated. Comparisons are done between fuzzy repetitive control strategy and PID control strategy, the results of which show that the fuzzy repetitive control strategy can achieve higher accuracy.In chapter 6, all achievements of the dissertation are summarized and the further research work is put forward.