Investigation Of The Control Technology For Resonant Electro-Hydraulic High Frequency Fatigue Testing Machine

Fatigue testing machine is the standard equipment used to measure the mechanical properties of the specimen material and to detect the fatigue life limit. With the rapid development of industrial production, requirements for fatigue test of specimen material are continuously increased. In the cases of high-speed railway locomotive components and spacecraft parts, the requirements for fatigue life need to be as high as107cycles or even more than1010cycles, which poses new challenges for the fatigue testing. Fatigue testing machine is a kind of tester with high energy consumption, the performance of which largely determines the testing cycle and ultimately impact the testing efficiency. The solution of2D high-frequency rotary valve controlled hydraulic cylinder effectively overcomes the bandwidth bottleneck in traditional servo valves, significantly increases the working frequency of fatigue testing, thus broadening the application range of electro-hydraulic fatigue testing machines.2D high-frequency rotary valve is a kind of rotary valve which is designed by taking advantage of the dual freedom of spool movement, where the rotational movement of spool changes periodically the co-ordination relationship between spool notches and sleeve windows, thus achieving the periodic on-offs of valve ports and then ensuring the periodic vibration of piston rod in the valve-controlled cylinder. The axial sliding movement of the spool could adjust the waveform amplitude of valve ports area, control the flow rate to the chamber of the hydraulic cylinder and then adjust the waveform amplitude of the load force output. However, with the increase of operating frequency of the2D high-frequency rotary valve controlled hydraulic cylinder, waveform amplitude attenuation of load force output occurs. In this dissertation, the solution of2D high-frequency rotary valve controlled hydraulic cylinder in resonant fatigue testing is presented. The approaches to alter the resonant frequency of the system, to control the waveform amplitude of the load force output in resonant condition and to realize the bias-control load force output are studied. In resonant operating conditions, the specimen almost completely release the energy absorbed, which satisfies the requirements of high frequency and high efficiency in fatigue testing. The main work and findings of this study are as follows:1. The principle of2D high-frequency rotary valve controlled hydraulic cylinder is studied. The mathematical models, including three-port2D high-frequency rotary valve, three-port2D high-frequency rotary valve controlled asymmetric cylinder, four-port2D high-frequency rotary valve and four-port2D high-frequency rotary valve controlled symmetric cylinder are established. The operating process of hydraulic power mechanism is studied in resonant working conditions.2. The structure of2D high-frequency rotary valve is optimized. The flow field in the valve chamber is simulated. The impact of spool structural geometry on the axial flow force is examined. The relationship between the groove numbers on the spool shoulder and the flow moment and the axial flow force impacted on the spool is studied. Cavitation phenomena in valve ports of2D high-frequency rotary valve are studied.3. The investigation to the dynamic characteristics of2D high-frequency rotary valve controlled hydraulic cylinder in resonant working condition is simulated. Simulation model of2D high-frequency rotary valve controlled asymmetric cylinder is established. Simulation model of2D high-frequency rotary valve controlled symmetric cylinder is established. Resonant working conditions of2D high-frequency rotary valve controlled cylinder (including asymmetric cylinder and symmetric cylinder) are studied, such as damping characteristics, bias-control of the load force output, amplitude control of the load force waveform and alteration of the resonant frequency of the system.4. Resonant mechanism of the fatigue test rig is analyzed. The fatigue test rig is divided into2D high-frequency controlled asymmetric cylinder test rig and2D high-frequency controlled symmetric cylinder test rig. The investigation results show that the pipeline volume between2D high-frequency rotary valve and hydraulic cylinder has a greater impact on the alternation control of the test rig resonant frequency than other factors. When the pipeline volumetric coefficient is more than50%, the initial position of piston rod has little influence on the alternation of the natural frequency of the system. Moreover, the natural frequency alternation of the fatigue test rig composed of the symmetric cylinder is more sensitive to the value of the pipeline volumetric coefficient.5. Both2D high-frequency rotary valve controlled asymmetric cylinder fatigue test rig and2D high-frequency rotary valve controlled symmetric cylinder fatigue test rig are established. For the fatigue test rig constructed by asymmetric cylinder, sweep frequency tests of the load force output, respectively selecting the initial position of the piston rod at5mm,10mm,40mm,45mm,95mm and145mm, are carried out, and resonant frequencies measured respectively are922Hz,870Hz,845Hz,768Hz,742Hz and718Hz. Meanwhile, the flow rate of the system is measured when the initial position of the piston rod is at10mm,40mm,95mm and145mm. The load force amplitude alternation is achieved by changing the axial valve port opening of2D high-frequency rotary valve, and the sweep frequency of the waveform amplitude of the load force output is carried out when the initial position of the piston rod is respectively at5mm,10mm,40mm,95mm and145mm. For the fatigue test rig constructed by the symmetric cylinder, sweep frequency tests of the load force output, respectively selecting the initial position of the piston rod at2mm,3mm,6.5mm,10mm,13.5mm,16mm and18mm, are carried out, and resonant frequencies measured respectively are1011Hz,998Hz,979Hz,949Hz,973Hz,992Hz and1005Hz. Meanwhile, the flow rate of the system is measured when the initial position of the piston rod is at10mm,40mm,95mm and145mm. The sweep frequency of the waveform amplitude of the load force output is carried out when the initial position of the piston rod is respectively at2mm,3mm,10mm,13.5mm and18mm. The bias-control load force output of the piston rod is carried out when the initial position of the piston rod is at3mm.