Design Of Hydraulic Catapult Mechanism And Research On Its Key Control Components

Catapult mechanism was used to accelerate the catapult object instantaneously by using the rapid release of pre-stored energy, it was widely used in modern military and general industry, such as aircraft catapult takeoff, missile ejection launch, ejection seat, car crash test device and so on. Depending on the power actuator, the catapult mechanism includes mechanical, gas-fired, compressed air, steam, electromagnetic and hydraulic. Hydraulic catapult mechanism has many advantages,for example the higher ratio of power to weight, the faster response ability, the high control precision, easy to cushion and lower noise. It has been taken seriously in high-performance applications. Hydraulic catapult mechanism takes the high speed movement of the hydraulic cylinder piston as its main characteristic. The key technical difficulties are energy supply of instantaneous high-power source, the seal and cushion design of high-speed hydraulic actuators, the catapult control valve with rapid response and large flow rate and its pilot servo valve with high frequency response. Hydraulic catapult mechanism belongs to the high-speed and high-power hydraulic system. As hydraulic equipment is developing towards high-speed, high power, low noise, the study on the above key technologies will have a wide range of application prospect.The summary and analysis of the development status and applications of hydraulic catapult mechanism was first done, and the research status about high-frequency response and high flow rate servo valve was discussed. On this basis, the hydraulic catapult mechanism and its key control elements-2D servo valve with high-frequency was studied, the main contents and results are as follows:1. For specific applications, the following several hydraulic catapult mechanism schemes were mainly proposed. The low speed hydraulic catapult mechanism, with its hydraulic cylinder piston maximum speed of8m/s, piston acceleration time is approximately70ms, and its improvement design. The high-speed hydraulic catapult mechanism, design index includes piston initial20mm piston stroke reached a velocity of2m/s, the maximum speed up to15m/s. The simultaneous ejection mechanism with double hydraulic cylinder, the two hydraulic cylinders eject synchronously with a maximum speed of6m/s. The ultra high-speed hydraulic catapult mechanism, with corresponding piston speed is above20m/s. The high-speed hydraulic ejection mechanism was taken as an example, the parameters calculation and structure design of the ejection mechanism was discussed.2. In order to solve the contradiction between high response and large flow rate of high-speed switch valve, a double throttle parallel output structure scheme was proposed. The theoretical analysis and experimental research of the zero leakage characteristic, the flow characteristic and the pressure characteristic of switch valve were done. The flow reaches3000L/min with a2MPa import and export pressure difference, the mathematical model of switch valve spool motion process was established, the influence of structure parameters on the dynamic characteristics of the switch valve was analyzed. Finally, a prototype was built and the experiment was completed, the valve open time is less than15ms with lOMPa working pressure. A structural scheme of sieve valve was also discussed.3. In order to improve frequency and flow of2D servo valve, circular hole type pilot control structure is changed into a full bow type structure, through the analysis of static characteristics of2D servo valve, the pilot stage of zero leakage at21MPa is about0.5L/min. The mathematical model of2D servo valve servo spiral mechanism was established. The influence of work pressure, initial bow height, area gradient on step response and frequency characteristic was analyzed by the linear and nonlinear simulation. Theoretical analysis showed that the frequency response of circular type structure at the-3dB amplitude attenuation is about250Hz, and the full bow type is800Hz, the measured full bow type step response time is about1.6ms, the circular type about3ms, proved that the full bow structure can significantly improve the dynamic characteristic of servo spiral mechanism. Lay the foundation for the development of high frequency, large flow2D servo valve.4. To solve the cushion of high speed hydraulic cylinder piston, a piston cushion structure was proposed. Theoretical analysis showed that the piston initial clearance is the most significant influencing factor to peak pressure. The mathematical model of piston cushion structure was established, and the dynamic characteristics of piston cushion process is studied by simulation and experiment, showed that it can reduce the piston speed from7m/s to0.6m/s in60mm cushion stroke. The influence of structure parameters on the cushion performance was studied, on this basis, put forward two stage and combined cushion structure, and the structure scheme of ultra high speed hydraulic cylinder was also discussed.5. The mathematical model of high speed hydraulic catapult mechanism which combined by the accumulator, switch valve and hydraulic cylinder was established. The structure parameters and operating parameters affect on dynamic characteristics of catapult mechanism was analyzed. The catapult mechanism prototype was built to test. Showed that the ejection mechanism can accelerate the load with150kg from0m/s to8m/s in100ms at8MPa working pressure, and the load can reaches speed of more than2.0m/s in just20mm piston initial stroke.6. The double hydraulic cylinder closed loop catapult mechanism and its key technology were analyzed. The double cylinder synchronous open-loop control characteristic was studied by establishing the mathematical model, Double cylinder synchronous closed loop control characteristic was also studied by establishing linear approximate transfer function. Theoretical studies showed that the closed-loop step response time is about50ms.