| In modern times, automobile has already been a part of people's lives. With the wide sharing of automobile and the increasing demands of automotive qualities, automatic transmission technology has become the focus of auto transmission system study. Among the ways to present automatic transmission, only CVT can realize the continuous variation of speed ratio, and achieve the best dynamic and economy performance.Metal Pushing V-belt CVT is an ideal gearbox in auto Transmission field. Its speed stepless effect can realize the relative independence between vehicle speed and engine speed, and make the engine always working in ideal speed area. The speed changing process in CVT is achieved by electro-hydraulic control system. The early CVT adopt servo control technology, because it responded quickly with high control precision. But this control technology has shortcomings, such as complex structure and control, high cost, and poor pollution resistance. Therefore, the electro-hydraulic proportional control emerged. Compared with electro-hydraulic servo valve, the electro-hydraulic proportional valve has low cost and high pollution resistance. But, using proportional solenoid as mechanical and electrical switching device, it also has some shortcomings, such as high hysteresis error, low precision. At the same time, it also needs continuous analog signals. The newly emerging electro-hydraulic digital hydraulic valves not only inherit the two valves'advantages but also overcome their disadvantages. Additionally, the digital hydraulic valves have simple control mode, and can be connected to computer without digital-to-analogue conversion device. With the rapid development of electronic technology, the use of digital control technology in CVT hydraulic control system can be the future development trend.Hence, according to the clamping force and speed ratio control requirements in CVT speed changing process, this thesis has designed digital relief valve, digital reducing valve, and the hydraulic integrated valve block equipped with above valves.The main contents are as follows:(1) We compare various Automatic transmissions, and emphatically introduce the working principles and developing process of metal pushing v-belt CVT with its core electro-hydraulic control system.(2)We analyze electro-hydraulic proportional and electro-hydraulic digital technology. On that basis, we also analyze the electro-hydraulic control system in current CVT.(3) According to the pressure and flow requirements of driven cylinder and active cylinder in CVT work process, we confirm the parameter design requirements in digital relief valve and digital reducing valve. According to parameter design requirements and pressure regulating methods, we establish the physical structures. According to static characteristics, we determine the size parameters. Finally, we manufacture the digital relief valve product, digital reducing valve product, and the hydraulic integrated valve block equipped with above valves and oil galleries.(4) We have built the models of digital relief valve and digital reducing valve in AMESim software. On the basis of model simulation, we analyze the influences of key parameters on the outputs in both digital relief valve and digital reducing valve , and optimize the key parameters in digital relief valve.(5) We take the static and dynamic characteristics tests of digital relief valve and digital reducing valve on Hydraulic experimental bench. Through the test results, we can obtain the relationship between pressure and step in digital relief valve. Simultaneously, the relationship among output pressure, input pressure and pilot-operated-stage duty cycle in digital reducing valve can also be obtained.The main innovations in this thesis are the designation of digital relief valve and digital reducing valve used for CVT speed control. Current speed control valve in CVT electro-hydraulic control system are mostly electro-hydraulic proportional valve.
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