| Supplying convenient transportation for people and materials, the vehicle has been closely related with people's work and life. It's inevitable to apply automatic transmission with people's requirement improving in mobility, economy, security and comfort of vehicles. There are several methods to realize automotive transmission shifting. For the moment, Automatic Transmission (AT), Electronically Controlled Automatic Mechanical Transmission (AMT) and Continuously Variable Transmission (CVT) are widely used all round the world. CVT is an ideal transmission device for it can realize the continuous change of transmission ratio, get the optimum matching between the transmission and the engine and improve the whole vehicle's fuel economy, mobility, the handing facility and comfort. Compared with AT and AMT, the efficiency of CVT is higher than that of AT, and the handling facility and comfort of CVT better than that of AMT. Therefore, automobiles equipped with CVT can meet the requirement of saving energy and protecting environment. Metal V-belt type CVT has such merits as bigger power transferring capacity, transmission efficiency and longer life, so it has been the most widely used. Early CVT was always controlled by the mechanical-hydraulic servo technology, its response is quick and its precision high, but its controlling module complicated and power loss (fall of valve pressure) big. Besides, it is sensible of oil pollution, and needs developing for a certain type as it can't be applied to other CVT, the development of such controlling module is so expensive that it hasn't been widely used. Therefore, CVT adopts electro-hydraulic proportional controlling technology. Like servo valve, electro-hydraulic proportional valve uses proportional electro-magnet as the driving mechanism. It is inevitable for CVT to adopt digital controlling technology with the development of the computer. Digital controlling technology combines both the superiority of big hydraulic power-weight ratio and flexible electronic-computer controlling, so it develops quickly in all industry fields. Digital controlling technology can meet the requirement of dynamic and static performance of CVT, and has such merits as low cost, strong anti-pollution ability, handling performance, easy automation, simple structure and low energy consumption. On the whole, it is beneficial to research and develop the digital electro-hydraulic controlling module to improve the performance of CVT and reduce its cost for economical and social benefit. Based on the demands above and aiming at electro-hydraulic controlling system of metal belt CVT, this paper is to research the controlling theory and digital proportional controlling technology on CVT. This paper has six chapters, and the main contents are following: 1) Based on lots of professional documents, the problems the automotive industry confronted, the corresponding solutions, the development and application history and research of CVT nowadays were introduced. 2) The structure and principle of operation of CVT were introduced. The theory of how to calculate CVT transmission ratio and clamp force and the policy, the matching of CVT with engine and the total vehicle on emphasis were also expounded. 3) The advanced digital proportional electro-hydraulic controlling technology was applied on CVT to improve the controlling system of CVT. The structure and principle of high-speed on-off valve were introduced. 4) The digital valve and its driver were designed, and the math model of the digital valve built and analyzed through computer simulation with MATLAB. The analysis shows that the digital valve has fine performance on dynamic and static state, and as the result, it can meet the requirement of electro-hydraulic controlling system of CVT.
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