A Study On Control Strategy Of Double State Continuously Variable Transmission

The two main themes of the contemporary world are economizing energy and protecting environment. Auto manufacturers are trying to apply new technologies to reduce the fuel consumption and minimize the exhaust gas. In order to achieve the goals mentioned above, the most effectual and promising way is introducing CVT into the powertrain.The metal pushing V-belt type continuously variable transmission (CVT) has the advantages of simple and compact structure, easy operation, low cost, fuel economy and etc. As an advanced technology to develop passenger car, it can meet the demands of developing transmission of passenger car in our country. So this technology attracts much more attention in recent years. Double state continuously variable transmission, a kind of CVTS, with a torque converter working as the start component, has better dynamic performance than any other kind of CVT, it could improve the passing topography capacity of vehicles on rough roads, and it could reduce the dynamic loads on the powertrain, so it is accepted by more and more auto manufacturers. Control strategy is the key technology of Double state continuously variable transmission. The dissertation mainly focused on the matching and the control strategy of Double-state continuously variable transmission. The contents are listed as follow:At the beginning the dissertation reviews the development process and the types of CVT, analyzes the characteristics and applications of metal pushing V-Belt type CVT technology. Compared with other transmissions, the metal pushing V-Belt type CVT can meet the currently demand of drivability, economics, emission and etc. It is the ideal transmission with prosperous future for passenger cars. Further more, several kinds of start components in CVT are compared with each other, and it could be concluded that the double state CVT with a hydraulic torque converter working as its start component has more advantages. In this paper the background and the necessity of this theme in science and practice are given.A dynamic simulation model is built for the vehicle with Double state continuously variable transmission. For the first time the efficiency model of CVT for real-time control is derived. The characteristics of hydraulic control system are analyzed, and the hydraulic control system is divided into four systems: clamping control system, ratio control system, torque converter control system and shifts control system. And then a simulation model is built for the electronic-hydraulic control system. The simulation model is verified by indoor rig tests. The test results show that the simulation model could be used to forecast the dynamic response of the vehicle with sufficient precision, and it also could be used to simulate or evaluate the effectiveness of control strategy used in the powertrain with Double state CVT.The characteristics of line pressure control valve and ratio control valve were studied on the test-rig. The test results show that both of the two valves are nonlinear, and their characteristics depend on the speed of engine.Further research was done on the match of engine, torque converter and CVT. Clutch engaging control strategy was presented at the first time, which depended on the match of engine, torque converter and CVT. In economic mode or driving mode, lock up speed was determined according to engine economic speed curve or driving speed curve. Slip control of the lock-up clutch was introduced to avoid the vibration that often occurred at low locking up speed, and driving comfort was improved. Simulation result showed that the control strategy could fulfill the requirements of vehicle.The concept of layer-based control is presented. The design of CVT control system is divided into the top layer control system and the bottom layer control system. The top layer control system determines the object velocity ratio according to the driver's expectation and the driving environment. The output of the top layer control system, the object velocity ratio works as the input signal to the bottom layer control system. And the bottom layer control system is designed to realize the control object specified by the top layer control system. A certain kind of control algorithm is taken, and control signals are output to the velocity ratio control valve and the clamp force control valve to control the real actual velocity ratio close to the given object velocity ratio. The developed CVT control system is versatile and easy to change.For a given driving condition, when the vehicle speed and load are constant, the velocity ratio of the powertrain with CVT changes continuously, so the engine could keep working in the best performance area. With considerations of determining points on the integrated control strategy, the control strategy of CVT powertrain has been improved. And the influence of the control strategy on CVT efficiency is also considered. Some typical conditions are discussed by simulation on the model mentioned above. The simulation results show that the model works well, and the simulation results are consistent with the test results. The new control strategy improved the driveability in transient state, while the fuel consumption in steady state is not affected.The electronic control system of double state CVT was introduced according to the analysis of the Fiat Palio electro-hydraulic control system. The microprocessor is the Infenion C164CI. The electronic control system (including hardware and software) is developed in this paper.The clamping force control and the ratio control is the most important parts of the CVT control technology. For the purpose of improving the efficiency of CVT, the requirement for clamping force control is minimizing clamping force while guaranteeing the transfer of torque. The requirement for ratio control is changing ratio speed at a certain value to the object and minimizing the steady state error. A feedback control system was introduced for the first time , in which the metal belt slip rate worked as the control variable. A displacement sensor was used to measure the movable sheave of primary pulley and then to estimate the slip rate. To control slip rate in a reasonable region can guarantee the transfer of torque while minimizing clamping force. Considering the nonlinearity of the control system, a fuzzy PID controller is introduced to the line ratio control system. The test results show that the two controllers can be used in the practical system.A CVT performance test rig was built. A series of tests were carried out. The test results showed that, compared with the original control strategy, the new control strategy could cut down the vehicle fuel consumption, so it is more practical.