| PrefaceElectric power steering (EPS), which firstly use an electric motor withan electronic controller in steering system, provide turning force with the motor. Until now, Many studies have been done on assistant and wheel return. However, there are little research concerned about the problems during assistance, especially the improvement of the effect of assistant in experimentation. In this study, research by experimentation and theory is developed,which is concerned about these problems. factors decreased the steering feel and dynamic response is mentioned. At the same time, compensation arithmetic, which is aimed at the improvement of effect of assistant, was formulated, the effect was validated by the bench test.Main ContentsThe paper contain 5 chapters, and the main contents as follows: In chapter one, firstly, the history of the EPS system and the latest Research&App.was introduced. Secondly, the difficulties in EPS was mentioned. At last, the construction and operation of this system, especially four type of EPS, was introduced.In the last section of chapter one, it is concluded that EPS systems is superior to conventional systems, it is also concluded the future of EPS system.In chapter two, research is mainly concerned about the basic control strategy and the control of DC motor.The goal of EPS is through the change of the electrical machinery output of force, makes the hand power change according to the steering wheel torque and the vehicle speed changes, provides road feeling for driver, and decrease the noise of sensor and the high frequency disturbance from road. To achieve the function of EPS, the control arithmetic is most important. The arithmetic can be divided two species, one is assistant, the other is arithmetic for returnablity. At the same time, control for damping, inertia and friction are also important, these arithmetic ensure the well steering feeling.Assistant is a control method for decreasing the hand force during steering. For this purpose, the motor torque(after speed reduce) add to pinion through gear box. The control method utilize the basic principle of DC motor, that is the motor torque in proportion to the motor current. According to the table of vehicle speed-steering torque-motor current, the signal of torque sensor and the signal of vehicle speed is send to ECU, after caculating,the aimed current will be send to motor. the feedback signal of motor current then compare to the aimed current through PID control. Modifying the ratio of PWM will then achieve the output of PID controller. By doing these, the steering force will be adjusted to a appropriate area.Return Control Strategy is used to improve the performance of returnablity. It mainly contain two function. one is prevent the incomplete return of steer, the other is prevent the over return of steer. Control for damping, rotational inertia and friction are aimed at improving the response ability and steering feeling. By adding these arithmetic, the period of assistant will becomes smoothness.The characteristic of control system is important for us to fulfill an exact control.DC motor is a important part of EPS. It help for steering with certain logic. the research of the characteristic of DC motor will help to write a better strategy. In the section of DC motor control, the basic characteristics and model of DC motor is referred, and three kinds of control method is also refered. At the end of this section,PWM (pulse width modulation)control technology, which can change the motor current through modifying the motor voltage, is introduced.In chapter three, the construction and details of EPS test-bed is introduced, as well as the ECU ,the drive of motor, the testing and control system building by VC++6.0.the filter method of signal is then introduced. At the end of chapter three, some parameter of the EPS system is measured.EPS test-bed include bench, steering gear, the simulation equipment of road force, steering wheel equipped torque and angle sensor. A D64 chip, produced by FreeScale,is used as the main chip for control system. It is responsibility for capturing and managing signal, outputting the PWM signal, executing the arithmetic. The principal part of the motor drive is composed of four MOSFET(metal-oxide semiconductor field-effect transistor),these four MOSFET make up of a H-bridge ambipolar and reversible circuit containing a current sensor.Using IR2130 to drive H-bridge.The testing and control system building for monitoring the status of system. With the data get from CAN card. it can observe the status of ECU,and debug the program easier. The control computer use USB to get data, whereas the ECU send data through CAN port, so it need add a CAN card to conversate the data. At the end of this section, some necessary parameter were measured, it consist of the rotational inertia of system and the rotational inertia of motor, the resistance torque from road and the parameter of motor.In chapter four, the basic strategy of assistant and some compensation strategy were tested by the bench test.At the beginning of this chapter, the basic, Power-Assisted strategy were tested by the bench test. Then, the guideline for evaluating the assistant effect were introduced. It also analyze the problem with the basic strategy. To solve these problem, some compensation arithmetic introduced, the effect of these compensation arithmetic were tested. the result proved in effect.Some problem were exposure during the test of the basic strategy, such as time lag, the steering feeling become some lock, libration of steering wheel, stabilization of system is poor. Despite the basic strategy makes the steering force decline, the steering feeling is coarse without smooth.The reason result in these problem were discussed in the other section. In section two, the friction compensation was introduced to reduce the response time and road feeling; Use the rotational inertia compensation to enhance dynamic and solve the lock feeling; torque constant compensation is used to insure the enough assist torque even in large current; use the derivative of torque to harmonize the conflicts exist between high assistance torque and harsh steering feel; introduced a kind of arithmetic, using dynamic search the steady torque region, to settle the shake.The effect of these compensation arithmetic were proved by bench test. these experience will be useful to build more efficient power assist arithmetic and return arithmetic.In chapter five, the contents and conclusions are summed up. Also some advice on the improvement of the later research is given.
|
PDF Full Text Request