Research on Electronic Control Unit of Automotive Electric Power Steering System

Research on electronic control unit of automotive electric power steering system1 Introduction

With the development of electronic control technology and its wide application in automotive field, electric power steering system Electric power steering (EPS) has increasingly become one of the hotspots of automotive electronic technology research. Compared with the traditional steering system, EPS system has simple structure and great flexibility, can obtain ideal handling stability, can dynamically adapt to the changes of vehicle driving conditions, and is also fully displayed in handling comfort, safety, environmental protection, energy saving and easy maintenance Its advantages [1] At present, electric power steering has partially replaced hydraulic power steering and has been widely used. For example, Dafa, Mitsubishi and Honda Motor Companies in Japan, Delphi Automotive Systems Company in the United States and ZF company in Germany have successively developed and assembled their own EPS. The research on EPS system in China started late, only Tsinghua University, Huazhong University of science and technology, Jilin University and Hefei University of technology The university has carried out research on system structure scheme design, system modeling and dynamic analysis, but it is in the stage of theoretical exploration and experimental research. Some domestic automobile manufacturers such as Chongqing Chang'an, Nanchang Changhe, Dongfeng, FAW and other joint research with colleges and universities are also in the primary stage of development and do not reach the practical level [2].

2. Hardware composition and working principle of EPS system2.1 hardware composition of EPSEPS is a power steering system that directly relies on electricity to provide auxiliary torque. Its structure is shown in Figure 2-1. It is assisted by the motor controlled by the electronic control unit (ECU). The system is mainly composed of electronic control unit, torque sensor, angle sensor and speed sensor (which can be shared with other systems) , DC motor, clutch, electromagnetic relay, deceleration mechanism and steering mechanism.

Figure 2-1 EPS system structure2.2 working principle of EPS

When the ignition switch of the vehicle is closed, the ECU will power on and start the self inspection of the EPS system. After the self inspection is passed, the relay and clutch will be closed, and the EPS system will start to work. When the steering wheel rotates, the angle sensor and torque sensor on the steering shaft will transmit the measured angular displacement and torque on the steering wheel to the ECU. The ECU will combine these two signals with vehicle speed and other signals Control the motor to generate corresponding power to achieve the best control in the full speed range: when driving at low speed, reduce the steering force to ensure the flexible and light steering of the vehicle. When driving at high speed, appropriately increase the damping control to ensure the stable and reliable operation of the steering wheel.

3 ECU system design based on PIC MCUFigure 3-1 schematic diagram of ECU system structure3.1 working principle of ECU

The control core of the system is PIC16F877 single chip microcomputer, and the structure of the control unit is shown in Figure 3-1. The whole system is powered by the on-board 12V battery. When the ECU works, the torque, angle, speed, temperature and other sensors process the collected signals through the input interface circuit and then send them to the corresponding port of the single chip microcomputer. The single chip microcomputer analyzes and processes these data according to the system power assisted characteristics and corresponding algorithms Determine the size and direction of the boost current, send the pulse command through the PWM port of the single chip microcomputer and the commutation command through the corresponding commutation control port, and control the operation of the DC motor through the driving circuit and H-bridge circuit. The driving circuit of the motor is equipped with a current sensor, which feeds back the detected actual working current of the motor to the single chip microcomputer through the current detection circuit The chip machine realizes the closed-loop control of the motor according to the corresponding control algorithm. If the EPS system works abnormally, the single chip microcomputer will drive the EPS light to give an alarm, disconnect the relay and clutch, exit the electric power assisted working mode and switch to the manual power assisted mode [3].

3.2 introduction to PIC16F877 single chip microcomputerThis model is an 8-bit RISC single chip microcomputer produced by microchip company of the United States. It has the characteristics of high-speed data processing (execution speed up to 120ns). PIC16F877 has its own watchdog timer, EEPROM with 256bytes, flash memory with 8K space, 8-way 10 bit AD conversion function, 2 pulse width modulation CCP modules and online burning debugging (ISP) Function, wide voltage operation and high reliability. PIC16F877 has an 8-level depth hardware stack, each byte bit of RAM area can be addressed, and there are 4 special bit operation instructions and 2 shift instructions.3.3 selection of DC motor

Brushless DC motor has obvious advantages in control characteristics, efficiency, torque pulse, manufacturing cost, etc. the project uses permanent magnet brushless DC motor as the driving source.3.4 selection of torque and angle sensorsIn this paper, the torque and position composite sensor of Italian Bi company is used. In addition to providing torque signal, the sensor also provides steering wheel position signal, which facilitates the development of centering and damping logic.

3.5 design of motor drive control circuit

The motor drive control circuit must be able to adjust the motor speed and output torque with high precision and fast, so as to meet the requirements of real-time and reliability of EPS system. The core control of backward channel in this project adopts pulse width modulation (PWM) Control H-bridge circuit. There are many PWM control modes for DC motor. According to the actual needs of motor work and the overall requirements of the system, the limited unipolar reversible PWM control mode is adopted in the project. The main advantages are that it can avoid the same arm conduction of switch tube, high operation reliability, no delay circuit and relatively high switching frequency. It is especially suitable for high power, large moment of inertia and reliable operation DC motor control occasions with high requirements.

3.5.1 motor drive circuitThe drive circuit of the motor mainly consists of FET bridge circuit, FET base drive circuit, current sensor and relay on the motor drive circuit.The FET bridge circuit is mainly composed of four high-power MOSFET power tubes, which are required to have good switching characteristics, bear large driving current and have long service life. According to the power parameters of the motor and the limit parameters and electrical characteristics of the power tube, we use four identical n-channel irfp250 power tubes to form the H-bridge circuit.

The MOSFET special gate integrated circuit ir2109 is selected as the core module of the FET base driving circuit. The chip is a single channel, gate driven, high-voltage and high-speed power device. The highly integrated level conversion technology is adopted, which greatly simplifies the control requirements of the logic circuit for the power device. The upper tube is powered on by an external bootstrap capacitor, which greatly reduces the number of driving power supplies and controls the circuit The volume of the board reduces the cost and improves the system reliability [4].

The driving circuit is shown in Figure 3-2. The in terminals of the two ir2109 are the signal pulse input terminals of the upper and lower power tubes driving the same arm of the H-bridge, which are respectively connected to the two PWM pulse output ports of the PIC16F877 single chip microcomputer through the 6n137 optocoupler with high-speed performance; the two SD terminals are respectively connected to an I / O port of the single chip microcomputer to control the motor shutdown operation; the Ho and lo terminals of each chip are respectively connected to the same bridge The power tube of the arm is connected to control the motor speed; the VB end is connected to the 12V power supply through the bootstrap diode uf1005. In order to block all the voltage borne in the special circuit, the diode uf1005 with ultrafast recovery characteristics is selected here.

Figure 3-2 motor drive circuit3.5.2 motor current sampling circuit

The current sampling of the system has two purposes: one is to provide protection for the motor; the other is to feed back the armature current signal through the current sensor for closed-loop control of the armature current. The standard resistance is a common current sensor. Because it is simple and reliable, the resistance value is stable, high precision, good frequency response and the output voltage is directly proportional to the current flowing, it is used in the PWM system The standard resistance is generally made of manganese copper or silicon manganese copper. In the sampling circuit, ad626 is selected to amplify the sampling signal by 10 times and send it to the corresponding port of the single chip microcomputer. The specific circuit is shown in Figure 3-3.

Fig. 3-3 motor current sampling circuit3.6 relay control circuitAs shown in Figure 3-4 below, after the CPU control signal is output through the CPU port psp0, the switch Q1 turns on and drives the power triode Q12 to power on the relay and close the node. After the relay node is closed, it can supply power to the motor and clutch. The high and low level signals output by the CPU control the closing and opening operation of the relay respectively.

Figure 3-4 relay control circuit design4 Conclusion

Based on the analysis of the principle and power control process of EPS system, this paper studies and designs the hardware circuit of EPS control system, puts forward the use of limited unipolar reversible PWM control mode to control DC motor, and explores the control method of low-voltage, low-speed and high current permanent magnet brushless DC motor in automotive electric power steering system After completing the hardware circuit design and software programming, the bench test is carried out on the EPS system according to the predetermined boost characteristic curve. The test results show that the electronic control unit has high real-time signal acquisition, good follow-up to the motor closed-loop control, and the whole system has good electrical performance Dynamic assistance characteristics, anti-interference ability and reliability of hardware are very high.

Innovation points1. PIC16F877 single chip microcomputer is used as the core of electronic control unit.2. The restricted unipolar reversible PWM control mode is used to control the DC motor.