Research on Electronic Control Unit of Automobile Electric Power Steering System

1 Introduction

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With the development of electronic control technology and its wide application in the automotive field, Electric Power Steering (EPS) has become one of the hotspots in automotive electronics technology research. Compared with the traditional steering system, the EPS system is simple in structure, flexible, and can achieve ideal steering stability. It can dynamically adapt to changes in driving conditions of the car, in terms of handling comfort, safety, environmental protection, energy saving, easy maintenance, etc. Aspects also fully demonstrate its superiority [1]. At present, electric power steering has partially replaced hydraulic power steering and has been widely used. For example, Japan's Daihatsu, Mitsubishi, Honda Motor Co., Delphi Automotive Systems of the United States, and ZF of Germany have successively developed their own EPS and assembled them. . Domestic research on EPS system started late. Only Tsinghua University, Huazhong University of Science and Technology, Jilin University, Hefei University of Technology and other universities have carried out research on system structure design, system modeling and dynamics analysis, but in theoretical exploration and experimental research. stage. Some domestic automakers such as Chongqing Changan, Nanchang Changhe, Dongfeng, FAW and other universities have also been in the initial stage of development, which has not reached the practical level [2].

2 EPS system hardware composition and working principle

2.1 EPS hardware components

EPS is a power steering system that directly relies on electric power to provide auxiliary torque. The structure is shown in Figure 2-1. It is powered by an electronic control unit (ECU). The system is mainly composed of electronic control unit, torque sensor, angle sensor, The vehicle speed sensor (which can be shared with other systems), DC motor, clutch, electromagnetic relay, speed reduction mechanism and steering mechanism.


Figure 2-1 EPS system structure

2.2 How EPS works

When the ignition switch of the car is closed, the ECU starts to self-test the EPS system. After the self-test passes, the relay and the clutch are closed, and the EPS system starts to work. When the steering wheel rotates, the angle sensor and the torque sensor on the steering shaft The angular displacement on the steering wheel and the torque acting on the steering wheel are transmitted to the ECU. The ECU controls the motor to generate corresponding assistance according to the two signals combined with the information such as the vehicle speed, so as to achieve optimal control in the full speed range: at low speed. It can reduce the steering force and ensure the steering of the car is flexible and light. When driving at high speed, the damping control is appropriately increased to ensure the steering wheel is stable and reliable.

3 ECU system design based on PIC microcontroller


Figure 3-1 Schematic diagram of ECU system structure

3.1 ECU working principle

The control core of the system is the PIC16F877 microcontroller, and the structure of the control unit is shown in Figure 3-1. The whole system is powered by the vehicle 12V battery. When the ECU is working, the torque, angle, speed, temperature and other sensors will send the collected signals to the corresponding port of the MCU through the input interface circuit. The MCU will use the data according to the system's assist characteristics and corresponding algorithms. Analyze and process to determine the magnitude and direction of the boosting current, and issue a pulse command through the PWM port of the microcontroller and the corresponding commutation control port to issue the commutation command, and control the DC motor through the drive circuit and the H-bridge circuit. A current sensor is arranged on the driving circuit of the motor, and the sensor feeds the detected actual working current of the motor to the single-chip microcomputer through the current detecting circuit, and the single-chip microcomputer realizes closed-loop control of the motor according to the corresponding control algorithm. If the EPS system works abnormally, the MCU will drive the EPS light to light the alarm prompt, and at the same time disconnect the relay and clutch, exit the electric assist working mode and switch to the manual manual assist mode [3].

3.2 Introduction to PIC16F877 Microcontroller

This model is an 8-bit RISC single-chip microcomputer produced by Microchip Corporation of the United States. It has high-speed data processing characteristics (execution speed of up to 120ns), PIC16F877 internal watchdog timer, 256Bytes EEPROM, 8k space FLASH Memory, 8-channel 10-bit AD conversion function, 2 pulse width modulation CCP modules, online programming and debugging (ISP) function, wide voltage operation, high reliability. The PIC16F877 has an 8-level deep hardware stack. Each Byte bit in the RAM area is addressable. There are 4 dedicated bit manipulation instructions and 2 shift instructions.

3.3 DC motor selection

Brushless DC motors have obvious advantages in terms of control characteristics, efficiency, torque pulses, and manufacturing costs. This project uses a permanent magnet brushless DC motor as the driving source.

3.4 Torque and angle sensor selection

In this paper, the torque and position composite sensor of Italian BI Company is used. In addition to providing the torque signal, the sensor also provides the steering wheel position signal, which facilitates the development of the return and damping logic.

3.5 Motor drive control circuit design

The motor drive control circuit must be able to adjust the motor speed and output torque with high precision and speed to meet the real-time and reliability requirements of the EPS system. The core control of the backward channel in this project uses pulse width modulation (PWM) to control the H-bridge circuit. There are many kinds of DC motor PWM control methods. According to the actual needs of the motor and the overall requirements of the system, this project adopts the limited single-pole reversible PWM control mode. The main advantage is that the switch can be prevented from being connected to the same arm, and the operation reliability is high. There is no need for additional delay circuit and relatively high switching frequency, especially suitable for DC motor control where high power, large moment of inertia and high reliability are required.

3.5.1 Motor drive circuit

The drive circuit of the motor mainly comprises a FET bridge circuit, a FET base drive circuit, a current sensor on the motor drive line, and a relay.

The FET bridge circuit is mainly composed of four high-power MOSFET power tubes, which requires good switching characteristics, can withstand large driving current, and has a long service life, according to the power parameters of the motor and the limits of the power tube. For the parameters and electrical characteristics, we use four identical N-channel IRFP250 power tubes to form the H-bridge circuit.

The FET base drive circuit uses MOSFET dedicated gate integrated circuit IR2109 as the core module. The chip is a single-channel, gate-driven, high-voltage high-speed power device. It uses highly integrated level shifting technology, which greatly simplifies the logic circuit power. The control requirements of the device, the upper tube is powered by an external bootstrap capacitor, which greatly reduces the number of driving power sources, controls the size of the circuit 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 IR2109s are the signal pulse input terminals for driving the upper and lower power tubes of the H-bridge and the upper arm. They are connected to the PIC16F877 MCU through the 6N137 optocoupler with high-speed performance. PWM pulse output port; two SD terminals are respectively connected with one I/O port of the single chip microcomputer to control the motor to stop operation; the HO and LO terminals of each chip are respectively connected with the power tube of the same bridge arm to control the motor speed; the VB end passes The bootstrap diode UF1005 is connected to the +12V power supply. In order to block all the voltages in the special circuit, the UF1005 with ultra-fast recovery characteristics is used here.


Figure 3-2 Motor drive circuit

3.5.2 Motor current sampling circuit

The system performs current sampling for two purposes, one is to provide protection for the motor; the other is to feed back the signal of the armature current through the current sensor to perform closed-loop control of the armature current. The standard resistance is a commonly used current sensor. Because of its simple and reliable, stable resistance, high precision, good frequency response, and direct output voltage proportional to the current flowing, it is widely used in PWM systems. Standard resistors are generally made of manganese or silicon manganese bronze. In the sampling circuit, the AD626 is selected to send n times of the sampling signal amplification 10 to the corresponding port of the single chip microcomputer. The specific circuit is shown in Figure 3-1.


Figure 3-3 Motor current sampling circuit

3.6 Relay Control Circuit

As shown in Figure 3-4 below, after the CPU control signal is output through the CPU port PSP0, the switch Q1 is turned on and drives the power transistor Q12 to energize the relay and close the node. After the relay node is closed, the motor and the clutch can be powered. The high and low level signals output by the CPU respectively control the closing operation of the relay.


Figure 3-4 Relay Control Circuit Design

4 Conclusion

Based on the analysis of the principle and power-assisted control process of EPS system, this paper studies and designs the hardware circuit of EPS control system, and proposes to control DC motor with limited unipolar reversible PWM control mode. Control method for low voltage, low speed, high current permanent magnet brushless DC motor in steering system. The method of sampling the motor current with precision resistors realizes closed-loop control of the output torque of the DC motor. After the hardware circuit design and software programming are completed, the EPS system is tested according to the predetermined power-assisted characteristic curve. The test results show that the real-time performance of the electronic control unit is higher, and the follow-up of the closed-loop control of the motor is better. Well, the whole system has good electric assist characteristics, and the anti-interference ability and reliability of the hardware part are very high.

Innovation

1. The PIC16F877 microcontroller is used as the core of the electronic control unit.

2. Control the DC motor with a limited single-pole reversible PWM control mode. a

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