Research on Automobile Electric Power Steering System Based on ARM Single Chip Microcomputer

Electric power steering system (EPS) is one of the hot topics in the field of automotive engineering. The main research content at present is the design of control rules and hardware controller (ECU) of EPS system, and the implementation of control rules must be based on a stable and reliable controller. Most of the existing controllers are based on 8-bit microcontrollers with enhanced functions, and some use DSP. At present, it is an inevitable trend for the development of embedded technology to use 32-bit processors as the core of high-performance embedded system development. ARM processors have been widely used in the field of 32-bit microcontrollers due to their outstanding advantages, and they are ranked first in the world in 32-bit embedded system applications. In the field of automotive electronic technology, from body control, chassis control, engine management, active and passive safety systems to in-vehicle entertainment and information systems, they are inseparable from the support of embedded technology. Therefore, ARM processors have good application prospects in the field of automotive electronics. This paper studies the structure and working principle of electric power steering system (EPS) and its controller (ECU), and on this basis, studies the electric power steering control system based on ARM S3C44B0X microcontroller.

1 EPS working principle 

Figure 1 is a typical electric power steering system schematic. When the car turns, the torque sensor measures the output torque of the steering wheel and sends it to the controller ECU. The controller then integrates the vehicle speed signal sent by the vehicle speed sensor and determines a target current according to the corresponding control strategy to control the rotation of the motor. The output torque of the motor is applied to the steering column output shaft through the clutch and reduction mechanism, and the steering mechanism such as the gear rack deflects the wheel to a certain angle, thereby assisting the steering system.

2 Structure and principle of controller

The controller is mainly composed of A/D acquisition circuit, H-bridge motor control circuit and system protection circuit, and its structure is shown in Figure 2. When the vehicle is started, the system receives the ignition signal and starts to work, collecting torque signal and vehicle speed signal and sending them to the single-chip microcomputer. According to the established control rules, the system determines a target current and the direction of motor rotation, and drives the motor to rotate through the H-bridge circuit in a PWM modulation manner. At the same time, the system samples the output current of the motor, compares the sampling result with the target current on the one hand to control the motor; on the other hand, it combines the vehicle speed signal to protect the system. When the motor current is greater than the set value or the vehicle speed is higher than the set value, in order to protect the safety of the motor and the system, the controller will send a control signal to the relay, disconnect the motor power supply, stop the power assist, and resume the power assist function after the system is normal.

3 Controller Design
　 　
The controller designed in this paper uses the 32-bit ARM S3C44B0X microcontroller as the core of the controller. Since the S3C44B0X microcontroller integrates rich hardware resources, the circuit design is greatly simplified, the reliability of the system is improved, and it also leaves room for future expansion and upgrade of the system. The direction control signal and the PWM signal are combined and loaded into the H-bridge drive circuit through the optocoupler to control the running state of the motor. The circuit is simple and easy. The pulse width modulation method uses unipolar PWM to avoid the possibility of MOS tube direct pass. It is not only reliable, but also easy to adjust the pulse width duty cycle.

3.1 Introduction to ARM S3C44B0X

SUMSUNG's S3C44B0X is based on the ARM7 TDMI architecture, and on this basis integrates a wealth of peripheral function modules, mainly including: 8KB Cache, external expansion storage controller, LCD controller, 2 UARTs, 5 PWM timers and 1 internal timer, 8-channel 10-bit ADC, 71 general programmable I/O ports, 8 external interrupt sources and watchdog timer. At the same time, ARM microcontrollers support C language development, which is conducive to the development and debugging of system control software.

3.2 A/D data acquisition

S3C44B0X uses a successive approximation 10-bit ADC with an input voltage range of 0~2.5V and a conversion accuracy of 2.5V/210=2.4mV. For the torque sensor, its output voltage range is 0~5V, so it only needs to perform low-pass filtering and voltage division on the signal. For the sampling current of the motor, since there is a difference between positive and negative, it should also be converted into a positive voltage through level conversion. The circuit is shown in Figure 3. Among them, the function of R1 is to convert the output signal (0~50mA) of the Hall sensor into a corresponding voltage signal.

3.3 Motor control circuit

The motor control circuit consists of a direction control circuit and an optocoupler-isolated MOSFET H-bridge motor drive circuit. The principle of the motor drive circuit is shown in Figure 4. T1, T2, T3, and T4 are optocoupler components, which are used to isolate the system's strong and weak currents on the one hand, and to drive MOSFET components on the other hand. Therefore, when selecting an optocoupler model, an optocoupler with a higher output power should be selected (such as TLP250, whose maximum output current can reach 1.5A). The motor's PWM control signal and direction control signal are both loaded onto the MOSFET component after passing through the optocoupler. When T1 and T4 are turned on and T2 and T3 are turned off, the corresponding Q1 and Q2 are turned on, and Q2 and Q3 are turned off. The motor current flows to the ground through Q1, MOTOR, and Q4, and the motor rotates forward at this time; when the motor is reversed, the on-off condition of the device is just the opposite.

The direction control circuit is mainly composed of AND gates and NOR gates, and its circuit principle is shown in Figure 5. U1, U2, U3, and U6 are AND gates, U4 and U5 are NOR gates, and the system adopts unipolar PWM modulation. D1 and D2 are direction control signals. There are four combinations of D1 and D2. The motor rotates forward at 11, reverses at 00, and stops at 01 and 10. When D1 and D2 are 11, the AND gates U1 and U2 output high levels, where the signal of U2 is used to drive the optocoupler T4 and turn on the MOS tube Q4. U3 drives the optocoupler T1 according to the PWM signal and the signal of U1, and turns on the MOS tube Q1, that is, Q1 and Q4 are turned on. At this time, the outputs of the NOR gates U4, U5 and the AND gate U6 are low levels, the optocouplers T2 and T3 are cut off, the MOS tubes Q2 and Q3 are turned off, and the motor rotates forward. When D1 and D2 are 00, the situation is just the opposite. T1 and T4 are cut off, Q1 and Q2 are turned off, T2 and T3 are turned on, Q2 and Q3 are turned on, and the motor reverses. [page]

3.4 Circuit protection design


The protection circuit is mainly composed of MOSFET buffer circuit and system relay protection circuit. Switching devices may be subjected to overvoltage, overcurrent, excessive di/dt, du/dt and excessive instantaneous power during the opening and closing process. The buffer circuit is a protective measure to protect the switching device during the switching process and suppress high voltage and high current. This design adopts RCD charging and discharging buffer circuit, as shown in Figure 6. When the MOSFET is turned off, the capacitor C is charged through the diode D. Since the voltage drop of the diode is very small when it is forward-conducted, the overvoltage absorption effect during the shutdown is equivalent to the absorption effect of the capacitor. When the MOSFET is turned on, the capacitor C discharges through the resistor R, limiting the opening peak current in the MOSFET. The RCD buffer circuit can effectively improve the switching characteristics of the switching device and reduce the power consumption and heat generation of the switching device itself.

The relay protection circuit is mainly used for overcurrent protection of the motor and to ensure that the EPS works within the set vehicle speed range. The ECU ensures that the motor works within the rated current range by sampling the motor current. Once the motor current is higher than the set protection value, or the vehicle speed exceeds the set range, the ECU will send a shutdown signal to the relay to cut off the power supply to the motor and stop assisting.

4 System control strategy and simulation

Domestic and foreign scholars have studied different EPS control strategies, such as PID control [5-6], H_∞ robust control, fuzzy control, etc. Since the input characteristics of the torque signal and the vehicle speed signal are very suitable for fuzzy control, and PD control has better control performance, this paper adopts the fuzzy PD control strategy by combining the characteristics of these two methods. The control structure block diagram is shown in Figure 7.

Here, the system input is the ground reaction torque, Tsw is the steering wheel holding torque, Kp is the proportional coefficient of PD control, Kd is the differential coefficient of PD, Ia is the target current, and Tm is the motor output torque. The fuzzy controller collects the torque sensor signal and adjusts the Kp and Kd parameters online for PD control, which then determines the target current of the system. The above control strategy is applied to simulate the in-situ steering of the EPS system in the MATLAB environment. Given the input curve of the steering wheel steering torque shown in Figure 8, the system current upper limit is set to 30A. The current response of the EPS system to the input obtained by simulation is shown in Figure 9.

It can be seen from the simulation results that by adopting this control strategy, the motor output current has a good tracking performance for the steering wheel input torque, which shows that the fuzzy PD control strategy studied in this paper has a good power-assisting effect.