Detailed explanation of the design of the electronic control unit of the automobile power steering system

   The steering system is an important part of the car, and its performance directly affects the stability and safety of the car. The early car steering system was a pure mechanical steering system without power assistance. The steering power was completely provided by the driver, and the driving experience was poor. Since the 1930s, power steering systems have gradually emerged. At present, there are three main forms of automotive power steering: hydraulic power steering system (HPS), electric hydraulic power steering system (EHPS) and electric power steering system (EPS). Compared with the first two, EPS is provided with auxiliary torque by the motor and has no oil system, which greatly reduces the complexity of the car steering system and has obvious advantages in fuel efficiency, modularity, power effect and environmental friendliness. According to the different positions of the EPS power motor on the gear and steering column assembly, the EPS system is divided into four types: steering column power-assisted type, rack power-assisted type, pinion power-assisted type and double pinion power-assisted type. Pinion and steering column power-assisted type are used in light vehicles, and double pinion power-assisted type is used in heavy vehicles. They all have three basic components: the Electric Control Unit (ECU), the power motor and the torque sensor installed on the steering column. In this paper, the EPS controller is designed for small cars with the 16-bit single-chip microcomputer MC9S12DP256 from Freescale Company of the United States as the core.

　　1 Electric power steering system structure and working principle

　　The structure of the electric power steering system is shown in Figure 1. It is mainly composed of a steering wheel, a torque sensor, an electronic control unit (ECU), a motor, an electromagnetic clutch, a reduction mechanism, and a rack and pinion steering gear. After the car engine is ignited, when the steering wheel is turned, the steering torque is measured by the torque sensor installed on the steering shaft and sent to the ECU. The ECU calculates the optimal current required by the motor based on the torque and vehicle speed through the pre-set power characteristic curve and control strategy, thereby controlling the motor output torque and rotation direction, and then applying it to the steering mechanism through the reduction mechanism, and finally obtaining a steering force that is adapted to the driving conditions to assist the driver in steering.

　　2 Control strategy

　　2.1 EPS model establishment

　　According to Newton's laws, a mathematical model of the steering system can be established.

　　Where: Th is the steering wheel input torque, Js is the moment of inertia of the steering column and wheel assembly, Bs is the input shaft damping coefficient, Ks is the torque sensor stiffness coefficient, Tm is the motor output torque, Km is the stiffness coefficient of the power-assisted motor and the reduction mechanism, Jm is the moment of inertia of the power-assisted motor, Bm is the power-assisted motor damping coefficient, M is the rack mass, Br is the rack and steering wheel viscous damping coefficient, Kr is the rack equivalent stiffness, G is the power-assisted mechanism transmission ratio, rp is the pinion radius, θs is the steering wheel angle, θm is the motor angle, xr is the rack displacement, and Fr is the steering resistance.

　　2.2 Design of power assist characteristic curve

　　The EPS power-assist characteristic is the relationship between the driver's input torque and the motor power-assist torque (power-assist current). When the car is driving, the steering resistance decreases as the speed increases. In order to obtain the ease of steering when the car is driving at low speeds and the stability when driving at high speeds, under the same driving conditions, the motor power-assist torque decreases as the speed increases, and when the speed exceeds a certain range, the motor does not assist. There are three common power-assist characteristic curves: linear, broken line, and curved. The linear power-assist characteristic curve is simple in form and easy to adjust and implement in practice. Therefore, this article uses the linear power-assist characteristic for controller design. The linear power-assist characteristic can be expressed as the following functional relationship:

　　Where: Th is the motor target torque, f(v) is the vehicle speed induction coefficient, Tmax is the maximum motor assist torque, Td0 is the minimum torque input by the driver when the assist starts, and Tdmax is the driver input torque when the motor provides maximum assist.

　　The power assist characteristic parameters are determined as follows: Td0=1 Nm, Tdmax=7.6Nm, Thmax=21Nm. The vehicle speed induction coefficient is determined according to the rules shown in Table 1 (finally, it needs to be corrected after actual vehicle testing). When the vehicle speed exceeds 80 km/h, the motor does not assist.

　　According to the above parameters, the power assist characteristic curve is designed as shown in Figure 2.

　　The motor target current can be obtained by formula (8):

　　Where ki is the motor torque coefficient, and G is the transmission ratio of the motor reduction mechanism.

　　2.3 Control Algorithm

　　EPS system control is the control of the motor current and direction. The quality of its control algorithm directly affects the performance of the steering system. This article adopts the PID control algorithm that is currently widely used in the field of industrial control. PID control has high stability and reliability, strong real-time performance, and simple control and debugging methods. It is easy to implement and is suitable for the control of automotive electric power steering systems. Therefore, PID control is the main control strategy of EPS control systems at this stage.

　　The expression of PID control can be expressed as:

　　Where: r(t) is the target current value of the power assist motor, y(t) is the actual output value of the control system, u(t) is the output signal of the PID controller, TI is the integral time constant, and Td is the differential time constant. The core of the EPS control system is a single-chip microcomputer that uses digital signals. Therefore, it is necessary to process equation (10) as follows:

　　In order to reduce the amount of calculation and improve the real-time performance of the steering system, this design adopts incremental PID control and uses the increment of the control amount △u as the output of the controller. The implementation method is as follows:

　　Assuming the target current of the assist motor is i and the actual current assist current is io, the control deviation is:

　　ek=i-io (12)

　　△u=u(k)-u(k-1) (13)

　　The target current of the power-assist motor can be calculated by the microcontroller according to the current vehicle speed, input torque, and power-assist characteristic curve. Then the corresponding PWM increment △u can be obtained by equations (11), (12), and (13).

　　PID parameters can be initially obtained by trial and error and then modified based on the test results.

　　3 Hardware Design

　　3.1 Overall Design

　　The microcontroller is the core of the controller, and its selection needs to consider many factors such as applicability, reliability, on-chip resources, price, etc. The appropriateness of the microcontroller selection directly affects the performance of the mechanism control system and the degree of design difficulty. This design uses Freescale's 16-bit high-precision MC9S12DP256 microcontroller. MC9S12DP256 has 5 built-in CAN modules, 2 8-channel 10-bit A/D conversion modules, 8 PWM channels, a bus speed of 25 MHz, a 5 V power supply, and a 112-pin LQFP package. This microcontroller has rich internal resources, which can greatly simplify the hardware circuit of the control system. It has high reliability and is very suitable for EPS control. The pins not used in the design are led to the circuit board for subsequent development.

　　The hardware design is shown in Figure 3. The vehicle speed, engine, and torque signals are processed and sent to the MC9S12DP256 microcontroller. After calculation by the microcontroller, the motor assist current value is obtained. After passing through the drive circuit, it acts on the assist motor to control the magnitude and direction of the motor output torque. At the same time, the motor current is sampled and sent back to the microcontroller to form a closed-loop control. Based on the assist control, the motor protection circuit and the fault diagnosis and prompt circuit are designed. Once a fault is detected, the clutch is immediately disconnected, pure manual steering is used, and a fault signal is issued, thereby ensuring driving safety.

　　3.2 Control system hardware circuit design

　　The hardware circuit design mainly includes power conversion circuit, torque signal processing circuit, vehicle speed signal processing circuit, CAN communication circuit, and clock circuit. The specific design is as follows:

　　Power conversion: Since the pin voltage of the microcontroller is +5 V when it is working, and the vehicle power voltage is +12 V, it is necessary to convert the +12 V voltage to +5 V. In this design, the 7805 voltage conversion chip is used for voltage conversion.

　　Torque signal processing Since the torsion sensor obtains some weak signals, which are easily disturbed, it needs to be filtered. This design adopts a filter circuit, and R12 takes a large resistor to increase the input impedance.

　　The vehicle speed processing circuit The vehicle speed signal is a +12 V unipolar square wave, which is too high to be used directly in a single-chip microcomputer and needs to be converted into a square wave within +5 V. The LM358 is used to process it, and after conversion, a square wave signal with a high level of 3.72 V and a low level of 0.01 V is obtained.

　　The CAN bus driver circuit MC9S12DP256 has a CAN bus controller integrated inside, and the CAN driver circuit only needs physical layer driver. This design uses the 82C250 chip for design.

　　Clock circuit is the basis of microcontroller operation. The MC9S12DP256 microcontroller integrates a voltage-controlled oscillator, which can be connected to a phase-locked loop circuit and a 16MHz crystal oscillator circuit at its 43, 44 and 46, 47 pins respectively. This constitutes the MC9S12DP256 clock circuit and provides a 25MHz clock signal.

　　The specific circuit design is shown in Figure 4.

　　4 System Software Design

　　The EPS control software adopts modular design, including system initialization, signal acquisition, control state judgment, control mode judgment, PWM duty cycle calculation, system state monitoring and protection, current closed-loop module, communication module, etc. The EPS control system needs to perform multiple tasks at the same time. In order to ensure the real-time and reliability of the system, the interrupt service method is adopted to divide the entire software into the main program and the interrupt service subroutine. The main program design process is shown in Figure 5.

　　5 Conclusion

　　This paper analyzes the working principle of the automotive electric power steering system. A linear power-assistance characteristic curve is designed, and an incremental closed-loop PID control strategy is established to reduce the amount of chip calculations and enhance the power-assistance follow-up of the system. By utilizing the rich internal resources of the MC9S1 2DP256 microcontroller, the EPS hardware circuit system is simplified, the interference between circuits is reduced, and thus the system reliability is improved. The EPS control system hardware circuit based on MC9S12DP256 is designed, and the software design process is given. The EPS system designed in this paper can write a variety of EPS control algorithms, which is conducive to subsequent in-depth research. The optimization of control performance will be carried out in further control strategy research and experiments.