Design of ABS/ASR/VDC fault diagnosis system based on dual MCU architecture

集成了防抱死制动系统ABS(Anti-lock Braking System)、驱动防滑控制系统ASR(Acceleration Slip Regulation System)与车辆动力学控制系统VDC(Vehicle Dynamic Control System)的ABS/ASR/VDC集成系统是汽车主动安全性控制系统的核心装置之一。该系统可显著提高车辆的制动性、驱动性、转向可操纵性和横向稳定性，减少轮胎磨损和事故风险，增加行驶安全性和驾驶轻便性[1]。

为提高系统的可靠性，世界各大汽车整车厂或零部件厂商在推出的ABS/ASR/VDC产品中都配有故障诊断系统。该系统通过有关电气元件状态参数的在线测试，监控ABS/ASR/VDC系统的工作状况，实现了系统自诊断。

ABS/ASR/VDC系统常见的主要故障发生在电磁阀、轮速传感器、电源、电子控制单元ECU（Electronic Control Unit）、电磁阀总开关等部位[2]。在ABS/ASR/VDC故障诊断系统中，要对稳压电源、轮速处理电路、电磁阀驱动电路、电磁阀总开关等进行监测。当ABS/ASR/VDC系统出现故障时，关闭电磁阀总开关，使ABS/ASR/VDC退出工作，恢复到常规制动与驱动，同时存储故障代码，供维修时使用。故障代码可以通过不同的方式显示：由仪表盘的故障警告灯闪烁故障代码；由仪表盘上的显示屏直接显示故障代码的数字和信息资料；用专用的故障检测仪连接到诊断座上，读取故障代码[3]。

现代汽车上装备的ABS/ASR/VDC系统的故障诊断过程一般可分为三个阶段[4]：(1)系统静态自检；(2)汽车起步时的动态自检；(3)汽车行驶中的定时动态自检。

1 ABS/ASR/VDC系统关键部件的故障诊断电路

ABS/ASR/VDC系统ECU主要实现轮速信号采集与处理、控制软件存储与运行、压力调节器电磁阀驱动以及与其他ECU或计算机进行通信等功能。目前国际上几大ABS/ASR/VDC系统生产厂商都采用了主、辅双MCU的总体设计方案：主MCU主要负责信号采集、计算处理，并根据控制逻辑产生相应的控制指令输出到系统执行机构；辅MCU主要负责检测主MCU运行状况，并具备一定故障检测和应急处理功能，当检测到主MCU不能正常工作或发现故障时，ABS/ASR/VDC及时退出控制并恢复常规制动与驱动。本文研究并设计了基于双MCU架构的ABS/ASR/VDC故障诊断系统。

1.1 电磁阀故障诊断电路

MCU对轮速输入数据进行分析、处理后，经一定的控制逻辑判断后输出相应的控制信号。控制信号必须经过功率放大后才能驱动执行机构。驱动电路的主要作用是把MCU输出的TTL电平转换为执行机构所需要的驱动电平，而且把很小的电流放大到足够驱动执行机构。另外，由于驱动执行机构动作时电流大、变化快，处理不当将对电源电压干扰很大、引起较大波动。为了减小干扰，在驱动电路和其他电路之间进行电气隔离。驱动电路附带有故障监测电路，实时监测电磁阀工作状态，及时将故障信息反馈给MCU。电磁阀驱动及其故障诊断电路如图1所示。

1.2 Wheel speed sensor fault diagnosis circuit

Static faults of magnetoelectric wheel speed sensors include short circuit and open circuit of the electromagnetic coil inside the sensor. The hardware fault diagnosis circuit can make judgments and monitor during the system self-check. This paper designs a voltage divider circuit, which reflects the internal resistance of the sensor by measuring the voltage divider value on the electromagnetic coil of the sensor, so as to judge whether there is a short circuit or open circuit fault. CD4066 (four-channel bidirectional analog switch) is selected to control the voltage divider circuit and the wheel speed signal output to work in time. The total voltage of the voltage divider circuit is +5 V, which is connected to the resistor R, the chip CD4066, the sensor internal resistance and the ground to form a loop. Figure 2 shows the wheel speed sensor fault diagnosis circuit diagram.

The working principle of the circuit is that when PA1 outputs a high level, pins 6 and 12 are high, and control pins 8 and 9 as well as pins 10 and 11 are all turned on. At this time, the +5 V power supply voltage passes through the internal resistance of RC101 and CD4066, the internal resistance of the sensor to the ground to form a loop. The voltage value at PAD01 indirectly reflects the internal resistance of the sensor, and is connected to the AD conversion channel of the auxiliary MCU. The conversion value is compared with the short-circuit limit voltage value 3.05 V and the open circuit limit voltage value 4.5 V to infer whether the sensor has a short circuit or open circuit fault; when PA1 outputs a low level, after the inverter, PA1 outputs a high level, which is input to pins 13 and 15, and control pins 1 and 2 as well as pins 3 and 4 are all turned on, so that the wheel speed signal output by the sensor enters the wheel speed processing circuit.

1.3 MCU fault diagnosis circuit design

In order to ensure the safe and reliable operation of the main MCU, the SPI (Serial Peripheral Interface) interface communication circuit is designed, and the auxiliary MCU monitors the main MCU through communication. SPI is a high-speed and efficient synchronous serial interface, mainly used for MCU to exchange data with external interface chips. By pulling up and pulling down the slave select (SS) pin respectively, the main MCU is set to the host mode and the auxiliary MCU is set to the slave mode. The specific SPI communication circuit is shown in Figure 3.

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2 Fault diagnosis interface circuit design

The current internationally common fault diagnosis interface and standard is OBD-II, which includes three forms: SAE J-1850 PWM, SAE J-1850 VPW and ISO 9141. LIN (Local Interconnect Network)[5] is a low-cost serial communication network that complies with the ISO9141 protocol specification. It is widely used in automotive distributed electronic system control and fault diagnosis. Its goal is to provide auxiliary functions for existing automotive networks. Therefore, the LIN bus is an auxiliary bus network. In situations where the bandwidth and multi-functions of the CAN bus are not required (such as communication between smart sensors and brake devices), the use of the LIN bus can greatly save costs. The LIN network has also become an international standard fault diagnosis protocol interface.

This paper adopts the ISO9141-2 protocol and selects the single-ended bus transceiver SI9243A[6] produced by Vishay Siliconix as the bidirectional communication chip. The chip design meets the requirements of the ISO9141 fault diagnosis system. It has a built-in K-line driver for bidirectional communication and an L-line receiver that wakes up before data transmission. The communication circuit is shown in Figure 4.

3 FAULT DIAGNOSIS SOFTWARE DESIGN

The software of ABS/ASR/VDC fault diagnosis system consists of two parts, namely the initial self-test when the system is powered on and the car starts, and the online test during driving.

During the system self-check, the fault indicator light will light up first, and the fault indicator light and its circuit can be checked for faults. If the self-check passes, the fault indicator light will go out after about 3 seconds, and the system self-check is over. If a fault is found in the system during the self-check, the fault information will be stored in the form of a fault code, and the fault indicator light will continue to light up to remind the driver that the ABS/ASR/VDC system has a fault. At the same time, the ABS/ASR/VDC system exits, and normal braking and driving are restored. If no fault is detected during the self-check, the software continues to run.

The initial self-check items mainly include:

(1) Detection of fault information already stored in the system and review of certain fault information;

(2) Detect the working status of the main and auxiliary MCUs through SPI communication;

(3) Check the main switch of the solenoid valve: Open and close the main switch of the solenoid valve, and determine the working condition of the main switch of the solenoid valve by measuring the value of the power supply voltage VBB of the solenoid valve driver chip;

(4) Check the function of the solenoid valve: drive the solenoid valve to work and determine whether it is working normally;

(5) Check for static wheel speed sensor failure and excessive wheel speed difference when the vehicle starts;

(6) Detection of key software parts to determine whether the program is running normally.

During the working process, the working status of key parts should be monitored in real time through the ABS/ASR/VDC fault diagnosis system. If a fault is found, it should be handled immediately. Online fault diagnosis mainly includes dynamic detection of wheel speed signals, real-time monitoring of solenoid valves and real-time monitoring of the main MCU.

The wheel speed real-time diagnosis program uses a certain algorithm to determine whether the wheel speed signal is abnormal. The program logic judgment is shown in Figure 5. When the absolute value of the front wheel speed difference and the rear wheel speed difference exceeds the set threshold value, the program logic is used to determine whether each wheel speed signal has a fault. In the figure, DWF, DWR, DWL, and DWP are the absolute values ​​of the front wheel speed difference, rear wheel speed difference, left wheel speed difference, and right wheel speed difference respectively; DW0 is the difference threshold value of the front wheel speed difference and the rear wheel speed difference, and DW1, DW2, DW3, and DW4 are the threshold values ​​of DWF, DWR, DWL, and DWP respectively. Taking into account road regulations and actual driving conditions of the car, each threshold is initially determined through theoretical calculation and then corrected through experiments. The corrected threshold values ​​are: DW0=2 km/h, DW1=6 km/h, DW2=5 km/h, DW3=7 km/h, DW4=7 km/h.

4 Fault diagnosis test verification

During the calibration test of the ABS/ASR/VDC system, when there is a sudden unexpected fault such as the solenoid valve or wheel speed, the fault indicator light can light up and exit the ABS/ASR/VDC control at the same time. This shows that the designed fault diagnosis system can accurately realize the fault diagnosis and processing of the solenoid valve, wheel speed sensor, etc. The communication between the ECU and the fault diagnosis instrument can realize the functions of reading, displaying or clearing the fault code.

The designed fault diagnosis system was applied to the independently developed ABS/ASR/VDC integrated system, and a real vehicle road test was carried out. The test results show that the developed fault diagnosis system can detect key component faults in time, store fault codes, exit ABS/ASR/VDC control, and ensure driving safety. The ECU design based on the dual MCU architecture enhances the system's fault diagnosis capability, and in some special cases, the auxiliary MCU can replace the main MCU to work, greatly reducing the failure probability of the ECU.