ABS control system for hybrid bus based on SAEJ1939

Developing electric vehicles (EV) is the ultimate choice to achieve automobile energy diversification and zero emissions. Since the performance of automotive power batteries is difficult to meet the use requirements, it has become a "bottleneck" that seriously restricts the application and development of electric vehicles. After the 1990s, many automobile manufacturers in the world turned their focus to the research and development of hybrid electric vehicles with strong feasibility. Hybrid-Electric Vehicle (HEV) uses traditional internal combustion engines and electric motors as power sources, and achieves the purpose of saving fuel and reducing exhaust pollution by mixing thermal energy and electricity systems. Compared with electric vehicles, this type of hybrid vehicle can not only maintain the advantages of ultra-low emissions of electric vehicles, but also give full play to the advantages of long duration and good power performance of traditional internal combustion engines. At the same time, it uses energy recovery during braking to reduce braking energy consumption and increase driving range.

In order to improve the fuel economy and energy recovery utilization rate of hybrid vehicles, and ensure the stability and safety of the driving direction during the braking process of the vehicle, the energy recovery strategy of hybrid vehicles and the anti-lock braking system (ABS) control strategy are integrated into a controller, called the ABS controller. Hybrid vehicles use two power sources, with complex structures, and a large number of electronic devices are used on the vehicle, such as the vehicle control unit (HECU), ATM controller, motor controller, battery controller, CAN instrument controller, ABS controller, etc. These complex controllers need to detect and continuously exchange a large amount of data. The traditional connection method is not only cumbersome and expensive, but also has poor reliability and high maintenance costs, and cannot meet the requirements of vehicle communication. Therefore, it is an inevitable trend in the development of modern automobiles to connect various electronic systems with the network to achieve communication. In hybrid vehicles, based on the J1939 communication protocol, the communication between the ABS controller and other controllers is realized to achieve energy recovery during vehicle operation or braking and keep the vehicle in the best control state.

1 Hybrid Bus ABS Control System

1.1 Overview of Hybrid Electric Vehicles

Hybrid-Electric Vehicle (HEV) generally refers to a vehicle that uses both battery power and gasoline (diesel) oil as two power sources. During the driving process of the vehicle, there is a process of energy distribution and energy storage between the battery power and the internal combustion engine mechanical energy, and generally there is a process of braking energy recovery. Hybrid vehicles use the internal combustion engine and motor driving force to drive the vehicle according to the operating conditions, so that each power source can make up for the shortcomings of the other power source while playing its own advantages. In this way, the two complement each other, which can increase the thermal efficiency of the vehicle by more than 10% and improve exhaust emissions by more than 30%. It is both a transitional model from fuel engine vehicles to electric vehicles and a relatively independent model.

The vehicle controller is the core of the hybrid electric bus. It determines the current state of the hybrid electric vehicle based on the input signal, and determines the value of the current control signal sent to each subsystem through certain control logic and control algorithm judgment and analysis. Since hybrid electric vehicles frequently operate between starting, driving, accelerating, decelerating and idling parking on urban roads, the engine and motor are allocated energy according to a certain strategy through the task allocation of the vehicle controller, so that the engine can work in the high-efficiency zone as much as possible and reduce the emission of automobile exhaust. Since hybrid electric vehicles use motors to provide power at startup and run at a higher idle speed after starting the engine, the exhaust emissions of the engine at startup are reduced; the engine can be automatically shut down at traffic lights to reduce idling time; when the vehicle speed is lower than the predetermined speed, pure electric drive can be used to avoid the engine working at low speed. When decelerating, the brake device can convert the mechanical energy during braking into electrical energy and recover part of the energy. These have improved the fuel economy of the vehicle and reduced emissions compared with similar engine models.

1.2 ABS control strategy for hybrid buses

Compared with traditional vehicles, hybrid vehicles can use the traction motor as a generator during braking, relying on the reverse drag of the wheels to generate electrical energy and wheel braking torque, thereby converting part of the kinetic energy into electrical energy while slowing down the vehicle, and recovering part of the energy lost during the traditional braking process for reuse. Therefore, the brake energy recovery system can improve the energy recovery rate of HEVs, effectively reduce vehicle emissions, and improve fuel economy and vehicle mileage.

The purpose of ABS participating in the braking process is to increase the energy recovery part of hybrid vehicles. Compared with traditional vehicles, the braking requirements of hybrid vehicles are different from those of traditional vehicles. In hybrid vehicles, when the driver lifts the accelerator pedal, it means that the driver has a braking demand. If the vehicle electronic control unit (HECU) determines that the battery does not need to be charged at this time, the motor is not required to participate and provide any braking torque and energy recovery. In this case, if the driver steps on the brake pedal, the entire braking process is achieved by the brake air pressure system. When the vehicle's deceleration reaches the threshold value for the activation of the ABS system, the ABS system is activated and independently adjusts the vehicle's braking process. If the vehicle's braking intensity is relatively weak and does not meet the conditions for the activation of the ABS system, the braking torque of the vehicle maintains the original vehicle's braking torque. The above braking process does not participate in energy recovery.

If the driver lifts the accelerator pedal and there is a need to brake, and the vehicle electronic control unit (HECU) determines that the battery is in a low-power state or the battery needs to be charged, the vehicle HECU sends information to the motor electronic control unit ECU, requiring the motor to participate in the braking process, and the maximum braking torque provided by the motor is 20% of the motor output torque at this time, that is, the motor provides constant torque braking. In this case, the motor can independently and fully recover energy during the low-intensity braking process from the driver lifting the accelerator pedal to stepping on the brake pedal. When the driver steps on the brake pedal, if the vehicle's deceleration threshold value does not reach the conditions for ABS system activation, and the braking torque provided by the motor remains unchanged at this time, and the original vehicle's conventional braking system also provides braking torque, the vehicle's braking torque = motor braking torque + original vehicle braking torque. In this medium-intensity braking situation, the motor can fully recover energy. When the driver steps on the brake pedal, if the vehicle's deceleration threshold value reaches the conditions for ABS system activation, the ABS system is activated at this time, and the ABS control unit sends information to the vehicle control unit HECU, requesting the motor brake to be released. In this high-intensity braking situation, the vehicle's braking process is completely independent of the ABS adjustment based on the road conditions the wheels are on at the time, completing the entire vehicle braking process. The above process can achieve vehicle stability, directional controllability and safety during braking, and effectively recover energy.

2 SAE J1939 Protocol

2.1 CAN bus content

The Controller Area Network (CAN) bus is a serial communication protocol developed by the German BOSH company in the early 1980s to solve the real-time data exchange problem between many control units and test instruments in modern cars. After several revisions, the technical specification version 2.0 was formed in September 1991, which includes 2.0A and 2.0B. It is a serial communication network that effectively supports distributed control and real-time control, with a rate of up to 1Mbit/s-1. In order to standardize the compatibility of communication systems with various systems, ISO issued the international standard for high-speed communication area network (CAN) for road vehicle data information exchange in November 1993. In 2000, the American Society of Automotive Engineers (SAE) proposed the J1939 communication protocol based on CAN2.0B, and it became the standard for controller area networks in trucks, buses, agricultural and construction machinery.

CAN complies with the OSI 7-layer reference model. According to the IEEE802.2 and IEEE802.3 standards, its communication interface integrates the physical layer and data link layer functions of the CAN protocol. According to the data type of the information carried, it can be divided into 4 frame formats: the data frame is the main body of the network information and is used for data transmission between nodes. The remote frame is sent by the node to request the transmission of data with the same identifier. The error frame can be sent by any node to detect bus errors. The overload frame is used to provide additional delay between the current and subsequent data frames and remote frames. The data frame consists of 7 different bit fields: frame start (SOF), arbitration field, control field, data field, cyclic redundancy check field (CRC), acknowledgement field (ACK), and end frame (EOF). The CAN protocol has a standard frame format CAN 2.0A and an extended frame format CAN2.0B. The standard frame format uses an 11-bit identifier and the control frame format uses a 29-bit identifier format. Its data frame format is shown in Figure 1. Since the CAN bus is a serial multi-master controller area network bus, it has high network security, communication reliability and real-time performance, is simple and practical, and has low network costs. Therefore, it is suitable for automotive computer control systems and working environments with harsh ambient temperatures, strong electromagnetic radiation and high vibration.

2.2 SAE J1939 Communication Protocol

J1939 is a network protocol that supports closed-loop control and high-speed communication between multiple ECUs. It is an industrial standard defined by the American Society of Automotive Engineers for vehicles and is mainly used in trucks and buses. Its purpose is to provide an open system for electronic systems so that there is a standard architecture for mutual communication between devices. The J1939 protocol is based on the CAN2.0B specification. It uses the 29-bit identification bit of the CAN standard to develop the coding system of the J1939 protocol and form the J1939 communication protocol, realizing a complete network definition. J1939 is customized with reference to the 7-layer reference model defined by the ISO open data interconnection model. It is an advanced CAN protocol standard. It has detailed provisions for the address configuration, naming, communication mode and message sending priority of the ECU inside the car, and detailed descriptions of the communication of each specific ECU inside the car. It uses multiplexing technology to provide standardized high-speed network connections based on the CAN bus for various sensors, actuators and controllers on the car, realizes high-speed data sharing between on-board electronic devices, effectively reduces the number of electronic wiring harnesses, improves the flexibility, reliability, maintainability and standardization of the vehicle's electronic control system, and maximizes the performance of CAN.

The J1939 protocol is implemented and encapsulated through PDU. PDU consists of 7 parts: priority P, reserved bit R, data page DP, PDU format PF, specific PDU target address PS, source address SA and data field DATA, which corresponds to the 29-bit identification code plus data field of the CAN protocol extension frame. Among them, priority P occupies 3 bits, and the smaller the value, the higher the priority. R is a reserved bit for extension use. DP is a data page, which is also used for control. The protocol data unit format (PDU Format) PF is an 8-bit data that indicates the format of the protocol data unit and provides a flag for a partial or complete parameter group. This data is also used to mark the CAN data field in the parameter group. The specific protocol data unit (PDU Specific) PS is an 8-bit data with a definite value of the data protocol unit format. This data is also used to mark the data field DA of the CAN data frame in the parameter group, and may also be the group control information GE. This data is also used to mark the data field of the CAN data frame in the parameter group. The source address (Source Adress, SA) is used to indicate the 8-bit data field of the source of the message. The source address field contains the address of the control unit that sends the message. The PDU unit message format is shown in Figure 2.

The core of J1939 protocol communication is the transmission protocol responsible for data transmission. Data splitting and packaging and reassembly. A J1939 message unit has an 8-byte data field, so only 8 bytes of data can be transmitted at a time. If the data to be sent exceeds 8 bytes, it must be split into small data packets, each with only 8 bytes of data, and sent in batches. The first byte of the data field starts from 1 as the message sequence number, and the following 7 bytes are used to store data. After the message is received, it is reassembled into the original data according to the sequence number. Connection management mainly manages the establishment and closing of connections between nodes and the transmission of data. Among them, 5 frame structures are defined: send request frame, send clear frame, end response frame, connection failure frame, and broadcast frame for global acceptance. Nodes establish a connection by sending a request frame to the destination address. After receiving the send request frame, if the node has enough space to receive data and the data is valid, it sends a clear frame to start data transmission. If there is insufficient storage space or the data is invalid, a connection failure frame is sent to close the connection. If data reception is complete, the node sends an end response frame to close the connection.

2.3 HEV vehicle data sharing mode

Figure 3 shows the HEV hybrid bus communication network topology, where HCU is the vehicle controller, TCU is the motor controller, ABS is the anti-lock braking system controller, and BCU is the battery controller. In this hybrid bus CAN bus topology network communication system, a dual bus structure is adopted. Each controller is connected to two CANL and CANH, and the communication rate of the bus is 250 kbit·s-1. The vehicle controller plays the role of a gateway. Each controller always reports the current status to the vehicle controller HCU or the HCU forwards control commands to each controller. At the same time, on this bus, each controller can receive or send message information from the bus in real time, thereby realizing the sharing of message information.

3 ABS control system implementation

The hybrid ABS control system designed is based on the demand analysis and control strategy of the hybrid vehicle braking force control system. The messages sent and received are in compliance with the SAE J1939 standard. Information messages are the main means of CAN communication. The SAE1939 standard stipulates that CAN communication messages can only use data frame format, and remote frames are not allowed. Its remote frame request function is realized through the parameters of the SAE J1939 standard. The SAEJ1939/71 substandard specifically describes parameters and parameter groups.

In order to share the parameters of the hybrid system and coordinate with other controllers when the ABS system is activated, the parameters and parameter groups of the SAEJ1939 standard are required, and their application in the control system is explained.

(1) ABS system self-check information parameter group. This parameter group is mainly used when the system is powered on. After the ABS system ECU completes the self-check of each component, it sends information to indicate whether the system is normal. If the ABS system is normal, the HCU notifies the driver that the system is normal and can be put into operation. If the ABS system is not normal, the HCU will not be put into operation and will not notify the driver. The message format is shown in Table 1.

(2) EBC1 brake control parameter group. This parameter group mainly sends control information from the ABS controller system to the vehicle controller HCU to inform the HCU of the vehicle's braking intensity and whether the ABS is in a monitoring state or an adjustment state. If the vehicle's deceleration value does not activate the ABS to put it in an adjustment state, the ABS control system notifies the HCU that the motor can participate in braking and recover energy. If the deceleration value is large at this time and the vehicle is in a strong braking state, the ABS controller notifies the HCU to release the motor braking, and the braking process is regulated by the ABS system. The message format is shown in Table 2.

(3) ABS system torque request parameter group. This parameter group is used to transmit the motor status parameters and the magnitude of the motor braking torque calculated by the HCU. If the motor is in normal working condition, the motor can participate in low-intensity or medium-intensity braking according to the control requirements. If the motor fails, the braking torque can be provided by the system's air supply valve. The message format is shown in Table 3.

4 Conclusion

SAE J1939 communication protocol is the most comprehensive communication protocol in the field of automotive electronic control. Understanding and mastering this protocol is helpful for developing my country's own automotive electronic control protocol. At present, many domestic scientific research institutions have carried out a lot of research on hybrid vehicles based on the J1939 protocol, and some manufacturers have trial-produced hybrid products, but none of them have been mass-produced. Hybrid ABS braking force control and energy recovery are still in the research and development stage. This research will further promote the development of hybrid vehicles.