Firstly, the structure and working principle of FlexRay bus protocol and on-line control braking system are briefly introduced, and then Flexray is applied to on-line control braking system. By making full use of the advantages of Flexray communication, the source address allocation, output parameters and parameter group definition of each node of the on-line control braking system are clearly defined, which provides a certain theoretical and experimental method for the application of Flexray bus in the field of on-line control vehicles. In recent years, the biggest hotspot in the development of automobile braking system is network.
In the future, the braking system of automobiles must be oriented to the transformation of Brake-By-Wire (BBW) without mechanical transmission and hydraulic devices. As the line-controlled braking system is a security-critical distributed real-time control system, it is bound to have higher requirements for the system bus. The original CAN bus based on event triggering can no longer meet the requirements. FlexRay, a class C network for wire control technology proposed by FlexRay Alliance, can effectively guarantee the safety and reliability of wire control system.
FlcxRay will gradually replace CAN and become the backbone of the future intra-vehicle network. Firstly, the composition and working principle of FlexRay protocol and online control braking system are briefly introduced. The source address allocation, thermostatic element output parameters and parameter group definition of each node of online control braking system are clearly defined. The structure principle and basic structure of BBW system are shown in Figure 1. It consists of six parts: pedal module (including displacement sensor and force sensor), sensor group (including wheel speed sensor, steering wheel angle sensor, side slip rate and lateral deceleration sensor), electronic control module ECU (main control node), four independent vehicles. The wheel brake module ECM (brake node), power module and communication network are composed. The working principle is as follows: when braking, the electronic information is sent from pedal node to central controller ECU, and combined with other sensor signal ECU, the optimal braking force is calculated and sent to wheel electronic control module ECM. ECM receives brake pedal signal and processes it into voltage signal, so that motor actuator can complete the necessary torque response. The motor actuator converts electrical energy into mechanical energy and transmits it to the brake pad through the reducer device.
The brake pad then applies the brake pressure to the brake disc, thus completing the whole braking process. The structure of the line control braking system is complex. Compared with traditional automobiles, battery modules and other auxiliary equipment are added. Through the analysis of network topology, the ideal network topology of the double redundancy fault tolerant line control braking system of BBW system based on FlexRay protocol is shown in Figure 2 below.
The structure has an important impact on the safety of automobile network system. Four wheel brake nodes are 1, 2, 3 and 4, which are connected to 5 and 6 main control nodes and 7 and 8 power management nodes respectively. The power node is mainly responsible for supplying power to other nodes. This dual redundancy fault-tolerant structure is characterized by: if the main control node (5) fails, the system can continue to operate under the function of another main control unit (6) after taking corresponding measures in the application software. In order to achieve the purpose of fault tolerance. FlexRay is a powerful communication network protocol developed by BMW, DaimlerChrysler, Philips and Motorola. It supports high throughput, certainty and fault tolerance, as well as event triggering and time triggering communication. FlexRay’s structure is divided into physical layer, transport layer, presentation layer and application layer. The definition of FlexRay bus only defines the frame specification transmitted on the bus, and the content of each frame is determined by the application layer. FlexRay’s application layer is the highest layer of FlexRay network. The application layer protocol is not defined in FlexRay specification, but many organizations have developed application layer protocols for different applications. Among them, the most famous one is the communication standard J1939 formulated by the American Automobile Engineering Association (SAE). Referring to SAEJ1939 protocol, FlexRay application layer protocol adapted to BBW system can be formulated. By defining different bits of identifier, the protocol defines the nature, length, meaning, type and period of data transmission. Through this protocol, bus braking system can form effective data transmission in this protocol, which makes the application of FlexRay bus communication in BBW system possible. 。 In this system, we adopt 8-node structure, which includes two main nodes (main control node), four wheel brake nodes (slave node) and two power management nodes. Each node sends and receives data differently. Before defining, the data type should be determined for each parameter transmitted. The data type can be response type, control type or data type. Table 1 lists the received and sent data tables of two types of nodes. Because the system uses FlexRay Bus to connect all nodes of BBW system, the data on FlexRay Bus can get bus possession through static fixed slot allocation or dynamic minimum time slice allocation, and then data transmission is carried out on the bus. The slot allocation is determined by the identifier of each data frame. FlexRay bus standard is applied in this design, which has 11-bit identifier. We express different functions of data through specific bits in identifiers, so that data can be sent timely and accurately. As shown in Table 2. In Table 2, the first two bits (ID10 and ID9) define three different data types, which are transmitted in accordance with the fixed time slots specified in FlexRay protocol. The transmitted information should be allocated slots according to priority, as shown in Table 3. Three data segments representing different meanings of data task attributes (ID10, ID9, ID8) in identifiers are described below. Task attributes of data refer to the representation of different data frames in the same data type. Wheel speed data and pedal data are different data task attributes in the same data type. The system will transmit data frame types into three different types: response type, control type and data type. In this design, we use ID8, ID7 and ID6 to define the transfer data types in BBW system, which participate in bus arbitration through fixed slot allocation to ensure the transmission of important information. As shown in tables 4, 5 and 6. Represents the slot in which the data is allocated to the bus in the whole system. That is, the address code of the destination node of the frame in the system. We give all nodes in the system a unique address. The address list of each node is shown in Table 7.
Used to extend the future functionality of identifiers. FlexRay protocol should first determine the parameter digits of three kinds of data information. According to SAE 193971, the parameter digits of each data can be defined, and then the minimum time slice length of the static part and the dynamic part can be determined according to the parameter digits.
The input/output signal list of the main control node is shown in Table 8. Data types include braking wheel speed information, pedal position and force information required by BBW system, which are output to four braking nodes; control type data refers to the main control node collecting relevant sensor signals from various sensors and issuing braking commands to four wheel braking nodes; response type refers to that each unit is the main unit. The control node can grasp the working status of each node of the system in time, collect the running status information of each node, ensure that the driver can grasp the running status of the vehicle at any time, avoid accidents, and facilitate vehicle maintenance. Table 9, 10, 11 lists the number of parameters of three types of frames. Firstly, the on-line control braking system is briefly introduced, and the Flexray bus application layer protocol is developed for the system. Wire-controlled braking system is a new system. In the future, it will integrate ABS, vehicle driving stability, automatic braking, braking power, traction control and other functions into a new generation of intelligent braking control system.
At the same time, FlexRay’s good performance will make it a new standard of automobile brake system. With the development of automotive electronic technology, more wire-controlled technologies will be introduced into automobiles in the future, such as wire-controlled throttle, wire-controlled suspension, wire-controlled shift and so on. FlexRay will undoubtedly eventually become the main control framework of the car and the main way to bring all the car buses together.
In this paper, the design of FlexRay bus protocol is used to develop a new idea for realizing a low-cost, low-power and easy-to-implement automobile safety system in the future.