Information integrated control system for city buses based on CAN bus

随着电子技术的不断发展，汽车电子技术也迅速的发展了起来，汽车上各种电子控制单元的数目也不断地增加，连接导线显著增多，因而提高控制单元间通信的可靠性和降低导线成本已成为迫切需要解决的问题。在20世纪80年代，以研发和生产汽车电子产品著称的德国bosch 公司针对此问题开发了can总线协议，这种多主网络协议，它的基础是无破坏性仲裁机制，使得总线能以最高优先权访问报文而没有任何延时。can作为标准车载网络技术，其在汽车网络化应用的进程中起着桥梁和纽带的作用，将城市客车信息集成采集提高到一个新的层次。

2 Overview of City Bus Information Integrated Control System

The city bus information integrated control system is based on the automobile network control technology. From the control object, the city bus information integrated control system can be divided into power transmission electronic system, safety and chassis electronic system and body electronic system. The control system block diagram is shown in Figure 1.

Figure 1 City bus information integration control block diagram

The power transmission electronic system is composed of EEC (Engine Electronic Control), ECT (Electronic Control Transmission), electronically controlled power steering, etc
.; the information displayed on the instrument panel comes from the power transmission electronic system, and the driving information is displayed on the instrument panel, showing the amount of information related to the driving of the bus during driving, such as vehicle speed, speed, mileage, fuel level, water temperature, fault alarm, etc.; the safety and chassis electronic system includes ABS (Anti-lock Brake System), ASR (Anti-slide Retractor), SAB (Safety Air Bag), CCS (Cruise Control System), retarder, suspension system control, etc.; signal control is related to the components of the body electronic system, and controls automatic doors and windows, lights, air conditioning, wipers, audio and video equipment, electronic monitors, electric rearview mirrors, and roof windows.

3 System structure design

3.1 Information integrated control system

The information integrated control system is the core of the entire city bus information integrated control system. Its task is to comprehensively apply automotive electronic control technology, vehicle network technology and intelligent control technology to achieve information sharing and related real-time control of various ECUs in city buses, so as to achieve the overall improvement of the safety and comfort of city buses and reduce dependence on driver skills.

From the perspective of information exchange, the city bus information integrated control system is divided into the powertrain control subsystem and the body control subsystem. The powertrain control subsystem includes the engine control system and the chassis control system. The body control subsystem includes the body electronic system and the instrument panel control system. The powertrain control subsystem has frequent internal information exchange and has extremely high requirements for the real-time nature of the control information; while the body control subsystem has relatively low real-time requirements compared to the powertrain control subsystem. There is also information exchange between the powertrain control subsystem and the body control subsystem. If a single bus structure is adopted, that is, all ECUs are connected to the same CAN bus, the information transmission of the two subsystems will be superimposed, which will inevitably increase the network load and reduce the real-time nature of the control information. We adopt a dual bus structure. The powertrain control subsystem adopts high-speed CAN with an information transmission rate of 500kb/s, and the body control subsystem adopts low-speed CAN with an information transmission rate of 100kb/s. A small amount of information exchange between the two is realized through the gateway. The gateway is the core of the city bus information integrated control system and the basis of comprehensive control. Its main function is to analyze and process various information, issue instructions, and coordinate the work of various control units and electrical equipment of the vehicle. The topological structure of the city bus information integrated control system is shown in Figure 2.

Figure 2 Topology of city bus information integrated control system

3.2 Gateway and bus interface

The gateway uses Philips' LPC2101 microcontroller, which is based on an ARM7TDM1-S CPU that supports real-time simulation and has 8KB and 32KB embedded high-speed flash memories. The 128-bit wide memory interface and unique acceleration structure enable 32-bit code to run at the maximum clock rate. This can improve the performance of important functions in interrupt service routines and DSP algorithms by 30% compared to thumb mode. Applications that have strict control over code size can use 16-bit thumb mode to reduce code size by more than 30% with little performance loss. It integrates two CAN controllers internally, with the following main features: data transfer rate up to 1MB/s on a single bus; 32-bit register and RAM access; compatible with CAN2.0B; global acceptance filter can recognize all 11-bit and 9-bit RX identifiers; acceptance filter provides full CAN style automatic reception for selected standard identifiers.

The CAN transceiver uses the philips tja1050 interface chip, which can provide differential transmission performance for the bus and differential reception performance for the CAN controller.

The LPC2101 microcontroller is connected to the two CAN buses through the optocoupler circuit and the high-speed CAN bus transceiver TJA1050. The connection method of the two CANs is basically the same, and the CAN bus drivers are powered separately by isolated DC/DC modules. This not only realizes the electrical isolation between the two CAN interfaces, but also realizes the isolation between the gateway and the CAN bus. The gateway and bus structure is shown in Figure 3.

Figure 3 Gateway and CAN bus interface structure

4 System Software Design

The main function of the CAN/CAN gateway is to filter and forward data between two CAN network segments. Due to the real-time communication requirements in the city bus information integrated control system, the data storage and forwarding time should be as short as possible during software design. In order to meet this requirement, the data is received in IRQ mode, and since the data communication volume of the powertrain control subsystem is significantly higher than that of the body control subsystem, the CAN1 connected to the powertrain control subsystem is set to the highest acceptance priority, and the CAN2 connected to the body control subsystem is set to the second priority. At the same time, the interrupt service program is simplified as much as possible to make the system response time as short as possible.

Due to the different transmission rates, the data transmission between high-speed and low-speed CAN networks is different. When high-speed CAN network data is transmitted to low-speed CAN, soft buffering needs to be added for temporary storage; when low-speed CAN network data is transmitted to high-speed CAN network, it can be directly transmitted. The overall process is shown in Figure 4.

Figure 4 High and low speed CAN gateway communication process

5 Conclusion

The CAN bus is gaining more and more attention for its high performance, high reliability and unique design, and is recognized as one of the most promising buses in the automotive control network. This paper presents a design scheme for an information integration control network for city buses with high and low speed CAN networks, and introduces the software and hardware design of the LPC2101 microcontroller as a high and low speed gateway in the CAN network. The automotive computer control unit can share all information and resources through the CAN bus, so as to simplify wiring, reduce the number of sensors, avoid duplication of control functions, improve system reliability and maintainability, reduce costs, and better match and coordinate various control systems.