Modern automotive core technology - CAN bus application layer protocol standard SAE J1939

First, the technical background

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Automotive bus technology is used in today's mid- to high-end cars. The car bus provides a unified data exchange channel for various complex electronic devices, controllers, measuring instruments, etc. inside the car. Some automotive experts believe that the introduction of data bus technology in the past 10 years will be a milestone in the development of automotive electronics technology, just as the introduction of integrated circuits in the 1970s and the introduction of microprocessors in the 1980s.

Since the 1990s, the number of components controlled by electronic control units (ECUs) has increased, such as electronic fuel injection devices, anti-lock brakes, airbag devices, electronically controlled doors and windows, active suspensions, etc. Wait. With the widespread use of integrated circuits and microcontrollers in automobiles, the number of ECUs on the car is increasing. Therefore, a new concept - the concept of the controller area network CAN (Controller Area Network) came into being. CAN was originally developed by the German company BOSCH to solve the data exchange between the control and test instruments in modern cars. According to the relevant ISO standards, the topology of CAN is bus type, so it is also called CAN bus. .

The data amount of each frame in the CAN protocol does not exceed 8 bytes, and the real-time performance of the data is realized in a short-frame multi-issue manner; the error correction capability of the CAN bus is very strong, thereby improving the accuracy of the data; With a rate of 1M bit/s, it is a true high-speed network. In summary, there are many advantages to using the CAN bus in a car:

(1) Replace the expensive wires in the vehicle body with a low-cost twisted pair cable, and greatly reduce the number of wires used; improve reliability, safety, and cost.

(2) It has fast response time and high reliability, and is suitable for applications with high real-time requirements such as brakes and airbags; interconnection platform of control platform, information platform and driving platform.

(3) CAN chip can withstand high temperature and high noise, and has a low price, open industry standard.

In the design of modern cars, CAN has become a must-have device. Mercedes-Benz, BMW, Volkswagen, Volvo, Renault and other vehicles have adopted CAN as a means of networking controllers. According to reports, China's first CAN network system hybrid car has been successfully installed in Chery and has undergone preliminary trial operation. The CAN bus technology was also introduced in Shanghai Volkswagen's Passat and POLO cars. But in general, the current application of CAN bus technology in China's automotive industry is still in the test and initial stage, and most of the cars have not adopted the design of the car bus, so there are many drawbacks.

For example, it is well known that the core equipment of an automobile is an engine, and operating parameters of the engine, such as engine speed, oil pressure, coolant temperature, etc., are closely related to automobile driving. The traditional car instrument is designed to convert mechanical signals into electrical signals, such as voltage, current, pulse signals, and then D/A conversion or counters, etc., through sensors placed inside automotive components (such as engines). The pointer signal converted to visible is displayed on the analog dashboard. With the development of automotive bus technology, many imported engines no longer provide sensor signals directly to the outside, but switch to the CAN bus communication interface. In the event of engine failure, due to the lack of CAN bus-based test and maintenance equipment, the current method used by our maintenance personnel can only be to drill holes in the engine, put the sensor directly into the engine for measurement, the operation is cumbersome, the equipment is complicated, and Not conducive to protecting the overall structure of the engine.

For example, some existing automobile instruments use a dedicated bus design. Because the hardware standards are not uniform and the communication protocol is not compatible, the instruments produced for the company A can not be used on the cars of Company B, and the production cost is difficult to reduce. Fault repair is very inconvenient. If you can unify the various dedicated buses to the CAN bus standard, you can solve the problem.

For example, in a manual car, the driver's shift is based on experience. It may happen that the gear should be raised and not upshifted in time. That is, low-speed high-speed driving is not conducive to reducing fuel consumption and easy. Causes the wear of automotive transmission components. If automatic shifting is enabled, the vehicle can always stay at an economical speed.

According to the OSI model defined by ISO (International Organization for Standardization), the CAN protocol defines the physical layer and data link layer specifications, which provides great convenience for different car manufacturers to develop application layer protocols that meet their own needs. If you need to build a more complete system, you need to choose the appropriate application layer protocol based on CAN. Such as CANopen, SAE J1939 and so on.

The J1939 protocol is currently the most widely used application layer protocol in large vehicles, reaching a communication rate of 250Kbps. The J1939 agreement is maintained and promoted by the US Society of Automotive Engineers (SAE). The J1939 protocol has the following characteristics:

(1) Based on the CAN2.0B protocol, the physical layer standard is compatible with the ISO11898 specification and uses a CAN controller and transceiver that conforms to the specification. Communication rates up to 250Kbps.

(2) The information is transmitted using PDU (Protocol Data Unit), and each PDU is equivalent to one frame in the CAN protocol. Since each CAN frame can transmit up to 8 bytes of data, the transmission of the PDU is highly real-time.

(3) The meaning of each PDU and the priority of the PDU are defined by the 29-bit identifier of the CAN2.0B extended frame format.

(4) The J1939 protocol is mainly used as a communication protocol for applications in automobiles, and various parameters applied to automobiles are regulated. The parameters are specified in accordance with the ISO11992 standard.

Second, the development of J1939 in China

China's single-chip public laboratory combined with Beijing Yingbei Multi-embedded Network Technology Co., Ltd. and domestic famous automobile design manufacturers, after more than two years of design and development, realized the single-chip J1939 gateway chip em2003 series products.

Em2003 can be used in automotive digital instrumentation, automotive J1939 gateway, the core unit of automotive multi-function electronic control, and has passed strict reliability testing and actual product verification, and has been put into mass production.

The indicators of em2003 have reached the international advanced level. In addition to supporting SAE J1939 firmware, SAE 1587 and ISO9141 standards can be supported to control the driving of automobile instruments. In China, it is the only network computing information platform that realizes the control of the whole LCD digital instrument of the car, the control of the stepper motor pointer instrument and the operating parameters of the multi-function vehicle sensor information. It passed the interconnection test with the J1939 engine of the United States, Germany and Italy, and obtained an international passport.

At present, the technical specifications and software and hardware development test platform of SAE J1939 have been imported and digested by the China MCU public laboratory and localized of some technologies, and provide technical support and solutions for the control computing platforms of dozens of domestic auto manufacturers. Consultancy services. As shown in the figure:

Figure: J1939 Network and LIN Network Simulation Test Platform

The system consists of 11 network nodes, with the J1939 network as the backbone, integrating the network essence of modern automotive technology. Including LINbus, GPRS / GSM (wireless TCP / IP network), Bluetooth (Bluetooth), USB, and embedded Ethernet and other latest technologies. The power unit of the car is realized by embedded hardware digital simulation technology. The introduction of each unit is as follows:

(1) Engine ECM simulation unit: (node ​​1)

Realize the (actual) engine bus simulation function, generate 10 to 20 kinds of electronic control time parameters of the engine, and simulate the actual running state of the automobile engine. Suitable for automotive EMC requirements.

(2) NMT / body electric control unit (node ​​2)

It can realize the network management function and diagnostic record function specified in J1939/81, issue alarm control information, and has 16 photoelectric isolation output interfaces (50V/500mA) and 8 digital signal (sensor) input interfaces and 4 analog sensor interfaces. Control functions can be modified in the field. Suitable for a variety of automotive EMC development requirements.

(3) retarder simulation unit: (node ​​3)

The drive interface of the electromagnetic retarder can be controlled according to the running state of the vehicle and the speed of the vehicle.

(4) ABS simulation unit: (node ​​4)

The ABS braking force and starting time are controlled according to the integrated parameters of the vehicle network.

(5) AMT simulation unit: (node ​​5)

The communication between the gearbox and the engine ECM can be simulated according to the design parameters.

(6) Asymmetric bridge (node ​​6)

Asymmetric bridging of high-speed network (power system) and low-speed network (instrument information electrical control system) can be realized to ensure the safety and electrical safety of the bus load rate.

(7) LIN BUS gateway (node ​​7)

Realize the interconnection of LIN-BUS sensors, electrical control systems and CAN-BUS systems. And comply with the J1939 agreement.

(8) J1939MFM (node ​​8)

J1939 multi-function vehicle integrated parameter meter (car information center), can realize real-time display of 14 kinds of vehicle operating parameters (Chinese LCD), programmable 300~5000Km historical vehicle condition record and fault alarm information display function. Suitable for automotive EMC requirements.

(9) J1939 car instrument (node ​​9)

Realize J1939 bus type car instrument. Can be adapted to a variety of domestic or imported automotive instrument assembly.

(10) J1939 to Ethernet/USB (Gateway Node 10)

Ethernet or USB interface can be connected to the general computer to enter the J1939 network, statistical analysis of the bus load rate, open API interface.

(11) J1939 running real-time parameter record (node ​​11)

Access to the J1939 network can record 200,000 operating parameters for real-time analysis of the operation of each ECU unit, test operating parameters in the actual running car, and access the Internet network server through the GMS/GPRS network. It has strong anti-interference ability and is suitable for automotive EMC requirements.

These node units in the above system can form a relatively complete network test environment, and most of the units have been commercialized. And suitable for automotive EMC requirements. Therefore, it can be interchangeably connected with actual automotive electronics. Based on this, an electronic control unit simulation test platform for automotive electronic bus technology can be formed. With a professional test instrument, the CAN-BUS laboratory can be built, which is standardized and scalable.

Beijing Yingbei Duo Company has developed and integrated the J1939 network system simulation platform V1.0 through the cooperation with German Phytec, IXXAT and other companies to achieve the international level of hardware ECU.

The network system has been working continuously for more than 10,000 hours under the 12 nodes (bus load rate up to 30%) according to J1939's physical layer, link layer and network layer. According to the J1939/71 vehicle application layer standard, the product testing of the MFM/J1939 multi-function gateway and the testing of the bus type digital automobile instrument were completed.

Third, the technical outlook

Hyundai is an intelligent network computing platform. Each unit of the vehicle network, that is, the control system, information system, driving system and sensing execution system, is interconnected by the control area LAN-BUS, and it is the key technology to master the application layer network standard and develop embedded software.

Control the vehicle's control network and information network, such as fault information detection system, automatic condition record system, real-time driving information display system (intelligent digital meter) and embedded Internet (support IPv4 and IPv6), so that each car has one Web pages will be the key core technology for future automotive computing platforms.

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