How to design the wiring harness to meet the requirements of vehicle CAN network layout

1. Introduction

The interaction effectiveness between the modules of electric vehicles directly affects the driving efficiency and driving experience of the vehicle. Plug-in hybrid vehicles can be divided into three categories according to the layout of the electric drive system and the engine power system: series, parallel, and series-parallel hybrid. This article discusses the parallel hybrid system solution, which has the characteristics of modularization of the original vehicle technology, convenient implementation, and wide applicability.

2. Terminology

Node: A collection of devices connected to a communication network that can communicate through the network according to a certain communication protocol.

Bus voltage: The bus voltage is the voltage from CAN_H or CAN_L of the CAN bus to the ground of the respective CAN node.

3. CAN bus network topology design

The CAN bus is a multi-master bus. Once the bus is idle, any CAN node can send data to the bus. The node that sends the message first in the CAN bus channel depends on the lossless bit-by-bit arbitration method. This method can save conflict time, especially when the network load is heavy, it can also run smoothly, ensure the utilization of the signal channel, and improve the real-time performance of the entire system.

As for the control system of plug-in hybrid electric vehicles, it is necessary to carry out detailed layering and planning of the vehicle control module (HCU), power system and safety system.

Generally speaking, the topological structure of the communication network is diverse, and the branches are complicated and cumbersome, while the bus often has strong detection and correction capabilities, good implementation signals, and guaranteed reliability. It can effectively determine the wrong nodes and automatically correct simple errors. Therefore, designing the communication network into a bus-type CAN network structure can meet the requirements of information transmission timeliness and reliability. According to the principle of system hierarchical control, the parallel hybrid CAN communication structure topology is designed, as shown in Figure 1.

4. CAN harness layout design principles

4.1 CAN Harness Requirements

The wiring harness layout design requires multiple on-board ECUs and offline tools in the network, as shown in Figure 2.

For CAN harness design, the following points are generally required:

1) The total length of the CAN bus is less than 40m;

2) ECU spacing < 20m;

3) The number of ECU nodes is less than 20;

4) The wiring harness uses shielded wire (high-speed CAN) or twisted pair wire (low-speed CAN);

5) The wiring harness requires that the branch line must be as short as possible; the diagnostic node is directly connected to the bus, and the port length should be as short as possible, with a maximum length of 1 meter. The connection between the diagnostic instruments from the connector should also be as short as possible, with a maximum length of no more than 5 meters.

6) When laying out the wiring harness, try to keep the CAN line smooth and avoid folding. Make sure that when each branch line is connected to the main line, the connection point of CAN_H and CAN_L is kept at the same position of the main line. In principle, the node distance D between each node is not allowed to be equal in length, and should be kept as far away from the power line and signal line as possible, and parallel routing should be avoided as much as possible;

7) Do not lay out the CAN bus near the interference source. If it cannot be avoided, use shielded wires. When using shielded wires, the shielding layer must be grounded at either end (or both ends). The requirements for the length of each trunk and branch line of the CAN line in the vehicle wiring harness are as shown in Table 1:

In order to avoid interference from cable power signals, the nodes of the CAN network should be as close to the trunk as possible. In practical applications, short branches should be used to connect to the bus as much as possible. In order to reduce standing waves, the ECUs in the CAN network should not be placed at equal distances and the tail of the cable should not all have the same length lead.

4.2 Terminal resistance

The terminal resistor can be placed adjacent to or in the two online ECUs with the longest distance. Non-terminal ECUs can be connected optionally. According to the standard requirements, the terminal resistor provided on the CAN bus is 60Ω, so the two farthest nodes on the vehicle CAN bus must each have a 120Ω terminal resistor (required between CAN_H and CAN_L).

The terminal resistance is required to be between 110Ω and 130Ω. The typical value is 120Ω.

5. Common fault tolerance of CAN line faults

The actual CAN detection circuit is more complicated. First, the fault type to be detected must be determined, and then the detection circuit is designed according to the fault type; the fault types are shown in Table 2.

VI. Conclusion

In summary, according to the vehicle system characteristics and functional requirements of plug-in hybrid vehicles, the vehicle CAN communication system of hybrid vehicles is designed, and its network structure layout is determined on this basis. According to the type of communication nodes, an effective network topology diagram of the vehicle is designed, which effectively meets the design requirements of the vehicle for the network system and is of great significance to the further development of plug-in hybrid vehicles.