Several methods of CAN bus interference location and interference elimination of new energy vehicles

Abstract: CAN bus is the bus standard for communication between Electronic control units of today’s automobiles, and is widely used in new energy vehicles. The CAN bus faults and hidden dangers of new energy vehicles will affect the driving experience and even driving safety. How to locate and eliminate CAN bus faults? This article offers a few suggestions for designers.

1. Industry status

At present, the number of domestic motor vehicles has exceeded 300 million. Due to the serious environmental problems brought about by a large number of fuel vehicles, the schedule of a complete ban on the sale of fuel vehicles has been brought up all over the world. Domestic new energy vehicles are mainly powered by lithium batteries, and the entire industry has entered a stage of rapid development. We have noticed that in addition to new energy vehicles and charging piles, extended industries such as unmanned driving and smart parking have also entered a period of rapid development. The above industries all have one thing in common – the use of CAN bus. Therefore, the application of CAN bus always runs through the development of the new energy industry.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 1 CAN bus application industry

2. Common faults of new energy vehicles

The CAN bus fault in new energy vehicles can be considered from two aspects, namely the communication application layer and the physical layer. Problems at the application layer rely more on software packet capture or simulation, while the physical layer relies more on hardware testing and simulation. At the physical layer, the causes of CAN bus faults are concentrated in the following aspects: interference problems, network topology problems, bus capacitive reactance impedance control, node design specifications and consistency, etc. This article focuses on introducing interference problems and interference solutions.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 2 Core design to ensure CAN stability

3. Interference positioning solution

How to locate the interference of the physical layer of the CAN bus? We need to use a professional interference analysis tool – spectrum analysis. CANScope is a professional CAN bus analysis tool. Its FFT analysis function can separately test CAN_L, CAN_H, CAN_differential, and CAN_common mode to locate the interference frequency. Generally, the interference source of CAN bus application in industrial communication is not fixed, and it is especially dependent on such frequency analysis tools. When CAN bus is applied to new energy vehicles, the interference source is relatively simple, mostly from motor drivers. At this time, the analyzer It can be used as a verification tool before and after rectification. Figure 4 is the actual test case of the FFT analysis function. CANScope locates the frequency of the interference source and the frequency of the Inverter through FFT spectrum analysis statistics.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 3 CAN bus waveform after interference

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 4 Using CANScope’s FFT function

4. Interference elimination solution

How to eliminate the interference after locating the interference? Let’s start with the source of the disturbance. There are two sources of interference, conduction and radiation. For the conducted interference, we adopt the isolation method to block the interference propagation; for the radiated interference, we adopt the shielding method to isolate the interference propagation.

The isolation scheme is to add an optocoupler or magnetic coupling device at the bus end of the CAN transceiver, and provide isolated power supply for the CAN transceiver at the same time. This isolation method is often referred to as a discrete device isolation scheme, as shown in Figure 5. For some occasions with high requirements on volume, protection level and stability, the integrated isolation solution is the best choice. In the new energy bus, the high-power air-conditioning compressor system market interferes with the bus communication, resulting in abnormal air-conditioning function. The quasi-vehicle-grade CTM1051(A)HQ? (Figure 6) is widely used in such scenarios.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 5 Discrete device isolation scheme

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 6 Quasi-vehicle-grade CAN bus isolation module CTM1051(A)HQ

The isolation discussed above is all board-level isolation, which is suitable for the vehicle development stage. If the fault caused by the interference occurs after the vehicle leaves the factory or in the after-sales stage, please use the CAN isolation bridge as the isolation method.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 7 Application of CANBridge+ in bus isolation

For the shielding scheme, the selected CAN communication cable should have at least one reliable shielding layer, and ensure that the outermost shielding layer is connected to the reference ground through a single point. If the shielding layer is inevitably cut off somewhere in the middle, a single-point grounding treatment should be performed for each section, as shown in Figure 8.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 8 Single-point grounding of shielding layer

In addition to the shielding layer, parameters such as the cross-sectional area, DC impedance value, parasitic capacitive reactance, and the number of twisted pairs per unit length of the communication line will affect the communication quality. How to balance the relationship between cost and communication distance, this article provides data in Figure 10 for Reader reference.

Several methods of CAN bus interference location and interference elimination of new energy vehicles

Figure 9 CAN bus dedicated twisted pair shielded cable

Figure 10 CAN bus wire parameters and communication distance

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