The CAN bus is a bus standard for communication between the various electric control units of the automobile, and is widely used in the new energy vehicle. The CAN bus fault and hidden dangers of the new energy vehicle will affect the driving experience or even driving, how to perform CAN bus fault positioning and interference exclusion? This article provides a few suggestions for designers.
First, industry status
At present, domestic motor vehicle insurance has exceeded 300 million. Since a large amount of fuel trucks bring severe environmental issues, the full-bunch of fuel vehicles has been filed worldwide. Domestic new energy vehicle power is based on lithium batteries, and the entire industry has entered the stage of rapid development. We noticed that in addition to the new energy vehicle, the extension industry such as unmanned, smart parking is also entered into the rapid development period. There is a common point in the above industry - using the CAN bus, therefore, the application problem of the CAN bus is always through the development of the new energy industry.
Figure 1 CAN bus application industry
Second, new energy vehicles are common faults
The CAN bus fault in the new energy vehicle can be considered from two aspects, namely communication application layer and physical layer. The problem of applying layers is relatively relying on software capture or simulation, and the physical layer is more dependent on hardware testing and simulation. In the physical layer, the cause of the CAN bus fault is concentrated in the following aspects: interference issues, network topology problems, bus capacitance control, node design specifications and consistency, etc., this film is focused on introducing interference issues and interference resolution.
Figure 2 guarantees CAN stable core design
Third, interference positioning solution
How to perform interference positioning of the CAN bus physics layer? We need to use professional interference analysis tools - spectrum analysis, can Canscope is a professional CAN bus analysis tool, and its FFT analysis function can be tested, CAN_common mode, respectively, and position interference frequency points. In general, the CAN bus application is not fixed in industrial communication, especially depending on such frequency analysis tools, and when the CAN bus is applied to new energy vehicles, the source is more than one, more from the motor driver, at this time the analyzer The verification tool after rectification and rectification can be made. Figure 4 is an actual test case of FFT analysis function, and Canscope is positioned to the interference source frequency with the inverter frequency via FFT spectrum analysis.
Figure 3 is interference CAN bus waveform
Figure 4 FFT function using CANSCOPE
Fourth, interference exclusion solution
How to interfere with interference after positioning to interference? We started from the source of interference. There are two sources of interference, and there are two, conductive and radiation. In response to the interference of the conducted, we take the insectional way blocking interference dissection; for radiation, we take the mask method to isolate interference spread.
The isolation scheme is photocoupled or magnetic coupling in the CAN transceiver bus, and is simultaneously supplied to the CAN transceiver, which is often referred to as a discrete device isolation scheme, as shown in Figure 5. For some applications with high volume, protection level, stability requirements, the integrated isolation scheme is optimal. In new energy passenger cars, high-power air conditioning compressor systems market interference to bus communication, which is abnormal, and the CTM1051 (A) HQ (Figure 6) of the depth of the car is widely used in such scenarios.
Figure 5 Disposer Isolation Scheme
Figure 6 Experts CAN bus isolation module CTM1051 (a) HQ
The isolation of the above discussion is a plate-level isolation, which is suitable for model development stages. If the failure caused by interference occurs after the factory or after the after-sales phase, use the CAN to isolate the bridge.
Figure 7 Application of Canbridge + in bus isolation
Shielding scheme, the selected CAN communication cable should have at least one reliable shield layer, and ensure that the outermost shielding layer is connected to the reference ground. If you encounter a shield, you should not avoid truncation from the middle, you should do a single ground processing for each paragraph, as shown in Figure 8.
Figure 8 Shielding single point grounding processing
In addition to the cross-sectional area of the communication line, the cross-sectional area, DC impedance value, parasitic resistance, and unit length of the two-ranks can affect the quality of communication, how to balance the cost and communication distance, this article provides Figure 10 data for Reader reference.
Figure 9 CAN bus special twisted shield line
Figure 10 CAN bus line parameters and communication distance
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