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“Engineers can control many ECU units on the vehicle by reading signals such as vehicle speed and rotation speed on the vehicle through the CAN bus. However, do you know how the shutdown of the CAN bus will affect the functions of new energy vehicles? This article will do a detailed analysis.
Engineers can control many ECU units on the vehicle by reading signals such as vehicle speed and rotation speed on the vehicle through the CAN bus. However, do you know how the shutdown of the CAN bus will affect the functions of new energy vehicles? This article will do a detailed analysis.
1. Why can CAN bus still play an important role in new energy vehicles?
Friends in the automotive-related industries all know the CAN bus, which is the bus standard for communication between Electronic control units in today’s automobiles. Now almost all automobile manufacturers choose to use CAN bus communication. CAN bus was born based on BOSCH company in order to solve many problems such as the contradiction of wiring space caused by the increase of electronic control units of automobiles and the increase of automobile weight. At the same time, the CAN bus connects the various electronic control units in the car into a local area network, which realizes the sharing of information and greatly reduces the wiring harness of the car, as shown in Figure 1:
Figure 1 Vehicle network
Compared with the current popular Ethernet and 485 bus, CAN bus has the following characteristics:
Bus access-non-destructive arbitrated carrier sense, multiple access, conflict avoidance;
Multi-host broadcast structure, automatic priority arbitration, and strong real-time performance;
Automatic retransmission of transmission errors, automatic CRC check and reception, extremely low data error rate;
Differential signal transmission has strong anti-interference ability, which is suitable for the environment with strong interference inside the car;
The hardware message filtering function reduces the burden on the CPU.
Based on the above characteristics, the CAN bus can ensure real-time and reliable data transmission and ensure the normal communication of the entire vehicle network. It has an irreplaceable position in the new energy vehicle industry.
2. What is CAN bus shutdown?
There are many ECU nodes in the car. When one of the nodes fails and enters the bus off state, it will greatly affect the communication of the vehicle’s CAN network. For example, when a car crashes, the sensor sends an electrical signal to the airbag ECU to process the signal. When it is determined that the airbag needs to be opened, the ECU will immediately send out an ignition signal, and the gas generator will fill the airbag, which is harmful to the driver. Passengers provide safety protection, as shown in Figure 2. If the airbag ECU is in the bus closed state at this time, the airbag cannot be ejected normally, which will cause serious consequences.
Figure 2 Schematic diagram of airbag ECU work
So, what is the CAN bus shutdown?
The CAN controller can determine whether the type of error is a temporary data error on the bus (such as external interference, etc.) or a continuous data error (such as unit internal failure, drive failure, disconnection, short Circuit, etc.). Therefore, when a continuous data error occurs on the bus, the error counter inside the CAN controller accumulates to the bus closing threshold, which can isolate the faulty unit from the bus and not participate in network communication with other nodes on the bus. As shown in Figure 3:
Figure 3 Error detection mechanism
Third, the cause analysis of CAN bus shutdown
The causes of errors are mostly caused by physical faults, mainly caused by CAN lines. Including: CAN_H open circuit, CAN_L open circuit, CAN_H short circuit to CAN_L/open circuit, CAN_L short circuit to VBAT, CAN_H short circuit to GND, CAN_L short circuit to GND, terminal resistance open circuit, etc.
In addition to physical layer wiring factors, it may also be caused by component failures such as CAN controllers or transceivers. At the same time, it is also possible that the CAN signal is incorrectly sent and received due to CAN bus signal interference. In severe cases, the message cannot be sent normally, which makes the CAN bus more likely to be closed.
For example, a new energy vehicle usually refers to a pure electric vehicle or a hybrid vehicle, which is characterized by using batteries and capacitors to store capacity, which is then converted into AC by means of inverter, driving the electric motor to drive the vehicle. The huge current generated by the inverter forms strong interference and crosstalks to the CAN bus, causing the controller to crash, damage or interrupt communication, and the vehicle runs unstable.
4. How to recover from CAN bus shutdown?
If BUS OFF occurs, the nodes on the bus need to do some actions, such as restarting the CAN controller or powering on again, but these are just some remedial measures. The most fundamental thing is to find the root cause of the BUS OFF.
When the CAN bus is closed, we can check the value of the BUS OFF register and initialize the drive and related registers of the CAN controller. After the initialization is completed, the CAN bus shutdown fault will be immediately removed. In order to avoid frequent bus shutdown problems of this node in the CAN network, it is recommended not to send CAN messages immediately after initialization.
As mentioned above, due to strong interference inside the car, the CAN bus will also be turned off. In response to this phenomenon, we deal with it in the following ways:
Due to the spatial magnetic interference generated by the strong current inside the car, the degree of twisting of the CAN cable should be increased;
CAN interface design adopts CTM1051 isolation transceiver and isolation limiter to prevent ECU from crashing;
CAN interface adds magnetic ring, common mode inductance and other effective inductive protective devices;
Connect a dedicated signal protector to eliminate interference, such as ZF-12Y2;
Use CANBridge, a bridge relay device, to isolate some strong interference sources.
Based on the above, when an automobile CAN bus shutdown fault occurs, the physical layer should be analyzed including CAN lines, CAN controllers and transceivers, CAN signal interference and other external factors, and the CAN registers and software processing should be analyzed at the same time, and the CAN driver should be reinitialized and restored to normal , Periodically try to send messages out.
Five, CAN network black box-CANDTU
When a new energy vehicle sends a bus off condition, we also hope to have the corresponding CAN message to analyze, especially for occasional failures, we need a reliable CAN message as a basis for judgment.
Guangzhou Zhiyuan Electronics Co., Ltd. launched the CAN network bus “black box”, we call it CANDTU. As shown in Figure 4, CANDTU integrates 2 or 4 independent CAN-bus channels that meet the ISO11898 standard, and can be equipped with standard storage media. The 32G high-speed SD card can perform multiple modes such as long-term recording, conditional recording, pre-trigger recording and timing recording, and can convert the recorded data into CSV format, or CANOE, CANScope and other mainstream analysis software formats for later Processing analysis.
Figure 4 Schematic diagram of CANDTU series products
CANDTU can record CAN messages for a long time so that engineers can find data, but it cannot locate and analyze network stability and occasional failures.