Metro screen door CAN bus fault troubleshooting process

Subway is an important means of transportation for people. With the comprehensive construction of subway lines, people are paying more and more attention to the safety performance of subways, especially the reliability of subway screen doors. So how to troubleshoot subway screen door CAN bus faults in a complex subway control system? This article will give a detailed introduction.

1. Subway screen door control system - application of CAN bus

At present, the subway uses automated technology to achieve all-round control. The subway comprehensive control system includes ATC (automatic train control), SCADA (power monitoring system), BAS (environmental monitoring system), FAS (fire alarm system), PSD (shield door/safety door system), etc. These systems form a network throughout the line and are uniformly controlled by the control center.

Among them, the subway screen door system PSD is implemented based on the CAN bus. As shown in Figure 1, the system includes the following sub-units:

Figure 1 Schematic diagram of subway platform screen door control system

PSC (central interface panel): the core part of the shielding/safety door control system. Each station is equipped with a PSC, which consists of two identical and independent subsystems;

PSA (remote alarm panel): used to monitor the status of the platform shield door, diagnose the fault and operation status of the platform shield door, etc.;

PSL (local control panel): It is installed on the train departure platform on each side of the platform, as shown in Figure 2. It is used for the staff to issue door opening and closing commands to each  DCU  to achieve platform-level control when the system-level control fails;

DCU (Door Control Unit): The control device of the sliding door motor. Each shield door is equipped with a door control unit. There are two DCUs (master and slave) for each pair of sliding doors of the safety door.

Figure 2 Schematic diagram of subway PSL

From the above introduction, we can find that the subway screen door system is controlled by PSC directly through the CAN bus to control the DCU door unit. At the same time, PSA monitors the switch status of DCU and feeds back to PSC through the CAN bus. Due to the error handling mechanism of the CAN-bus, it can ensure that when any node in the network fails, it will not affect the operation of the entire network, and it is also convenient to locate the faulty node. At the same time, because the message of the CAN-bus is sent to the bus in a broadcasting manner, it can ensure the safe closing or opening of the screen door, and improve safety and stability.

If there is a CAN communication error between the PSC and the DCU, it will directly cause the subway screen door to malfunction, which will seriously cause the subway train system to fail to operate normally and even threaten the safety of passengers. So, how to solve the problem when a fault occurs? Or how to avoid the occurrence of screen door failure? The following is a brief introduction.

2. PSC and DCU communication failure - bus branches are too long/too many

From the subway control topology diagram in Figure 1, we can know that once a subway screen door fails, we can consider whether it is caused by improper wiring between PSC and DCU. As shown in Figure 3, it is the waveform generated by the long bus branch captured by the CANScope analyzer. If the bus branch between PSC and DCU is too long, it will cause the rising and falling edges to produce "steps", which is prone to bit width imbalance, thus causing communication errors between PSC and DCU.

In this case, you can refer to the following solutions:

Figure 3 Bus branch line is too long waveform

The standard “hand in hand” interface wiring specification as shown in Figure 4 is used between the PSC and DCU, and the transceiver should be placed close to the interface;

Figure 4 “Hand in hand” wiring specification

 As shown in Figure 5, different branch distance specifications are specified according to different baud rates;

Figure 5 Relationship between baud rate and branch line distance

According to the principle that the longer the branch, the smaller the matching resistance, the matching resistance is between 120-680 ohms, and the total parallel resistance is between 30-60 ohms;

CANBridge+ can be used for device branch networking.

3. PSC and DCU communication failure - bus capacitance is too large

When designing the PSC and DCU communication circuit, the impact of capacitance should be taken into account. Whether it is line capacitance or node internal capacitance, it will affect the communication of the entire network and cause shielding door failure. As shown in Figure 6, the waveform collected by the CANScope analyzer when the capacitance is too large. The larger the capacitance, the slower the edge, which is easy to cause bit sampling errors.

You can refer to the following solutions:

Figure 6 Waveform caused by excessive capacitance

Reduce the terminal resistance and speed up the capacitor discharge, as shown in Figure 7;

Figure 7 Relationship between terminal resistance and voltage amplitude

Replace with low capacitance wire;

Use CANBridge+ for waveform rectification.

Check the baud rate setting issue, starting with SJW.

5. PSC and DCU communication failure - excessive bus interference

The on-site environment of the subway control system is relatively complex, with numerous internal lines and excessive passenger flow. During peak hours, it is easy for people and bags to be pinched, doors to be forced to open, etc., causing great interference to the subway shield doors. Therefore, interference is inevitable on the bus formed by PSC and DCU, which is also one of the important reasons for the failure of shield door communication.

In order to better improve the anti-interference ability of PSC and DCU and ensure the communication quality, you can refer to the following solutions:

To ensure that each DCU node is electrically isolated, the isolated CAN transceiver CTM1051 can be used;

The bus between the shielded doors uses shielded twisted pair cables, and the degree of twisting is strengthened to effectively shield common mode interference;

Add signal protector to improve surge pulse resistance;

Add protection circuits such as magnetic rings and common-mode inductors.

The above content mentions some simple error solutions. However, the most difficult part in solving errors is how to find the error. Usually, the easiest way is to connect the DCU nodes one by one until an error occurs. Or use the CANScope analyzer developed by Zhiyuan Electronics, connect it to the subway control system, and analyze it from the bottom layer of CAN, which can more easily locate the error node and analyze the cause of the error through waveform.

6. CANScope Bus Comprehensive Analyzer Series

When the subway screen door sends a communication failure, it is difficult to locate the cause of the error. At this time, engineers can consider using the CANScope analyzer to quickly diagnose and locate. As shown in Figure 8, the CANScope bus comprehensive analyzer is a comprehensive professional tool for CAN bus development and testing, integrating a mass storage oscilloscope, a network analyzer, a bit error rate analyzer, a protocol analyzer and a reliability test tool, and organically integrating and linking various instruments; redefining the development and testing methods of the CAN bus, it can evaluate the correctness, reliability and rationality of CAN network communication from multiple angles and in an all-round manner; helping users quickly locate faulty nodes and solve various problems in CAN bus applications.

Figure 8 CANScope analyzer schematic diagram

7. CAN Network Black Box - CANDTU

In order to facilitate engineers to detect the operation of CAN devices or systems in real time, Guangzhou Zhiyuan Electronics Co., Ltd. launched the CAN network bus "black box", which we call CANDTU. As shown in Figure 9, CANDTU integrates 2 or 4 independent CAN-bus channels that comply with the ISO11898 standard, and can be equipped with a standard storage medium of 32G high-speed SD card. It can perform multiple modes such as long-term recording, conditional recording, pre-trigger recording and timed recording.

At the same time, CANDTU can collect CAN bus data and positioning information in real time, provide real-time cloud curves, provide visual analysis of CAN message data, and upload it to the designated cloud server in real time through 4G communication. In addition, users can directly perform standard UDS diagnosis on the vehicle, and cloud operation is more convenient and quick; users can log in to the cloud through mobile phones and other terminals, and can flexibly configure CAN channels, LIN channels, etc., view the Beidou/GPS trajectory positioning of the car in real time, monitor the real-time positioning of the equipment, and realize artificial intelligence big data processing of user terminals.

Figure 9 CANDTU product diagram