Subway is an important means of transportation for people to travel. With the comprehensive laying of subway lines, people pay more and more attention to the safety performance of subway, especially the reliability of metro platform screen doors. In the complex subway control system, how to carry out the CAN bus fault diagnosis of metro platform screen doors? This article will make a detailed introduction.

Control system of metro platform screen door — Application of CAN bus

At present, Metro adopts automation technology to realize all-round control. Metro integrated control system includes ATC (automatic train control), SCADA (electric power monitoring system), bas (Environmental Monitoring System), Fas (fire alarm system), PSD (platform screen door / safety door system), etc. these systems form a network in the whole line and are controlled by the control center.

PSD of metro platform screen door system is based on CAN bus, as shown in Figure 1. The system includes the following sub units:

Control diagram of Metro screen door 1

PSC (central interface panel): the core part of PSD / PSD control system. Each station will be equipped with a set of PSC, which is composed of two identical and independent subsystems;

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

PSL (local control panel): it is set on the starting platform of the train on each side of the platform, as shown in Figure 2. It is used for the staff to send the command of opening and closing the door to each DCU in case of system level control failure, so as to realize platform level control,;

DCU (door control unit): the control device of sliding door motor. Each PSD is equipped with a door control unit. Each pair of sliding doors has two DcuS (master and slave).

Figure 2 Schematic diagram of Metro PSL

From the above introduction, we can find that the PSC directly controls the DCU door unit through CAN bus in metro platform screen door system. At the same time, PSA monitors the switch state of DCU, and feeds back to PSC through CAN bus. Due to the error handling mechanism of CAN bus, it can ensure that when any node in the network fails, it will not affect the operation of the whole network, and it is convenient to locate the wrong node. At the same time, because the message of CAN bus is sent to the bus in the form of broadcast, it can ensure the safe closing or opening of the screen door and improve the safety and stability.

If can communication error occurs between PSC and DCU, it will directly lead to the failure of metro platform screen door, seriously lead to the failure of metro train system, and even threaten the safety of passengers. So, when the fault occurs, how to solve it? Or how to avoid PSD failure? The following is a brief introduction.

Communication fault between PSC and DCU: too long / too many bus branches

It can be seen from Figure 1 subway control topology diagram that in case of Metro PSD failure, we can consider whether it is caused by nonstandard wiring between PSC and DCU. As shown in Fig. 3, it is the waveform generated by too long bus branch line captured by canscope analyzer. If the bus branch between PSC and DCU is too long, it will lead to the “step” phenomenon of rising edge and falling edge, which is easy to cause bit width imbalance, which will cause communication error between PSC and DCU.

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

Figure 3 too long waveform of bus branch line

The standard “hand in hand” interface wiring specification as shown in Figure 4 shall be used between PSC and DCU, and the transceiver shall 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 is, the smaller the matching resistance is, the matching resistance is between 120-680 Ω, and the total parallel resistance is between 30-60 Ω;

Canbridge+ can be used for the branch network of equipment.

Communication fault between PSC and DCU: bus capacitance is too large

When designing the communication circuit between PSC and DCU, the influence of capacitance should be considered. Whether the capacitance between lines or the internal capacitance of nodes will affect the communication of the whole network and cause the failure of PSD. As shown in Fig. 6, the waveform when the capacitance is too large is collected by the canscope analyzer. The larger the capacitance is, the slower the edge is, which is easy to cause bit sampling error.

You can refer to the following solutions:

Figure 6 waveform generated by excessive capacitance

Reduce the terminal resistance and accelerate the discharge of capacitor, as shown in Fig. 7;

Figure 7 Relationship between terminal resistance and voltage amplitude

Replace with low capacitance conductor;

Use Canbridge + to rectify the waveform.

Check the baud rate setting problem, starting with SJW.

Communication fault between PSC and DCU: excessive bus interference

The site environment of metro control system is more complex, there are many internal lines, and the flow of people is too large. When it comes to the peak period, it is easy to clamp people and bags, force the door to open, which will cause great interference to the metro platform screen door. Therefore, the bus composed of PSC and DCU will inevitably be interfered, which is one of the important reasons for the failure of PSD communication.

In order to improve the anti-interference ability of PSC and DCU and ensure the communication quality, the following schemes can be referred to:

Ensure that each DCU node is electrically isolated and can use isolation can transceiver ctm1051;

The bus between the screen doors uses shielded twisted pair, which can effectively shield the common mode interference;

Add signal protector to improve the ability of anti surge pulse;

Protection circuits such as magnetic ring and common mode inductance are added.

As mentioned above, some simple error solutions are mentioned. However, when solving errors, the most difficult thing is how to find them. Usually, the simplest way is to connect the DCU nodes one by one until an error occurs. Or use the canscope analyzer developed by Zhiyuan electronics, access to the subway control system, and analyze from the bottom layer of can, it is more convenient to locate the wrong node and analyze the cause of the error through the waveform.

Canscope bus integrated analyzer series

When the metro platform screen door sends communication failure, it is difficult to locate the cause of the error. At this time, engineers can consider using canscope analyzer to quickly diagnose and locate. As shown in Figure 8, canscope bus comprehensive analyzer is a comprehensive professional tool for CAN bus development and testing, which integrates mass storage oscilloscope, network analyzer, bit error rate analyzer, protocol analyzer and reliability testing tools, and integrates and correlates various instruments organically. Redefining the development and test method of CAN bus can improve the accuracy of can network communication, improve the accuracy of can network communication, and improve the reliability of can network It can help users locate fault nodes quickly and solve various problems of CAN bus application.

Figure 8 schematic diagram of canscope analyzer

Can network black box candtu

In order to facilitate engineers to detect the operation of can equipment or system in real time, Guangzhou Zhiyuan Electronic Co., Ltd. has 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 meet the iso11898 standard, and can be equipped with a 32g high-speed SD card as the standard storage medium, which can be used for long-term record and condition record Pre trigger recording and timing recording.

At the same time, candtu can collect real-time can bus data and positioning information, real-time cloud curve, provide visual analysis of can message data, and upload to the designated cloud server through 4G communication. In addition, users can directly carry out standard UDS diagnosis on vehicles, making cloud operation more convenient and fast; users can log in to the cloud through mobile phones and other terminals, flexibly configure can channel, Lin channel, etc., view real-time vehicle Beidou / GPS track positioning, real-time positioning and monitoring of equipment, and realize artificial intelligence big data processing of user terminal.

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