Research on Fault Diagnosis Using PLC Instruction Tables in Industrial Automation Control
I remember a few days ago when I went to a pharmaceutical factory to address a production line fault. As I entered the control room, a sweating engineer named Xiao Li rushed towards me, saying, “Wen Ge, it’s great that you’re here! The entire filling line has been down for two hours, and we can’t find the problem!” Looking at the thick stack of electrical diagrams he was flipping through, I smiled and patted his shoulder, saying, “Don’t worry, let’s first check the status of the PLC instruction table.” Ten minutes later, we found the issue—a faulty signal from a cylinder limit switch had caused the entire production line to be in a waiting state.
The PLC instruction table is the “health report” of an automation control system, recording the execution status and data information of various instructions in the current system. Mastering this tool is like having an “industrial CT scanner” that can visualize the internal operating conditions of the equipment and quickly identify fault points. So, how can we utilize this powerful tool for fault diagnosis?
First, we need to understand what a PLC instruction table is. Simply put, it is a real-time monitoring window for the PLC program’s operation, displaying the status of various input/output points, counter values, timer statuses, data register contents, and more. For different brands of PLCs like Siemens and Mitsubishi, this window may be referred to as the “variable monitoring table,” “device monitoring,” or “status monitoring.”
The first step in PLC fault diagnosis is to check the input signals. I remember once when Xiao Wang complained that the photoelectric switch on the assembly line was clearly blocked by an object, yet the PLC did not respond. I asked him to open the instruction table and check the corresponding input point X0, only to find that its status was always OFF. At this point, the problem became clear—either the photoelectric switch was faulty, the wiring was loose, or the PLC input point was damaged. The instruction table narrowed down the fault range to a few possible points.
The core of fault diagnosis is “status comparison”, which involves comparing the actual operating status with the expected status. For example, if a cylinder is supposed to start after the limit switch is triggered but does not move, we can check the instruction table for:
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Whether the limit switch input point (e.g., X5) is ON
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Whether the cylinder control output point (e.g., Y3) is ON
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Whether the intermediate relay in the program controlling this process (e.g., M10) is activated
Once we find that a certain status does not match the expectation, the problem is basically locked in.
When using the instruction table, there is also a small trick—the “force function.” Suppose we suspect that a certain sensor is faulty; we can “force ON” the corresponding input point in the instruction table. If the system operates normally, then the problem lies with the sensor. This method is particularly suitable for diagnosing intermittent faults. However, the force function must be used with caution and should be released immediately after debugging, or it may cause abnormal actions in the equipment.
Real-time data monitoring is key to preventive maintenance. For instance, abnormal values in timers (T), counters (C), and data registers (D) often indicate that equipment is about to have issues. I once encountered an automatic assembly line where monitoring the cumulative counts in the instruction table revealed that a certain cylinder was operating far more frequently than others. We promptly replaced the cylinder that was nearing its wear limit, thus avoiding a production halt.
In PLC fault diagnosis, the network communication status is often overlooked. In modern factories, PLCs are often connected to host computers and other devices via industrial Ethernet or fieldbus. When communication anomalies occur, we can check specific communication status bits in the instruction table (different brands of PLCs have dedicated communication status areas). If we find that the communication error flag is set, the problem is likely with the network devices or communication configuration.
When diagnosing using the instruction table, pay special attention to manually added monitoring points, and key node variables in the program should be closely monitored. Experienced engineers will set “status indicator bits” in the program to indicate which working stage the system is currently in, so that if the system stalls, we can immediately see where it is stuck from the instruction table.
Finally, I want to say that PLC fault diagnosis is a combination of technology and experience. Although the instruction table is powerful, it still needs to be combined with electrical schematics, equipment manuals, and on-site conditions for comprehensive judgment. I recommend beginners start with simple simulation projects to gradually accumulate experience and develop their own fault diagnosis approach. I remember when I first entered the industry, I also started with simple indicator light fault diagnosis and gradually mastered the diagnostic methods for complex systems.
Once you have mastered the instruction table diagnostic techniques, don’t forget to take it a step further by learning advanced skills such as online modification of PLC programs and remote monitoring, so you can truly become a versatile expert in the field of automation.
