How to judge the daily failure of PLC

As an indispensable part of industrial control, PLC has been widely used in industrial production, but its maintenance and repair methods and usage skills are still not well understood by many engineers. This article summarizes some experience and skills in the process of using PLC.
A small PLC flexibly controls a complex system. What you can see are two rows of staggered input and output relay terminals, corresponding indicator lights and PLC numbers, just like an integrated circuit with dozens of pins. . If anyone checks the faulty equipment without looking at the schematic diagram, he will be at a loss what to do, and the speed of finding the fault will be very slow.
In view of this situation, we draw a form based on the electrical schematic diagram, paste it on the console or control cabinet of the equipment, and indicate the electrical symbol and Chinese name corresponding to each PLC input and output terminal number, that is, similar to each tube of an integrated circuit. Description of the function of the foot. With this input and output table, electricians who understand the operation process or are familiar with the ladder diagram of this equipment can carry out maintenance. But for those electricians who are not familiar with the operation process and don't know how to read the ladder diagram, they need to draw another table: PLC input and output logic function table. This table actually illustrates the logical correspondence between input circuits (trigger elements, associated elements) and output circuits (actuator elements) during most operations. Practice has proved that if you can skillfully use the input-output correspondence table and input-output logic function table, you can easily check and repair electrical faults without drawing.
To judge the quality of an input circuit such as a button, limit, line, etc., you can press the button (or other input contacts) when the PLC is powered on (preferably in a non-running state to prevent misoperation of the equipment), and then When the corresponding PLC input point terminal is short-circuited with the common terminal, the PLC input indicator corresponding to the button is on, indicating that the button and the circuit are normal. If the light is not on, the button may be broken, the line may be in poor contact, or the line may be broken.
For the PLC output point (here we only talk about the relay output type), if the indicator light corresponding to the action object is not on, and the PLC is in the running state, it means that the PLC input and output logic function of the action object is not satisfied, that is to say, the input If the circuit fails, check the input circuit as mentioned above. If the corresponding indicator light is on, but the corresponding actuators such as solenoid valves and contactors do not operate, first check the solenoid valve control power supply and fuse. The easiest way is to use an electric pen to measure the common terminal of the corresponding PLC output point. . If the electric pen does not light up, it may correspond to a power failure such as a blown fuse. If the electric pen lights up, it means that the power supply is good, and the corresponding solenoid valve, contactor, and circuit are faulty.
After troubleshooting the solenoid valve, contactor, line and other faults, if it is still abnormal, use a test lead of the multimeter to connect one end to the corresponding output common terminal and the other end to the corresponding PLC output point. At this time, the solenoid valve and the like still do not operate. Indicates that the output line is faulty. If the solenoid valve acts at this time, then the problem is on the PLC output point. Because the electric pen sometimes makes false reports, another method can be used to analyze. Use a multimeter to measure the voltage between the PLC output point and the common terminal. If the voltage is zero or close to zero, it means that the PLC output point is normal and the fault point is in the periphery. If the voltage is high, it means that the contact resistance of this contact is too large and has been damaged. In addition, when the indicator light is not on, but the corresponding solenoid valve, contactor, etc. are activated, it may be that the output point is burned due to overload or short circuit. At this time, the external wire of this output point should be removed, and then use the multimeter resistance file to measure the resistance between the output point and the common terminal. If the resistance is small, it means that the contact is broken. If the resistance is infinite, it means that the contact is good. , it should be that the corresponding output indicator is broken.
There are many kinds of PLCs that are often used in industry. For low-end PLCs, the ladder diagram instructions are similar. For mid-to-high-end machines, such as S7-300, many programs are compiled in language tables. Practical ladder diagrams must be annotated with Chinese symbols, otherwise it will be difficult to read. If you can roughly understand the equipment process or operation process before looking at the ladder diagrams, it seems easier. For electrical fault analysis, the reverse search method or reverse deduction method is generally used, that is, according to the input-output correspondence table, the output relay corresponding to the PLC is found from the fault point, and the logical relationship that satisfies its action is started to reverse search. Experience shows that if one problem is found, the fault can basically be eliminated, because there are not many fault points with two or more faults occurring at the same time.
Generally speaking, PLC is an extremely reliable device with a low failure rate, but it can also be damaged due to external reasons.
A proximity switch with a working power supply of 220V, the two leads of its input PLC signal contact share a 4-core cable with the 220V power line of the proximity switch. Once the proximity switch was damaged, the electrician replaced the zero wire of the power supply by mistake. The common line with the input PLC was wrongly adjusted, which caused the 3-way PLC input points to be burned out during power transmission.
The neutral line of the primary system power supply transformer was interrupted due to corrosion, which caused the 220V power supply connected to the PLC to rise to 380V, and burned out the power module at the bottom of the PLC. After the rectification, a 380/220V isolation control transformer was added.
Siemens S7-200’s PLC output public terminals are labeled 1L, 2L, etc., the working computer is AC L1 N, and the +24V power supply is L+M, which is easy for beginners or inexperienced to make mistakes. If L+M is mistakenly regarded as the 220V power supply terminal, the PLC24V power supply will be burned out at the moment of power transmission
The probability of PLC, CPU and other hardware damage or software operation error is almost zero. If the PLC input point is not caused by strong current intrusion, it will hardly be damaged. The normally open point of the PLC output relay is not short-circuited by the peripheral load or the design is unreasonable. , the load current exceeds the rated range, and the life of the contacts is also very long.
Therefore, when we look for electrical fault points, we should focus on the peripheral electrical components of the PLC. Don’t always suspect that there is a problem with the PLC hardware or program. This is very important for quickly repairing the faulty equipment and quickly restoring production. Therefore, the PLC control loop The focus of electrical troubleshooting is not on the PLC itself, but on the peripheral electrical components in the loop controlled by the PLC.