PLC has been widely used in industrial production, but its maintenance and maintenance methods and usage skills have made many engineers not understand. This article summarizes some experiences and techniques in the process of using PLC.
PLC input and output
A small PLC flexibly controls a complex system. What you can see is the staggered input and output relay terminals, corresponding indicator lights and PLC numbers, just like a integrated circuit with dozens of pins. Anyone who does not look at the schematic to repair the faulty equipment will be helpless and the speed of finding faults will be particularly slow.
In view of this situation, we draw a table based on electrical schematic , paste it on the device's console or control cabinet , indicating the electrical symbol corresponding to the number of each PLC input and output terminal, the Chinese name, that is, the function description of each pin of the integrated circuit is similar. 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 electricians who are not familiar with the operation process and cannot 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 the input loop (trigger component, associated component) and the output loop (actuation component) during most operations. Practice has proved that if you can skillfully use the input and output corresponding tables and input and output logic tables, repair electrical faults, and you can also be relaxed and free without drawings.
Input circuit maintenance
determines the quality of a certain input circuit, such as button, limit, line, etc., and can press the button (or other input contacts) when the PLC is powered on (preferably in a non-operating state to prevent the equipment from erroneously operating). At this time, the corresponding PLC input point terminal is short-connected to the common end, and the PLC input indicator light corresponding to the button is on, indicating that the button and line are normal. If the light does not light, it may be broken, poor line contact or disconnection.
Output loop maintenance
For the PLC output point (I will only talk about relay output type), if the indicator light corresponding to the action object is not lit, and when it is determined that the PLC is in operation, it means that the PLC input and output logic function of this action object is not satisfied, that is, the input loop has a fault. As mentioned earlier, check the input loop. If the corresponding indicator light is on, but the corresponding actuators such as solenoid valve and contactor do not operate, check the solenoid valve control power and safety device first. The easiest method is to use a pen to measure the common terminals of the corresponding PLC output point. If the pen does not light up, it may correspond to power failures such as fuse blown. The electric pen is on, which means that the power supply is good and the corresponding solenoid valve, contactor, and line are faulty.
After troubleshooting the fault of solenoid valve, contactor, line, etc., it is still abnormal. Use a multimeter meter to connect the corresponding output common terminal at one end and contact the corresponding PLC output point at the other end. At this time, the solenoid valve, etc. still do not operate, indicating that the output line is faulty. If the solenoid valve operates at this time, the problem lies at the PLC output point. Since the pen sometimes falsely reports, another method can be used to analyze it. The voltage at the output point of the multimeter voltage level PLC and the common terminal, and the voltage is zero or close to zero, indicating that the PLC output point is normal and the fault point is on the periphery. If the voltage is high, it means that the contact resistance of this contact is too large and it is damaged. In addition, when the indicator light does not light up, but the corresponding solenoid valve, contactor, etc. may be used, this output point may be burned due to overload or short circuit. At this time, the external wiring of this output point should be removed, and then the multimeter resistance level should be used to measure the resistance of 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, and the corresponding output indicator light should be broken.
Program logic inference
There are many types of PLCs commonly used in the industry. For low-end PLCs, ladder diagram instructions are similar. For mid-to-high-end machines, such as S7-300, many programs are compiled in language tables.The practical ladder diagram must have Chinese symbol annotations, otherwise it will be difficult to read. If you can roughly understand the equipment process or operation process before looking at the ladder diagram, it will look easier. If electrical fault analysis is performed, the reverse search method or reverse push method is generally used, that is, according to the input and output corresponding table, the output relay corresponding to the PLC is found from the fault point, and the logical relationship that meets its actions is reversely checked. Experience shows that if a problem is found, the fault can be basically eliminated, because there are not many fault points in the equipment at the same time.
PLC's own fault judgment
Generally speaking, PLC is an extremely reliable device with a very low failure rate, but due to external reasons, it can also cause PLC to be damaged.
A proximity switch with a working power supply of 220V. The two leads of the input PLC signal contacts share a 4-core cable with the 220V power cable of the proximity switch. Once the proximity switch is damaged, the electrician mistakenly adjusts the neutral line of the power supply and the common line of the input PLC when changing it, resulting in the 3 PLC input points burning during power supply.
primary system power transformer neutral line was interrupted due to corrosion, causing the PLC220V power supply to rise to 380V, burning down the power module at the bottom of the PLC. After the rectification, an isolation control transformer of 380/220V was added.
The PLC output common terminal of Siemens S7-200 is 1L, 2L, etc. The working computer is AC L1 N, and the +24V power supply is L+M means that it is easy to make mistakes for beginners or those who are not experienced. If you mistakenly treat L+M as a 220V power supply terminal, the PLC24V power supply will be burned out immediately when the power is sent.
The probability of hardware damage to PLC, CPU, etc. or software operation errors is almost zero. If the PLC input point is not caused by strong power intrusion, it will hardly be damaged. If the normal opening point of the PLC output relay is not short-circuited or unreasonable design, the load current exceeds the rated range, and the life of the contact point 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 focus of the electrical fault maintenance of the PLC 1 control loop is not on the PLC itself, but on the peripheral electrical components in the loop controlled by the PLC.
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