Ingenious test of engineer experience sharing plc input and output

PLC input and output

A small PLC flexibly controls a complex system. What you can see is the two rows of staggered input and output relay terminals, corresponding indicators and PLC numbers, just like an integrated circuit with dozens of feet. . Anyone who does not look at the schematic to troubleshoot the faulty device will be at a loss, and the speed of finding the fault will be particularly slow. In view of this situation, we draw a table according to the electrical schematic diagram, attached to the console or control cabinet of the equipment, indicating the electrical symbol corresponding to each PLC input and output terminal number, Chinese name, that is, similar integrated circuit tubes Description of the function of the foot. With this input and output form, the electrician who understands the operation process or is 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 will not look at 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 (actuator) during most of the operation. Practice has proved that if you can skillfully use the input and output correspondence table and the input and output logic function table, it is easy to repair electrical faults without drawings.

Input loop overhaul

Judging whether the input loop of a button, limit, line, etc. is good or bad, you can press the button (or other input contact) when the PLC is powered (preferably in a non-operational state to prevent the device from malfunctioning). When the corresponding PLC input point terminal and the common end are shorted, the PLC input indicator corresponding to the button is illuminated, indicating that the button and the line are normal. The light does not illuminate, it may be broken, the line is not well connected, or the line is broken.

Output loop overhaul

For the PLC output point (here only the relay output type), if the corresponding indicator of the action object is not lit, in the determination of the PLC in the running state, then the PLC input and output logic function of this action object is not satisfied, that is, the input The loop is faulty. Check the input loop as described above. If the corresponding indicator light is on, but the corresponding actuators such as solenoid valve and contactor do not work, first check the solenoid valve control power supply and fuse, the easiest way to use the electric pen to measure the common terminal of the corresponding PLC output point. . The electric pen does not light, and may correspond to a power failure such as a blown fuse. The electric pen is bright, indicating that the power supply is good, and the corresponding solenoid valve, contactor, and line are faulty. After eliminating the malfunction of the solenoid valve, contactor, circuit, etc., it is still not normal. Use a multimeter of the multimeter to connect the corresponding output common terminal to one end, and the other end to the corresponding PLC output point. At this time, the solenoid valve still does not move. The output line is faulty. If the solenoid valve is activated at this time, the problem is at the PLC output point. Since the electric pen sometimes falsely reports, it can be analyzed by another method. The voltage of the PLC output point and the common terminal is measured by the voltage of the multimeter. The voltage is zero or close to zero, indicating that the PLC output point is normal and the fault point is at the periphery. If the voltage is high, the contact resistance of this contact is too large and it is damaged. In addition, when the indicator light is not lit, but the corresponding solenoid valve, contactor, etc., this output point may be burnt due to overload or short circuit. At this time, the external wiring of this output point should be removed, and then the resistance of the output point and the common terminal should be measured by the multimeter resistance file. If the resistance is small, the contact is broken. If the resistance is infinite, the contact is good. , the corresponding output indicator should be broken.

Program logic inference

There are many kinds of PLCs that are often used in the industry. For low-end PLCs, the ladder instructions are similar. For medium and high-end machines, such as the S7-300, many programs are written in language. The practical ladder diagram must have Chinese symbol annotation, otherwise it is very difficult to read. If you can understand the equipment process or operation process before looking at the ladder diagram, it seems easier. If the electrical fault analysis is carried out, the reverse check method or the inverse push method is generally applied, 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 satisfying the action is started. Experience has shown that when a problem is found, the fault can be basically eliminated, because there are not many fault points where the device has two or more simultaneous occurrences.

PLC self failure judgment

In general, PLC is an extremely reliable device with a low failure rate, but it can also cause PLC damage due to external causes.

A working power supply is a 220V proximity switch. The two input wires of the input PLC signal contact share a 4-core cable with the 220V power supply line of the proximity switch. Once the proximity switch is damaged, the electrician replaces the neutral line of the power supply. The error is adjusted with the input PLC's common line, causing the three PLC input points to burn out during power transmission.

The zero line of the system power transformer was interrupted due to corrosion, which led to the PLC220V power supply rising to 380V, which burned out the power module at the bottom of the PLC. After the rectification, the 380/220V isolation control transformer was added.

Siemens S7-200 PLC output common end standard 1L, 2L, etc., the working computer is AC L1 N, +24V power supply is L+M, which means that it is easy for beginners or inexperienced people to make mistakes. If L+M is mistaken as a 220V power supply terminal, the PLC24V power supply will be burned out immediately after the power transmission.

The probability of PLC, CPU and other hardware damage or software running error is almost zero. If the PLC input point is not caused by strong power intrusion, it will hardly be damaged. The normally open point of PLC output relay is not short-circuited or improperly designed. The load current is out of the rated range and the contact life is also very long. Therefore, we look for electrical fault points, the focus is on the peripheral electrical components of the PLC, do not always suspect that there is a problem with the PLC hardware or the program, which is very important for quickly repairing the faulty equipment and quickly recovering the production, so the PLC control loop The electrical fault diagnosis, the focus is not on the PLC itself, but on the peripheral electrical components in the loop controlled by the PLC.