The Daily Maintenance and Troubleshooting of PLC

1. Guarantee of operating environment
Because PLC is a precision electronic product, the automatic control system requires long-term uninterrupted operation, so the operating environment of PLC is extremely demanding, dustproof, fireproof, Waterproof, anti-high temperature, anti-lightning, so the electrical control room is generally arranged on the second floor with double glass and installed with water-type refrigeration and air-conditioning. The temperature of the electrical control room is generally controlled at about 28 degrees; The PLC is installed in a 1M*2.2M*0.6M control cabinet with a lockable front panel and a glass door. An exhaust system with a dust cover is installed on the upper part of the cabinet to cool down the cabinet.
2. Do a good job in daily work
1. Daily preparations: firstly, familiarize yourself with the process flow, secondly, familiarize yourself with the explanation materials of various PLC modules, and then understand the on-site layout. Finally, make sure that your various testing tools are intact.
2. Daily inspection work: regularly check the voltage of the battery of the CPU, which is 3V under normal circumstances, regularly inspect and maintain the related equipment that constitutes the PLC system, such as regular maintenance of UPS, use the opportunity of shutdown, The control cabinets of PLC carry out manual dust removal, cooling, regular manual backup of PLC programs and battery backup, and replacement of related bad devices.
3. Diagnosis of PLC faults
(1) General rules:
For the fault detection method of PLC system: one touch, two look, three smell, four listen, five press Trace tracing method, six replacement method.
First touch, check whether the temperature of the CPU is high, the normal operating temperature of the CPU does not exceed 60°C, because the temperature acceptable to the hand is 37-38°C of the human body temperature, it is suitable for hand feeling; Whether the module indicator light is normal; the third smell, whether there is any peculiar smell, whether the electronic components or cables are burned; the fourth listen, whether there is any abnormal movement, loose screws, whether the relay is working normally, and listen to the feedback of the on-site staff; 5. If there is a failure, find the location of the failure according to the drawings and process flow; 6. Use the component replacement method for the uncertain parts to determine the failure.
(2) Specific steps:
When the software of the PLC is abnormal, it mainly depends on whether the RUN state of the CPU is normal. If it is not normal, clear the CPU and download the control program again.
When the PLC hardware is not normal, check the work in the following order:
1. Check whether the PLC power supply has electricity: if there is electricity, measure whether the voltage is within the range of ±5% of +24V, and if there is electricity and If it is normal, proceed to the next step; if the power is abnormal, check the output and input terminals of the power module. If the output terminal is abnormal but the input terminal is normal, replace the module; if the input terminal is abnormal, perform the reverse flow rule of the input terminal. Carry out corresponding checks, such as checking the ±10% of the AC voltage 220V at the input voltage terminal of the 24V AC-DC transformer, if it is normal, then change theReplace the DC 24V transformer. If there is no electricity, follow the traces, and check whether the wiring of the output terminals of the various electrical devices that supply power to the power module is correct with the help of the schematic diagram + general layout diagram of the site + wiring drawings. Whether there is normal power supply at the incoming line terminal and outgoing line terminal, if there is no normal power supply, find out whether it is external or its own cause, if it is external, it is insufficient voltage or no voltage at all, or the load is too heavy, or the analysis of severe overcurrent, etc. , until the accident is ruled out and the normal power supply is reached; if the device itself is broken, replace it.
2. Understand the CPU working mode and priority: high priority includes STOP, HOLDUP, STARTUP (WARMRESTART, COLDRESTART); low priority includes: RUN, RUN-P (PG/PC online reading and writing program). Check whether the CPU is in RUN mode, or in STOP mode, or the hold mode or debug mode in which the flashing state of RUN mode and STOP mode are combined. If it is only in RUN mode, the CPU and each board are normal and proceed to step 3. If the hold mode occurs, it may be that the user program has a breakpoint during the running process and is in the state of debugging the program, or the breakpoint appears in the startup mode. In this case, re-debug the program and download the control program to the CPU again.
If it is in STOP mode, visually analyze the cause of STOP: A. No power, analyze the cause of no power, whether it is because of a problem with the power supply department, or abnormal power failure (because there is a 1K3AH UPS to ensure that abnormal power failures rarely occur ), under normal circumstances, the power is pulled for the maintenance, and the power is sent manually after the maintenance is completed. Then use the online function of the PLC to switch the working mode of the CPU from STOP to RUN; B. If the CPU is broken, replace it with a new good CPU of the same type and version; C. If a board is broken, replace the board in an orderly manner. When replacing the hardware, pay attention to using the same model and version of the original device, otherwise the actual PLC configuration will be different from the hardware configuration in the hardware configuration database in the corresponding programming software, and the normal cycle of the user control program cannot be performed. implement.
3. Check the communication cables of each main board and expansion board and check the LED lights of each module to see if there is a fault light on a bad module. If so, the module is abnormal. For each point on the digital output module, it has the same function as the light switch in real life and is a normally open point. Therefore, when any point of the module is repaired online, as long as there is no connection and the address is not output in the control program It is enough to check whether the signal is connected or not. If it is connected, the point is abnormal, and if it is not connected, it is normal; when it is abnormal, it is necessary to reconnect the other point of the hardware connection line; in addition, we can also replace it with a new module , Measure the on-off status of the point of the replaced module. For the digital input module, the point is equivalent to the conduction coil, which is in the normally closed state. It can be detected online or offline, and can be detected with a meter.If it is a bad point, it is in a state of failure, then change the point and rewire; if it is a good point, it is in a state of communication. As long as the hardware wiring is re-changed and re-connected, the corresponding programming software must be used to replace the 0X or 1X address of the control software. The analog input module is the same as the digital input module, each channel is equivalent to a wire form, that is to say, it is equivalent to a normally closed point, so the method to detect whether the channel is good or bad is to use the on-off function of the meter to detect , when the state is on, it is good, and when it is off, it is a bad channel; the detection method of the analog output module is the same as that of the digital output module. If the channel is broken, the hardware wiring needs to be replaced and the corresponding 3X or 4X address in the control program should be replaced at the same time; in addition, for the analog module, it is necessary to check the selection of the range block and check whether the fuse is disconnected. Whether the software configuration is normal, generally the voltage is 1-5V or the current is 4-20mA, which is selected according to the type of sensor and smart converter used. After the hardware point or channel replacement work, if the conditions permit, stop the CPU of the PLC, and then re-download the program. If the conditions do not allow, directly use the update change to download the changed program without stopping the CPU. For unused input modules, the good channel/good point is connected in series with the last used good channel/good point or specially set in the software.
4. The power module on the board of a large number of output modules cannot be powered off during normal production status, because if the power is cut off at this time, the normally open relay in the relay cabinet will become normally open, which is prone to accidents. Error, so when testing this type of output module, it is necessary to contact the on-site operator. After manual operation of this part of the relevant equipment, remove the power supply line of the digital output module and then work on the module measurement point.
5. The detection work of various types of switches: the detection work of devices such as relays, proximity switches, and air switches is distinguished according to whether the type of switch is normally closed or normally open, and a meter is used to detect whether it is on or off. If the state is unreasonable, its state is opposite to that of a good device, then the device is broken and needs to be replaced. For most of the circuit, the normally open type is used, and they are used to manually control or automatically control the on and off of the current; for the normally closed type, it is mainly used in the protection circuit. In this way, it is possible to know the normal state of the switch and protection devices and identify whether the devices are good or bad.
6. The detection of the communication module is to simply replace it with a good new communication module to identify whether the module in use on the board is normal.
7. The measurement method of the wire: the wire is also carried out by detecting the on-off method. You can use the wires known to be connected to detect whether the wires are good or bad. The method is to connect the good wires to the unknown wires and measure the on-off status.
8. Resistance detection: detect the voltage when it is charged, and detect the corresponding resistance when it is not charged.
4. Summary
Most of the faults in the work can be eliminated through detection. In addition, because this work involves AC single-phase electricity 220V andWhen working with 24V direct current, pay attention to accumulating knowledge and common sense of safe electricity use, as well as safety precautions and gas safety regulations during work to ensure safe operation.

Leave a Reply

Your email address will not be published. Required fields are marked *