PLC is widely used in industrial control industry, and it often fails to work normally. Through this article, let you solve most PLC problems proficiently.
Various brands of PLCs have self-diagnostic functions, but the skill of PLC repair is to make full use of this function for analysis and then accurately find the problem. We have sorted out 8 common types of errors that PLC repair personnel need to know when the PLC presents abnormal alarms.
Years of debugging experienceA set of system software is solidified inside the PLC, so that you can start to initialize and configure the hardware. PLC start-up settings, watchdog, interrupt settings, communication settings, and I / O module address identification are all performed in the PLC system software.
Each PLC has its own programming software as a programming tool for application programs. The commonly used programming language is the ladder diagram language, as well as ST, IL and other languages. How to use the programming language for programming will not be elaborated here.
However, using a programming language to compile a very optimized program is an expression of the engineer's programming level. Each PLC programming language has its own characteristics, and the instruction design and layout ideas are different. If you are very familiar with the instructions of a PLC, you can compile a very simple, beautiful and smooth program. For example, for the design of the same program of the same PLC, if the programming engineer is not familiar with the instructions and the programming skills are poor, 1000 statements are required; but an engineer with excellent programming skills may only need 200 statements. To achieve the same function. The conciseness of the program not only saves memory, but also makes the probability of errors much smaller, the execution speed of the program is much faster, and it is much easier to modify and upgrade the program in the future.
Therefore, although the ladder logic of all PLCs is similar, as long as an engineer is familiar with the programming of a PLC, and then learn the PLC of the second brand, you can get started quickly. However, when an engineer uses a new PLC, he should carefully read the programming manual of the new PLC carefully, look at the special features of the instructions, especially the instructions that he may need, and consider how to use these special Way to optimize your own program.
The instruction design and interface design of each PLC's programming language are different. There is no problem of which is better, mainly because of the different styles. We cannot arbitrarily say that Mitsubishi PLC ’s programming language is not as good as Siemens ’STEP7, nor can it be said that STEP7 is better than ROCKWELL ’s RSLOGIX. The so-called good and bad are mostly due to the programming habits formed by engineers and the design style of the programming language. .
The scene often needs to modify the already compiled program. The reason for the modification may be that the user's needs have changed, it may be that an original programming error was found, or a power interruption occurred while the PLC was running, and some status data will be lost, such as the non-holding timer will be reset, and the input mapping area will be Refresh, the output mapping area may be cleared, but all the configuration data and accidental events of the status file such as the accumulated value of the counter will be saved.
At this time, the engineer may need to program the PLC so that some memory can be restored to the default state. When the program does not need to be modified, you can design and apply the default way to restart, or use the function of the first scan bit.
All intelligent I / O modules, including analog I / O modules, lose their configuration data after entering the programming mode or after the power supply is interrupted. The user program must confirm that the configuration data can be Rewrite the intelligent I / O module.
One problem that is often overlooked when the on-site modification has been run is that the engineer forgets to switch the PLC to programming mode. Although this error is not difficult to find, the engineer often mistakes the PLC for a failure when negligent, and therefore delays a lot of time.
In addition, when the PLC is downloading programs, many PLCs are not allowed to interrupt the power supply, because at this time, the old program has been partially rewritten, but the new program has not been completely written, so if the power is interrupted, it will cause the PLC Unable to run, at this time, it may be necessary to reload the bottom software of the PLC, and many manufacturers are not allowed to perform this operation on site. Most new PLCs have separated the user program from the PLC system program to avoid this problem.
Practical experience sharing
1. Abnormal CPU
When the CPU alarms abnormally, you should check all the equipment connected to the internal bus of the CPU unit. The specific method is to replace the problematic units in sequence, find out the problematic units, and deal with them accordingly.
When the memory is abnormally alarmed, if it is a problem of program memory, it cannot be solved after reprogramming. This situation may be caused by the disturbance of noise caused by the program change, otherwise the memory should be replaced.
When this type of alarm occurs, you should first check the connection status of the input / output unit and the expansion unit connector, and the cable connection status, determine the unit where the problem occurs, and then replace the unit.
Under normal conditions, you can follow the process of input-program execution-output
(1) Input viewing is to use the input LED indicator to identify, or use the input monitor formed by the writer to view. When the input LED does not light up, it can be concluded that the external input system is a problem, and then check with the multimeter. If the output voltage is abnormal, it can be concluded that the input unit is the problem. When the LED is on and the internal monitor is not visible, it can be considered a problem with the input unit, CPU unit, or expansion unit.
(2) The program is viewed through the monitor on the writer. When the contact status of the ladder diagram is inconsistent with the result, it is a program error (such as the use of two layers of internal relays, etc.), or there is a problem in the calculation part.
(3) Output view can be identified by output LED indicator. When the calculation result is correct and the output LED indicates an error, it can be considered as a problem of the CPU unit and the I / O interface unit. When the output LED is bright and there is no output, it can be determined that the output unit is a problem, or there is a problem with the external load system.
Due to different PLC models, I / O and LED connection methods are different (some are connected to the I / O unit interface, and some are connected to the I / O unit). Therefore, the scale of the problem based on LED discrimination is also different.
5. Some procedures are not executedThe checking method is the same as the previous item, but if the input time of the counter, step controller, etc. is too short, there will be no echo problem. At this time, you should check whether the input time is large enough. The maximum response time of the unit is calculated by multiplying the scan time by 2).
(1) First check whether there is a problem with the battery.
(2) Check by repeatedly turning on and off the power of the PLC itself. In order to make the microprocessor start correctly, the PLC is provided with a circuit for saving the reset point and the program saved when the power is turned off. When there is a problem with this circuit, the program cannot be saved. So you can check the power on and off.
(3) If the abnormal battery alarm still appears after replacing the battery, it can be determined that the leakage current of the memory or external circuit is abnormally increased.
(4) The on and off of the power supply always occur synchronously with the machine system. At this time, you can check the noise effect of the machine system. Because the disconnection of the power supply is often a problem that occurs simultaneously with the operation of the machine system, most of it is caused by the strong noise generated by the motor or the winding.
Seven, PROM does not workFirst check whether the PROM connection is good, and then determine whether the chip needs to be replaced.
This problem can be considered to be caused by noise disturbance or poor internal contact within the PLC. The cause of noise is generally caused by the reduction of the small capacitance in the circuit board or the poor function of the component. The cause of poor contact can be checked by tapping the PLC body gently, and the connection status of the cable and the connector.
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