Explanation of nine common problems in PLC application

1. PLC's own fault judgment






Generally speaking, PLC is an extremely reliable device with a very low failure rate. The probability of PLC CPU hardware damage or software operation error is almost zero; the PLC input point is almost not damaged if it is caused by strong electrical intrusion; the normally open point of the PLC output relay, if it is not a peripheral load short circuit or unreasonable design , The load current exceeds the rated range, and the life of the contact is also very long.


Therefore, we look for electrical fault points, and the focus should be on the peripheral electrical components of the PLC. Do n’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, so the author talks about The electrical fault maintenance of the PLC control loop of the PLC is not focused on the PLC itself, but on the peripheral electrical components in the loop controlled by the PLC.


2. Selection of input / output (I / O) module






The output module is divided into transistor, bidirectional thyristor, and contact type. The switching speed of the transistor type is the fastest (generally 0.2ms), but the load capacity is the smallest, about 0.2 ~ 0.3A, 24VDC. It is suitable for fast switching and signal contact equipment. It is generally connected to signals such as frequency conversion and DC devices. The effect of current on the load.


The advantages of the thyristor type are non-contact, AC load characteristics, and low load capacity.


The relay output has the characteristics of AC and DC loads, and the load capacity is large. In conventional control, the relay contact type output is generally selected first. The disadvantage is that the switching speed is slow, generally around 10ms, and is not suitable for high-frequency switching applications.


3. Grounding problem






The grounding requirements of the PLC system are relatively strict. It is better to have an independent dedicated grounding system. It should also be noted that other equipment related to the PLC must also be reliably grounded. When multiple circuit ground points are connected together, unexpected currents can be generated, resulting in logic errors or damage to the circuit.


The reason for the different ground potentials is usually because the ground points are too far apart in the physical area. When devices that are far apart are connected by communication cables or sensors, the current between the cable and the ground is Will flow through the entire circuit, even within a short distance, the load current of large equipment can change between it and the ground potential, or directly generate unpredictable current through electromagnetic action.


Between power sources with incorrect grounding points, there may be a devastating current in the circuit that could damage the equipment.


The PLC system generally selects one-point grounding. In order to improve the ability to resist common mode interference, shielding floating technology can be used for analog signals, that is, the shielding layer of the signal cable is grounded at one point, the signal loop is floating, and the insulation resistance from the ground should not be less than 50MΩ.


4. Eliminate the capacitance between lines to avoid misoperation


There is capacitance between each conductor of the cable, and a qualified cable can limit this capacitance to a certain range. Even for a qualified cable, when the cable length exceeds a certain length, the capacitance value between the lines will exceed the required value. When this cable is used for PLC input, the capacitance between the lines may cause the PLC to malfunction , There will be many incomprehensible phenomena.


These phenomena are mainly manifested as: the wiring is correct, but the PLC has no input; the input that the PLC should have is not, but the others should not, that is, the PLC inputs interfere with each other. To solve this problem, you should do:


1. Use cables with cable cores twisted together;


2. Try to shorten the length of the cable used;


3. Separate cables that interfere with each other and use cables;


4. Use shielded cables.


5. Anti-interference treatment






The environment at the industrial site is relatively harsh and there are many high and low frequency interferences. These interferences are generally introduced into the PLC through the cable connected to the field device. In addition to grounding measures, in the design selection of cables and installation work, some anti-interference measures should be taken:


1. The analog signal is a small signal and is easily affected by external interference. Double-layer shielded cables should be used; high-speed pulse signals (such as pulse sensors, counting code disks, etc.) should be shielded cables to prevent external interference and Interference of high-speed pulse signal on low-level signal;


2. The frequency of communication cables between PLCs is relatively high. Generally, the cables provided by the manufacturer should be selected. In the case of low requirements, shielded twisted pair cables can be used;


3. Analog signal lines and DC signal lines cannot be routed in the same trunking as AC signal lines;


4. The shielded cables introduced in and out of the control cabinet must be grounded, and should be directly connected to the equipment without going through the wiring terminals;


5. The AC signal, DC signal and analog signal cannot share the same cable, the power cable should be laid separately from the signal cable;


6. During on-site maintenance, the methods to solve the interference are: use shielded cables for the interfered lines and re-lay them; add anti-interference filtering code to the program.






6. Mark input and output for easy maintenance






PLC controls a complex system, all you can see are the upper and lower rows of staggered input and output relay terminals, corresponding indicator lights and PLC numbers, just like an integrated circuit with dozens of feet. If anyone does not look at the schematic diagram to repair the faulty equipment, they will be helpless and the speed of finding faults will be particularly slow.


In view of this situation, we draw a table based on the electrical schematic diagram and paste it on the console or control cabinet of the equipment, indicating the electrical symbol corresponding to the number of each PLC input and output terminal, and the Chinese name, that is, similar to each tube of the integrated circuit Functional description of the foot.


With this input and output table, an 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 unfamiliar 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. The table actually illustrates the logical correspondence between the input loop (trigger element, associated element) and the output loop (executive element) during most operations.


Practice has proved that if you can skillfully use the input-output correspondence table and the input-output logic function table to troubleshoot electrical faults, without drawings, you can easily.


7. Infer faults through program logic


There are many types of PLCs commonly used in industry now. For low-end PLCs, ladder diagram instructions are similar. For mid- to high-end machines, such as S7-300, many programs are written in language tables. Practical ladder diagrams must be annotated with Chinese symbols, otherwise reading is very difficult. If you can roughly 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 search method or reverse inversion method is generally applied, that is, the output relay corresponding to the PLC is found from the fault point according to the input and output correspondence table, and the reverse search is performed to satisfy the logical relationship of its actions. Experience shows that if one problem is found, the fault can be basically eliminated, because there are not many failure points of two or more simultaneous occurrences of the equipment.


8. Make full and reasonable use of software and hardware resources






1. Commands that have not participated in the control cycle or have been input before the cycle may not be connected to the PLC; when multiple commands control a task, they can be connected in parallel outside the PLC before connecting to an input point;


2. Try to use the PLC internal function software components to fully call the intermediate state, so that the program has complete coherence and is easy to develop. At the same time, it also reduces hardware investment and reduces costs;


3. When conditions permit, it is better to isolate each output independently to facilitate control and inspection, and also protect other output circuits; when an output point fails, it will only cause the corresponding output circuit to lose control;


4. If the output is a load controlled by forward / reverse, not only interlock from the PLC internal program, but also take measures outside the PLC to prevent the load from moving in both directions;


5. PLC emergency stop should be cut off with an external switch to ensure safety.


9. Other matters needing attention






1. Do not connect the AC power cord to the input terminal to avoid burning the PLC; the ground terminal should be independently grounded, not connected in series with the ground terminal of other equipment, and the cross-sectional area of the ground wire is not less than 2mm2


2. The auxiliary power supply has a small power and can only drive low-power devices (photoelectric sensors, etc.);


3. Some PLCs have a certain number of occupied points (ie empty address terminals), do not connect the line;


4. When there is no protection in the PLC output circuit, a fuse and other protective devices should be used in series in the external circuit to prevent damage caused by the short circuit of the load.