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Explanation of nine common problems in PLC application

Last Updated on 2020-01-03 Hits:5403

1.PLC fault diagnosis
Generally speaking, PLCs are extremely reliable devices with very low failure rates. The probability of hardware damage or software error of the CPU of the PLC is almost zero; if the PLC input point is not caused by a strong electric invasion, it will hardly be damaged; if the normally open point of the PLC output relay is not a short circuit of the external load or the design is unreasonable The load current exceeds the rated range, and the life of the contact is also very long.


Therefore, when we look for electrical fault points, the focus should be on the peripheral electrical components of the PLC. Do not always suspect that there is a problem with the PLC hardware or program. This is very important to quickly repair the faulty equipment and quickly resume production. Therefore, the author talked about The electrical troubleshooting of the PLC control loop is not focused on the PLC itself, but on the peripheral electrical components in the PLC control loop.


2. Selection of input / output (I / O) modules
The output module is divided into a transistor, a triac, and a contact type. The transistor type has the fastest switching speed (usually 0.2ms), but has the smallest load capacity, about 0.2 ~ 0.3A, 24VDC. It is suitable for fast switching and signal connection equipment. It is generally connected to signals such as frequency conversion and DC devices. Pay attention to transistor leakage. The effect of current on the load.


The SCR type has the advantages of no contact, AC load characteristics, and small load capacity.


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

3. Grounding problems

PLC system grounding requirements are relatively strict, it is best to have an independent dedicated grounding system, but also pay attention to other equipment related to the PLC must also be reliably grounded. When multiple circuit grounds are connected together, they can generate unexpected currents that can cause logic errors or damage the circuit.


The reason for the different ground potentials is usually because the ground points are separated too far in the physical area. When the devices that are far away 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 its potential and the ground, or directly generate unpredictable current through electromagnetic action.


Between incorrectly grounded power sources, there can be devastating currents in the circuit that can damage the equipment.


PLC systems generally use a little grounding. In order to improve the ability to resist common mode interference, shielded floating technology can be used for analog signals, that is, the shielding layer of the signal cable is grounded at one point, the signal circuit is floating, and the insulation resistance with the ground should not be less than 50MΩ.


4. Eliminate the capacitance between lines to avoid malfunction


There is a capacitance between the wires of the cable. A qualified cable can limit this capacitance to a certain range. Even if the cable is qualified, 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 line capacitance may cause 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 PLC should have some inputs but not the other, that is, the PLC inputs interfere with each other. To solve this problem:


1. Cables twisted together using cable cores;


2. Minimize the length of the cable used;


3. Separate the mutually interfered input cables;


4. Use shielded cables.


5.Anti-interference processing

The industrial site environment is relatively harsh, and there are many high and low frequency interference. These interferences are generally introduced into the PLC through cables connected to field devices. In addition to grounding measures, attention should be paid to taking some anti-interference measures in the design and installation of cables.


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


2. The communication cable frequency between PLCs is high. Generally, the cable provided by the manufacturer should be selected. When the requirements are not high, the shielded twisted pair cable can be used.


3. Analog signal lines and DC signal lines cannot be routed in the same trough with AC signal lines;


4. The shielded cable that is led in and out of the control cabinet must be grounded and connected directly to the device without going through the terminal;


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


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






6. Mark input and output for easy maintenance

The PLC controls a complex system. What you can see is the staggered input and output relay terminals, the corresponding indicator lights, and the PLC number, 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 fault finding speed will be particularly slow.


In view of this situation, we draw a table according to 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, the Chinese name, which is similar to the various integrated circuits Functional description of feet.


With this input and output form, electricians who understand the operation process or are familiar with the ladder diagram of this equipment can start overhaul. But for those electricians who are unfamiliar with the operation process and do not look at the ladder diagram, they need to draw another table: PLC input and output logic function table. This table actually explains the logical correspondence between the input circuit (trigger element, associated element) and output circuit (actuator 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, you can easily repair electrical faults without drawings.


7. Infer faults through program logic


Nowadays, there are many types of PLCs often used in the industry. For low-end PLCs, the ladder instructions are similar. For mid-to-high-end machines, such as the S7-300, many programs are written in language tables. Practical ladder diagrams must be annotated with Chinese symbols, otherwise it is difficult to read. If you can roughly understand the equipment process or operation process before looking at the ladder diagram, it looks easier.


If electrical fault analysis is performed, the reverse check method or reverse push method is generally applied, that is, according to the input-output correspondence table, the corresponding output relay of the PLC is found from the fault point, and the reverse check is performed to satisfy the logical relationship of its action. Experience shows that when a problem is found, the fault can be basically eliminated, because the equipment has two or more fault points at the same time.


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

1. The instructions that do not participate in the control cycle or have been input before the cycle may not be connected to the PLC; when multiple instructions control a task, they can be connected in parallel outside the PLC before being connected to an input point;


2. Make the best use of the PLC's internal functional devices and make full use of the intermediate state, so that the program has complete coherence and is easy to develop. It also reduces hardware investment and costs;


3. It is better to allow each output independently when conditions permit, to facilitate control and inspection, and to 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 in forward / reverse direction, not only must be interlocked from the PLC internal program, but also measures must be taken outside the PLC to prevent the load from acting 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 grounding terminal should be grounded independently and not connected in series with the grounding terminal of other equipment.


2. Auxiliary power is small and can only drive low-power equipment (photoelectric sensors, etc.);


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


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

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