In recent years, with the development of society, PLC programmable logic controllers have been widely used in industrial production. At the same time, technical personnel are increasing their use requirements year by year. As a result, requirements for the normal and stable operation of the system are also increasing.
The reliability of the PLC product itself can be guaranteed, but some incorrect operations in the application will have a certain impact. For everyone, we have compiled some 9 tips for everyday PLC applications, hoping to help everyone in the daily use of PLC.
One, PLC own fault judgment
In general, PLC is an extremely reliable device with a low failure rate. The probability of PLC hardware damage such as PLC or software running is almost zero; if PLC input point is not caused by strong power intrusion, it will not be damaged; PLC output relay normally open point, if not external load short circuit or unreasonable design The load current exceeds the rated range and the contact life is also very long.
Therefore, to find the points of electrical failure, the focus should be on the external electrical components of the PLC. Do not always suspect that the PLC hardware or program has problems. This is very important for quickly repairing faulty equipment and quickly recovering production. Therefore, the author has talked about it. The maintenance of electrical faults in the PLC control loop is not focused on the PLC itself, but on the peripheral electrical components in the PLC controlled loop.
Second, the input and output (I / O) module selection
Output modules are divided into transistors, triacs, and contact types.
Transistor-type switching speed is fastest (usually 0.2ms), but the smallest load capacity, about 0.2 ~ 0.3A, 24VDC, suitable for fast switching, signal contact equipment, generally with the frequency conversion, DC devices and other signal connections, should pay attention to the transistor leakage Current impact on the load.
The advantages of thyristor type are non-contact, AC load characteristics, and small load capacity.
Relay output has AC and DC load characteristics and large load capacity. In the conventional control, the relay contact type output is generally used first. The disadvantage is that the switching speed is slow, generally about 10ms, and it is not suitable for high frequency switching applications.
Third, the grounding problem
PLC system grounding requirements are more stringent, it is best to have a separate dedicated grounding system, but also pay attention to other equipment related to PLC but also reliable grounding. When multiple circuit grounds are connected together, unexpected currents are generated, resulting in logic errors or circuit damage.
The reason for the different ground potentials is usually due to the fact that the ground points are separated too far in the physical area. When the equipments far apart are connected together by communication cables or sensors, the current between the cable and the ground will be Flow through the entire circuit.
Even within a short distance, the load current of a large device can change between it and the ground potential, or it can directly generate unpredictable current through electromagnetic action.
Between the power supplies at the incorrect ground point, there is the potential for catastrophic electrical currents in the circuit that could damage the equipment.
The PLC system generally uses one point grounding method. 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 with the earth should not be less than 50 MΩ.
Fourth, eliminate the line capacitance to avoid malfunction
Capacitance exists between the wires of the cable, and qualified cables can limit this capacitance within a certain range. Even if the cable is qualified, when the cable length exceeds a certain length, the capacitance between the lines will exceed the required value.
When this cable is used for PLC input, the line capacitance may cause malfunction of the PLC and many unintelligible phenomena will occur.
These phenomena are mainly manifested as follows: the wiring is correct, but the PLC has not been input; PLC should have no input, but should not have, that PLC input interfere with each other. To solve this problem, you should do:
1, the use of cable core stranded together;
2, as short as possible to use the cable length;
3. Separate the use of cables from inputs that interfere with each other;
4. Use shielded cables.
Fifth, anti-jamming treatment
The environment at the industrial site is rather harsh and there are many high- and low-frequency interferences. These disturbances are generally introduced into the PLC through cables connected to field devices.
In addition to grounding measures, some anti-jamming measures should be taken during cable design and installation.
1. The analog signal belongs to small signal and is easily affected by outside interference. Double-layer shielded cable should be used;
2, high-speed pulse signal (such as pulse sensor, counting code disk, etc.) should use shielded cable, both to prevent external interference, but also to prevent high-speed pulse signal on low-level signal interference;
3, PLC communication cable between the higher frequency, generally should use the manufacturer's cable, in the case of low demand, you can use shielded twisted pair cable;
4. The analog signal line and the DC signal line cannot be routed in the same slot with the AC signal line.
5. The shielded cables introduced into the control cabinet must be grounded and should not be directly connected to the equipment through the terminals;
6, AC signal, DC signal and analog signal can not share a cable, power cable should be laid separately from the signal cable.
7. During field maintenance, the methods for resolving interference include: using shielded cables for the disturbed lines and re-laying them; adding anti-interference filter code to the program.
Sixth, mark input and output, convenient maintenance
The PLC controls a complex system. What can be seen is the upper and lower rows of staggered input and output relay terminals, corresponding indicators, and PLC numbers, just like an integrated circuit with dozens of feet.
Any person who does not look at the schematic to troubleshoot a faulty device will be unable to find a solution. The speed at which faults are found will be particularly slow.
In this case, draw a table according to the electrical schematic drawing and stick it on the console or control cabinet of the equipment, and indicate the electrical symbol corresponding to each PLC input and output terminal number, the Chinese name, that is similar to each pin of the integrated circuit. Function Description.
With this input and output form, an electrician who understands the operation process or is familiar with the ladder diagram of the device can perform the overhaul. But for those electricians who are not familiar with the operation process and do not look at the ladder diagram, it is necessary to draw a table: PLC input and output logic function table.
Use the table to actually illustrate the logical correspondence between the input loop (trigger element, associated element) and output loop (actuator element) during most of the operation. Practice has proved that if you can skillfully use the input and output correspondence table and input and output logic function table, and repair electrical faults, without drawing, you can also easily and freely.
VII. Inferring faults through program logic
Nowadays PLCs are widely used in industry. For low-end PLCs, the ladder diagram instructions are much the same. For high-end machines, such as S7-300, many programs are compiled using language tables.
The practical ladder diagram must have a Chinese symbol note, otherwise it is difficult to read. If you look at the ladder diagram before you can understand the equipment process or operation process, it seems easier.
If the electrical fault analysis is performed, the reverse inspection method or reverse estimation method is generally applied, that is, according to the input and output correspondence table, the output relay corresponding to the PLC is found from the fault point, and the logical relationship that satisfies the action is started.
Experience shows that if a problem is found, the fault can be basically eliminated because there are not many faults occurring at the same time and two or more faults.
VIII. Making full and reasonable use of hardware and software resources
1. Instructions that have not been involved in the control cycle or have been put in before the cycle may not be connected to the PLC;
2. When multiple instructions control a task, they can be connected in parallel outside the PLC and then connected to an input point.
3, make full use of PLC internal functional software components, fully call the intermediate state, so that the program has a complete coherence, easy to develop, but also reduce hardware investment, reduce costs;
4. When the conditions permit, it is better to independently output each channel to facilitate control and inspection, and also to protect other output circuits; when an output point fails, it will only lead to the corresponding output circuit out of control;
5, if the output is a positive / negative control of the load, not only from the PLC internal interlocking, and to take measures outside the PLC to prevent the load in both directions of action;
6, PLC emergency stop should be cut off using an external switch to ensure safety.
Nine, other matters needing attention
1. Do not connect the AC power cord to the input terminal to avoid burning the PLC.
2. The grounding terminal should be grounded independently, not in series with the grounding terminal of other equipment, and the cross-sectional area of ​​the grounding wire should not be less than 2mm2.
3. The power of the auxiliary power supply is small, and it can only drive low-power devices (photoelectric sensors, etc.);
4, some PLC has a certain number of possession points (ie empty address terminal), do not connect the line;
5. 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 due to short circuit of the load.
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