Hollysys PLC - Electrical troubleshooting of PLC control loop

In general, PLCs are extremely reliable devices with a very low failure rate. The probability of hardware damage such as the CPU of the PLC or software running errors is almost zero; If the PLC input point is not caused by strong current intrusion, it will hardly be damaged; The normally open point of the PLC output relay is not for the external load short circuit or unreasonable design, the load current exceeds the rated range, and the contact life is also very long.

Therefore, we find the electrical fault point, 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, which is very important for the rapid repair of faulty equipment and the rapid recovery of production, so the electrical fault repair of the PLC control loop is not focused on the PLC itself, but the peripheral electrical components in the PLC control loop.

　　Selection of input-output (I/O) modules

The output modules are divided into transistors, triacs, and contact types.

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, suitable for fast switching, signal contact equipment, generally connected with frequency conversion, DC devices and other signals, should pay attention to the influence of transistor leakage current on the load. The advantages of the thyristor type are that it has no contacts, has AC load characteristics, and has a small load capacity.

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

　　Grounding issues

The grounding requirements of the PLC system are relatively strict, and it is best to have an independent dedicated grounding system, and it should also be noted that other equipment related to PLC should also be reliably grounded. When multiple circuit grounding points are connected together, unexpected currents can be generated, causing logic errors or damaging circuits.

The reason for the different ground potentials is usually due to the fact that the ground points are too far apart from each other in the physical area, and when devices that are very far apart are connected together by communication cables or sensors, the current between the cable and ground flows through the entire circuit.

Even over very short distances, the load current of a large device can vary between it and the ground potential, or it can directly generate unpredictable currents through electromagnetic action. Between power supplies at incorrect grounding points, there is a risk of a devastating current being generated in the circuit to the point of destroying the device.

The PLC system generally uses a one-point grounding method. In order to improve the ability to resist common mode interference, the 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 ground should not be less than 50MΩ.

　　Eliminate line-to-line capacitance to avoid malfunction

There is a capacitance between the wires of the cable, and 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 wires will exceed the required value, and when the cable is used for PLC input, the capacitance between the lines may cause PLC malfunction, and many incomprehensible phenomena will occur.

These phenomena are mainly manifested as: the bright wiring is correct, but the PLC has no input; The input that the PLC should have is not, and the input that should not be there is, that is, the PLC input interferes with each other. To solve this problem

You should do the following:

Cables that are twisted together using cable cores; Keep the length of the cable used as short as possible

Separate cables for interfering inputs; Use shielded cables

　　Anti-interference processing

The environment of the industrial site is relatively harsh, and there are many high and low frequency interference. These interferences are typically introduced into the PLC via cables connected to field devices. In addition to grounding measures, some anti-interference measures should be taken in the design selection and laying of cables:

The analog signal is a small signal and is very susceptible to external interference, so double-layer shielded cable should be selected; High-speed pulse signals (such as pulse sensors, digital disks, etc.) should be shielded cables, which not only prevent external interference, but also prevent high-speed pulse signals from interfering with low-level signals;

The frequency of communication cables between PLCs is high, and the cables provided by the manufacturer should generally be selected, and in the case of low requirements, twisted pair cables with shielding can be selected; Analog signal lines and DC signal lines cannot be routed in the same trunking as AC signal lines;

The shielded cable introduced and drawn out in the control cabinet must be grounded, and should be directly connected to the equipment without passing through the terminal; AC signals, DC signals and analog signals cannot share a single cable, and the power cable should be laid separately from the signal cable.

In the on-site maintenance, the methods to solve the interference are: the interference line is shielded and re-layed; Add anti-interference filtering code to the program.

　　Marked inputs and outputs for easy access

The PLC controls a complex system, and what can be seen is the upper and lower rows of staggered input and output relay terminals, the corresponding indicator lights and PLC numbers, like an integrated circuit with dozens of legs. Anyone who doesn't look at the schematic diagram to troubleshoot faulty equipment will be helpless and will be particularly slow to find faults.

　　Faults are inferred from program logic

Nowadays, there are many kinds of PLCs often used in the industry, for low-end PLCs, ladder instructions are similar, for high-end machines, such as S7-300, many programs are programmed in language tables. The practical ladder diagram must have Chinese symbol annotations, otherwise it is difficult to read, 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 check method or the reverse deduction method is generally applied, that is, according to the input and output corresponding tables, the output relay of the corresponding PLC is found from the fault point, and the logical relationship that satisfies its action is started to be reversed. Experience shows that when a problem is found, the fault can basically be eliminated, because there are not many points of failure of the equipment with two or more failures at the same time.

　　Rational use of software and hardware resources

Instructions that do not participate in the control cycle or have been put in before the cycle can 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; Try to make use of the internal functional soft components of the PLC, 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 costs;

If conditions permit, it is best to have each output independently, which is convenient for control and inspection, and also protects other output circuits; When one output point fails, it only causes the corresponding output loop to get out of control; If the output is a load controlled by forward/reverse control, it is necessary not only to interlock from the internal program of the PLC, but also to take measures outside the PLC to prevent the load from moving in both directions; The PLC emergency stop should be cut off using an external switch to ensure safety.

　　Other considerations

Do not connect the AC power cord to the input terminal, so as not to burn out the PLC; The grounding terminal should be grounded independently, not connected in series with the grounding terminal of other equipment, and the cross-sectional area of the grounding wire should not be less than 2mm²; The auxiliary power supply is small and can only drive small power equipment (photoelectric sensors, etc.); Some PLCs have a certain number of occupancy points (i.e., empty address terminals), do not connect the wires;

When there is no protection in the PLC output circuit, protection devices such as fuses should be used in series in the external circuit to prevent damage caused by short circuit of the load.

Hollysys PLC - Electrical troubleshooting of PLC control loop

Hollysys PLC - Electrical troubleshooting of PLC control loop

Sections viewed

Contact

working hours