Megmeet PLC – 10 PLCs integrated failure analysis

1. Grounding problems

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 separated too far from the physical area, when the devices that are very far apart are connected by communication cables or sensors, the current between the cable and the ground will flow through the entire circuit, even if it is a short distance, the load current of a large device can change between it and the ground potential, or an unpredictable current can be directly generated by 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.

2. 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:

(1) The analog signal is a small signal, which is very susceptible to external interference, and a double-layer shielded cable should be selected;

(2) High-speed pulse signals (such as pulse sensors, digital disks, etc.) should be shielded cables, which not only prevent external interference, but also prevent the interference of high-speed pulse signals to low-level signals;

(3) 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;

(4) Analog signal lines and DC signal lines cannot be routed in the same trunking as AC signal lines;

(5) The shielded cable introduced and led out in the control cabinet must be grounded, and should be directly connected to the equipment without passing through the terminal;

(6) AC signals, DC signals and analog signals cannot share a single cable, and the power cable should be laid separately from the signal cable.

(7) In the on-site maintenance, the methods to solve the interference are: the interfered line adopts shielded cables and relays; Add anti-interference filtering code to the program.

3. Eliminate the capacitance between the lines 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 address this problem, it is necessary to:

(1) Cables twisted together using cable cores;

(2) Minimize the length of the cable used;

(3) Use cables separately for inputs that interfere with each other;

(4) Use shielded cables.

4. Selection of output module

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

(1) 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.

(2) The advantages of the thyristor type are that there is no contact, it has the characteristics of AC load, and the load capacity is not large.

(3) 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.

5. Overvoltage and overcurrent treatment of inverter

(1) When reducing the given motor to slow down the operation, the motor enters the regenerative power generation braking state, and the energy fed back by the motor to the inverter is also higher, and these energies are stored in the filter capacitor, so that the voltage on the capacitor rises, and soon reaches the setting value of DC overvoltage protection and trips the inverter.

The treatment method is to take measures to add a braking resistor outside the inverter, and use the resistor to consume the regenerative electrical energy fed back to the DC side of the motor.

(2) The inverter has a plurality of small motors, and when one of the small motors has an overcurrent fault, the inverter will alarm the overcurrent fault, causing the inverter to trip, which will cause other normal small motors to stop working.

Solution: Install a 1:1 isolation transformer on the output side of the inverter, when one or several small motors have an overcurrent fault, the fault current DC impact transformer instead of the inverter, so as to prevent the inverter from tripping. After the experiment, it worked well, and there was no failure that the previous normal motor was shut down.

6. Mark the input and output for easy maintenance

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

With this I/O table, it is time for an electrician who understands the operation process or is familiar with the ladder diagram of the equipment.

But for those electricians who are not familiar with the operation process and do not know how to read 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 loops (trigger elements, associated components) and output loops (actuators) during most operations.

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

7. Infer faults through 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.

8. PLC self-fault judgment

Generally speaking, PLC is an extremely reliable equipment, the failure rate is very low, the probability of hardware damage or software operation error such as PLC and CPU is almost zero, the PLC input point is almost not damaged if it is not caused by strong current intrusion, the normally open point of the PLC output relay, if it is not a short circuit of the peripheral 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 find electrical fault points, we should focus on the peripheral electrical components of the PLC, and do not always suspect that there is a problem with the PLC hardware or program, which is very important for quickly repairing the faulty equipment and resuming production quickly.

Therefore, the focus of the electrical troubleshooting of the PLC control loop that the author talks about is not on the PLC itself, but on the peripheral electrical components in the PLC control loop.

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

(1) Instructions that do not participate in the control cycle or have been put into the cycle before the cycle can 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) 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;

(4) If conditions permit, it is best to independently each output, 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;

(5) If the output is a load controlled by forward/reverse control, it is not only necessary 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;

(6) The PLC emergency stop should be cut off with an external switch to ensure safety.

10. Other precautions

(1) Do not connect the AC power cord to the input terminal, so as not to burn the PLC;

(2) 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;

(3) The auxiliary power supply is small and can only drive equipment with small power (photoelectric sensors, etc.);

(4) Some PLCs have a certain number of occupancy points (i.e., empty address terminals), do not connect the wires;

(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 caused by short circuit of the load.

Megmeet PLC – 10 PLCs integrated failure analysis

Megmeet PLC – 10 PLCs integrated failure analysis

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