Practical PLC Daily Application Skills

1. Grounding Problem

PLC system grounding requirements are strict, it is best to have an independent special grounding system, related equipment also should reliable grounding.

When multiple circuit ground points are connected together, unexpected currents can occur, causing logic errors or damaging the circuit.

Causes of different grounding potential, usually due to ground on the physical part is separated too far, when far away from the device is communication cable or sensors connected together, between the cable and ground current will flow through the circuit, even within a short distance, large equipment of the load current is also available in between with the potential to produce change, Or an unpredictable current can be generated directly by electromagnetic action.

Between power supplies at incorrect grounding points, there is the potential for a devastating current in the circuit to damage the device.

PLC system generally chooses one point grounding mode. In order to improve the ability of resisting common-mode interference, the shielded floating ground technology can be adopted for analog signals, that is, the shielding layer of the signal cable is grounded at one point, the signal loop is floating in the air, and the insulation resistance with the earth should not be less than 50M ω.

2. Interfering Process

The environment of industrial site is bad, there are many high and low frequency interference. These interferences are generally introduced into PLC through cables connected with field equipment.

In addition to grounding measures, some anti-interference measures should be taken in the design selection and laying of cables:

(1) Analog signal belongs to small signal and is easily affected by external interference, so double-layer shielded cable should be selected;

(2) Shielding cables should be used for high-speed pulse signals (such as pulse sensors and digital meter disks) to prevent interference from external sources and interference from high-speed pulse signals to low-level signals;

(3) The communication cable frequency between PLC is high, generally should choose the cable provided by the manufacturer, in the case of low requirements, can choose shielded twisted-pair cable;

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

(5) The shielded cable introduced into the control cabinet must be grounded, and should be directly connected to the equipment without going through the wiring terminal;

(6) AC signals, DC signals and analog signals cannot share one cable. Power cables should be laid separately from signal cables.

(7) In the field maintenance, the methods to solve the interference are as follows: the shielded cable is used for the interfered line, and the line is re-laid; Add anti-jamming filter code in the program.

3. Eliminate Capacitance Between Wires to Avoid Misoperation

There is capacitance between the wires of the cable, and qualified cable can limit this capacitance to a certain range.

Even qualified cable, when the cable length exceeds a certain length, the capacitance between the lines will exceed the required value, when the cable used for PLC input, the capacitance between the lines is likely to cause PLC error action, there will be many incomprehensible phenomena.

These phenomena are mainly manifested as: the wiring is correct, but the PLC has no input; PLC input should not have, and should not have, that is, PLC input interference. To solve this problem, it should:

(1) cables with cable cores twisted together;
(2) shorten the length of the cable as far as possible;
(3) Separate the interfering inputs from the cables;
(4) Use shielded cables.

4. Selection of Output Module

Output module is divided into transistor, bidirectional thyristor, contact type:

(1) transistor type switching speed is the fastest (generally 0.2ms), but the load capacity is the smallest, about 0.2A to 0.3a, 24VDC, suitable for fast switching, signal connection equipment, generally connected with frequency conversion, DC devices and other signals, should pay attention to the transistor leakage current on the load.

(2) SCR has the advantage of no contact, ac load characteristics, 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-type output is generally selected first. The disadvantage is that the switching speed is slow, generally about 10ms, which is not suitable for high-frequency switching applications.

5. Converter Overvoltage and Overcurrent Processing

(1) When the motor is decelerated for a given operation, the motor enters the regenerative power generation braking state, and the energy returned to the inverter by the motor is also high. This energy is stored in the filter capacitor, which increases the voltage on the capacitor and soon reaches the setting value of DC overvoltage protection, causing the inverter to trip.

The treatment method is to add brake resistance outside the frequency converter, and use the resistance to consume the regenerative electric energy fed back to the DC side of the motor.

(2) The frequency converter has multiple small motors. When one of the small motors has overcurrent fault, the frequency converter will give an overcurrent fault alarm, resulting in the frequency converter switching off, resulting in other normal small motors also stop working.

Treatment method: 1:1 isolation transformer is installed on the output side of the frequency converter. When one or several small motors have overcurrent fault, the fault current is DC impact transformer, rather than impact frequency converter, so as to prevent the inverter from falling off. After the experiment, the work is good, there is no previous normal motor shutdown failure.

6. Mark Input and Output for Easy Maintenance

PLC controls a complex system, what can be seen is the upper and lower two staggered input and output relay wiring terminals, the corresponding indicator light and PLC number, like a piece of integrated circuit with dozens of feet. Anyone who does not look at the schematic diagram to repair the faulty equipment will be at a loss, the speed of finding the fault will be very 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, Chinese name, that is, the function description of each pin similar to integrated circuit.

With this input and output form, for understanding the operation process or familiar with the equipment ladder diagram electrician can start maintenance.

But for those who are not familiar with the operation process, will not see the trapezoidal diagram electrician, you need to draw a form: PLC input and output logic function table. The table actually describes the logical correspondence between the input loop (trigger element, associated element) and the output loop (executive element) in most operations.

Practice has proved that: if you can skillfully use the input and output corresponding table and the input and output logic function table, repair electrical faults, without drawings, also can easily.

7. Infer Faults Through Program Logic

Now the industry often use a wide variety of PLC, for the low end of the PLC, ladder diagram instructions are similar, for the high-end machine, such as S7-300, many programs are language table.

A practical ladder diagram must be annotated with Chinese symbols, otherwise it is difficult to read. It is easier to look at the ladder diagram if you have a general understanding of the equipment process or operation process.

If the electrical fault analysis, generally is the application of reverse lookup method or reverse push method, that is, according to the input and output corresponding table, from the point of failure to find the corresponding PLC output relay, start to reverse lookup to meet the logical relationship of its action.

Experience shows that when a problem is found, the fault can be eliminated basically, because the equipment has two or more fault points at the same time is not much.

8. PLC Fault Judgment

In general, the PLC is extremely reliable equipment, the failure rate is low, PLC, damage to the CPU and other hardware, or software runs the probability of error is almost zero, the PLC input point such as is not caused by high voltage invasion, almost will not be damaged, PLC output relay normally open point, if not short circuit or peripheral load design is unreasonable, beyond the rated load current, Contact life is also long.

Therefore, we find the electrical fault point, the focus should be on the peripheral electrical components of PLC, do not always suspect that PLC hardware or program has a problem, which is very important for fast maintenance of faulty equipment, rapid recovery of production.

Therefore, the author of the PLC control loop of the electrical fault repair, the focus is not PLC itself, but the PLC control loop in the peripheral electrical components.

9. Make Full and Reasonable Use of Software and Hardware Resources

(1) Instructions that do not participate in the control cycle or have been invested before the cycle may not be connected to PLC;
(2) When multiple instructions control a task, they can be connected in parallel outside PLC and then access an input point;
(3) try to use PLC internal function soft components, fully call the intermediate state, so that the program has complete coherence, easy to develop. At the same time, it also reduces hardware investment and costs;
(4) conditions allow the best independent output each way, easy to control and check, but also protect other output loop; When an output point failure will only lead to the corresponding output loop out of control;
(5) if the output is positive/reverse control of the load, not only to interlock from the PLC internal procedures, and to take measures outside the PLC to prevent the load in the two directions of action;
(6) PLC emergency stop should be cut off by external switch to ensure safety.

10. Other Notes

(1) Do not connect the AC power cable to the input terminal, so as not to burn PLC;
(2) The grounding terminal shall be grounded independently, not in series with the grounding terminal of other equipment, and the cross-sectional area of the grounding cable shall not be less than 2mm²;
(3) The power of the auxiliary power supply is small, which can only drive the equipment with low power (photoelectric sensors, etc.);
(4) some PLC has a certain number of points (that is, empty address wiring terminal), do not connect the line;
(5) when there is no protection in the PLC output circuit, fuses and other protective devices should be used in series in the external circuit to prevent damage caused by short circuit of load.

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