Programmable controller (PLC) is a new type of general automatic control device, which integrates traditional relay control technology, computer technology and communication technology, and has the advantages of strong control function, high reliability, flexible and convenient use, easy to expand and so on. However, due to the harsh working environment of the industrial production site, there are many sources of interference, such as the start or stop of high-power electrical equipment caused by the fluctuation of the grid voltage to form low-frequency interference, welding machines, EDM machine tools, motor brushes, etc. through electromagnetic coupling generated power frequency interference, etc., will affect the normal operation of PLC.

Although PLC is a control device specially used in the field, many measures have been taken in the design and manufacture to make it more suitable for the industrial environment, but in order to ensure that the whole system is stable and feasible, it is still necessary to make the PLC have good working environment conditions and take necessary anti-interference measures.

1. PLC should pay attention to problems during installation and maintenance

1.1 Installation of PLC

PLC is suitable for most industrial sites, but it still has certain requirements for use occasions, ambient temperature, etc. Controlling the working environment of PLC can effectively improve its work efficiency and life. When installing PLCs, avoid the following places:

(1) The ambient temperature exceeds the range of 0 ~ 50°C;

(2) The relative humidity exceeds 85% or there is dew condensation (caused by sudden temperature changes or other factors);

(3) Direct sunlight;

(4) Corrosive and flammable gases, such as hydrogen chloride, hydrogen sulfide, etc.;

(5) There are iron filings and dust;

(6) Frequent or continuous vibration, vibration frequency of 10 ~ 55Hz, amplitude of 0.5mm (peak-peak);

(7) Shock exceeding 10g (acceleration due to gravity).

There are mounting holes in the four corners of the small programmable controller housing. There are two installation methods, one is fixed with screws, and different units have different installation sizes; The other is DIN (German Republican Standard) orbital fixation. DIN track is equipped with mounting plates, one pair on the left and right. On the track, first install the left and right splints, install the PLC, and then tighten the screws. In order to make the control system work*, the programmable controller is usually installed in a control cabinet with a protective housing to prevent dust, oil and water splashes. In order to ensure that the temperature of the programmable controller is kept within the specified ambient temperature range in the working state, the installation machine should have sufficient ventilation space, and there should be a gap of more than 30mm between the basic unit and the expansion unit. If the surrounding environment exceeds 55C, install an electric fan and force ventilation.
In order to avoid electrical interference from other peripheral devices, the programmable controller should be as far away as possible from the high-voltage power line and high-voltage equipment, and the distance between the programmable controller and the high-voltage equipment and power line should be at least 200mm.
When the programmable controller is installed vertically, it is necessary to prevent the wire head, iron filings, etc. from falling into the programmable controller from the ventilation window, causing a short circuit circuit of the printed circuit board, making it unable to work normally or even permanently damaged.

1.2 Power wiring

The PLC power supply is 50Hz, 220V±10% AC.

FX series programmable controllers have DC 24V output terminals. This terminal provides DC 24V power for input sensing, such as photoelectric switches or proximity switches.
If the power supply fails and the interruption time is less than 10ms, the PLC operation is not affected. If the power supply is interrupted for more than 10ms or the power supply drops more than the allowable value, the PLC stops working and all output points are disconnected at the same time. When power is restored, if the RUN input is switched on, the operation is automatic.

For interference from the power line, the PLC itself has sufficient resistance ability. If the power disturbance is particularly severe, an isolation transformer with a conversion ratio of 1:1 can be installed to reduce the interference between the device and ground.

1.3 Grounding

Good grounding is an important condition to ensure that PLC can work, which can avoid accidental voltage shock hazards. The grounding wire is connected to the grounding terminal of the machine, and the basic unit is grounded. If an extension unit is to be used, its grounding point should be connected with the grounding point of the base unit. In order to suppress the interference added to the power supply, input and output, the programmable controller should be connected to a special ground wire, and the grounding point should be separated from the grounding point of the power equipment (such as a motor). If this requirement is not met, it must also be common ground with other equipment, and it is forbidden to connect in series with other equipment. The grounding point should be as close to the PLC as possible.

1.4 DC 24V terminal

When using an input device with passive contacts, the PLC's internal 24V supply supplies 7mA per point to the input through the input device.

A 24V terminal block on the PLC can also supply current to external sensors such as proximity switches or photoelectric switches. When the 24V terminal is used as the sensor power supply, the COM terminal is the DC 24V ground. If an extended crew is adopted, the 24V end of the base unit and the expansion unit should be connected. In addition, any external power supply cannot be connected to this terminal.

If an overload occurs, the voltage will automatically drop and this point input will not work for the programmable controller.

The number of input points for each model of PLC is specified. It consumes no power for each unused input point, so in this case, the ability of the 24V power terminals to supply power to the outside can be increased.

The vacant terminals of the FX series PLCs cannot be used under any circumstances.

1.5 Input wiring

PLCs generally accept switching signals from input such as stroke switches and limit switches. The input terminal block is the port where the PLC converts signals to the external sensor load. Input wiring generally refers to the wiring of external sensors and input ports.

The input device can be any passive contact or an open-collector NPN tube. When the input device is on, the input terminal is turned on, the input line is closed, and the LED indicated by the input is on.

The primary circuit and the secondary circuit at the input end are isolated by photoelectric coupling. The secondary circuit has an RC filter to prevent PLC malfunction due to input contact jitter or electrical noise coming in from the input line.

If the diode is connected in series with the input contact circuit, the voltage across the series diode should be less than 4V. If a reed switch with light-emitting diodes is used, the number of series diodes cannot exceed two.

In addition, the input wiring should pay special attention to the following points:

(1) The input wiring should generally not exceed 30m. However, if the environmental interference is small and the voltage drop is not large, the input wiring can be appropriately longer.

(2) The input and output lines cannot be used with the same cable, and the input and output lines should be separated.

(3) The width of the pulse signal that the programmable controller can accept should be greater than the time of the scanning cycle.

1.6 Output wiring

(1) The programmable controller has three forms: relay output, thyristor output and transistor output.

(2) The output terminal wiring is divided into independent output and common output. When the output relay or thyristor of the PLC is active, the two output terminals of the same number are switched on. Different types and voltage levels of output voltages can be used in different groups. However, the output in the same group can only use the same type and voltage level of the same power supply.

(3) Since the output element of the PLC is packaged on the printed circuit board and connected to the terminal board, if the load connecting the output component is short-circuited, the printed circuit board will be burned, so the fuse should be used to protect the output element.

(4) When the relay output is used, the size of the inductive load affected by the relay working life, so the relay working life is required to be long.

(5) The output load of the PLC may produce noise interference, so measures should be taken to control it.

In addition, for dangerous loads that can cause injury to the user, in addition to being considered in the control program, an external emergency shutdown circuit should be designed so that when the programmable controller fails, the load power that causes the injury can be cut off.

AC output line and DC output line should not use the same cable, output line should be as far away as possible from the high voltage line and power line to avoid parallelism.
2 .Conclusion

PLC is widely used in industrial control with its significant advantages, and its practical application involves many problems, this article puts forward some precautions on its on-site installation and maintenance problems, for reference only.