1. Introduction
Programmable Logic Controllers (PLCs) are a new type of general-purpose automation control device that integrates traditional relay control technology, computer technology, and communication technology. They are increasingly widely used due to their advantages such as strong control functions, high reliability, flexible and convenient use, and easy expansion. However, in industrial production sites, the harsh working environment and numerous interference sources, such as voltage fluctuations caused by the starting or stopping of high-power electrical equipment, generating low-frequency interference, and power frequency interference generated by electromagnetic coupling from welding machines, EDM machine tools, and motor brushes, can all affect the normal operation of PLCs.
Although PLCs are control devices specifically designed for field use and are designed and manufactured with many measures in place to make them more adaptable to industrial environments, in order to ensure the stability and reliability of the entire system, it is still necessary to provide the PLC with good working environment conditions and take necessary anti-interference measures.
2. Issues to be aware of during PLC installation and maintenance
2.1 PLC Installation
PLCs are suitable for most industrial environments, but they do have certain requirements regarding the operating conditions and ambient temperature. Controlling the PLC's working environment can effectively improve its efficiency and lifespan. When installing a PLC, avoid the following locations:
(1) The ambient temperature exceeds 0~50℃;
(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 and hydrogen sulfide;
(5) There are a lot of iron filings and dust;
(6) Frequent or continuous vibrations with a frequency of 10~55Hz and an amplitude of 0.5mm (peak-to-peak).
(7) Impact exceeding 10g (gravitational acceleration).
The small programmable controller has mounting holes at each of its four corners. There are two mounting methods: one is screw fixing, with different mounting dimensions for different units; the other is fixing via a DIN (German Republican Standard) rail. The DIN rail comes with a pair of mounting plates on each side. First, install the left and right plates on the rail, then install the PLC, and finally tighten the screws. To ensure reliable control system operation, the programmable controller is usually installed in a protective control cabinet to prevent dust, oil, and water splashes. To ensure the programmable controller's temperature remains within the specified ambient temperature range during operation, the installation machine should have sufficient ventilation space, with a minimum distance of 30mm between basic and expansion units. If the ambient temperature exceeds 55°C, a fan should be installed for forced ventilation.
To avoid electrical interference from other peripheral devices, the programmable controller should be kept as far away as possible from high-voltage power lines and high-voltage equipment. A distance of at least 200mm should be maintained between the programmable controller and high-voltage equipment and power lines.
When a programmable logic controller (PLC) is installed vertically, care must be taken to prevent wire ends, metal shavings, and other debris from falling into the PLC through the ventilation window, which could cause a short circuit on the printed circuit board, rendering it malfunction or even permanently damaged.
2.2 Power supply wiring
The PLC is powered by 50Hz, 220V±10% AC power.
The FX series programmable controllers have a DC 24V output terminal. This terminal can provide DC 24V power to input sensors such as photoelectric switches or proximity switches.
If a power failure occurs and the interruption time is less than 10ms, the PLC operation will not be affected. If the power interruption exceeds 10ms or the power drop exceeds the allowable value, the PLC will stop working, and all output points will be disconnected simultaneously. When the power is restored, if the RUN input is connected, the operation will resume automatically.
The PLC itself has sufficient immunity to interference from the power line. If the power interference is particularly severe, an isolation transformer with a 1 : 1 turns ratio can be installed to reduce interference between the equipment and ground.
2.3 Grounding
Proper grounding is crucial for the reliable operation of a PLC, preventing damage from accidental voltage surges. The grounding wire should be connected to the machine's grounding terminal, and the basic unit should be grounded. If expansion units are used, their grounding points should be connected together with the basic unit's grounding point. To suppress interference applied to the power supply, input terminals, and output terminals, the programmable controller should be connected to a dedicated grounding wire, with the grounding point separate from the grounding points of power equipment (such as motors). If this requirement cannot be met, it must at least share a common ground with other equipment; series grounding with other equipment is prohibited. The grounding point should be as close to the PLC as possible.
2.4 DC 24V terminals
When using passive contact input devices, the PLC's internal 24V power supply provides 7mA of current to each input terminal through the input device.
The 24V terminal on the PLC can also supply current to external sensors (such as proximity switches or photoelectric switches). When the 24V terminal is used as a sensor power supply, the COM terminal is the DC 24V ground terminal. If an expansion crew is used, the 24V terminals of the basic unit and the expansion unit should be connected together. Furthermore, no external power supply should be connected to this terminal.
If an overload occurs, the voltage will drop automatically, and this input will have no effect on the programmable controller.
The number of input points for each PLC model is fixed. Each unused input point does not consume power, therefore, the ability of the 24V power supply terminals to supply current can be increased in this case.
The unused terminals of the FX series PLC should never be used under any circumstances.
2.5 Input Wiring
PLCs typically accept switching signals from limit switches, travel switches, and other similar inputs. Input terminals are the ports through which the PLC converts signals to external sensor loads. Input wiring generally refers to the wiring between external sensors and the input ports.
The input device can be any passive contact or open-collector NPN transistor. When the input device is turned on, the input terminal is closed, the input line is closed, and the input indicator LED lights up simultaneously.
The primary and secondary circuits at the input terminals are optocoupled for isolation. The secondary circuit includes an RC filter to prevent PLC malfunctions caused by input contact bounce or electrical noise introduced from the input lines.
If a diode is connected in series in the input contact circuit, the voltage across the diode should be less than 4V. If a reed switch with an LED is used, the number of diodes connected in series should not exceed two.
In addition, the following points should be paid special attention to when wiring the input:
(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) Input and output lines cannot use the same cable; input and output lines must be separated.
(3) The width of the pulse signal that the programmable controller can accept should be greater than the time of the scan cycle.
2.6 Output wiring
(1) Programmable controllers have three forms: relay output, thyristor output, and transistor output.
(2) Output wiring is divided into independent output and common output. When the PLC's output relay or thyristor operates, the two output terminals with the same number are connected. Different types and voltage levels of output voltage can be used in different groups. However, outputs in the same group can only use the same type and voltage level of power supply.
(3) Since the output components of the PLC are encapsulated on the printed circuit board and connected to the terminal block, if the load connected to the output components is short-circuited, the printed circuit board will be burned. Therefore, fuses are used to protect the output components.
(4) When using relay output, the size of the inductive load affects the service life of the relay, so the service life of the relay is required to be long.
(5) The output load of the PLC may generate noise interference, so measures should be taken to control it.
In addition, for dangerous loads that could cause harm to users, in addition to considering them in the control program, an external emergency stop circuit should be designed so that the power supply to the load causing the harm can be cut off when the programmable controller fails.
Do not use the same cable for AC output lines and DC output lines. Output lines should be kept as far away as possible from high-voltage lines and power lines, and should not run in parallel.
3. Conclusion
PLCs are widely used in industrial control due to their significant advantages. However, their practical applications involve many issues. This article only raises some points to note regarding their on-site installation and maintenance for reference by PLC design and application personnel.
