PLC application should pay attention to the problem First, the working environment 1. The temperature PLC requires the ambient temperature to be 0~55°C. It should not be placed below the heat generating components under installation. The space for ventilation and heat dissipation should be large enough. There must be an interval of more than 30mm between the basic unit and the extension unit. The upper and lower parts of the switchgear should be Ventilated shutters to prevent direct sunlight; if the ambient temperature exceeds 55 °C, install an electric fan to force ventilation.
2. Humidity To ensure the insulation of the PLC, the relative humidity of the air should be less than 85% (no condensation).
3. The vibration should keep the PLC away from the strong vibration source and prevent frequent or continuous vibration with the frequency of 10~55Hz. When the use of the environment can not avoid vibration, we must take measures to reduce vibration, such as the use of shock absorption rubber.
4. Air is protected from corrosive and flammable gases such as hydrogen chloride, hydrogen sulfide, and the like. For environments where there is more dust or corrosive gas in the air, the PLC can be installed in a closed control room or control cabinet and an air purification device installed.
5. Power supply PLC power supply is 50Hz, 220 (1 ± 10%) V of AC, for the interference from the power line, PLC itself has enough resistance. For environments with high reliability requirements or where the power supply interference is particularly severe, an isolation Transformer with a 1:1 transformer ratio can be installed to reduce the interference between the equipment and the ground. It is also possible to cascade the LC filter circuit at the power input.
FX series PLCs have a DC 24V output terminal that provides DC 24V power for input sensors such as photoelectric switches or proximity switches. When the input uses an external DC power supply, a DC stabilized power supply should be used. Because of the common rectification and filtering power supply, due to the influence of ripple, it is easy for the PLC to receive error information.
Second, installation and wiring The power line, control line, PLC power supply line and I/O line should be wired separately. The isolation transformer should be connected with PLC and I/O by double glue line.
2. PLCs should be kept away from strong sources of interference such as welders, high-power silicon rectifiers, and large power equipment. They must not be installed in the same switchgear with high-voltage appliances.
3. The input and output of the PLC are preferably routed separately, and the switch quantity and analog quantity are also laid separately. Shielded wires should be used for the transmission of analog signals. The shielding layer should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the shielding layer resistance.
4. The connection cables of the PLC basic unit, extension unit and function module should be laid separately to prevent external signal interference.
5. Do not use the same cable for the AC output line and the DC output line. The output line should be as far away from the high voltage line and the power line as possible to avoid parallel operation.
Third, the I / O terminal wiring 1. Input wiring (1) Input wiring should not exceed 30 meters. However, if the ambient interference is small and the voltage drop is not large, the input wiring can be longer.
(2) The input/output line cannot use the same cable, and the input/output lines should be separated.
(3) As far as possible, use a normally open contact form to connect to the input terminal so that the programmed ladder diagram is consistent with the relay's schematic diagram for easy reading.
2. The output connection (1) output terminal wiring is divided into independent output and common output. In different groups, different types and voltage levels of output voltage can be used. However, the output in the same group can only use the same type of power supply with the same voltage level.
(2) Since the output element of the PLC is packaged on the printed circuit board and connected to the terminal board, if the load connected to the output element is short-circuited, the printed circuit board will be burned. Therefore, the fuse protection output element is applied.
(3) When using a relay output, the size of the inductive load that is subjected to it will affect the service life of the relay. Therefore, when using an inductive load, the operating life of the relay is longer.
(4) PLC output load may cause interference, so take measures to control, such as the DC output of the free flow tube protection, AC output RC absorption circuit, transistor and bidirectional thyristor output bypass resistance protection.
Fourth, the external safety circuit In order to ensure that the entire system can work reliably in a safe state, to avoid the external power supply failure, PLC abnormalities, misuse and accidental output caused by major economic losses and personal injury accidents, PLC should install the necessary protection outside Circuit.
(1) Emergency stop circuit. For dangerous loads that can cause harm to the user, in addition to being considered in the control program, an external emergency stop circuit should also be designed so that when the PLC fails, the load-causing power source that causes the damage can be reliably cut off. (2) Protection circuit. The control system of the reversible operation, such as forward and reverse operation, should be set with external electrical interlocking protection; the control system for reciprocating operation and lifting movement should be provided with an external limit protection circuit.
(3) The programmable controller has a self-check function such as a watchdog timer. When an abnormality is detected, the outputs are all turned off. However, when the programmable controller CPU fails, the output cannot be controlled. Therefore, in order to ensure that the device operates in a safe state, dangerous electrical loads that can cause damage to the user must be designed to protect the circuit.
(4) Power overload protection. If the PLC power supply fails and the interruption time is less than 10 seconds, the PLC operation is not affected. If the power interruption exceeds 10 seconds or the power supply drops below the allowable value, the PLC stops working and all output points are disconnected at the same time; when the power is restored , If the RUN input is turned on, the operation is automatically performed. Therefore, the necessary current limiting protection circuit should be set for some overloaded input devices.
(5) Major fault alarm and protection. For locations that are prone to major accidents, in order to ensure reliable control and protection of the control system in the event of a major accident, signals related to major faults should be output through an external circuit so that the control system can operate under safe conditions.
V. Grounding of the PLC A good grounding is an important condition for ensuring the reliable operation of the PLC. It can avoid accidental voltage surge hazards. The grounding wire of the PLC is connected to the grounding end of the machine. The cross-sectional area of ​​the grounding wire should not be less than 2mm2, and the grounding resistance should be less than 100Ω. If the expansion unit is to be used, the grounding point should be connected to the grounding point of the basic unit. In order to suppress the interference on the power supply and the input and output terminals, the PLC should be connected to a special ground wire. The grounding point should be separated from the grounding point of the power equipment (such as the motor). If this requirement cannot be met, it must be done. Other devices are commonly grounded and are not allowed to be connected to other devices. The grounding point should be as close to the PLC as possible.
VI. Redundant systems and hot standby systems Certain systems in the petroleum, chemical, and metallurgical industries require extremely high reliability of control devices. If the control system fails, it will result in production stoppages, a lot of waste of raw materials, or equipment damage, causing great economic losses to the company. However, it is far from enough to improve the reliability of the control system hardware to meet the above requirements, because the reliability of the PLC itself has a certain limit. The use of a redundant system or hot standby system can more effectively solve the above problems.
1. Redundant Control System In a redundant control system, the entire PLC control system (or the most important part of the system, such as a CPU module) consists of two identical systems. The two CPU modules work in parallel using the same user program, one of which is the primary CPU and the other is the standby CPU; the primary CPU is working and the output of the standby CPU is disabled. When the primary CPU fails, the standby CPU is automatically put into operation. This switching process is controlled by the redundancy processing unit RPU. The switching time is between 1 and 3 scanning cycles. The switching of the I/O system is also completed by the RPU.
2. Hot Standby System In a hot standby system, two CPUs are connected together using a communication interface and are all powered on. When the system fails, the standby CPU is notified by the primary CPU to put the standby CPU into operation. This switching process is generally not very fast, but its structure is simpler than the redundant system
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