First, the selection of output module
The output modules are divided into transistors, triacs, and contact types:
(1) The switching speed of the transistor type is fast (generally 0.2ms), but the load can be small, about 0.2-0.3A, 24VDC, suitable for fast switching, signal contact equipment, generally connected with frequency conversion, DC equipment and other signals, should pay attention to the impact of transistor leakage current on the load.
(2) The advantage of the thyristor type is that it has no contact and has the characteristics of communication load, so the load can not be large.
(3) The relay output has the characteristics of AC and DC load, and the load can be large. In conventional control, relay contact output is generally used, and the disadvantage is that the switching speed is slow, generally about 10ms, which is not suitable for high-frequency switching applications.
2. Other matters to be paid attention to
(1) Do not connect the communication power cord to the input terminal, so as not to burn out 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 the 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 maintenance in the PLC output circuit, the fuse and other maintenance equipment should be used in series in the external circuit to avoid damage caused by short circuit of the load.
3. Disturbance treatment
The environment of the industrial site is relatively harsh, and there are many high and low frequency disturbances. These disturbances are typically introduced into the PLC via cables connected to the field device.
In addition to grounding measures, attention should be paid to the adoption of anti-disturbance measures in the design selection and laying of cables:
(1) The imitation signal is a small signal, which is very susceptible to the influence of external disturbance, 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 avoid external disturbance, but also avoid the disturbance of high-speed pulse signals to low-level signals;
(3) The frequency of communication cables between PLCs is high, and the cables supplied by the manufacturer should generally be selected, and in the case of low requirements, twisted pair cables with shielding can be selected;
(4) The imitation signal line and DC signal line cannot be routed in the same line slot as the communication signal line;
(5) It is necessary to grounding the shielded cable introduced in the control cabinet, and it should not be directly connected to the equipment through the terminal;
(6) Communication signals, DC signals and analog signals cannot share a single cable, and the power cable should be laid separately from the signal cable.
(7) During on-site maintenance, the solutions to the disturbance are: the disturbed line is shielded and laid from scratch; Participate in the anti-disturbance filtering code in the program.
Fourth, eliminate the capacitance between the lines to avoid malfunction
There is a capacitance between the wires of the cable, and a qualified cable can constrain this capacitance within 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 appear.
These phenomena are mainly manifested as: the wiring is correct, but the PLC has no input; There are no inputs that the PLC should have, but there are no inputs that should not have, that is, the PLC inputs disturb each other. To address this problem, it is necessary to:
(1) Cables twisted together with cable cores;
(2) Shorten the length of the cable as much as possible;
(3) Separate the input cables that disturb each other;
(4) Use shielded cables.
5. Grounding problem:
The grounding requirements of the PLC system are relatively strict, and it is good to have an independent special grounding system, and it is also necessary to pay attention to other equipment related to PLC to be reliably grounded. When multiple circuit addresses are connected together, unexpected currents can occur, causing logic errors or damage to the circuit. When devices that are far apart are connected by communication cables or sensors, the current between the cable and the ground will flow through the entire circuit, and even over a short distance, the load current of a large device can change between it and the ground potential, perhaps through electromagnetic action. Between incorrectly connected power supplies, there is a potential for a devastating current to occur in the circuit, which can damage the device. The PLC system generally uses a one-point grounding method. In order to improve the ability to resist common mode disturbance, the shielding floating skill can be used for the imitation signal, 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.
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