The little common sense that you must know to learn PLC programming, learn together
1
PLC fault judgment
Generally speaking, PLC is an extremely reliable device with a very low failure rate. The probability of hardware damage such as PLC CPU or software running error is almost zero; PLC input point is almost not damaged if it is not caused by strong current intrusion; PLC output relay normally open point, if it is not a short circuit of external load or unreasonable design , The load current exceeds the rated range, and the life of the contact is also very long.
Therefore, when we look for electrical failure points, we should focus on the peripheral electrical components of the PLC. Don’t always suspect that there is a problem with the PLC hardware or program. This is very important to quickly repair the faulty equipment and quickly resume production. Therefore, the author is talking about The focus of the electrical troubleshooting of the PLC control loop is not on the PLC itself, but on the peripheral electrical components in the loop controlled by the PLC.
2
Selection of input and output (I/O) modules
Output modules are divided into transistors, triacs, and contact types.
Transistor type has the fastest switching speed (usually 0.2ms), but the smallest load capacity, about 0.2~0.3A, 24VDC, suitable for fast switching and signal connection equipment, generally connected with frequency conversion, DC equipment and other signals, should pay attention to transistor leakage The effect of current on the load. The advantage of the SCR type is that it has no contact, has AC load characteristics, and has a small load capacity.
The relay output has the characteristics of AC and DC load, and the load capacity is large. In conventional control, the 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.
3
Grounding problem
The PLC system grounding requirements are relatively strict. It is best to have an independent dedicated grounding system. Also, note that other equipment related to the PLC must be grounded reliably. When multiple circuit grounding points are connected together, unexpected current will be generated, resulting in logic errors or circuit damage.
The reason for the different ground potentials is usually that the grounding points are separated too far in the physical area. When the devices that are far apart are connected by communication cables or sensors, the current between the cable and the ground will be Flow through the entire circuit.
Even in a short distance, the load current of large equipment can change between it and the ground potential, or directly generate unpredictable currents through electromagnetic effects. Between power sources at incorrect grounding points, a destructive current may be generated in the circuit, which may damage the equipment.
The PLC system generally chooses one point grounding. In order to improve the ability to resist common mode interference, shielding floating technology can be used for analog signals, 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 earth should not be less than 50MΩ.
4
Eliminate line capacitance to avoid malfunction
There is a capacitance between the wires of the cable, and a qualified cable can limit this capacitance within a certain range. Even if it is a qualified cable, when the cable length exceeds a certain length, the capacitance value between the lines will exceed the required value. When this cable is used for PLC input, the line capacitance may cause the PLC to malfunction. , There will be many incomprehensible phenomena.
These phenomena are mainly manifested as: the connection is correct, but the PLC has no input; the input that the PLC should have does not have, but the input that should not be there is there, that is, the PLC inputs interfere with each other. To solve this problem
Should do:
Use cables with cable cores twisted together; try to shorten the length of the cable used
Separate the interfering inputs with cables; use shielded cables
5
Anti-interference processing
The environment of the industrial site is relatively harsh, and there are many high and low frequency interferences. These interferences are generally introduced into the PLC through cables connected to field devices. In addition to grounding measures, some anti-interference measures should be taken in the design and selection of cables and installation work:
The analog signal is a small signal and is easily affected by external interference. Double shielded cables should be used; high-speed pulse signals (such as pulse sensors, counting code discs, etc.) should be shielded cables to prevent external interference and high-speed pulses. Signal interference to low-level signals;
The frequency of communication cables between PLCs is relatively high. Generally, the cables provided by the manufacturer should be selected. If the requirements are not high, shielded twisted-pair cables can be used; analog signal lines and DC signal lines cannot be in the same position as AC signal lines. Wire in the trunking;
The shielded cables introduced and drawn in the control cabinet must be grounded and should be directly connected to the equipment without connecting terminals; AC signals, DC signals and analog signals cannot share the same cable, and the power cable should be laid separately from the signal cable.
During on-site maintenance, the methods to solve the interference are: use shielded cables for the interfered lines and re-lay them; add anti-interference filtering codes to the program.
6
Mark input and output for easy maintenance
The PLC controls a complex system. What you can see is the two rows of staggered input and output relay terminals, the corresponding indicator lights and the PLC number, just like an integrated circuit with dozens of pins. Anyone who does not look at the schematic diagram to repair the faulty equipment will be at a loss, and the speed of finding the fault will be extremely slow.
7
Infer failure through program logic
There are many types of PLCs that are often used in the industry. For low-end PLCs, the ladder diagram instructions are similar. For mid-to-high-end computers, such as S7-300, many programs are compiled in language tables. Practical ladder diagrams must have Chinese symbol annotations, otherwise it is difficult to read. If you can roughly understand the equipment technology or operation process before looking at the ladder diagram, it seems easier.
For electrical fault analysis, the reverse check method or reverse push method is generally used, that is, according to the input and output correspondence table, find the corresponding PLC output relay from the fault point, and start to check the logical relationship that satisfies its action. Experience has shown that if a problem is found, the fault can basically be eliminated, because there are not many failures that occur in the equipment at the same time or more.
8
Reasonable use of software and hardware resources
Instructions that do not participate in the control cycle or have been input before the cycle may not be connected to the PLC;
When multiple instructions control a task, you can connect them in parallel outside the PLC and then connect to an input point; try to use the PLC internal function soft elements to fully call the intermediate state, so that the program has complete continuity and is easy to develop. At the same time, it also reduces hardware investment and reduces costs;
When conditions permit, it is best to separate each output for easy control and inspection, and also protect other output circuits; when one output point fails, it will only cause the corresponding output circuit to lose control; if the output is a load with forward/reverse control, not only Interlock from the PLC internal program, and take measures outside the PLC to prevent the load from moving in both directions; the PLC emergency stop should be cut off with an external switch to ensure safety.
9
Other considerations
Do not connect the AC power cord to the input terminal to avoid burning the PLC; the ground terminal should be grounded independently, not in series with the ground terminal of other equipment, and the cross-sectional area of the grounding wire should not be less than 2mm²; the auxiliary power supply is small and can only drive low-power Equipment (photoelectric sensors, etc.); some PLCs have a certain number of occupied points (namely, empty address terminal), do not connect the line;
When there is no protection in the PLC output circuit, a protective device such as a fuse should be used in series in the external circuit to prevent damage to the load caused by a short circuit.