1. Peripheral circuit component failure
This type of failure often occurs in the failure of the PLC after a certain period of work. In the PLC control loop, if there is a component damage fault, the PLC control system will immediately and automatically stop working.
The input circuit is the port of the PLC to accept the input signals such as switching quantity and analog quantity, and the quality of its components, wiring mode and whether it is reliable are also important factors affecting the reliability of the control system.
For the switching output, the output of the PLC has three forms: relay output, thyristor output, transistor output, and the specific choice of which form of output should be decided according to the load requirements, and the improper selection will reduce the reliability of the system When it is serious, the system can not work normally.
In addition, the load capacity of the output terminal of the PLC is limited. If the stated maximum limit is exceeded. An external relay or contactor must be connected. to work properly.
The quality of actuators such as external relays, contactors, and solenoid valves is an important factor affecting the reliability of the system. Common faults include short circuits in the coil, mechanical failures that cause immobility of the contacts or poor contact.
2. Poor terminal wiring contact
This kind of fault occurs after the PLC has been working for a certain period of time with the increase in the frequency of equipment action. Due to the defects in the wiring of the control cabinet or the intensification of vibration in use and the mechanical life, the terminal posts of the terminals or components are easy to loosen and cause poor contact.
The method of troubleshooting this kind of fault is to use a multimeter, with the help of the control system schematic diagram or PLC logic ladder diagram for fault diagnosis and maintenance.
For the wiring of some important peripheral terminals, in order to ensure reliable connection, the method of soldering cold-pressed sheets or cold-pressed pins is generally used.
3. Functional failure caused by PLC interference
The various types of PLCs used in automation systems are control devices specially designed for industrial production environments. In the design and manufacturing process, multi-level anti-interference and selected component measures are adopted, so it has a strong ability to adapt to harsh industrial environments, operational stability and high reliability. Therefore, it can generally be used directly in an industrial environment without any special measures. The interference experienced by the PLC can be divided into external interference and internal interference.
In the actual production environment, external interference is random, independent of the system structure, and the source of interference cannot be eliminated and can only be limited on a case-by-case basis.
Internal interference is related to the structure of the system. It is mainly caused by the AC main circuit and analog input signal in the system, and the internal interference can be suppressed to the greatest extent through careful design of system circuits or system software filtering.
Anti-interference technical measures at PLC production site - usually from four aspects: power supply and grounding protection, wiring arrangement, shielding and noise resistance:
(1) Power supply and grounding protection
The anti-interference ability of the PLC itself is generally very strong. Usually. Separate the power supply of the PLC from the power supply of the system for the interference of the power line, which generally has a strong enough suppression ability.
However, the power supply interference is particularly serious, and a shielded isolation transformer can be added to reduce the interference between the equipment and the ground and improve the reliability of the system. If a system contains expansion units, their power supply must share a switch control with the base unit, i.e., they must be powered on and off at the same time.
In order to suppress the interference attached to the power supply and the input and output terminals, the PLC should be connected to the special ground wire The diameter of the ground wire should be thick enough. The ground resistance should be less than 4 Ω, the ground point should be as close to the PLC as possible, and the ground point should be separated from other equipment. For the strong current equipment in the power supply system, the metal components such as the shell, cabinet, frame, base and operating handle must be protected and grounded.
The internal circuitry of the PLC includes the CPU, memory and other interfaces that are connected to the digital ground. The external circuit includes A/D, D/A, etc., which are co-connected to analog ground, and the PLC base plate is connected with the central grounding point star with a thick copper wire to prevent dole interference. When the PLC is not grounded, the mounting bracket of the PLC should be capacitively grounded to suppress electromagnetic interference.
(2) Wiring arrangement
Routing of wiring in the electrical cabinet
(1) Only the shielded analog input signal line can be installed in the same line slot as the digital signal line, and the DC voltage digital signal line and the analog signal line cannot be in the same line slot as the AC voltage line.
(2) Only shielded 220V power cables can be installed in the same trunking as signal cables.
(3) The shielding of the cable plug of the electrical cabinet must be reliably grounded.
Electrical cabinet external wiring arrangement
(1) The digital signal line and analog signal line of DC and AC voltage must use independent cables and shielded cables.
(2) The signal cable can be installed in the first line slot together with the power cable, and it is recommended to ensure that the interval is more than 10cm in order to improve the noise immunity.
(3) Shielding
The shielding of the PLC shell should generally ensure that it floats with the electrical cabinet. Install an isometric shielding plate (generally use galvanized sheet) on the bottom plate of the PLC shell, and use a copper wire to maintain a point connection with the bottom plate, and its cross-sectional area should not be less than 1 0mm, so as to constitute an isometric shielding body and effectively eliminate the interference of external electromagnetic fields.
The shielded bus for the analog signal insulates and connects the central point to the reference potential or ground (GND). Grounding at both ends of the cable for the digital signal line ensures good rejection of high-frequency interference.
(4) Anti-noise measures
The part in the strong magnetic field (such as the transformer) should be shielded with metal, and fluorescent lamps should not be used for lighting in the electric control cabinet. The power supply of the PLC control system should also adopt corresponding anti-interference measures.
There are three methods of anti-interference of power supply of PLC control system: isolation transformer, low-pass filter and spectrum equalization method. Among them, the isolation transformer is the most commonly used, because the power supply of the I/O module of the PLC is usually DC24V, which must be stepped-down by the isolation transformer, and then rectified by the rectifier bridge, or directly supplied by switching power supply.
PLC crashes periodically
PLC periodic crash is characterized by the PLC crashing or chaotic program every certain amount of time of operation, or different interrupt fault displays, and everything is normal after restarting. According to practical experience, the most common cause of this phenomenon is caused by the long-term dust accumulation of the PLC body.
Therefore, the PLC rack slot interface should be purged regularly. When purging, you can first use compressed air or a soft brush to blow the dust on the control board and each slot, and then wipe the slots and control board plugs with 95% alcohol. After cleaning, check it carefully, and resume the boot to run normally.
The loss of PLC programs is usually caused by several reasons such as poor grounding, incorrect wiring, operation errors and interference:
1. The PLC host and module must have good grounding.
2. The phase wire and neutral wire of the main engine power cord must be wired correctly.
3. Prepare the package in advance for backup.
4. When using a handheld programmer to find faults, the locking switch should be placed in a vertical position, and pulling it out can play a role in protecting the memory.
5. PLC program is lost due to interference. The treatment method can refer to the handling of PLC interference caused by failure.
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