In order to extend the life of the plc control system, in the system design and production use, it is necessary to have a clear estimation of the equipment consumption of the system and the occurrence of component equipment failure, that is, to know which parts of the whole system are most likely to fail, In order to take measures. Taking the PLC process control system of our special cement line 1 as an example, the fault distribution law of PLC process control system is analyzed. It is hoped that it can help the system design and U maintenance of PLC process control system.
1. The concept of system failure System failure generally refers to the sum of the failure of the entire production control system, which can be divided into two parts: PLC failure and on-site production control equipment failure. The PLC system includes a central processing unit, a main chassis, an expansion chassis, I/O modules, and related network and external devices. On-site production control equipment includes I/O ports and field control testing equipment such as relays, contactors, valves, motors, and more.
2. System fault statistics and analysis and treatment (1) Introduction to the process control system of special cement No. 1 line of our factory
In 2000, the system was transformed with the PLC process control system consisting of A2 series PIC from Japan's Mitsubishi Corporation.
The system has two centralized control rooms: the kiln tail control room and the kiln head control room, in which the kiln head control room is the main station; two on-site workstations: the kiln tail raw material automatic batching station and the kiln tail ball plate automatic water adding ball station 2 TV monitoring systems: preheater import blanking monitoring and kiln head TV watching fire. The on-site workstation is an independent microcomputer automatic control system, which only performs analog communication and switching interlocking with the main station. Frame synchronization full-duplex communication between the master station and the slave station:
(2) Statistics of system fault data According to statistics, system faults total 126 times, of which PLC fault ratio is about 4.7%, and part of the field fault ratio is about 95.3%, compared with other PLC process control system fault data. And considering that the system running time is not very long, the ratio is closer to the fault distribution law of the general PLC process control system, and has certain universality. Generally speaking, the failure ratio of the PIC part is about 5%, and the failure ratio of the field control equipment is about 95%.
(3) System failure analysis and processing
PLC host system
The most prone to failure of the PLC host system is generally in the power system and communication network system. The power supply is in continuous operation and heat dissipation, and the fluctuation of voltage and current is inevitable. Communication and networks are highly susceptible to external interference, and the external environment is one of the biggest factors causing communication external equipment failure. The damage of the system bus is mainly due to the fact that the PLC is mostly a plug-in structure. The long-term use of the plug-in module will cause bus damage to the local printed board or the backplane, the connector interface, etc. Under the influence of air temperature changes and humidity changes, the plastic of the bus is aged. The aging of printed circuits and the oxidation of contact points are all causes of system bus losses. Therefore, when the system is designed and the system is faulty, the damage of the equipment should be taken into consideration by factors such as air, dust and ultraviolet rays. At present, most of the PLC main memory uses rewritable ROM, and its service life is mainly related to the manufacturing process, but also related to the power supply of the backplane and the CPU module. The central processing unit of PLC currently uses high-performance processing chips, and the failure rate has been greatly reduced. The prevention and treatment of the failure of the PLC host system is mainly to improve the management level of the centralized control room, install the cooling measures, regularly remove the dust, so that the external environment of the PLC meets the requirements of its installation and operation; at the same time, in the system maintenance, strictly follow the operating procedures. Operation, beware of damage to the host system.
PLC I/O port
The biggest weakness of PLC is in the I/O port. The technical advantage of PLC lies in its I/O port. In the case that the technical level of the host system is almost the same, the I/O module is a key component that reflects the performance of the PLC, so it is also a prominent link in the PLC damage. To reduce the failure of I/O modules, it is necessary to reduce the impact of external interferences. First, they should be used according to the requirements of their use. The external protection equipment should not be reduced at will. Secondly, the main interference factors should be analyzed. Isolation or processing.
Field control equipment is the most susceptible to fault location in the entire process control system. Table 2 lists the most vulnerable aspects of the field.
1) The first type of fault point (also the location with the most faults) is in relays and contactors. For routine maintenance of the PLC control system of the production line, the largest amount of electrical spare parts is the various types of relays or air switches. The main reason is that the site environment is harsh except for the product itself. The contactor contacts are easy to be ignited or oxidized, and then deformed by heat until they cannot be used. All the control boxes on the production line are made of better sealed cabinets, and the internal components are obviously longer than those of other open-type cabinets. Therefore, to reduce such faults, high-performance relays should be used as much as possible to improve the environment in which components are used and reduce the frequency of replacement to reduce the impact on the operation of the system.
2) The second type of faults are often found on equipment such as valves or gates. Because of the critical displacement of such equipment, the relative displacement is generally large, or several steps such as electrical conversion can be used to complete the valve or gate. The position of the board is changed, or the position of the valve or the gate of the electric actuator is pushed and pulled, and the mechanical, electrical, hydraulic and other links are slightly out of place, which may cause errors or malfunctions. Long-term use lacks maintenance. Mechanical and electrical failures are the main causes of faults. Therefore, when the system is running, it is necessary to strengthen the inspection of such equipment and find problems in a timely manner. Our factory has established a strict inspection system for such equipment, often checking whether the valve is deformed, whether the actuator is flexible and available, and whether the controller is effective, etc., which ensures the effectiveness of the entire control system.
3) The third type of fault point may occur on some components or equipment in the switch, limit position, safety protection and on-site operation. The reason may be due to long-term wear or rust and aging due to long-term use. For example, the fabric walking vehicle on the kiln tailings ball storage tank moves frequently back and forth, and the dust on the site is large, so the deformation of the proximity switch contacts, oxidation, dust clogging, etc., resulting in poor contact contact or insensitive mechanism action. The handling of such equipment failures is mainly reflected in regular maintenance, so that the equipment is always in good condition. For limit switches, especially for limit switches on heavy equipment, in addition to regular maintenance, multiple protection measures must be added during the design process.
4) The fourth type of fault point may occur in sub-devices in the PLC system, such as junction boxes, wire terminals, bolts and nuts. The cause of this type of failure is related to the installation process in addition to the manufacturing process of the device itself. For example, some people think that the wire and screw connection are pressed as tightly as possible, but it is easy to cause disassembly during secondary maintenance. It is easy to cause damage to the connector and its nearby parts. Long-term fire, rust, etc. are also the cause of the failure. According to engineering experience, such faults are generally difficult to find and repair. Therefore, in the installation and maintenance of the equipment must be carried out in accordance with the installation requirements of the installation process, leaving no hidden dangers.
5) The fifth type of fault point is the sensor and the instrument. Such fault is generally reflected in the control system and the signal is abnormal. When the equipment is installed, the shielding layer of the signal line should be grounded reliably at one end, and should be laid separately from the power cable as much as possible, especially the high-interference inverter output cable, and software filtering should be performed inside the PIC. The discovery and handling of such faults are also related to daily inspections, and problems should be dealt with in a timely manner.
6) The sixth type of fault is mainly the noise (interference) of the power supply, ground and signal lines. The solution or improvement of the problem mainly lies in the experience in engineering design and the observation and analysis in daily maintenance.
To reduce the failure rate, it is important to pay attention to the factory process and safe operating procedures, to comply with the process and safety operating procedures in daily work, and to strictly enforce some relevant regulations, such as maintaining the environment of the centralized control room, etc. At the same time, we must strengthen these aspects in production.
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