PLC process control common failure analysis and maintenance

In order to prolong the life of the PLC control system, in the system design and production use, it is necessary to have a clear estimate of the system’s equipment consumption and component equipment failure points, that is, to know which parts of the entire system are most likely to fail, in order to take action. Taking the PLC process control system of No. 1 special cement line of our factory as an example, the fault distribution law of the PLC process control system is analyzed, hoping to help the system design and regular maintenance of the PLC process control system.
1. The concept of system failure
System failure generally refers to the sum of failures 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 on-site control and testing equipment, such as relays, contactors, valves, motors, etc.
2. System failure statistics and analysis and processing
(1) Introduction to the process control system of our special cement No. 1 line
In 2000, when the system was transformed, the A2 series PIC of Japan Erling Company was used as the core composition PLC process control system.
The system has 2 centralized control rooms: the kiln tail control room and the kiln head control room, of which the kiln head control room is the main station; 2 on-site workstations: the kiln tail raw material automatic batching workstation and the kiln tail ball forming disc automatic water addition Ball forming workstation; 2 TV monitoring systems: preheater inlet feeding monitoring and kiln head TV watching fire. The on-site workstation is an independent microcomputer automatic control system, which only communicates with the master station and interlocks with the analog quantity. The frame synchronous full-duplex communication method is adopted between the master station and the slave station:
(2) System
Statistics of fault data
According to statistics, there are 126 system faults in total, of which the proportion of PLC faults is about 4. 7%, and the proportion of partial faults on site is about 95.3%. Compared with the fault data of other PLC process control systems, and considering that the running time of this system is not very long, this ratio is relatively close to the fault distribution law of general PLC process control systems. A 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
  PLC mainframe systems are most likely to fail in the power supply system and communication network system. During the continuous operation and heat dissipation of the power supply, the impact of voltage and current fluctuations is inevitable. The communication and network are likely to be disturbed by the outside world, and the external environment is one of the biggest factors that cause the failure of the external communication equipment. The damage of the system bus is mainly due to the fact that most PLCs are now plug-in structures. Long-term use of plug-in modules will cause damage to the bus at local printed boards or bottom plates, connector interfaces, etc. Under the influence of air temperature changes and humidity changes, the plastic of the bus will age. , Aging of printed lines, oxidation of contact points, etc. are all reasons for system bus loss. Therefore, when designing the system and dealing with system failures, it is necessary to take into account the damage to the equipment caused by factors such as air, dust, and ultraviolet rays. At present, the main memory of PLC mostly adopts rewritable ROM, and its service life is not only related to the manufacturing process, but also related to the power supply of the backplane and the process level of the CPU module. At present, the central processing units of PLC all use 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, add cooling measures, and regularly remove dust, so that the external environment of the PLC meets its installation and operation requirements; at the same time, during system maintenance, strictly follow the operating procedures. Operation, beware of man-made damage to the host system.
The I/O port of PLC
The biggest weak link of PLC lies in the I/O port. The technical advantage of PLC lies in its I/O port. When the technical level of the host system is almost the same, the I/O module is a key component that reflects the performance of PLC, so it is also a prominent link in PLC damage. To reduce the failure of the I/O module, it is necessary to reduce the influence of various external interferences on it. First, use it according to the requirements of its use, and do not reduce its external protection equipment at will. Secondly, analyze the main interference factors and conduct Quarantine or dispose of.
On-site control equipment
The most prone to failures in the entire process control system are on-site. Table 2 lists the most prone to failures in the field.
1) The first type of fault point (also the place with the most faults) is relays and contactors. For example, in the daily maintenance of the PLC control system of the production line, the largest consumption of electrical spare parts is various relays or air switches. The main reason is that besides the product itself, the on-site environment is relatively harsh, and the contacts of the contactor are easily ignited or oxidized, and then become heated and deformed until they cannot be used. All the on-site control boxes on this production line are panel cabinets with better airtightness, and the service life of the internal components is obviously longer than that of other open panel cabinets. Therefore, to reduce such failures, high-performance relays should be selected as much as possible to improve the use of components.Environment, reduce the frequency of replacement to reduce its impact on system operation.
2) The second type of faults often occur on equipment such as valves or gates, because the relative displacement of key execution parts of such equipment is generally large, or it can only be completed after several steps such as electrical conversion The position conversion of valves or gates, or the position conversion of pushing and pulling valves or gates by electric actuators, if the mechanical, electrical, hydraulic and other links are not in place, errors or failures will occur. Long-term use lacks maintenance, and mechanical and electrical failures are the main causes of failures. Therefore, it is necessary to strengthen inspections of such equipment during system operation, and deal with problems in a timely manner. Our factory has established a strict point inspection system for such equipment, and often checks 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 failure point may occur on some components or equipment in switches, limit positions, safety protection and on-site operations. The reason may be due to long-term wear, or rust and aging due to long-term use. For example, the cloth trolley on the kiln tailings ball storage of the production line moves back and forth frequently, and the site is dusty, so the contact of the proximity switch is deformed, oxidized, and dust is clogged, which leads to poor contact of the contact or malfunction of the mechanism
min. The treatment of such equipment failures is mainly reflected in regular maintenance to keep the equipment in good condition at all times. For limit switches, especially those on heavy equipment, in addition to regular maintenance, multiple protection measures must be added to the design process.
4) The fourth type of failure point may occur in sub-equipment in the PLC system, such as junction boxes, wire terminals, bolts and nuts, etc. The reason for this kind of failure is not only the manufacturing process of the equipment itself, but also the installation process. For example, some people think that the tighter the connection of wires and screws, the better, but it is easy to cause difficulty in disassembly during secondary maintenance. It is easy to cause damage to the connector and its nearby components. Long-term ignition, corrosion, etc. are also the reasons for the failure. According to engineering experience, this kind of failure is generally difficult to find and repair. Therefore, in the installation and maintenance of equipment, it must be carried out in accordance with the installation process required by the installation, so as not to leave hidden dangers in the equipment.
5) The fifth type of fault points are sensors and instruments. This type of fault is generally reflected in the abnormal signal in the control system. When this type of equipment is installed, the shielding layer of the signal line should be reliably grounded at one end, and it should be laid separately from the power cable as much as possible, especially the output cable of the inverter with high interference, and software filtering should be performed inside the PIC. The discovery and treatment of this kind of failure is also related to the daily point inspection, and the problem should be dealt with in time.
6) The sixth type of fault is mainly the noise (interference) of power supply, ground wire and signal wire. The solution or improvement of the problem mainly lies inExperience in engineering design and observation and analysis in daily maintenance.
To reduce the failure rate, it is very important to pay attention to the factory process and safety operating procedures, to abide by the process and safety operating procedures in daily work, and to strictly implement some relevant regulations, such as maintaining centralized control room The environment, etc., at the same time, we must strengthen the management of these aspects in production.