PLC stands for Programmable Logic Controller, but it's commonly referred to simply as PLC. To enhance your understanding of PLCs, this article will introduce PLC malfunctions. PLC malfunctions include two aspects: hardware malfunctions and software malfunctions. If you are interested in PLCs, please continue reading.
In industry, programmable logic controllers (PLCs) are comparable to or nearly equivalent to the mainframe of a compact computer. Their advantages in scalability and reliability have led to their widespread application in various industrial control fields. Whether in computer-directed control systems, centralized distributed control systems (DCS), or fieldbus control systems (FCS), PLCs are used extensively. Many manufacturers produce PLCs, such as Siemens, Schneider Electric, Mitsubishi, and Delta; almost every company involved in industrial automation offers PLC products.
PLC malfunctions can be categorized into two types: hardware malfunctions and software malfunctions.
I. PLC software malfunction
PLCs have self-diagnostic capabilities. When a module malfunction occurs, it can often alarm and react according to a pre-programmed procedure, which can be determined through fault indicator lights. When the power supply is normal, all indicator lights are also normal, especially the input signals are normal, but the system function is abnormal (no output or erratic output), following the principle of starting with the easy and then moving to the difficult, and starting with software and then moving to hardware, first check whether there is a problem with the user program.
The user program is stored in the PLC's RAM, which is volatile when power is lost. When the backup battery fails and the system power supply is interrupted, there is a high possibility that the program will be lost or corrupted. Strong electromagnetic interference can also cause program errors.
II. PLC Hardware Failure
① PLC main unit system failure
A. Power system failure. During continuous operation and heat dissipation, voltage and current fluctuations are unavoidable in the power supply.
B. Communication network system failure. Communication and networks are highly susceptible to external interference, and the external environment is one of the biggest factors causing failures of external communication equipment. Damage to the system bus is mainly due to the fact that PLCs are mostly plug-in structures. Long-term use of plugging and unplugging modules can cause damage to the bus at local printed circuit boards, baseboards, connector interfaces, etc. Under the influence of changes in air temperature and humidity, the aging of the plastic on the bus, the aging of printed circuits, and the oxidation of contact points are all causes of system bus wear.
② PLC I/O port failure.
The main causes of I/O module failures are the effects of various external interferences. First, it should be used in accordance with the requirements and its external protection devices should not be reduced arbitrarily. Second, the main interference factors should be analyzed and the main interference sources should be isolated or dealt with.
③ Failure of field control equipment
A. Relays and contactors. To reduce such failures, high-performance relays should be selected whenever possible, and the operating environment of components should be improved to reduce the frequency of replacement. If the field environment is harsh, contactor contacts are prone to arcing or oxidation, which can then cause them to overheat, deform, and eventually become unusable.
B. Valves or gates and similar equipment. Long-term use without maintenance, mechanical and electrical malfunctions are the main causes of failure. Because the key actuators of this type of equipment generally have large relative displacements, or require several steps such as electrical conversion to complete the position change of the valve or gate, or use electric actuators to push and pull the valve or gate to change its position, slight deficiencies in the mechanical, electrical, and hydraulic components can lead to errors or failures.
C. Failures in some components or equipment related to switches, limit positions, safety protection, and on-site operation may be caused by long-term wear or corrosion due to prolonged disuse. The main solution for these equipment failures is regular maintenance to ensure the equipment is always in good working order. For limit switches, especially those on heavy equipment, in addition to regular inspections, multiple protective measures should be incorporated into the design process.
D. Faults in sub-devices of the PLC system, such as junction boxes, terminals, bolts, and nuts. These faults are mainly caused by the manufacturing and installation processes of the equipment itself, as well as long-term arcing and corrosion. Based on engineering experience, these faults are generally difficult to detect and repair. Therefore, the installation and maintenance of the equipment must be carried out in strict accordance with the installation requirements to avoid leaving any potential hidden dangers.
E. Sensor and instrument malfunctions. These malfunctions are generally reflected in abnormal signals in the control system. When installing such equipment, the shielding layer of the signal line should be reliably grounded at one end and should be laid separately from the power cable as much as possible, especially the output cable of the high-interference frequency converter. In addition, software filtering should be performed inside the PLC.
F. Noise (interference) faults in power supply, ground, and signal lines.
