Part 1: Common Causes and Troubleshooting Methods for PLC Faults During Operation
(I) Faults in peripheral circuit components
Such faults frequently occur after a PLC has been operating for a certain period of time. If a component in the PLC control loop fails, the PLC control system will immediately and automatically stop working.
The input circuit is the port through which the PLC receives input signals such as digital and analog signals. The quality of its components, the wiring method, and its reliability are all important factors affecting the reliability of the control system.
For digital outputs, PLCs offer three types of outputs: relay outputs, thyristor outputs, and transistor outputs. The specific type of output to choose depends on the load requirements. An improper choice can reduce system reliability and, in severe cases, cause the system to malfunction.
In addition, the load capacity of the PLC's output terminals is limited. If the specified maximum limit is exceeded, an external relay or contactor must be connected for it to work properly.
The quality of external actuators such as relays, contactors, and solenoid valves is a crucial factor affecting system reliability. Common faults include coil short circuits and mechanical failures causing contacts to remain stationary or making poor contact.
(ii) Poor contact in terminal wiring
This type of fault appears after the PLC has been operating for a certain period of time as the frequency of equipment operation increases. Due to defects in the control cabinet wiring, increased vibration during use, and mechanical lifespan, the wiring terminals or component terminals are prone to loosening, causing poor contact.
The troubleshooting method for this type of fault is to use a multimeter and refer to the control system schematic diagram or PLC logic ladder diagram for fault diagnosis and repair.
For wiring of certain important peripheral terminals, in order to ensure reliable connection, the method of soldering cold-pressed plates or cold-pressed pins is generally used.
(III) Functional failures caused by PLC interference
The various types of PLCs used in automation systems are control devices specifically designed for industrial production environments. They employ multi-layered anti-interference measures and carefully selected components during the design and manufacturing process, thus possessing strong adaptability to harsh industrial environments, operational stability, and high reliability. Therefore, they generally can be used directly in industrial environments without requiring any special measures. Interference experienced by PLCs can be categorized into external interference and internal interference.
In actual production environments, external interference is random, unrelated to the system structure, and the sources of interference cannot be eliminated; they can only be restricted according to specific circumstances.
Internal interference is related to the system architecture. It is mainly caused by the AC main circuit and analog input signals within the system. Through careful design of the system circuit or system software filtering, internal interference can be suppressed to the greatest extent.
Anti-interference measures in PLC production sites typically involve four aspects: power supply and grounding protection, wiring arrangement shielding, and noise immunity.
(1) Power supply and grounding protection
PLCs generally have strong anti-interference capabilities. Typically, separating the PLC's power supply from the system's power equipment wiring provides sufficient suppression of interference from the power lines.
However, power interference is particularly severe. A shielded isolation transformer can be added to reduce interference between the equipment and ground, improving system reliability. If a system contains expansion units, their power supply must share a single switch control with the basic unit; that is, their power-on and power-off must occur simultaneously.
To suppress interference from the power supply, input terminals, and output terminals, a dedicated grounding wire should be connected to the PLC. The grounding wire diameter should be sufficiently thick, the grounding resistance should be less than 4Ω, and the grounding point should be as close to the PLC as possible, and the grounding point should be separate from other equipment. For high-voltage equipment in the power supply system, its casing, cabinet, frame, base, and operating handles, and other metal components must be protectively grounded.
The PLC's internal circuitry, including the CPU, memory, and other interfaces, shares a common digital ground. External circuitry, including A/D and D/A converters, shares a common analog ground. The PLC baseplate is connected to the central grounding point in a star configuration using short, thick copper wire to prevent electromagnetic interference. When the PLC is not grounded, its mounting bracket should be capacitively grounded to suppress electromagnetic interference.
(2) Wiring arrangement
Wiring layout inside the electrical cabinet
① Only shielded analog input signal lines can be installed in the same slot as digital signal lines. DC voltage digital signal lines and analog signal lines cannot be installed in the same slot as AC voltage lines.
② Only shielded 220V power cables can be installed in the same cable tray as signal cables.
③ The shielding of the cable plugs in the electrical cabinet must be reliably grounded.
External wiring arrangement of electrical cabinet
① Digital and analog signal lines for DC and AC voltages must each use independent cables, and shielded cables are required.
② Signal cables can be installed together with power cables in a cable tray. To improve noise immunity, it is recommended to ensure a spacing of more than 10cm.
(3) Shielding treatment
The shielding of the PLC enclosure should generally be ensured to be floating above the electrical cabinet. An equipotential shielding plate (usually galvanized sheet) should be installed on the bottom plate of the PLC enclosure. The protective ground should be connected to the bottom plate at one point using a copper wire with a cross-sectional area of not less than 10 mm² to form an equipotential shield, effectively eliminating interference from external electromagnetic fields.
For analog signals, the shielded bus can be insulated and its center point connected to a reference potential or ground (GND). For digital signal lines, grounding both ends of the cable can effectively eliminate high-frequency interference.
(4) Noise reduction measures
Parts exposed to strong magnetic fields (such as transformers) should be metal-shielded. Fluorescent lighting should not be used inside the electrical control cabinet. Appropriate anti-interference measures should also be taken for the power supply of the PLC control system.
There are three methods for power supply anti-interference in PLC control systems: using isolation transformers, low-pass filters, and applying spectrum equalization. Among them, isolation transformers are the most commonly used because the power supply of PLC I/O modules is usually DC24V, which must be stepped down by an isolation transformer and then rectified by a rectifier bridge, or directly supplied by a switching power supply.
(iv) Periodic PLC crashes
The characteristic of periodic PLC crashes is that the PLC crashes or malfunctions its program after running for a certain period of time, or displays different interrupt faults, and then everything returns to normal after a restart. Based on practical experience, the most common cause of this phenomenon is long-term dust accumulation on the PLC unit.
Therefore, the PLC rack slot interfaces should be cleaned regularly. During cleaning, first use compressed air or a soft brush to blow away dust from the control board and slots, then wipe the slots and control board plugs clean with 95% alcohol. After cleaning, carefully inspect the system, and then power it on for normal operation.
PLC program loss is usually caused by several factors, including poor grounding, incorrect wiring, operational errors, and interference.
1. The PLC host and modules must have good grounding.
2. The live wire and neutral wire of the host power cord must be connected correctly.
3. Prepare the program package in advance for backup.
4. When using a handheld programmer to troubleshoot, the locking switch should be placed in the vertical position. Pulling it out will protect the memory.
5. The PLC program is lost due to interference. The troubleshooting method can refer to the troubleshooting of PLC faults caused by interference.
Part Two: PLC Fault Troubleshooting Flowchart
NO.1 Overall Inspection
Based on the overall inspection flowchart, identify the general direction of the fault points, and gradually refine it to find the specific faults, as shown in the figure below.
NO.2 Power Supply Fault Inspection
NO.3 Operational Fault Check
If the power supply is normal but the operation indicator light is not lit, it means that the system has terminated normal operation due to some kind of abnormality. The troubleshooting flowchart is shown below.
NO.4 Input/Output Fault Check
Input/output (I/O) is the channel through which the PLC exchanges information with external devices. Its proper functioning depends not only on the I/O units themselves, but also on the condition of components such as wiring, terminals, and fuses. The inspection flowchart is shown below.
