A Programmable Logic Controller (PLC) is a digital electronic device specifically designed for industrial applications. It uses programmable memory to store instructions for performing logical, sequential, timing, counting, and arithmetic operations, and can control various types of machinery or production processes through digital or analog inputs and outputs. However, due to the complexity of industrial environments, PLC troubleshooting is a key aspect of instrument and equipment maintenance. This article shares common PLC faults and their solutions to improve PLC maintenance skills.
1. Common Fault Locations (PART)
In the entire PLC control system, the most likely place for failure is in the field, and failures are most likely to occur in the following aspects.
The first type of fault point, which is also the location with the most faults, is in relays and contactors.
In the daily maintenance of a PLC control system on a certain production line, the electrical spare parts consumed in the largest quantities are various relays or air switches. Besides product quality issues, this is mainly due to the harsh on-site environment. For example, contactor contacts exposed to the production environment are prone to sparking or oxidation, gradually overheating and deforming until they become unusable. All control boxes on this production line use well-sealed cabinets, whose internal components have a significantly longer lifespan than those in open cabinets. Therefore, to avoid such failures, high-performance relays should be selected whenever possible, and the operating environment of components should be improved to reduce replacement frequency and minimize the impact on system operation.
The second type of failure often occurs in equipment such as valves or gates.
Because the actuators of these types of equipment have relatively large displacements, or complex transmission structures, even slight deficiencies in the mechanical, electrical, and hydraulic components can lead to errors or malfunctions. Under long-term operation, a lack of maintenance can easily cause valve components to jam, become clogged, or leak. Therefore, it is crucial to strengthen the inspection of these devices during system operation and address any problems promptly. Our factory has established a strict inspection system for these devices, regularly checking for valve deformation, the flexibility and usability of actuators, and the effectiveness of controllers, thus ensuring the effectiveness of the entire control system.
The third type of failure point may occur in some components or equipment related to switches, limit positions, safety protection devices, and field operations.
The cause could be long-term wear and tear, or corrosion and aging due to prolonged disuse. For example, the material distribution trolley on the kiln tail ball storage silo of this production line moves back and forth frequently, and the site has a lot of dust, so the proximity switch contacts may deform, oxidize, or become clogged with dust, resulting in poor contact or sluggish mechanism operation. The main solution for this type of equipment failure 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.
The fourth type of failure point may occur in the sub-devices of the PLC system.
These types of equipment include junction boxes, wire terminals, bolts, and nuts. The causes of these malfunctions, besides issues with the equipment's manufacturing process, are also related to installation techniques. For example, some believe that wire and screw connections should be tightened as much as possible, but this can easily lead to difficulty in disassembly during secondary maintenance, and forceful disassembly can damage the connectors and nearby components. Long-term arcing and corrosion are also causes of malfunctions. Based on engineering experience, these types of malfunctions are generally difficult to detect and repair. Therefore, during equipment installation and maintenance, it is essential to follow the installation requirements and procedures to avoid leaving any potential hidden dangers.
The fifth type of failure point is sensors and instruments.
In control systems, this type of fault typically manifests as abnormal signals. When installing such equipment, the shielding layer of the signal cable should be reliably grounded at one end and laid separately from power cables as much as possible, especially for high-interference inverter output cables. Furthermore, software filtering should be performed within the PIC (Physical Input Circuit). The detection and handling of this type of fault are also related to routine inspections; any problems discovered should be addressed promptly.
The sixth type of fault mainly involves noise (interference) in the power supply, ground wire, and signal lines.
The solution or improvement of the problem mainly lies in the experience gained during engineering design and the observation and analysis during routine maintenance.
To reduce the failure rate, it is crucial to prioritize factory processes and safe operating procedures. In daily operations, it is essential to adhere to these procedures and strictly enforce relevant regulations, such as maintaining a clean environment in the central control room. Furthermore, management in these areas should be strengthened during production.
A process control system is a complete system, so a systemic approach must be taken into account when analyzing or troubleshooting faults. Optimizing only one part sometimes fails to improve the overall system performance. For example, excessively pursuing component precision without considering actual needs and matching the precision of related equipment will only increase system costs unnecessarily. In routine maintenance, there have been instances of systems becoming increasingly complex, such as using complex control methods and equipment to achieve controls that could be achieved with simple devices. This violates the principles of economy, simplicity, and practicality and may increase the failure rate; this is also something to be mindful of.
2. Troubleshooting PLC Faults
The reliability of PLC products themselves can be guaranteed, but some incorrect operations during application can have certain effects.
1. PLC self-fault diagnosis
Generally speaking, PLCs are extremely reliable devices with a very low failure rate. The probability of damage to PLC hardware such as the CPU or software malfunctions is almost zero; PLC input points are unlikely to be damaged unless caused by a high-voltage intrusion; and the normally open contacts of PLC output relays have a long lifespan unless there is a short circuit in the external load or an unreasonable design causing the load current to exceed the rated range.
Therefore, when locating electrical faults, we should focus on the peripheral electrical components of the PLC, rather than always suspecting problems with the PLC hardware or program. This is crucial for quickly repairing faulty equipment and restoring production. Thus, the focus of the electrical fault diagnosis of the PLC control circuit discussed in this article is not on the PLC itself, but on the peripheral electrical components in the circuit controlled by the PLC.
2. Selection of Input/Output (I/O) Modules
Output modules are categorized into transistor, triac, and contact types. Transistor types offer the fastest switching speed (typically 0.2ms), but have the lowest load capacity, approximately 0.2~0.3A, 24VDC. They are suitable for devices requiring fast switching and signal communication, typically connected to frequency converters, DC power supplies, etc. Attention should be paid to the impact of transistor leakage current on the load. Thyristor types offer the advantage of being contactless and having AC load characteristics, but their load capacity is relatively low.
Relay outputs have AC/DC load characteristics and a large load capacity. In conventional control, relay contact type outputs are generally preferred. The disadvantage is that the switching speed is slow, generally around 10ms, making it unsuitable for high-frequency switching applications.
3. Grounding issues
PLC systems have strict grounding requirements and should ideally have a dedicated, independent grounding system. It's also important to ensure that other equipment related to the PLC is reliably grounded. Connecting multiple grounding points together can generate unexpected currents, leading to logic errors or circuit damage. The reason for differing grounding potentials is usually that the grounding points are physically too far apart. When devices far apart are connected by communication cables or sensors, current flows between the cable and ground throughout the entire circuit. Even over short distances, the load current of large equipment can cause changes in its potential relative to ground, or unpredictable currents can be generated directly through electromagnetic interactions. Between power supplies with incorrect grounding points, potentially generating devastating currents in the circuit, even damaging the equipment.
PLC systems generally prefer a single-point grounding method. To improve common-mode interference immunity, shielded floating ground technology can be used for analog signals, where the shielding layer of the signal cable is grounded at one point, the signal loop floats, and the insulation resistance to the ground should not be less than 50MΩ.
4. Eliminate inter-line capacitance to avoid malfunctions.
All conductors in a cable possess capacitance, which a qualified cable can limit to a certain range. Even with a qualified cable, when the cable length exceeds a certain limit, the capacitance between the conductors will exceed the required value. When this cable is used for PLC input, the inter-conductor capacitance may cause malfunctions in the PLC, resulting in many inexplicable phenomena. These phenomena mainly manifest as: correct wiring on the surface, but no input to the PLC; inputs that should be present are absent, while inputs that shouldn't be present are present, i.e., PLC inputs interfere with each other.
To solve this problem, the following should be done:
1. Cables with cable cores twisted together;
2. Minimize the length of the cable used;
3. Use separate cables for mutually interfering inputs;
4. Use shielded cables.
5. Anti-interference processing
Industrial environments are harsh, with numerous high- and low-frequency interferences. These interferences are typically introduced into the PLC through cables connected to field equipment. In addition to grounding measures, anti-interference measures should be taken during cable design, selection, and installation.
Analog signals are small signals and are highly susceptible to external interference; therefore, double-shielded cables should be used.
High-speed pulse signals (such as pulse sensors, counting codes, etc.) should use shielded cables to prevent both external interference and interference from high-speed pulse signals to low-level signals;
Communication cables between PLCs operate at high frequencies, so cables provided by the manufacturer should generally be used. If the requirements are not high, shielded twisted-pair cables can be used.
Analog signal lines and DC signal lines must not be routed in the same cable tray as AC signal lines;
The shielded cables leading into and out of the control cabinet must be grounded and should be connected directly to the equipment without going through terminals.
AC signals, DC signals, and analog signals cannot share a single cable; power cables should be laid separately from signal cables.
During on-site maintenance, methods to resolve interference include: using shielded cables for the affected lines and re-laying them; and adding anti-interference filtering code to the program.
6. Mark inputs and outputs for easy maintenance.
A PLC controls a complex system, and what you see are two rows of staggered input/output relay terminals, corresponding indicator lights, and the PLC number, much like an integrated circuit with dozens of pins. Anyone trying to troubleshoot a faulty device without consulting the schematic diagram will be at a loss, and troubleshooting will be extremely slow. Therefore, it is advisable to create a table based on the electrical schematic diagram and affix it to the control panel or cabinet, clearly labeling each PLC input/output terminal number with its corresponding electrical symbol and Chinese name—similar to the function description of each pin on an integrated circuit.
With this input/output table, electricians familiar with the operation process or the ladder diagram of the equipment can begin troubleshooting. However, for electricians unfamiliar with the operation process and unable to read ladder diagrams, another table needs to be drawn: the PLC Input/Output Logic Function Table. This table actually illustrates the logical correspondence between input circuits (trigger elements, associated elements) and output circuits (execution elements) in most operations. Practice has shown that if you can skillfully use the input/output correspondence table and the input/output logic function table, you can easily troubleshoot electrical faults even without drawings.
7. Inferring faults through program logic
There are many types of PLCs used in industry today. For low-end PLCs, the ladder diagram instructions are largely the same. For mid-to-high-end PLCs, such as the S7-300, many programs are written using language tables. Practical ladder diagrams must have Chinese symbol annotations; otherwise, they are very difficult to read. If you have a general understanding of the equipment's process or operation before looking at the ladder diagram, it will be easier to understand.
Electrical fault analysis typically employs a reverse lookup method, or reverse deduction method. This involves using an input/output mapping table to locate the corresponding PLC output relay at the fault point and then tracing back the logical relationships that enable its operation. Experience shows that once a problem is identified, the fault can generally be eliminated, as it is rare for equipment to experience two or more fault points simultaneously.
8. Make full and reasonable use of software and hardware resources
Instructions that do not participate in the control cycle or have been input before the cycle do not need to be connected to the PLC;
When multiple instructions control a single task, it is advisable to connect them in parallel externally to the PLC before connecting them to a single input point.
Maximize the use of PLC's internal functional components and fully utilize intermediate states to ensure program continuity and ease of development. This also reduces hardware investment and lowers costs.
Where conditions permit, each output should be independent for easier control and inspection, and to protect other output circuits; when a fault occurs at one output point, only the corresponding output circuit will become uncontrollable.
If the output is a load controlled in both forward and reverse directions, interlocking must be implemented not only in the PLC's internal program but also externally to prevent the load from moving in either direction.
For PLC emergency stop, an external switch should be used to disconnect the circuit to ensure safety.
9. Other precautions
Do not connect the AC power cord to the input terminals, as this may damage the PLC.
The grounding terminal should be grounded independently and not connected in series with the grounding terminals of other equipment. The cross-sectional area of the grounding wire should not be less than 2mm².
The auxiliary power supply has low power and can only power low-power devices (such as photoelectric sensors);
Some PLCs have a certain number of unused address terminals (i.e., empty address terminals), do not connect the wires to these terminals;
When there is no protection in the PLC output circuit, it is advisable to use a fuse or other protective device in series in the external circuit to prevent damage caused by a short circuit in the load.
