Common PLC Faults and Solutions

In recent years, with the development of society, PLC (Programmable Logic Controller) has been widely used in industrial production, and the requirements for its use by technical personnel have been increasing year by year. Therefore, the requirements for the normal and stable operation of the system are also getting higher.

The Programmable Logic Controller (PLC) is an electronic device designed specifically for digital operation in industrial environments. It uses programmable memory to store instructions for executing logic operations, sequential operations, timing, counting, and arithmetic operations internally, and can control various types of machinery or production processes through digital or analog inputs and outputs. However, due to the complex environment in industrial sites, PLC fault handling is also one of the key points in instrument equipment maintenance. This article shares common PLC faults and their solutions to enhance PLC maintenance skills.

In the daily maintenance of a certain production line’s PLC control system, the electrical spare parts that are consumed the most are various types of relays or air switches. Apart from the quality of the products themselves, it is mainly due to the harsh environmental conditions on-site. For example, the contacts of contactors exposed to the production environment are prone to arcing or oxidation, gradually heating and deforming until they can no longer be used. All control boxes on this production line are selected to be of better sealing type; their internal components have a significantly longer lifespan compared to those using open-type cabinets. Therefore, to avoid such faults, it is advisable to select high-performance relays and improve the usage environment of the components to reduce the frequency of replacements and minimize the impact on system operation.

This type of equipment has larger relative displacement in its actuators; or the transmission structure is complex, and any slight misalignment in mechanical, electrical, or hydraulic aspects can lead to errors or faults. Under long-term operating conditions, if maintenance is lacking, it can easily cause phenomena such as jamming, blockage, or leaks in valve body components. Therefore, during system operation, it is necessary to strengthen the inspection of such equipment and address any issues promptly. Our factory has established a strict point inspection system for this type of equipment, regularly checking whether the valves are deformed, whether the actuators are flexible and usable, and whether the controllers are effective, thereby ensuring the overall effectiveness of the control system.

The cause may be due to long-term wear or rusting and aging from long disuse. For instance, the material handling vehicle at the kiln tail of this production line moves back and forth frequently, and there is a lot of dust on-site, causing the proximity switch contacts to deform, oxidize, or become blocked with dust, leading to poor contact or unresponsive mechanisms. The handling of faults in such equipment mainly focuses on regular maintenance to keep the equipment in good condition at all times. For limit switches, especially those on heavy equipment, in addition to regular inspections, multiple protective measures should be incorporated during the design process.

These devices include junction boxes, terminal blocks, bolts, and nuts. The causes of faults in these areas are not only related to the manufacturing process of the equipment itself but also to the installation process. For example, some believe that the tighter the connection of wires and screws, the better, but this can lead to difficulties during secondary maintenance, and excessive force during disassembly can damage the connecting parts and nearby components. Long-term arcing, rusting, etc., are also causes of faults. Based on engineering experience, these types of faults are generally difficult to detect and repair. Therefore, during the installation and maintenance of equipment, it is essential to follow the installation process requirements to avoid leaving hidden dangers for the equipment.

Such faults generally reflect as abnormal signals in the control system. When installing this type of equipment, the shielding layer of the signal line should be reliably grounded at one end and laid separately from power cables, especially the output cables of frequency converters that are highly susceptible to interference, and software filtering should be performed internally within the PLC. The detection and handling of these faults are also related to daily point inspections, and issues should be addressed promptly.

The resolution or improvement of these issues mainly relies on the experience from engineering design and observational analysis during daily maintenance.

To reduce the fault rate, one important point is to pay attention to factory processes and safety operating procedures. In daily work, it is essential to adhere to these processes and safety operating procedures and strictly implement relevant regulations, such as maintaining the environment of the central control room, etc. At the same time, management in these areas should be strengthened during production.

The process control system itself is a complete system. Therefore, when analyzing or handling faults, it is important to pay attention to the system’s integrity. Simply optimizing one part may not improve the overall performance of the system. For instance, excessively pursuing the precision of components without considering actual needs and matching with the precision of related equipment can unnecessarily increase system costs. In daily maintenance, there are also instances where the system becomes increasingly complex, such as using complicated control methods and devices to achieve controls that could have been accomplished with simpler devices, violating the principles of economy, simplicity, and practicality, which may also increase the fault rate. This is an aspect to be cautious about.

The reliability of PLC products themselves can be guaranteed, but some incorrect operations during application can have a certain impact.

Generally speaking, PLCs are extremely reliable devices with a very low failure rate. The probability of hardware damage such as the PLC’s CPU or software running errors is almost zero; PLC input points are unlikely to be damaged unless they are caused by strong electrical intrusions; the normally open points of PLC output relays have a long lifespan unless there is a short circuit in the external load or unreasonable design that exceeds the rated load current.

Therefore, when searching for electrical fault points, emphasis should be placed on the peripheral electrical components of the PLC rather than always suspecting that the PLC hardware or program has issues. This is crucial for quickly repairing faulty equipment and restoring production. Thus, the electrical fault inspection of the PLC control circuit focuses not on the PLC itself but on the peripheral electrical components in the control circuit that the PLC controls.

The output modules can be classified into transistor, bidirectional thyristor, and contact types. The transistor type has the fastest switching speed (generally 0.2ms) but the smallest load capacity, about 0.2-0.3A, 24VDC, and is suitable for fast switching and signal contact devices, generally connected with variable frequency and DC devices. Care should be taken regarding the impact of transistor leakage current on the load. The thyristor type has the advantage of being contactless and has AC load characteristics, although the load capacity is not large.

Relay outputs have characteristics for both AC and DC loads with a larger load capacity. In conventional control, it is generally advisable to first select relay contact-type outputs, but the drawback is that the switching speed is slow, generally around 10ms, making it unsuitable for high-frequency switching applications.

The grounding requirements for PLC systems are quite strict, and a dedicated grounding system should be established. Additionally, attention should be paid to ensure that other devices related to the PLC are also reliably grounded. When multiple circuit grounding points are connected together, unexpected currents may arise, leading to logical errors or damage to the circuit. The reasons for different grounding potentials usually stem from grounding points being too physically distant. When devices that are far apart are connected via communication cables or sensors, the current between the cables and the ground can flow through the entire circuit; even over short distances, the load current of large devices can cause variations in potential between them and the ground, or generate unpredictable currents through electromagnetic effects. Between incorrect grounding points, destructive currents may arise in the circuit, potentially damaging equipment.

It is generally advisable to use a single-point grounding method for PLC systems. To enhance the ability to resist common-mode interference, it is recommended to use shielded floating ground technology for analog signals, meaning that the shielding layer of the signal cable is grounded at one point while the signal circuit floats, with an insulation resistance to ground of not less than 50MΩ.

There is capacitance between the conductors of cables, and qualified cables can limit this capacitance value within a certain range. Even with qualified cables, when the length exceeds a certain limit, the capacitance value between the wires may exceed the required value. When such a cable is used for PLC inputs, inter-line capacitance may lead to PLC malfunctions, resulting in many incomprehensible phenomena. These phenomena mainly manifest as: connections are correct, but the PLC does not register inputs; inputs that should be present are absent, while inputs that should not be present are there, indicating mutual interference among PLC inputs.

To solve this issue, the following should be done:

Use twisted pairs of cables;

Minimize the length of the cables used;

Separate cables of interfering inputs;

Use shielded cables.

The industrial environment is often harsh, with many high and low-frequency interferences. These interferences are generally introduced into the PLC through cables connected to on-site devices. In addition to grounding measures, attention should be paid to taking some anti-interference measures during the design, selection, and installation of cables:

Analog signals are small signals and are easily affected by external interference; therefore, double-shielded cables should be used;

High-speed pulse signals (such as pulse sensors, counting disks, etc.) should also use shielded cables to prevent external interference and to protect low-level signals from interference from high-speed pulse signals;

For communication cables between PLCs, which have higher frequencies, it is generally advisable to use cables provided by manufacturers; if requirements are not high, shielded twisted pair cables can be used;

Analog signal lines and DC signal lines should not be run in the same cable tray as AC signal lines;

Shielded cables introduced and exited from the control cabinet must be grounded and should connect directly to the equipment without passing through terminal blocks;

AC signals, DC signals, and analog signals should not share the same cable; power cables should be laid separately from signal cables.

During on-site maintenance, solutions to interference include: using shielded cables for disturbed lines, and re-laying cables; adding anti-interference filtering code in the program.

The PLC controls a complex system, and what can be seen are two rows of staggered input and output relay terminal blocks, corresponding indicator lights, and PLC numbers, resembling an integrated circuit with dozens of pins. If someone does not refer to the schematic to troubleshoot the faulty equipment, they will be at a loss, and the speed of fault finding will be particularly slow. In light of this situation, it is advisable to create a table based on the electrical schematic and place it on the control panel or cabinet, indicating the number of each PLC input/output terminal corresponding to the electrical symbols and their Chinese names, similar to the functional descriptions of the pins of an integrated circuit.

With this input/output table, electricians familiar with the operation process or the ladder diagram of the equipment can carry out repairs. However, for those who are not familiar with the operation process and cannot read the ladder diagram, a logical function table for PLC input/output should also be created. This table actually explains the logical correspondence between the input circuits (triggering components, associated components) and the output circuits (executing components) during most operational processes. Practical experience shows that if you can skillfully use the input/output correspondence table and the input/output logical function table, you can troubleshoot electrical faults without needing a schematic.

Currently, a wide variety of PLCs are commonly used in the industry. For low-end PLCs, ladder diagram instructions are quite similar, while for mid to high-end models, such as S7-300, many programs are written in instruction lists. Practical ladder diagrams must have Chinese symbol annotations; otherwise, they are difficult to read. If one can roughly understand the equipment process or operation before looking at the ladder diagram, it will be relatively easy to comprehend.

When conducting electrical fault analysis, the reverse search method or reverse deduction method is generally applied, which involves finding the corresponding PLC output relay from the fault point based on the input/output correspondence table, and then starting to backtrack the logical relationships that satisfy its operation. Experience shows that if one problem is found, the fault is mostly eliminated, as it is rare for equipment to have two or more fault points occurring simultaneously.

Instructions that do not participate in the control loop or have already been input into the PLC can be excluded from the PLC;

When multiple instructions control a task, it is advisable to first parallel them externally before connecting them to one input point;

Make full use of the internal functional soft components of the PLC, fully utilize intermediate states, so that the program has complete coherence and is easy to develop. This also reduces hardware investment and lowers costs;

Where conditions permit, it is advisable to isolate each output to facilitate control and inspection, also protecting other output circuits; when one output point fails, only the corresponding output circuit will lose control;

If the output is a load with forward/reverse control, not only should interlocks be implemented in the PLC’s internal program, but also measures should be taken externally to prevent the load from acting in both directions;

The emergency stop of the PLC should use an external switch to cut off for safety.

Do not connect AC power lines to input terminals to avoid damaging the PLC;

Ground terminals should be independently grounded and not connected in series with other device grounds; the grounding wire’s cross-sectional area should not be less than 2mm²;

Auxiliary power supplies have relatively low power and can only drive low-power devices (such as photoelectric sensors);

Some PLCs have a limited number of occupied points (i.e., empty address terminals); do not connect wires to these;

When there is no protection in the PLC output circuit, it is advisable to use fuses or other protective devices in the external circuit to prevent damage from load short circuits.