Have you ever encountered a power board failure? Have you ever experienced output or input failures? Have you ever had a program lost?
Perhaps in your more than ten years of industrial control system maintenance, you've only encountered a PLC output point failure once, and haven't had any other PLC hardware problems yet! Perhaps you've encountered various problems with electronic boards after welding. Perhaps the programming port of your Siemens 200 series has burned out countless times, the output relays are stuck together, the transistor outputs are short-circuited, and so on.
Are you struggling with these issues? Below, TianTuo shares nine important points to note when using PLCs.
It's said that following these nine principles for using PLCs can effectively prevent and save you from making mistakes!
Nine things to note when using a PLC:
I. Troubleshooting PLC Faults
Generally speaking, PLCs are extremely reliable devices with a very low failure rate. The probability of PLC hardware such as the CPU being damaged or software malfunctioning is almost zero; PLC input points will almost never be damaged unless caused by a high-voltage intrusion; and the normally open contacts of PLC output relays have a very long lifespan unless there is a short circuit in the external load or an unreasonable design, or the load current exceeds 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.
II. Selection of Input/Output (I/O) Modules
Output modules are categorized into transistor, bidirectional thyristor, and contact type.
Transistor-type transistors have the fastest switching speed (typically 0.2ms), but the smallest load capacity, approximately 0.2~0.3A, 24VDC. They are suitable for equipment requiring fast switching and signal communication, and are generally connected to frequency converters, DC devices, etc. Attention should be paid to the impact of transistor leakage current on the load.
The advantages of thyristor-type circuits are that they are contactless, have AC load characteristics, but have a limited load capacity.
Relay outputs have AC/DC load characteristics and a large load capacity. In conventional control, relay contact type outputs are generally the first choice. The disadvantage is that the switching speed is slow, generally around 10ms, which is not suitable for high-frequency switching applications.
III. Grounding Issues
PLC systems have strict grounding requirements and ideally should have a dedicated, independent grounding system. It's also important to ensure that other devices related to the PLC are reliably grounded. Connecting multiple grounding points together can generate unexpected currents, leading to logic errors or circuit damage.
The reason for different grounding potentials is usually that the grounding points are separated too far apart in the physical area. When devices that are far apart are connected by communication cables or sensors, the current between the cable and the ground will flow through the entire circuit. Even over a short distance, the load current of large equipment can change between its potential and the ground, or generate unpredictable current directly through electromagnetic effects.
Between power sources with incorrect grounding, a devastating current can be generated in the circuit, potentially damaging the equipment.
PLC systems generally use a single-point grounding method. To improve common-mode interference immunity, shielded floating ground technology can be used for analog signals, that is, the shielding layer of the signal cable is grounded at a single point, the signal loop is floating, and the insulation resistance to the ground should not be less than 50MΩ.
IV. Eliminate inter-line capacitance to avoid malfunctions
There is capacitance between each conductor in a cable, and a qualified cable can limit this capacitance within 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 as a PLC input, the capacitance between the conductors may cause malfunctions in the PLC, resulting in many inexplicable phenomena.
These phenomena mainly manifest as follows: the wiring is correct, but the PLC has no input; some inputs that should be present are missing, while others are present, indicating mutual interference between PLC inputs. 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.
V. 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, several anti-interference measures should be taken during cable design, selection, and installation.
1. Analog signals are small signals and are easily affected by external interference, so double-shielded cables should be used; high-speed pulse signals (such as pulse sensors, counting codes, etc.) should use shielded cables to prevent external interference and to prevent high-speed pulse signals from interfering with low-level signals.
2. 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.
3. Analog signal lines and DC signal lines must not be routed in the same cable tray as AC signal lines;
4. Shielded cables leading into and out of the control cabinet must be grounded and should be connected directly to the equipment without passing through terminals;
5. AC signals, DC signals, and analog signals cannot share a single cable; power cables should be laid separately from signal cables.
6. 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.
VI. 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 who tries to troubleshoot a faulty device without consulting the schematic will be at a loss, and the troubleshooting process will be extremely slow.
Given this situation, we draw a table based on the electrical schematic and attach it to the control console or control cabinet of the equipment, indicating the electrical symbol and Chinese name corresponding to each PLC input/output terminal number, which is similar to the function description of each pin of an integrated circuit.
With this input/output table, electricians who understand the operation process or are familiar with the ladder diagram of this equipment can begin maintenance. However, for electricians who are unfamiliar with the operation process and cannot read ladder diagrams, it is necessary to draw another table: 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 are proficient in using input/output mapping tables and input/output logic function tables, you can easily troubleshoot electrical faults without drawings.
VII. 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 rational use of software and hardware resources
1. 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 task, they can be connected in parallel outside the PLC before being connected to a single input point.
2. 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.
3. Ideally, each output should be independent if conditions permit, which facilitates control and inspection and protects other output circuits; when one output point fails, it will only cause the corresponding output circuit to lose control.
4. 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.
5. An external switch should be used to cut off the PLC emergency stop to ensure safety.
IX. Other Precautions
1. Do not connect the AC power cord to the input terminal to avoid burning out the PLC; the grounding terminal should be grounded independently and not connected in series with the grounding terminal of other equipment, and the cross-sectional area of the grounding wire should not be less than 2mm2;
2. The auxiliary power supply has low power and can only drive low-power devices (such as photoelectric sensors).
3. Some PLCs have a certain number of unused address terminals (i.e., empty address terminals), do not connect the wires to these terminals;
4. When there is no protection in the PLC output circuit, a fuse or other protective device should be connected in series in the external circuit to prevent damage caused by a short circuit in the load.
