In the daily application of PLCs, practical tips can greatly assist in their effective utilization. Let's delve into some:
- Grounding Issues
The grounding requirements for PLC systems are quite stringent. Ideally, there should be a dedicated ground system, and attention should be paid to ensuring reliable grounding for other devices associated with the PLC.
When multiple ground points are connected, unexpected currents may arise, leading to logic errors or circuit damage.
The reason for different ground potentials typically stems from ground points being physically separated in different areas. When distant devices are connected via communication cables or sensors, current can flow through the entire circuit between the cable and ground, even over short distances. Load currents from large equipment can also cause variations between their potential and ground, or unpredictable currents may be directly induced through electromagnetic effects.
Incorrect grounding points between power sources in the circuit may lead to destructive currents, resulting in equipment damage.
PLC systems generally employ single-point grounding. To enhance immunity to common-mode interference, shielded floating ground technology can be applied to analog signals, where the shield layer of the signal cable is grounded at one point, and the signal loop is floating, with the insulation resistance to ground not less than 50 MΩ.
- Interference Handling
The industrial environment often harbors various high and low-frequency interferences. Typically, these interferences are introduced into the PLC through cables connected to on-site equipment.
In addition to grounding measures, it's important to implement some anti-interference measures during cable selection and laying:
Analog signals, being weak signals, are highly susceptible to external interference and should be transmitted through double-layer shielded cables.
High-speed pulse signals (such as from pulse sensors, encoders, etc.) should be transmitted via shielded cables to prevent external interference and interference of high-speed pulse signals with low-level signals.
Communication cables between PLCs operate at high frequencies. Generally, cables provided by the manufacturer should be used. In less critical situations, shielded twisted-pair cables can be employed.
Analog signal lines and DC signal lines should not be routed in the same cable duct as AC signal lines.
Shielded cables introduced into and out of control cabinets must be grounded and should be connected directly to devices 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.
When dealing with interference during on-site maintenance, methods include using shielded cables for affected lines and reinstallation, as well as incorporating anti-interference filtering code into the program.
- Eliminating Inter-Wire Capacitance to Avoid Misoperation
There exists capacitance between the conductors of cables, and qualified cables can limit this capacitance within a certain range.
Even with qualified cables, when the cable length exceeds a certain limit, the capacitance between the conductors may surpass the required value. When such a cable is used for PLC inputs, inter-wire capacitance can potentially cause misoperations in the PLC, leading to various inexplicable phenomena.
These phenomena mainly manifest as follows: correct wiring connections are made, but the PLC does not register inputs; inputs expected by the PLC are missing, while unexpected inputs are present, indicating mutual interference among PLC inputs. To address this issue, the following steps should be taken:
- Use cables where the cores are twisted together.
- Minimize the length of cables used.
- Separate cables for inputs that may interfere with each other.
- Employ shielded cables.
- Selection of Output Modules
Output modules are classified into transistor, bidirectional thyristor, and relay types:
Transistor-type switches have the fastest switching speed (typically 0.2ms), but the lowest load capacity, approximately 0.2~0.3A at 24VDC. They are suitable for rapid switching and signal connection devices, often linked with frequency converters, DC devices, etc. Attention should be paid to the leakage current of transistors and its impact on the load.
The advantage of thyristor-type modules is their absence of contact and their capability to handle AC loads, although their load capacity is not high.
Relay outputs exhibit characteristics suitable for both AC and DC loads, with high load capacity. In conventional control applications, relay contact-type outputs are typically chosen initially. However, their disadvantage lies in their slower switching speed, usually around 10ms, making them unsuitable for high-frequency switching applications.
5.Handling Overvoltage and Overcurrent in Frequency Converters
Overvoltage Management: When reducing the speed of a motor, it enters a regenerative braking state, feeding back energy to the frequency converter. This regenerative energy increases the voltage on the capacitors of the filtering circuit, quickly reaching the set threshold for DC overvoltage protection, causing the frequency converter to trip.
Solution: Implement an external braking resistor connected to the frequency converter to dissipate the regenerative energy from the motor back to the DC side.
Overcurrent Management: In systems where a frequency converter drives multiple small motors, if one of these motors experiences an overcurrent fault, it triggers an overcurrent alarm in the frequency converter, leading to a trip and consequently stopping the operation of other normally functioning motors.
Solution: Install a 1:1 isolation transformer on the output side of the frequency converter. When one or several small motors experience an overcurrent fault, the fault current impacts the transformer instead of directly affecting the frequency converter, thus preventing the frequency converter from tripping. After experimental validation, this solution effectively prevented the occurrence of faults where normally functioning motors were also stopped.
6.Labeling Inputs and Outputs for Easy Maintenance
In a PLC-controlled complex system, what you see resembles the pins of an integrated circuit, with input and output relay terminals arranged in two rows, interspersed with corresponding indicator lights and PLC identifiers. Without referring to the electrical schematics, troubleshooting a faulty device becomes challenging and time-consuming. To address this, we've created a table based on the electrical schematics, displayed on the control panel or cabinet of the equipment. This table indicates the correspondence between each PLC input/output terminal and its electrical symbol, Chinese name, akin to the functionality description of various pins on an integrated circuit.
With this input/output table, electricians familiar with the operational processes or ladder diagrams of the equipment can proceed with troubleshooting. However, for those less familiar with operational processes and unable to read ladder diagrams, a separate table is needed: the PLC input/output logic function table. This table provides practical insights into the logical relationships between input circuits (triggering components, associated elements) and output circuits (executing components) in most operational processes.
Experience has shown that with proficiency in using the input/output correlation table and the input/output logic function table, troubleshooting electrical faults becomes manageable even without drawings.
- Fault Inference through Program Logic
In today's industrial landscape, there's a wide variety of PLC types in use. For lower-end PLCs, ladder logic instructions are largely similar, while for mid-to-high-end models like the S7-300, many programs are coded using language tables.
Practical ladder diagrams must be annotated with Chinese symbols; otherwise, they are challenging to interpret. Having a general understanding of the equipment process or operational procedures before examining the ladder diagram can make it easier to comprehend.
For electrical fault analysis, the common approach is the application of a backward analysis method, also known as the reverse inference method. This involves using the input/output correlation table to trace back from the fault point to the corresponding PLC output relay and then initiating a reverse search for the logical relationships that trigger its operation.
Experience has shown that identifying one problem usually leads to resolving the fault, as instances where multiple faults occur simultaneously are relatively rare in equipment systems.
- PLC Self-Fault Diagnosis
Generally speaking, PLCs are highly reliable devices with a very low failure rate. The probability of hardware damage to PLCs, CPUs, and other components, as well as software errors, is almost negligible. PLC input points are unlikely to be damaged unless caused by high-voltage intrusion, and the normally open contacts of PLC output relays have a long lifespan unless subjected to external factors like peripheral load short circuits or improper design leading to current exceeding the rated range.
Therefore, when searching for electrical fault points, the focus should be on the peripheral electrical components of the PLC. It's essential not to always suspect PLC hardware or software issues, as this is crucial for quickly repairing faulty equipment and restoring production promptly.
Thus, the electrical fault diagnosis of PLC control circuits discussed here focuses not on the PLC itself but on the peripheral electrical components within the circuits controlled by the PLC.
9.Fully Utilizing Software and Hardware Resources
- Instructions that are not involved in control loops or have been activated before the loop can be left disconnected from the PLC.
- When multiple instructions control a task, they can be connected externally in parallel before being connected to an input point on the PLC.
- Make the most of PLC internal function blocks, utilize intermediate states fully to ensure program integrity and coherence, making it easier to develop. This approach also reduces hardware investment, lowering costs.
- When conditions allow, it is preferable to have each output independently controlled for easier control and inspection, as well as to protect other output circuits. If a single output point fails, it will only affect the corresponding output circuit.
- For outputs controlling loads in both positive and negative directions, interlocking should be implemented internally within the PLC program and externally to prevent the load from moving in both directions.
- For emergency stops of the PLC, external switches should be used to cut off power to ensure safety.
10.Other Considerations:
- Avoid connecting AC power lines to input terminals to prevent damaging the PLC.
- Ground terminals should be individually grounded and not connected in series with grounding terminals of other devices. The cross-sectional area of the grounding wire should be no less than 2mm².
- Auxiliary power supplies have limited power and can only drive small devices (such as photoelectric sensors).
- Some PLCs have a certain number of reserved points (i.e., empty address terminals). Avoid connecting wires to these points.
- When PLC output circuits lack protection, external circuit protection devices such as fuses should be used in series to prevent damage from load short circuits.