10 Essential PLC Practical Tips
10 Essential PLC Practical Tips
In daily PLC applications, mastering these practical tips can enhance your efficiency and effectiveness. Here are ten key techniques to keep in mind:
1. Grounding Issues
PLC systems have stringent grounding requirements. An independent, dedicated grounding system is recommended, and all related equipment should be properly grounded. Connecting multiple circuit grounding points can cause unexpected currents, leading to logic errors or circuit damage. This often occurs when grounding points are physically separated and connected via communication cables or sensors.PLC systems typically use single - point grounding. To enhance common - mode interference resistance, shielded floating ground technology can be used for analog signals. This involves single - point grounding of the signal cable shield and floating the signal loop, with an insulation resistance from ground of no less than 50MΩ.
2. Dealing with Interference
Industrial environments are prone to high - and low - frequency interference, often introduced through cables connected to on - site equipment. In addition to proper grounding, the following anti - interference measures should be taken in cable design, selection, and installation:
For analog signals, use double - shielded cables.
For high - speed pulse signals, use shielded cables to prevent external interference and to avoid interference with low - level signals.
For PLC communication cables, manufacturer - provided cables are recommended. In less critical applications, shielded twisted - pair cables can be used.
Do not route analog signal lines, DC signal lines, and AC signal lines in the same conduit.
Shielded cables entering or exiting control cabinets must be directly grounded to equipment without passing through terminals.
AC signals, DC signals, and analog signals should not share the same cable. Power cables should be routed separately from signal cables.
To address on - site interference, use shielded cables for affected lines and reinstall them. Alternatively, add anti - interference filtering code to the program.
3. Eliminating Line - to - Line Capacitance to Prevent Misoperation
Capacitance exists between the conductors of any cable. Even qualified cables have a certain capacitance range. However, when cable length exceeds recommended limits, line - to - line capacitance can cause PLC misoperations. This may result in inexplicable phenomena, such as correct wiring but no PLC input response, or PLC inputs interfering with each other. To resolve this:
Use cables with twisted cores.
Minimize cable length.
Separate interfering inputs with dedicated cables.
Use shielded cables.
4. Selecting Output Modules
Output modules are available in transistor, triac, and relay types:
Transistor - type modules offer the fastest switching speed (typically 0.2 ms) but have the lowest load capacity (0.2 - 0.3 A, 24 VDC). They are suitable for fast - switching and signal - related devices and are commonly used with frequency converters and DC devices. Note the impact of transistor leakage current on loads.
Triac - type modules are contact - less and suitable for AC loads but have limited load capacity.
Relay - type modules support AC and DC loads and have high load capacity. They are typically the first choice for conventional control but have a slower switching speed (around 10 ms), making them unsuitable for high - frequency applications.
5. Handling Inverter Over - Voltage and Over - Current
When reducing the speed by lowering the set value, the motor may enter regenerative braking mode. The energy fed back to the inverter raises the voltage across the filter capacitor, potentially triggering over - voltage protection. To address this, add an external braking resistor to dissipate the regenerative energy.
When an inverter drives multiple small motors, an over - current fault in one motor can cause the inverter to trip, stopping all connected motors. To prevent this, install a 1:1 isolation transformer on the inverter's output side. This ensures fault currents are confined to the transformer, protecting the inverter from tripping.
6. Labeling Inputs and Outputs for Easy Maintenance
PLCs control complex systems with numerous input and output relay terminals, indicator lights, and PLC numbering. To simplify troubleshooting:
Create a table based on the electrical schematic and place it on the equipment control panel or cabinet. List each PLC input and output terminal number along with corresponding electrical symbols and Chinese names.
For those unfamiliar with the operation process or ladder diagrams, develop a PLC input - output logic function table. This table outlines the logical relationships between input and output circuits during operation.
7. Fault Diagnosis Using Program Logic
With the wide variety of PLCs available, ladder - diagram instructions for low - end PLCs are generally similar. For high - end PLCs like the S7 - 300, many programs are written in structured text. Practical ladder diagrams should include Chinese symbol annotations for easier understanding. When analyzing electrical faults, the reverse - lookup method is commonly used. Starting from the fault point, locate the corresponding PLC output relay and trace back the logical relationships required for its activation. Experience shows that identifying one issue typically resolves the fault, as multiple simultaneous faults are rare.
8. Judging PLC Faults
PLCs are highly reliable with a low failure rate. Hardware failures such as PLC or CPU damage, or software errors, are almost non - existent. PLC input points are unlikely to fail unless subjected to high - voltage interference. Similarly, PLC output relay contacts have a long lifespan unless overloaded due to peripheral load short circuits or design flaws. When troubleshooting electrical faults, focus on peripheral electrical components rather than suspecting PLC hardware or software issues. This approach is crucial for quick repairs and minimizing production downtime.
9. Making Full Use of Software and Hardware Resources
Commands not involved in the control loop or activated before the loop can be excluded from the PLC.
When multiple commands control a single task, they can be connected in parallel externally before being linked to an input point.
Utilize internal soft components of the PLC and intermediate states to ensure program integrity and continuity, making development easier and reducing hardware costs.
Where possible, keep each output separate for easier control and inspection, and to protect other output circuits. A fault in one output point will only affect the corresponding output circuit.
For outputs controlling bidirectional loads, implement interlocking both in the PLC program and externally to prevent bidirectional load movement.
Emergency stops for PLCs should use external switches to ensure safety.
10. Other Precautions
Never connect AC power lines to PLC input terminals to avoid damaging the PLC.
Grounding terminals should be independently grounded, not connected in series with other equipment. The grounding wire should have a cross - sectional area of no less than 2mm².
Auxiliary power supplies have limited capacity and can only power low - power devices like photoelectric sensors.
Some PLCs have a certain number of unused address terminals. Do not connect wires to these.
If there is no protective device in the PLC output circuit, include fuses or other protective devices in the external circuit to prevent load short - circuits from damaging the system.