In today's industrial production environment, a multitude of digital and analog control devices are utilized, ranging from motor control (start/stop), solenoid valve activation/deactivation, product counting, to temperature, pressure, and flow settings. Programmable Logic Controllers (PLCs) have become one of the most effective solutions for automating these processes. Here are some key considerations when designing a PLC control system:
Firstly, choosing the right PLC and programmer is crucial. There are numerous PLC brands available in the market, both domestic and international, including Japanese brands like OMRON, Mitsubishi, Fuji, IDEC, Hitachi, and Panasonic, as well as German brands like Siemens, and South Korean brands like LG. Selecting the appropriate PLC involves several factors:
1. **System Requirements**: Determine if the system will use a standalone PLC or a networked setup. This decision will help calculate the necessary Input/Output (I/O) points. Always add a buffer of around 10% over the actual required points when purchasing.
2. **Load Type**: Depending on whether the load at the PLC output is AC or DC, large or small current, and the frequency of the output points, decide whether to use relay outputs, transistor outputs, or thyristor outputs. Choosing the correct output method ensures stable system performance.
3. **Execution Speed**: The storage capacity and execution speed of commands are critical in selecting a PLC. Higher capacity and faster execution come at a higher cost, and while foreign brands may offer similar features, there are subtle differences.
4. **"COM" Points**: Different PLCs have varying numbers of "COM" points. Some have fewer output points per COM point, while others may have more. For systems with multiple load types and higher current demands, using a COM point with fewer output points might be preferable. Conversely, for smaller loads, a COM point with more output points is ideal.
5. **Product Compatibility**: Since each manufacturer’s development software differs, system compatibility is vital. While fully compatible products are yet to be discovered, it's essential to select PLCs based on the overall system requirements.
6. **Programmer Selection**: There are various ways to program a PLC:
- **Handheld Programmers**: These allow programming using statement tables specific to the manufacturer. They are compact, low-cost, and useful for small-scale systems.
- **Graphical Programmers**: These provide a visual interface, making programming straightforward and intuitive. However, they tend to be pricier.
- **IBM Compatible PCs with Software Packages**: This is the most efficient method, commonly used but often costly.
7. **Large Company Products**: Opting for products from reputable brands ensures quality, robust technical support, and reliable after-sales services, facilitating future upgrades.
Secondly, designing the input and output circuits is equally important:
1. **Power Circuit**: PLCs typically operate on AC85-240V (or DC24V). To mitigate interference, incorporate power purification components like filters and isolation transformers.
2. **DC24V Power Supply**: Most PLCs come with a built-in DC24V power supply, though its capacity is limited. Be mindful of this capacity and implement short-circuit protection.
3. **External DC24V Power Supply**: If the input circuit requires an external DC24V power source (e.g., from proximity switches or photoelectric sensors), ensure it doesn't overload the PLC's internal power supply. Avoid connecting the external power supply's "COM" terminal to the PLC's internal power supply.
4. **Input Sensitivity**: Each PLC brand specifies input voltage and current limits. Exceeding these can lead to malfunctions and reduced sensitivity. Weak current inputs and protective measures against leakage currents are recommended.
5. **Inductive Load Handling**: When dealing with inductive loads, connect a snubber circuit or a diode in parallel with the load, ensuring the diode’s cathode connects to the positive voltage side.
6. **External Interlocks and Grounding**: For motor control in forward/reverse directions, external interlock circuits should be established to prevent accidents. The GR terminal is a grounding point, and a dedicated grounding wire should be used. If noise levels are high, short the LG and GR terminals.
7. **PLC External Drive Circuits**: If the PLC cannot directly drive the load, use an external driver circuit, such as a solid-state relay or thyristor circuit. Protective and surge-absorbing circuits should also be included.
Thirdly, the selection of expansion modules is essential for larger systems:
For systems under 80 points, expansion may not be necessary. However, for larger systems, expansion modules are required. Manufacturers impose limits on the total system points and the number of expansion modules. If expansion still falls short, consider adopting a network structure. Additionally, some manufacturers’ instructions may not support expansion modules, so software programming should be carefully considered. Analog modules, like temperature sensors, also come with specific guidelines from each manufacturer.
Fourthly, when designing a PLC network:
Designing a networked PLC system is more complex than standalone control. Familiarity with the chosen PLC model, its instructions, execution speed, and memory capacity is crucial. Communication interfaces, protocols, and data transfer speeds must also be evaluated. Seeking detailed technical support from PLC manufacturers is advisable, and the number of workstations depends on the system size.
Lastly, software preparation:
Before programming, thoroughly study the PLC manual. If using a graphical programmer or software package, direct programming is possible. With handheld programmers, drawing a ladder diagram first minimizes errors and enhances efficiency. After completing the program, test it in a simulated environment before deploying it on the actual equipment.
By adhering to these guidelines, a robust and efficient PLC control system can be developed, ensuring seamless integration into modern industrial operations.
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