With the development of PLC technology, the variety of PLC products is also increasing. Different models of PLCs vary in structure, performance, capacity, instruction set, programming method, price, and other aspects, and are suitable for different applications. Therefore, the rational selection of a PLC is of great significance for improving the technical and economic indicators of a PLC control system.
The selection of a PLC should be based on a comprehensive consideration of factors such as the PLC model, capacity, I/O modules, power supply modules, special function modules, and communication networking capabilities.
PLC Model Selection
The basic principle for selecting a PLC model is to strive for the best performance-price ratio while meeting functional requirements, ensuring reliability, and facilitating maintenance. The following points should be considered when making a selection:
1. The structural form of reason
PLCs mainly come in two structural types: integrated and modular.
The average price of each I/O point of an integrated PLC is cheaper than that of a modular PLC, and its size is relatively smaller. It is generally used in small control systems with relatively fixed system processes. On the other hand, modular PLCs offer flexible and convenient function expansion, with a wide range of choices in terms of the number of I/O points, the ratio of input points to output points, and the types of I/O modules. They are also easy to maintain and are generally used in more complex control systems.
2. Installation method selection
PLC systems can be installed in three ways: centralized, remote I/O, and distributed (multiple PLCs networked together).
Centralized systems do not require the installation of remote I/O hardware, resulting in fast system response and low cost. Remote I/O systems are suitable for large systems with a wide distribution of devices. Remote I/O can be installed near the field devices with short wiring, but additional drivers and remote I/O power supplies are required. Distributed systems with multiple PLCs networked are suitable for situations where multiple devices need to be controlled independently but also communicate with each other. Small PLCs can be selected, but a communication module must be added.
3. Corresponding functional requirements
Small (low-end) PLCs generally have functions such as logic operation, timing, and counting, which can meet the needs of equipment that only requires on/off control.
For systems that primarily use digital control with a small amount of analog control, an enhanced low-end PLC with A/D and D/A conversion units and addition/subtraction arithmetic operations and data transmission functions can be selected.
For more complex control applications requiring PID calculations, closed-loop control, and network communication, a mid-range or high-end PLC can be selected depending on the size and complexity of the control system. However, mid-range and high-end PLCs are more expensive and are generally used in large-scale process control and distributed control systems.
4. Response speed requirements
PLCs are general-purpose controllers designed for industrial automation. The response speed of different levels of PLCs can generally meet the needs of their respective applications. If a PLC needs to be used across a wider range of applications, or if certain functions or signals have special speed requirements, the response speed of the PLC should be carefully considered. In such cases, a PLC with high-speed I/O processing capabilities, or a PLC with fast response modules and interrupt input modules, can be selected.
5. System reliability requirements
For most systems, the reliability of the PLC is sufficient. For systems with very high reliability requirements, consideration should be given to whether to use a redundant system or a hot standby system.
6. Keep the model as consistent as possible.
A company should strive to standardize the types of PLCs it uses. This is mainly due to the following three considerations:
1) The models are standardized and their modules can be used as backups for each other, which facilitates the procurement and management of spare parts.
2) The models are standardized, and their functions and usage methods are similar, which is conducive to the training of technical personnel and the improvement of technical level.
3) The machine model is uniform, its external equipment is universal, resources can be shared, it is easy to network and communicate, and it is easy to form a multi-level distributed control system after being equipped with a host computer.
