The choice of PLC should be considered from the aspects of PLC model, capacity, I/O module, power module, special function module, communication networking capability and so on.
PLC model selection
The basic principle of PLC model selection is to strive for the best performance-price ratio under the premise of satisfying functional requirements and ensuring reliability and convenient maintenance. The main points to consider when choosing:
(1) Reasonable structure type
PLC mainly has two types of structure: integral type and modular type.
The average price of each I/O point of the integrated PLC is cheaper than that of the modular type, and the volume is relatively small. It is generally used in a small control system with a fixed system process; and the function of the modular PLC is flexible and convenient. The choice of I/O points, the ratio of the number of input points to the number of output points, and the type of I/O modules are large, and the maintenance is convenient, generally in a more complicated control system.
(2) Selection of installation methods
The installation method of PLC system is divided into centralized, remote I/O type and distributed of multiple PLCs.
Centralized type does not need to be set to drive remote I/O hardware, system response is fast and low cost; remote I/O type is suitable for large-scale systems, and the system has a wide range of devices. Remote I/O can be installed in the vicinity of field devices. The line is short, but the driver and remote I/O power supply need to be added; the distributed of multiple PLCs is suitable for multiple devices to be independently controlled, and when it is necessary to connect with each other, a small PLC can be selected, but a communication module must be added.
(3) Corresponding functional requirements
Generally, the small (low-end) PLC has functions such as logic operation, timing, counting, etc., and can be satisfied for devices that only require switching control.
For systems with switching control and a small amount of analog control, an enhanced low-end PLC with A/D and D/A conversion units, plus arithmetic and data transfer functions can be selected.
For the control is more complex, it is required to realize the functions of PID calculation, closed-loop control, communication networking, etc., visual control scale and complexity, and select mid-range or high-grade PLC. However, medium and high-end PLCs are more expensive and are generally used in large-scale process control and distributed control systems.
(4) Response speed requirements
PLC is a universal controller designed for industrial automation. The response speed of PLCs of different grades can generally meet the needs of its application range. If you want to use the PLC across the range, or if certain functions or signals have special speed requirements, you should carefully consider the response speed of the PLC. You can use a PLC with high-speed I/O processing function, or choose a module with fast response and interrupt input. Module PLC, etc.
(5) System reliability requirements
The reliability of the general system PLC can be satisfied. For systems with high reliability requirements, consider whether to use redundant systems or hot standby systems.
(6) Models should be unified as much as possible
A company should try to achieve the uniformity of the PLC model. The main considerations are the following three aspects:
1) The models are unified, and the modules can be used interchangeably to facilitate the procurement and management of spare parts.
2) The unit is unified, its function and usage are similar, which is conducive to the training of technical strength and the improvement of technical level.
3) The model is unified, its external equipment is common, the resources can be shared, and it is easy to network communication. It is easy to form a multi-level distributed control system with the upper computer.
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