There are a wide variety of PLC products. The different models of PLC correspond to various structural forms, performance, capacity, instruction systems, programming methods, and prices, and they are also suitable for different application scenarios. Therefore, choosing the appropriate PLC is of great significance for improving the technical and economic indicators of the PLC control system. [5]
PLC Model
The selection of PLC should mainly be based on comprehensive consideration of factors such as PLC model, capacity, I/O module, power module, special function module, communication networking capability, etc. The basic principle for selecting PLC model is to strive for the best performance-price ratio while meeting functional requirements and ensuring reliability and ease of maintenance. When choosing, the main factors to consider include reasonable structural type, installation method, corresponding functional requirements, response speed requirements, system reliability requirements, and the uniformity of model, etc. [5]
Structural types
PLC mainly has two structural types: integrated type and modular type. [5]
The average price of each I/O point in an integrated PLC is lower than that of a modular PLC, and it is relatively smaller in size. It is generally used in small control systems where the process of the system is relatively fixed. On the other hand, modular PLC offers flexible and convenient functionality expansion. It has a wide range of options in terms of I/O points, the ratio of input points to output points, and types of I/O modules, and is also easy to maintain. It is generally used in more complex control systems. [5]
Installation method
The installation methods of PLC systems can be categorized as centralized, remote I/O type, and distributed in a networked manner involving multiple PLCs. [5]
Centralized mode does not require setting up drivers for remote I/O hardware, and it has fast system response and low cost. Remote I/O mode is suitable for large systems where the devices are widely distributed. Remote I/O can be installed separately near the field devices, with short connections, but additional drivers and remote I/O power supplies are needed. Distributed mode with multiple PLCs connected is suitable for situations where multiple devices need to be independently controlled while also being interconnected. Small PLCs can be selected, but a communication module must be added. [5]
Functional Requirements
Most small-sized (low-end) PLCs have functions such as logical operations, timing, and counting. They can meet the control requirements for equipment that only requires switch quantity control. [5]
For systems that mainly use on-off control with a small amount of analog control, an enhanced low-end PLC with A/D and D/A conversion units, as well as addition and subtraction arithmetic operations and data transmission functions, can be selected. For systems with more complex control requirements, such as implementing PID calculations, closed-loop control, and communication networking functions, depending on the size and complexity of the control, medium or high-end PLCs can be chosen. However, medium and high-end PLCs are more expensive and are generally used in large-scale process control and distributed control systems and other scenarios. [5]
Response speed
PLC is a general-purpose controller designed for industrial automation. The response speed of different grades of PLCs generally can meet the requirements within their application scope. If PLCs are to be used across different ranges, or if certain functions or signals have special speed requirements, then the response speed of the PLC should be carefully considered. One can choose PLCs with high-speed I/O processing capabilities, or PLCs with fast response modules and interrupt input modules, etc. [5]
Reliability
The reliability of general system PLCs can all meet the requirements. For systems with extremely high reliability demands, it is necessary to consider whether to adopt a redundant system or a hot standby system. [5]
Uniformize the model of the equipment
For an enterprise, it is advisable to ensure the uniformity of PLC models. The main considerations include the following three aspects: [5]
1) Uniform model allows the modules to serve as backups for each other, facilitating the procurement and management of spare parts. [5]
2) The models are uniform, and their functions and usage methods are similar, which is conducive to the training of technical personnel and the improvement of technical proficiency. [5]
3) The models are uniform, their external devices are common, resources can be shared, it is easy to connect and communicate, and when equipped with a host computer, it is easy to form a multi-level distributed control system.