The selection of PLC should be mainly considered from the PLC model, capacity, I/O module, power module, special function module, communication networking ability and other aspects. The basic principle of PLC model selection is to meet the functional requirements and ensure reliability and convenient maintenance, and strive for the best performance-price ratio. The selection should mainly consider reasonable structural type, installation method selection, corresponding functional requirements, response speed requirements, system reliability requirements, model as much as possible unified and other factors.
There are two structural types: main module and extended module.
The average price of each I/O point of the integral PLC is cheaper than that of the module type, and the volume is relatively small, which is generally used in the small control system with a fixed system process; The function expansion of module plc is flexible and convenient, and there is a large choice in the number of I/O points, the ratio of input points to output points, the type of I/O modules...etc, and the maintenance is convenient, generally in more complex control systems.
Selecting the installation method
The installation mode of plc system is divided into centralized, remote i/o and distributed with multiple plc networking. Centralized hardware does not need to be set to drive remote i/o hardware, the system response is fast and cost is low; Remote I/O type is suitable for large systems, the system device distruibution range is very wide, remote i/o can be scattered installed near the field device, the connection is short, but need to add drivers and remote i/o power supply; The distrbuted network of multiple plc is suitable for the occasions where multiple devices are independently controlled and interconnected, and small plc can be selected, but communication module must be attaced.
Gemerally, small plc have functions such as logic operation, timing and counting, which can be met for equipment that only needs to be controlled by switching quantity. For systems with switching control as the main and a small number of analog control, an enhanced plc with A/D and D/A conversion units and functions of addition and subtraction arithmetic operations and data transmission can be selected. For complex control, it is required to realized PID calculation, closed-loop control, communication networking and other functions, visual control scale and complexity, and choose mid-range or high-grade plc. However, medium and large PLC are more expensive and are generally used in large-scale process control and distributed control system.
Response Speed Requirements
PLC is a general controller designed for industrial automation, and the response speed of different grades of PLC can generally meet the needs of its application range. If you wanna use plc across the range, or some functions or signals have special speed requirements, you should carefully consider the response speed of the plc, you can choose a plc with high-speed i/o processing function, or choose a plc with a fast response module and an interrupt input module.
The reliability of the general system PLC can be met. For systems with high reliability requirements, consider whether to use redundant systems or hot standby systems.
PLC should be unified possible
Mainly consider the following three aspects:
1. The model is unified, and its modules can be used as backup for each other, which is convenient for the procurement and management of spare parts. 2. The model is unified and its function and use method are similar, which is conducive to the training of technical forces and the improvement of technical level. 3. The model is unified, its external equipment is universal, resources can be shared, easy to network communication, and it is easy to form a multi-level distributed control system after being equipped with a computer.