PLC type selection procedures and principles

With the development of PLC, PLC product range is also increasing. Different types of PLC, its structure, performance, capacity, instruction set, programming methods, prices are also different, also have focused on appropriate occasions. Therefore, the rational use of PLC, is important for improving PLC control system for technical and economic indicators of significance.


PLC selection from the PLC model, capacity, I/O modules, power modules, special function module, communication networking capacity are considered.


PLC type selection


Basic principles of PLC type selection is to meet functional requirements and under the premise of ensuring reliable, easy to maintain and strive for the best performance/price ratio. Main options considered the following:


(A) reasonable type of structure


PLC there are monolithic and modular structure type.


Overall type PLC of each a a I/O points of average price than module type of cheap, and volume relative smaller, General for system process process more fixed of small control system in the; and module type PLC of function extended flexible convenient, in I/O points, and entered points and output points of proportion, and I/O module of type, aspects select room big, and maintenance convenient, General Yu more complex of control system.


(B) selection of installation methods


PLC system was installed into a centralized, remote multiPLC networking I/O and distributed.


Concentrated type not need set drive remote I/O hardware, system reaction fast, and cost low; remote I/O type applies Yu large system, system of device distribution range is wide, remote I/O can dispersed installation in site device near, line short, but need additional drive and remote I/O power; more Taiwan PLC networking of distributed applies Yu more Taiwan equipment respectively independent control, and mutual contact of occasions, can selection small PLC, but must to additional communications module.


(C) the appropriate functional requirements


Small (low) PLC logic operations, timing and counting functions need only to switch control device can be met.


For switch control, with a small amount of analog control system can be used with a/d and d/a conversion unit, with addition and subtraction arithmetic operations, data transfer functions for enhanced low-end PLC.


To control more complex, demanding realization of PID functions such as operations, closedloop control, communications networking, Visual control of size and complexity, use mid-range or high-end PLC. But the medium and high-grade PLC are less expensive, generally used for large scale distributed control systems for process control and other occasions.


(D) the response speed required


General controller of the PLC is designed for industrial automation, different grades of speed of PLC can meet the needs within the scope of its application. If you want to use PLC, or some functions or signals with special requirements, you should consider the PLC‘s response speed, you can choose with high speed I/O processing function of PLC, or selected for their rapid response modules, and interrupt the PLC input modules, and so on.


(E) the reliability of the system requirements


For general system reliability of PLC can meet. High reliability requirements of the system, you should consider using redundant systems or hot standby system.


(F) model uniform


A company should try to attain the PLC model to unify. Taking into account the following three main aspects:


1) model uniform, the module can be used as a spare, spare parts procurement and management.


2) model uniform, its functions and using methods similar to help improve the technical training and skill levels.


3) model uniform, its external equipment, resource sharing, easy network communication, with host computer and is easy to form a distributed control system.

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