What are the types of PLCs and their Application

Introduction To PLC : 

PLC stands for Programmable Logic Controllers. They are basically used to control automated systems in industries. They are one of the most advanced and simplest forms of control systems which are now replacing hard wired logic relays at a large scale.

Programmable Logic Controller (PLC) also known as Industrial Computer is the major component in the industrial automation sector. Due to its robust construction, exceptional functional features like PID controllers, sequential control, timers and counters, ease of programming, reliable controlling capabilities and ease of hardware usage – this PLC is more than a special-purpose digital computer in industries as well as in other control-system areas. Different types of PLCs from vast number of manufacturers are available in today’s market. Therefore, in the subsequent paragraphs, let us study about PLCs and their types.

Types Of PLC :

Programmable Logic Controllers (PLCs) are integrated as either single or modular units.

An integrated or Compact PLC is built by several modules within a single case. Therefore, the I/O capabilities are decided by the manufacturer, but not by the user. Some of the integrated PLCs allow to connect additional I/Os to make them somewhat modular.

A modular PLC is built with several components that are plugged into a common rack or bus with extendable I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from same manufacturers or from other manufacturers. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.

Modular PLCs are further divided into small, medium and large PLCs based on the program memory size and the number of I/O features.

Small PLC is a mini-sized PLC that is designed as compact and robust unit mounted or placed beside the equipment to be controlled. This type of PLC is used for replacing hard-wired relay logics, counters, timers, etc. This PLC I/O module expandability is limited for one or two modules and it uses logic instruction list or relay ladder language as programming language.

Medium-sized PLC is mostly used PLC in industries which allows many plug-in modules that are mounted on backplane of the system. Some hundreds of input/ output points are provided by adding additional I/O cards – and, in addition to these – communication module facilities are provided by this PLC.

Large PLCs are used wherein complex process control functions are required. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules, and so on. Mostly, these PLCs are used in supervisory control and data acquisition (SCADA) systems, larger plants, distributed control systems, etc.

plc system basic construction
plc system basic construction

PLC Systems Types
Based on the system types following is the sample of PLCs
• Quantum, Modicon 984, 884, 584, 484, 384, 184
• Allen Bradley SLC-500, PLC-5, , PLC-3, PLC-2 PLC
• Texas Instruments 5T1/Siemens 530
• Hitachi
• Omron
• Mitsubishi
• Square D Symax
• General Electric 90/70, 90/30, Series One, Series Three, and Series Six
• Networks – Modbus, Ethernet, Data Highway Plus
• Operator Interface- Panelmate, Panelview, Dataliner.

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