Programmable Logic Controller(PLC):Basics,Types & Application
What is Programmable Logic Controller(PLCs)?
PLC stands for “Programmable Logic Controller”. A PLC is a computer specially designed to operate reliably under harsh industrial environments – such as extreme temperatures, wet, dry, and/or dusty conditions. PLCs are used to automate industrial processes such as a manufacturing plant’s assembly line, an ore processing plant, or a wastewater treatment plant.
PLCs share many features of the personal computer you have at home. They both have a power supply, a CPU (Central Processing Unit), inputs and outputs (I/O), memory, and operating software (although it’s a different operating software).
The biggest differences are that a PLC can perform discrete and continuous functions that a PC cannot do, and a PLC is much better suited to rough industrial environments. A PLC can be thought of as a ‘ruggedized’ digital computer that manages the electromechanical processes of an industrial environment.
PLCs play a crucial role in the field of automation, using forming part of a larger SCADA system. A PLC can be programmed according to the operational requirement of the process. In the manufacturing industry, there will be a need for reprogramming due to the change in the nature of production. To overcome this difficulty, PLC-based control system were introduced. We’ll first discuss PLC basics before looking at various applications of PLCs.
PLC Basics and History
PLCs were invented by Dick Morley in 1964. Since then PLC has revolutionized the industrial and manufacturing sectors. There is a wide range of PLC functions like timing, counting, calculating, comparing, and processing various analog signals.
The main advantage of PLC over a “hard-wired” control system is that you can go back and change a PLC after you’ve programmed it, at little cost (just the cost of the programmer’s time). In a hard-wired control system, you’re essentially having to rip out wires and start from scratch (which is more expensive and takes longer).
PLC Block Diagram
Programmable Logic Controllers continuously monitors the input values from various input sensing devices (e.g. accelerometer, weight scale, hardwired signals, etc.) and produces corresponding output depending on the nature of production and industry. A typical block diagram of PLC consists of five parts namely:
- Rack or chassis
- Power Supply Module
- Central Processing Unit (CPU)
- Input & Output Module
- Communication Interface Module
Rack or Chassis
In all PLC systems, the PLC rack or chassis forms the most important module and acts as a backbone to the system. PLCs are available in different shapes and sizes. When more complex control systems are involved, it requires larger PLC racks.
Small-sized PLC is equipped with a fixed I/O pin configuration. So, they have gone for modular type rack PLC, which accepts different types of I/O modules with sliding and fit in concept. All I/O modules will be residing inside this rack/chassis.
Power Supply Module
This module is used to provide the required power to the whole PLC system. It converts the available AC power to DC power which is required by the CPU and I/O module. PLC generally works on a 24V DC supply. Few PLC uses an isolated power supply.
CPU Module and Memory
CPU module has a central processor, ROM & RAM memory. ROM memory includes an operating system, drivers, and application programs. RAM memory is used to store programs and data. CPU is the brain of PLC with an octal or hexagonal microprocessor.
What is Input Module?
The module which interacts with the input signal is called as Input Module. It is required to connect input devices like different types of switches.
What is the Output Module?
The module which interacts with the output signal is called as Output Module. The output module is required to connect output devices like electric applications.
The input device provides a signal to an input module. This input module is connected with the CPU for the initial automated processes. CPU processes all the input data.
After processing by CPU, it gives output data to the output module. The output module provides a signal to the output device. The singles can be anything like activating or deactivating output devices.
Types of PLCs
The two main types of PLC are fixed / compact PLC and modular PLC.
Compact PLC
Within a single case, there would be many modules. It has a fixed number of I/O modules and external I/O cards. So, it does not have the capability to expand the modules. Every input and output would be decided by the manufacturer.
Modular PLC
This type of PLC permits multiple expansion through “modules”, hence referred to as Modular PLC. I/O components can be increased. It is easier to use because each component is independent of each other.
PLC are divided into three types based on output namely Relay output, Transistor output, and Triac Output PLC. The relay output type is best suited for both AC and DC output devices. Transistor output type PLC uses switching operations and used inside microprocessors.
According to the physical size, a PLC is divided into Mini, Micro, and Nano PLC.
PLC Applications
PLCs have a variety of applications and uses, including:
- Process Automation Plants (e.g. mining, oil &gas)
- Glass Industry
- Paper Industry
- Cement Manufacturing
- In boilers – Thermal Power Plants
PLC Programming
When using a PLC, it’s important to design and implement concepts depending on your particular use case. To do this we first need to know more about the specifics of PLC programming.
A PLC program consists of a set of instructions either in textual or graphical form, which represents the logic that governs the process the PLC is controlling. There are two main classifications of PLC programming languages, which are further divided into many sub-classified types.
- Textual Language
- Instruction list
- Structured text
- Graphical Form
- Ladder Diagrams (LD) (i.e. Ladder Logic)
- Function Block Diagram (FBD)
- Sequential Function Chart (SFC)
0 Comments
Leave your comments