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PLC & ACCESSORIES

PLC & ACCESSORIES

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Programmable Logic Devices

Programmable Logic Devices (PLD) are programmable systems and are generally used in manufacturing automation to perform different control functions, according to the programs written in its memory, using low level languages of commands. There are following three types of PLDs are being employed in mechatronics systems.

1. Microprocessor :- It is a digital integrated circuit which carries out necessary digital functions to process the information obtained from measurement system.

2. Microcomputer :- It uses microprocessor as its central processing unit and contains all functions of a computer.

3. Programmable Logic Controller (PLC) :- It is used to control the operations of electro-mechanical devices. especially in tough and hazardous industrial environments.

Any computer having input and output interfaces can be used to control external devices. However most of the computers are not industrially hardened. Input / Output devices of general-purpose microcomputers are not engineered to handle line-voltages and currents     above transistor-transistor logic (TTL) levels. Also they are not designed to with-stand the temperature, humidity, and vibration on shop floors. These drawbacks of a general purpose computer have been rectified by developing a Programmable Logic Controller. (PLC) with built-in isolation into their inputs and outputs.

Any computer having input and output interfaces can be used to control external devices. However most of the computers are not industrially hardened. Input / Output devices of general-purpose microcomputers are not engineered to handle line-voltages and currents above transistor-transistor logic (TTL) levels. Also they are not designed to with-stand the temperature, humidity, and vibration on shop floors. These drawbacks of a general purpose computer have been rectified by developing a Programmable Logic Controller (PLC) with built-in isolation into their inputs and outputs.

The programmable logic controller is defined as a digital electronic device that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic words to control machines and processes.

PLCs are generally used for incorporating automation in open loop systems where processes are to be performed in a sequential manner. PLCs are used for automation of assembly lines in industries. They are generally designed for multiple input multiple output (MIMO) systems. In PLCs, instructions are saved in nonvolatile memory.

Programming a PLC: PLCs are programmed through concept of ladder logic. In general there exists a graphical user interface (GUI) to program a PLC that makes it different from other processers. Ladder logic comprises of two columns. Left column shows input devices like switches, sensors while in output column is at right side which shows actuators like cylinders, motors.

In the other type of the control system, precise control over output of system is to be obtained. Therefore a continuous monitoring of such system is essential. For example there is a necessity to continuously monitor and control the fuel tank used in a Boiler based power plant. This type of control is also called modulating control. Feedback systems and Proportional-integral-derivative (PID) controllers are employed in these systems

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OMRON PLC