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PLC- A Very brief Overview
Posted Date: 28 Feb 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: denni thompson Member Level: Bronze
Rating:  Points: 4
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Introduction
In the earlier past power flow to various devices in a process were manually controlled by switching ON and OFF mechanical switches and the progress of the process were handled exclusively by humans. Later on electrical relays replaced the mechanical switches to control the power flow. This enabled the implementation of simple logic control decision by appropriate electrical circuits. By 1970’s with the development of computers, logic control were implemented by programmable devices and programmable logic controllers came into existence.
Definition
According to NEMA standards Programmable Logic Controller (PLC) is a digitally operating electronic apparatus that uses a programmable memory for the internal storage of instructions that implement specific functions such as logic, sequencing, timing, counting and arithmetic to control machines and processes.
Operation
The PLC receives its inputs from the process to be controlled through various digital as well as analog devices like Limit Switches, Proximity Switches, Pressure Transducers, Temperature sensors, Position sensors, On/Off Push buttons, Potentiometers, timers, counters etc. The method of receiving the inputs is known as Input scanning. Computations on the input information from the feed back devices are performed as per the logic instructions stored in its memory to generate the necessary outputs. The method of actuating the outputs is known as Output scanning. The outputs from the PLC switches on or switches off devices like solenoids, relays, contactors, servo motors, display devices, timers, counters etc. The change in the outputs brings about changes in the process being controlled. This in turn changes the input signal status. The input information is received by the PLC is used again to perform logic computations, which again changes the output status. Therefore control is performed cyclically. This method of execution of process control by a PLC is known as Scan Cycle.
Merits
Faster developments and implementation of complex control systems.
Implementation of changes and correction of errors can be easily done by changing the software.
Lower cost.
Computational capability of the controller allow sophisticated control.
Diagnostics and Trouble shooting is easier and reduces down time.
Reliability of the system is high.
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