Q. What is PLC..??
It is Programmable Logic Controller, a Solid-state device with digital processing capabilities designed for Real-Time control of processes or a Digital computer used for Automation of Electromechanical Processes such as control of machinery on factory assembly lines.
e.g. an induction motor,solenoid actuator,limit switch,ON/OFF or push buttons,etc.
Detailed Definition of a PLC : A programmable controller is a small industrial strength computer used to control real world actions, based on its program and real world sensors. The PLC replaces thousands of relays that were in older electrical panels, and allows the maintenance technician to change the way a machine works without having to do any wiring. The program is typically in ladder logic, which is similar to the wiring schematics maintenance electricians are already accustomed to working with. Inputs to a PLC can be switches, sensors, bar codes, machine operator data, etc. Outputs from the PLC can be motors, air solenoids, indicator lights, etc.
Q. Why PLC had been developed..??
To replace the hard wired relay control in industrial process automation by electronic control.The basic idea was to replace very complicated,unreliable,difficult for up gradation,time consuming,difficult to implement,limited scope and slowly operating Hard Wired Relay Control by a simple or easy to implement or understand,general purpose,reliable,fast operating Electronic Control.
Q. History of PLC..??
Demanded by GM Hydramatic and proposed by Bedford, Massachusetts in 1968.First PLC name 084.Bedford started new company for PLC called Modicon. Modicon brand acquired by Gould Electronics then by AEG and then by Schneider Electric.
Presently major players are Siemens,Allen-Bradley,Omron,Schneider,Mitsubishi,etc.
Q. How did the Ladder Logic Programming come..??
1. Earlier PLCs were developed to replace the relay control or relay logic. This program notation had been used to reduce/avoid the training required for existing technicians as Ladder logic resembles the relay logic.
2. Modern PLCs can be programmed using Ladder logic or BASIC or C language or State Logic. But because of phenomenon of graphical representation of program logic, Ladder logic programming became common.
Q. How PLC programming is done..??
PLCs are programmed using application/programming software on personal computers. The computer is connected to PLC through Ethernet,RS-232,RS-485 or RS-485 cabling.
e.g. RSLogix 5000 is a software/platform for PLC Programming.
