PLCs_part1_printout

# PLCs_part1_printout - 1 LAB INTRODUCTION TO PLCs...

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Unformatted text preview: 1 LAB LAB 11 11 INTRODUCTION TO PLCs INTRODUCTION TO PLCs Rajesh Rajamani ME 4231 Department of Mechanical Engineering University Of Minnesota PLCs A PLC (Programmable Logic Controller) works by looking at its inputs, and depending upon their state, turning ON/OFF its outputs The logic that determines the outputs is entered by the user, usually through a software program. User Entered Program Inputs Outputs 2 PLCs PLCs were invented to replace the sequential relay circuits used in machine control. What is a relay ? A relay is an electrically operated switch. INTRODUCTION TO RELAYS This circuit would not work too well for starting your car +- Starter motor Consider the following starter circuit 3 INTRODUCTION TO RELAYS Problem: A massive amount of current is required to turn the starter motor and crank your engine That much current would burn out the contacts of an ordinary switch Also You want shortest path from battery to starter You don’t want that many amps near you +- INTRODUCTION TO RELAYS Solution: Use a relay The contacting plate is big enough to carry the amps. +- switch Starter motor solenoid 4 INTRODUCTION TO RELAYS A relay is an electrically operated switch It is made up of a coil that can be energized with just a little power a moving armature(plunger) of some sort that pulls in when the coil is energized. one (or more) pairs of contacts that the plunger makes an electrical connection with. These contacts usually carry the high power. PLCs - HISTORY PLCs were introduced in the late 1960s Primary Motivation Eliminating the large cost involved in replacing the complicated relay based machine control systems In machine control systems, when production requirements changed, so did the control system. Changing the control system was time consuming and expensive when relay circuits were being used 5...
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## This note was uploaded on 02/07/2012 for the course ME 4231 taught by Professor Staff during the Fall '08 term at Minnesota.

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PLCs_part1_printout - 1 LAB INTRODUCTION TO PLCs...

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