Ch 1 Introduction 1 Chapter 1 INTRODUCTION Introduction to PLCs Programmable logic controllers (PLCs) were introduced to industry between 1968 and 1970 as a way to replace large expensive panels of relays, timers, and counters. Automotive manufacturers were looking for ways to simplify start-up of new car lines after model changeovers each year and save money in the cost of manufacturing installations. Historically, relays have been used since the late 1800's to control simple processes. They were used in the early days to control railroad crossings. Before simple relay logic was introduced to control railroad crossing arms and alarm lights, accidents at these crossings were contributing to a high toll on human life. The term “relay” was coined as the name of the dev ice invented by Samuel F. B. Morse who invented the telegraph. The relay was invented as a device to extend the signal or “relay” the signal of the telegraph more than the 20 mile limit of electrical signals at the time of the invention of the telegraph (1836). Relays, timers, and counters had been the favored choice for electrical and systems engineers to manufacturing facilities, especially in facilities with a large number of machines making discrete parts. Automotive manufacturers top the list of this type of manufacturing. At the same time that costs continued to rise for the engineering and construction of automotive assembly lines, computers were becoming more numerous and less costly. There was, however, a general discomfort among engineers to replace relays with computers. Most were reluctant to place the computer on the plant floor. A compromise was necessary for the engineer that he would be willing to accept. A computer that appeared to be relay-ladder logic to the electrician but able to use the computing capabilities of a computer was the device envisioned. The result of this vision is what is known today as the PLC (Programmable Logic Controller). Relays as well as timers and counters were the first devices replaced by the PLC. Relays are electromechanical devices that use magnetism caused by power flow through the circuit's coil to energize a core and move a plunger with contacts attached. Contacts change state when the coil is magnetized. Normally open contacts close while normally closed contacts open. Changing contacts combine to complete other circuits. Combinations of relay contacts energizing coils form the basis of Boolean logic. Boolean logic deals with the combination of discrete on-off states to turn on or off other outputs. The principle of using PLCs as substitutes for relays to reduce the wiring, panel fabrication, and engineering cost looked very appealing to the early PLC user. Like most electronic devices appearing in the early '70's, cost of the early PLC was high and functionality was not well developed. Early PLCs were developed around a mini-computer or special purpose control board. It was not unusual to pay $50,000 or more for a single PLC complete with I/O and still use relays for the most critical circuits.
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