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Automating Manufacturing Systems with PLCs Control engineering has evolved over time. In the past humans were the mainmethod for controlling a system. More recently electricity has been used for control andearly electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logicalcontrol decisions. The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC). The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls.PLCs have been gaining popularity on the factory floor and will probably remainpredominant for some time to come. Most of this is because of the advantages they offer. 1.1 Ladder logic Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced. Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure 1.1. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. Figure 1.1 Simple Relay Layouts and Schematics Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure 1.2. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applie