PLC Introduction
PLC History
PLC (Programmable Logic Controller) has become a major component in today industrial automation world. PLC was introduced in late 1960 to replace the relay based controlling systems. Before PLC was invented all the controlling was done by using relays. Relay is an electromechanical device that consists of a coil and mechanical contacts (Fig 1.1). By energizing the coil the output contacts can be controlled. Since relays have mechanical components it is subjected to wear and malfunctioning. The main problem engineers were faced was the difficulty to change the design of relay logic once it had been done and it consumed a lot of manpower. So to overcome these problems, the first PLC, designated as “084” was introduced by Bedford Associates. Then Bedford Associates started a new company, Modicon, to develop and manufacture the PLCs. Since then Programmable Logic Controller has evolved and has become a key component in the automation industry.
What is a PLC
A Programmable Logic Controller, or PLC for short, is simply a special computer device that is used in industrial applications where it is needed to control a machine or a plant. They are used in many industries such as oil refineries, manufacturing lines, conveyor systems and so on. PLC provides a flexible way to connect,”softwire”, components together.
PLC is comprised of a central processing unit (CPU), inputs, outputs, and power supply. Figure 1.3 illustrates the basic structure of a PLC. CPU is the main part of PLC that makes it so special. PLC‘s CPU is very reliable and fast. They are usually programmed in relay ladder logic and are designed to operate in an industrial environment. Input modules are used to grab sensor data and user commands to the CPU and there several types of them. Output modules are used to give signals to actuators, motors and other devices.
PLC working principle
Let’s look into how PLC is working. PLC’s CPU continuously scans its program which is programmed by basically ladder logic. Unlike other programming languages, ladder programming scanning sequence is a little bit different. First, CPU scans the inputs and then it executes the program. After executing the program, the CPU updates the outputs. This scanning sequence is illustrated in Figure 1.4.
When it comes to ladder logic scanning, PLC scans its program top to bottom and rungs in the program are scanned from left to right order. This scanning pattern is shown in figure 1.5.
