Programmable logic controller
While the instruction formats may not be the same for different brands, the hardware structures and programming concepts are very similar. PLC operation and ladder logic programs are discussed. This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials. At the conclusion of this course, the student will: This course is the first part of a two-part course on Programmable Logic Controllers.
There are a total of six modules in the two-part course.
A Beginner’s PLC Overview, Part 1 of 4: Introduction to PLCs | theranchhands.com
Part I covers Modules 1 through 4. The presence of PLCs helped simplify the design and implementation of industrial automation. PLCs can be described as small industrial computers with modular components designed to automate control processes.
PLCs are the controllers behind almost all modern industrial automation. There are many components to a PLC, but most of them can be put in the following three categories:. PLCs are complex and powerful computers.
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But, we can describe the function of a PLC in simple terms. We will get into more detail later but for now, think of it like this:. Many dishwashers have microprocessors that function similarly to PLCs. The dishwasher has inputs, outputs and, of course, a CPU.
Some of the inputs into the dishwasher controller would be the buttons on the front, the water sensors and the door switch. Some of the dishwasher outputs would be the water valves, the heat elements and the pumps. Remember, the CPU is the processor in the dishwasher that is programmed to make all the decisions we will see below. In the the dishwasher example above, we treated every input and output as a discrete or digital signal. Discrete signals are signals that can only be on or off.
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We will cover this in more detail in part 3 of this series. The term was first coined by the market research firm ARC in to differentiate the original PLCs from the newer, more powerful, more flexible controllers that were entering the market. There is disagreement about the definition differences between PAC and PLC, and often the terms are used interchangeably in the industry. I often use the terms interchangeably myself. A PLC is an example of a "hard" real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.
Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire the relays to change their operational characteristics. Digital computers, being general-purpose programmable devices, were soon applied to control industrial processes.
Early computers required specialist programmers, and stringent operating environmental control for temperature, cleanliness, and power quality. Using a general-purpose computer for process control required protecting the computer from the plant floor conditions. An industrial control computer would have several attributes: The response time of any computer system must be fast enough to be useful for control; the required speed varying according to the nature of the process.
In GM Hydra-Matic the automatic transmission division of General Motors issued a request for proposals for an electronic replacement for hard-wired relay systems based on a white paper written by engineer Edward R. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the because it was Bedford Associates' eighty-fourth project, was the result.
One of the people who worked on that project was Dick Morley, who is considered to be the "father" of the PLC. One of the very first models built is now on display at Modicon's headquarters in North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service. Modicon used the 84 moniker at the end of its product range until the made its appearance.
Early PLCs were designed to replace relay logic systems. These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. Another method is state logic, a very high-level programming language designed to program PLCs based on state transition diagrams.