What is a PLC?

2022-02-02 16:03:29
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Complete industrial automation has been the goal for some time now. Slowly but surely, we are getting closer. In 1785 the automatic flour mill was invented, marking the first fully automated industrial process. From 1968 to today, programmable logic controllers have made their way into everything from simple thermostats to complex heavy machinery. They have proven to be more than capable of automating a significant number of industrial processes and production lines. What is this indispensable tool? Here's everything you need to know.

What is a programmable automaton?

A programmable logic controller is an industrial computer primarily used in factories and industrial environments that rely heavily on automation. It is an autonomous unit that controls, automates and regulates various industrial processes, as well as all the elements that go into these processes. In other words, it supervises and controls the input devices to produce the desired result. To better understand what a PLC is, let's imagine a thermostat – it could be compared to a simplified PLC, and here's why. With a thermostat, you set the temperature to a specific degree (the desired output). The controller uses data from its temperature sensor (input) to determine whether the heating unit should operate or not. When the temperature is below the desired output, the controller sends a signal to the heating unit, which operates in response to maintain the desired temperature. This is how you get a permanently warm room.

For industrial PLCs, the basic principle is the same, but since it is applied on a large scale to control complex equipment, controllers are equipped with a large number of specialized functions. Also, by connecting a PLC to other units, one can easily control an entire production line without needing as many workers as without a PLC. Used on a large scale, PLCs ensure efficiency and save a lot of money, time and resources. For these reasons, the PLC and its programmers are highly sought after by manufacturing companies.

Components of a PLC

Inputs/outputs

The I/O or input/output section of a PLC is the hub that connects external machines/devices to the processor to collect input data and control output data. This I/O section is primarily responsible for translating real-world variables into electronic signals that the processor can understand. This section is also responsible for translating programming languages ​​into machine commands. By connecting machines and devices, such as temperature sensors, weight sensors, and tachometers, PLCs record variables that are used by software to manage the manufacturing process. Thanks to its I/O section, a controller can control motors, valves and various elements of machinery. If the PLC was a human being, the input/output section would be the body. It senses external stimuli, sends them to the brain, and follows commands as they arrive.

The processor

Speaking of the brain, its counterpart is the central processing unit (CPU) or processor. This is where the magic happens. It is the component in charge of analyzing and sorting all the data coming from the specified inputs in order to take the appropriate actions (predetermined by the program). If certain conditions are met, the CPU starts, stops, speeds up, etc. the current process. You can see this in an elevator. For the motor to run (output), the doors must be closed (input 1) and a button must be pressed (input 2). If the central unit receives neither of the two inputs, it does not trigger an output. Once all the conditions are met, the motor starts and the elevator begins to run.

Programming device

Just like a programmable automaton, humans operate according to a specific program, except we don't call it that. We call it instinct. We eat when we are hungry and sleep when we are tired. Imagine being able to rewrite instinct. That's what a programming device does. It is a computer, laptop, or handheld device equipped with PLC programming software that allows the programmer to dictate how a machine thinks and reacts. This is where you write the instructions, essential inputs, desired outputs, etc. Once the programmer has written a program, he transfers it to memory for the CPU to access.

Memory

As the name suggests, this is where all data is sent/written by a programmer/input device and stored to be read by the CPU. Keep in mind that when we talk about CPU memory, we are talking about two types: RAM and ROM. ROM is “read only memory”. It is the place where the fixed data, mainly the operating program, is stored only to be read and not modified by the CPU. RAM, on the other hand, is short for "random-access memory", and it's where everything can be read or written. The data stored therein is variable in nature. They include ladder logic programs and sensor/switch data, such as temperature, voltage, current, and pressure values. Like any memory unit,

The more complex your program is, and the more inputs and outputs you have, the more memory you will need.

power supply

 The power supply is what allows the controller to operate. Depending on the scale of your operation, you will need to provide enough power for your controller to operate properly. The power supply works by converting 240 volts AC (alternating current) to 24 volts DC (direct current). That said, it's important to note that power supplies come in different capacities. If the voltage stays the same, the capacitance is rated based on the current (measured in amps). A 2 amp supply is considered suitable for small scale use, while a 50 amp supply is considered better suited for large scale productions.

How is it programmed?

To write a PLC program, you must first understand the project you are going to write. Once you understand the process and determine the best approach to writing the program, the next step is to choose a programming language. There are five officially recognized languages, each with its advantages and disadvantages. As a programmer, you don't need to know them all, as most companies only use one, but the more you know, the more problem-solving you will be.

The Ladder (LD)

It is by far the most popular language because it is easy to read and edit online. It is written in the form of horizontal rungs and is read from left to right. The inputs/conditions that must be satisfied are written on the left side, and the desired outputs on the right side. If you're new to programming PLCs, Ladder (LD) is a great place to start, especially because it's easy to learn. The only downside you need to be aware of is the limitation of this language. Although great for basic projects, the Ladder can be limited for some commands like motion commands. In summary, it's a great place to start, but you'll have to look to other languages ​​if you want to excel in this area.

Instruction List (IL)

Unlike Ladders, Instruction List (IL) is a text-based language, which means you'll be writing lines of code instead of drawing your program. The textual nature of language is also what gives it two crucial advantages. First, text takes up less space than graphics, which means you can write as large a program as you want without having to worry about memory space. Second, the text is easier for the CPU to read, allowing for faster execution. Despite these advantages, it is not as common as other languages ​​because most people prefer to depend on graphics because it avoids the risk of critical errors.

Structured Text (ST)

ST is a high-level textual language which means two things. First, it is much closer to human languages ​​than others. Second, it's extremely powerful and more than capable of running complex projects. In terms of versatility, think of it like the C++ of industrial automation. Of course, for people inexperienced in C, ST won't be as easy to learn, but it's guaranteed to make you a strong candidate for any job. What's more, it has no limitations unlike LD and SFC. The main disadvantages of this language are that it is difficult to debug and edit online.

Function blocks (FB)

 Function blocks are where PLC programmers divide. Some consider the graphical language to be extremely useful, others think it is rather too complicated. Diagrams are made up of numbered input and output boxes linked together by lines. The CPU then loops through the diagrams to determine the order of execution. The main drawback is that there is no particular structure or order for the squares to be placed. It can be confusing when tackling a large project with lots of moving parts. However, for small repetitive tasks, FB has proven to be one of the best tools for the job.

Sequential Function Charts (SFC)

 It is a graphical language quite similar to flowcharts. When writing, you write vertically, using steps to indicate actions to take (outputs). The required inputs/conditions are written as transitions. The program then moves from one stage to the next meeting the requirements of the transition. As you can imagine, the language is writeable and readable, which helps a lot when it comes to editing the program or diagnosing an error. In the field, most people prefer to use SFC when programming multiple parallel processes or drawing an overview of a program.

Types of PLCs

 There are two main types of programmable logic controllers: compact or monoblock PLCs and modular PLCs. They are classified according to the number of inputs/outputs available. Compact PLCs are manufactured with a fixed number of inputs and outputs. This number is decided by the manufacturer. Some extensions remain possible but are still limited. Modular controllers, on the other hand, allow the addition of additional input/output sections and more. It all depends on how they are structured. The components of a modular PLC are divided into several modules which, when connected, form the complete PLC system. To increase the number of inputs, simply add additional I/O modules.

Advantages

 There are many reasons why it is necessary to use a programmable logic controller. In large-scale productions where uniformity is a necessity, PLCs ensure desirable and consistent production by supervising and controlling the production process. They also reduce the need to hire too many human workers. Although the company still has to spend money on the system and a trained technician, it saves money that would otherwise have been spent on salaries, insurance and liability for possible human error. What is even more advantageous is that the PLCs play a role in preventive maintenance. When a machine is overloaded, the quality of its performance decreases. Since PLCs collect huge amounts of data about the machines they are connected to, a technician can easily program a PLC to be on the lookout for any signs of a drop in performance. When the machine reaches a specific point defined by the technician, the system displays a warning message indicating the need for maintenance. Thus, there will be no need to stop production due to a sudden breakdown or unplanned maintenance. Also, with constant maintenance and supervision, a business can ensure efficiency, which results in higher profit margins, increased production and lower costs. a technician can easily program a controller to be on the lookout for any signs of declining performance. When the machine reaches a specific point defined by the technician, the system displays a warning message indicating the need for maintenance. Thus, there will be no need to stop production due to a sudden breakdown or unplanned maintenance. Also, with constant maintenance and supervision, a business can ensure efficiency, which results in higher profit margins, increased production and lower costs. a technician can easily program a controller to be on the lookout for any signs of declining performance. When the machine reaches a specific point defined by the technician, the system displays a warning message indicating the need for maintenance. Thus, there will be no need to stop production due to a sudden breakdown or unplanned maintenance. Also, with constant maintenance and supervision, a business can ensure efficiency, which results in higher profit margins, increased production and lower costs. Thus, there will be no need to stop production due to a sudden breakdown or unplanned maintenance. Also, with constant maintenance and supervision, a business can ensure efficiency, which results in higher profit margins, increased production and lower costs. Thus, there will be no need to stop production due to a sudden breakdown or unplanned maintenance. Also, with constant maintenance and supervision, a business can ensure efficiency, which results in higher profit margins, increased production and lower costs.