In this video, I'll explain:

• what a PLC is,
• what components make up a PLC, and
• how a PLC controls a process

Let's start with the basics.

What is a PLC?

A PLC, or Programmable Logic Controller, is a specialized industrial computer designed to control automated equipment and processes.

If you're new to industrial automation, you might wonder why we need PLCs at all. Why can't a regular computer control an automated process?

There are three core reasons PLCs are used in industrial automation.

First, they're easy to integrate with other devices. A PLC can communicate with hundreds or even thousands of different sensors and actuators, and connecting a device is often as simple as wiring it directly to the PLC.

Second, PLCs are incredibly durable by design.

They use a specialized operating system that doesn't need updates or patches.

That operating system runs on solid-state components with no moving parts to wear out.

And those components sit inside a ruggedized housing that is designed to withstand the harsh environments that are found in the manufacturing industry.

All of these things combined allow PLCs to run for 10, 15, or even 20 years without stopping.

Finally, PLCs are simple to program and troubleshoot.

As you'll see later in this video, they're programmed using high-level, visual programming languages that are easy for technicians, electrical engineers, and process engineers to understand. That’s really important in manufacturing environments, where the people operating and maintaining equipment are electricians, technicians, and operators, not software engineers.

Now that we know what a PLC is and why we use them, let’s talk about the components that make up a PLC.

PLC Architecture

PLCs come in many shapes and sizes, but they're all built from the same core components.

Processor

The processor is the brain of the PLC. It's responsible for storing and executing the project downloaded to it.

I/O Modules

Input and output modules, or I/O modules, let the PLC interact with the physical world.

Input modules take electrical signals from devices like pushbuttons and sensors and convert them into digital signals the processor can understand.

Output modules do the reverse, taking digital signals from the processor and converting them into electrical signals that actuate devices like lights and motors.

Communication Modules

Communication modules let a PLC talk to other devices on industrial networks like EtherNet/IP and PROFINET.

Power Supply

Finally, a power supply provides power to every component in the PLC.

Form Factors

In the example above, each component is its own physical module with its own housing, and these modules install into a chassis to build a PLC system. This is known as a modular PLC system, and it's the form factor used for large, powerful systems.

The advantage of modular PLCs is that they can be customized to fit a project's needs, and since individual modules can be replaced without touching the rest of the system, they are  also easy to maintain.

Other PLCs integrate every component into a single housing. This is called a compact PLC, and it's often used for smaller machines and process skids.

How Does a PLC Control a Process?

A PLC controls a process by monitoring inputs, executing its program, and controlling outputs. These steps run in an infinite loop, and each pass through that loop is called a scan, or a cycle.

Let's look at a simple example of a PLC controlling a conveyor, where an operator starts the conveyor by pressing the Start button, and it keeps running until the sensor at the end of the conveyor is triggered.

At the start of every scan, the PLC checks the status of its inputs, in this case the Start button and the sensor.

The PLC then executes its program. Simple programs like this one are typically written in Ladder Diagram, a high-level, visual programming language that resembles electrical schematics.

After executing the program, the PLC turns its outputs on or off to control the process.

This cycle repeats in an infinite loop to keep the process under control.

Wrap Up

If you want more information on how a PLC is programmed, check out this video that explains how ladder diagram, the most common PLC programming language, works.

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