An introduction to PLCs, or Programmable Controllers, and how they are used to control automated processes.
For a limited time only, use the code "startfree" at checkout to get access to your first beginner PLC programming course for free.
For a limited time only, use the code "startfree" at checkout to get access to your first beginner PLC programming course for free.
A PLC, or Programmable Logic Controller, is a specialized industrial computer designed to control automated processes and equipment. If you've ever wondered how modern factories, water treatment plants, or even traffic lights operate so reliably, the answer often lies in these robust devices.
Since controls engineers, PLC programmers, and maintenance technicians spend so much time working with PLCs, it's essential that anyone wanting to work in these fields understands:
These are the exact topics we'll cover in this post. Let's dive in.
PLC stands for Programmable Logic Controller. These devices are specialized computers designed specifically for industrial automation to control automated processes and equipment. Think of a PLC as the brain of an automation system.
In almost all applications, a PLC follows a simple three-step process:
You'll find PLCs controlling everything from assembly lines in automotive plants to conveyor systems in distribution centers, from water treatment facilities to food processing operations.
PLCs are preferred over traditional computers for several key reasons:
Easy Integration with Field Devices: Connecting a sensor like a photoelectric cell or an actuator like a signal light to a PLC is straightforward. Integrating these same devices with a standard computer requires significant additional hardware and software.
Exceptional Durability: PLCs are solid-state devices designed for harsh industrial environments. With no moving parts to wear out, properly installed PLCs can run continuously for ten, fifteen, or even twenty years. Anyone who's experienced frequent computer crashes knows you can't get this type of reliability from a traditional computer.
Simplified Programming: While PLCs are becoming more sophisticated, they remain much easier to program than traditional computers. Someone without extensive software engineering experience can learn to write effective PLC programs, whereas developing applications for standard computers requires significantly more technical expertise.
If you've talked to anyone about industrial automation, you may have heard various terms for the device that controls automation processes:
These terms are used interchangeably and all refer to the same thing: the industrial computer that controls a process. Regardless of whether someone mentions a PLC, PAC, controller, processor, or CPU, they're referring to the device that manages the automation process.
Regardless of size or manufacturer, all PLCs contain the same essential components that enable them to function effectively.
The processor is the heart of the PLC, responsible for executing the program downloaded to the device. Just as the PLC serves as the brain of an automation system, the processor serves as the brain of the PLC.
The processor contains:
Input Modules: PLCs gather information about the process through input devices like pushbuttons, sensors, and switches. Input modules convert electrical signals from these devices into digital signals the processor can understand.
Output Modules: PLCs control the process through output devices like motors, lights, and horns. Output modules perform the opposite function of input modules, converting digital signals from the processor into electrical signals that drive the actuators.
For integration into larger systems, PLCs need to communicate on industrial networks. Communication modules enable communication between the PLC and other devices via industrial networks like Profinet or Ethernet/IP. Most commonly, this takes the form of an RJ45 jack or similar port for Ethernet-based industrial networks.
All PLC components require power to operate. While PLCs and many control devices run on 24V DC power, the incoming power supply is typically between 120 and 400V AC. A power supply converts the incoming power to the appropriate voltage level for the PLC and its components.
These components can be integrated into a single housing or exist as separate modular components, depending on the PLC system.
In smaller PLC systems, all components are integrated into one housing. The PLC becomes a single piece of hardware that includes processor, I/O, communication module, and power supply. These compact PLCs are designed to control small, standalone machines or simple processes.
For example, a small PLC might have:
PLCs controlling large automation systems like full factories or production lines are typically modular. In these systems, each component is a separate physical device connected to the PLC through a chassis or backplane that creates electrical connections between modules.
Modular systems offer significant advantages:
A typical modular PLC system includes separate modules for power supply, processor, communications, and multiple input/output modules.
You might wonder why manufacturers like Rockwell Automation produce multiple series of controllers (Micro, CompactLogix, ControlLogix) or why Siemens offers LOGO!, S7-1200, and S7-1500 PLCs. The answer lies in the diverse requirements of automation applications.
Different automated processes require different performance levels. Controlling a single conveyor belt requires much less processing power and fewer I/O points than managing a fully automated warehouse. Similarly, motion control applications demand faster processors, while safety systems require redundant components.
By offering PLCs with varying performance levels and features, manufacturers enable users to select the right hardware for their specific applications. While this variety can seem overwhelming initially, manufacturers provide selection tools to help choose the appropriate PLC for each job.
PLCs can control many different types of automated processes, but the four most important automation disciplines are:
Modern PLCs are capable of multi-discipline control, meaning one PLC can perform discrete, process, motion, and safety control functions. This capability provides huge benefits for end users, who no longer need multiple specialized controllers and can train their engineers on a single platform.
In discrete control, actuators like motors and valves are turned on and off to create individual products. Discrete industries include:
An easy way to identify discrete control is if you can count the end product. If you can say "we manufactured 500 cars," you're dealing with discrete control in a discrete industry.
Process control involves adjusting setpoints to maintain process stability. The actual state of a process feeds into a control algorithm that adjusts setpoints to keep the process stable.
For example, in a brewery, you might need to control liquid temperature in a tank. The PLC would adjust a heater's setpoint and monitor temperature until the liquid reaches the target temperature. The control algorithm would then continue adjusting the heater setpoint to maintain stable temperature.
You can identify process control when you measure the end product. If you can say "we produced 10,000 liters of beer," you're dealing with process control in a process industry.
Historically, automation manufacturers focused on either discrete or process control, leading to completely different suppliers, system architectures, and technologies for each industry.
Today's multi-discipline PLCs bridge this gap, though many engineers still specialize in one or two disciplines. Interestingly, many process industries also have discrete control applications. A brewery controls a process application when brewing beer but switches to discrete control when bottling and packaging. Companies with mixed process and discrete control benefit most from multi-disciplinary controllers and engineers.
A PLC doesn't inherently know how to control a process. Controls engineers must program the PLC, defining exactly how to control the process by specifying:
Let's examine PLC programming basics through a simple example.
Imagine a conveyor that transports pallets across a warehouse with these requirements:
How would we write a PLC program to control this process?
PLC programs are written in proprietary development environments created by manufacturers. In these environments, you specify inputs to monitor, outputs to control, and logic to execute.
As a controls engineer, you write logic in high-level programming languages. When you download the program to the PLC, your logic compiles into efficient, binary machine code that the PLC can understand and execute.
One of the most common PLC programming languages is Ladder Diagram, a high-level, visual programming language that resembles electrical schematics. This language uses familiar electrical symbols, making it intuitive for engineers with electrical backgrounds.
Function Block Diagram is another high-level, visual language popular in process industries. It represents logic as interconnected function blocks, making it excellent for complex process control applications.
Structured Text is a text-based programming language useful for mathematical operations and complex algorithms. It resembles traditional programming languages and is powerful for computational tasks.
Most PLCs support multiple programming languages within a single project. This flexibility allows you to use the most appropriate language for each task. For example, you might write high-level logic in Ladder Diagram while handling complex calculations in Structured Text.
Language names may vary between manufacturers, but the core concepts remain consistent across platforms.
In this post, we've explored the fundamentals of PLCs, covering what they are, their essential components, and how these components are arranged in both compact and modular configurations.
We've also examined the automation disciplines PLCs handle and the different types of processes they control, from discrete manufacturing to process industries.
Finally, we've previewed the programming languages used to create PLC programs, showing how engineers translate process requirements into executable logic.
You now have a solid high-level understanding of PLCs and their capabilities. From here, you can begin learning PLC programming through hands-on courses where you'll discover how to transform process requirements into working automation solutions.
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