PLCs Explained: Programmable Logic Controller Basics

What is a PLC?

One of the simplest definitions of a PLC (Programmable Logic Controller) is a digital computer used for automation of industrial electromechanical processes, such as control of machinery on factory assembly lines. PLCs are the backbone of automation environments, offering centralized control with built-in input/output modules which allow for sophisticated operational sequences. In this post, we will delve into the fundamental components of a PLC, their programming languages, and various types of PLCs and their future applications.

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Traffic lights are a classic example of PLC application, automating what would otherwise require manual intervention. From automatic car washes to elevators and amusement park rides, PLCs invisibly orchestrate many modern conveniences.

PLC in automation equipment

History of PLCs

Originating in the 1960s, PLCs were developed as a solution to replace hardwired relay systems that were bulky and error-prone. Those older systems required excessive space and frequent maintenance, causing substantial downtimes for system rewiring and tackling relay contact issues. The advent of PLCs streamlined these processes, offering a more reliable and efficient solution.

Downsides of relay-powered machines:

• Hard-wiring specificity
• Labor-intensive troubleshooting
• Frequent relay contact issues
• Rigid maintenance schedules
• Bulky and cumbersome design
• Complicated rewiring for system changes

Engineers like Richard Morley paved the way for the first PLC, the MODICON 184, which integrated these features into a compact and user-friendly system.

PLC Advantages

PLCs have since evolved, offering key advantages such as reliability, flexibility, cost effectiveness, speed, better communication and advanced data processing functions. One of the most significant benefits is their reprogrammability, allowing them to adapt to changes in operational or monitoring requirements.

The diagnostic tools offered by modern PLCs simplify troubleshooting:

• Report generation
• Fault logs
• Step-by-step program simulation
• Manually run inputs/outputs

PLC Components

Understanding PLCs involves breaking down their primary components: the Central Processing Unit (CPU) and the Input/Output (I/O) interface system.

PLC CPU

The CPU is the "brain" of the PLC, hosting memory and communication systems that facilitate data processing and diagnostics. Key components include communication ports, chassis or racks, and programming devices like external computers. Compact modules may integrate these components, offering pre-set I/O and communication ports.

PLC I/O

I/O modules bridge inputs and outputs with the CPU, relaying data and executing tasks in a continuous loop. Inputs often include switches and sensors, whereas outputs can range from motor starters to lights and valves. The power supply, whether integrated or separate, typically operates within 12-24VDC or 110/220VAC ranges.

PLC control panel

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PLC Programming

PLCs are programmed using specific software, which is then downloaded to the PLC's memory. The program guides the CPU to interpret input data and perform the corresponding output actions. The IEC 61131 standard outlines five programming languages for PLCs:

• Ladder Diagram or Ladder Logic (LD)
• Function Block Diagram (FBD)
• Structured Text (ST)
• Sequential Function Chart (SFC)
• Instruction List (IL)

Ladder Logic

Ladder Logic is widely used due to its visual clarity, making it easy to read and implement. It translates symbols into relay operations, timers, counters, and mathematical functions.

PLC Operation

In understanding a typical PLC operation, consider the constant loop the controller runs through: input scan, program scan, output scan, and housekeeping. This loop ensures that the system efficiently processes inputs and produces the desired outputs based on the programmed instructions.

The four basic steps in PLC operation are:

1. Input Scan - Detects state of input devices
2. Program Scan - Executes user-created program
3. Output Scan - Operates all output devices
4. Housekeeping - Runs diagnostics and communicates with other devices

Types of PLCs

Unitary PLC

Unitary PLCs are self-contained units best for simple processes. They feature a fixed number of I/O points and an integrated CPU with connection ports.

Modular PLC

Modular PLCs allow for flexible configurations with a base module that houses the CPU, inputs, and power supply. You can add various I/O modules to customize the system as needed.

How Do I Choose Which PLC Type I Need?

Choosing the right PLC involves several considerations, such as whether it’s for a new or existing system, the environment it will be in, the complexity of the processes, and the communication needs.

Six questions to consider:

1. Is the PLC for a new or existing system?
2. What type of environment will it operate in?
3. How many I/O points are required?
4. How complex are the control processes?
5. What are the communication needs?
6. What programming language will be used?

PLCs Moving Forward

As industries evolve with the advent of big data, robotics, and cloud computing, PLCs must adapt to new challenges like working in extreme conditions, enabling remote operations, and integrating with the Industrial Internet of Things (IIoT) for better data analytics and decision-making. Enhanced security measures will also be a priority as cybersecurity becomes increasingly vital.

We hope this overview has strengthened your appreciation for PLCs, highlighting their crucial role in modern automation. If you’re intrigued by the potential of PLCs and need more information, feel free to reach out to us.

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