What Is A PLC?

A PLC (Programmable Logic Controller) is simply a computer adapted for industrial use. This means that a regular computer has been beefed up and given additional capabilities so that it can control manufacturing processes. It was designed to handle applications that demand straightforward programming, high reliability and process fault diagnosis. These applications include machinery, assembly lines and robotic devices. 

PLCs have been widely adopted across industry as high-reliability automation controllers adapted to many working conditions, including harsh environments. They’re capable of many more advanced functions and are commonly used to control the complex processes of an automation environment.

What Does A PLC Do?

A PLC monitors inputs and outputs in order to control automated industrial processes. These inputs can be from human interfaces such as buttons or switches, or from automated data capture points like sensors. A PLC controls the functions of an automated system using the internal logic that’s been programmed into it. 

The definition and function of PLCs are sometimes confused with the PCs or personal computers developed for industrial use. Industrial PCs are now being designed with performance accelerators that provide the supervisory control offered by PLCs. But PLCs are also able to consolidate more workloads like AI applications, gateways, etc. This reduces their hardware footprint in the workplace. 

With the technological advances of today’s Industrial Internet of Things (IIoT), modern PLCs are able to perform more advanced functions. They can connect to databases via SQL (Structured Query Language), communicate data via web browsers and even connect to cloud data via MQTT (Message Queuing Telemetry Transport). 

What Sets PLCs Apart?

PLCs provide a few key features that set them apart from other industrial control solutions:

• Input and output modules can be either digital or analogue and connect the PLC to the rest of its working components. While the CPU is responsible for storing and processing program data, it’s the I/O modules that provide the CPU with information and trigger specific responses. Input devices might include sensors, meters and switches, with outputs including lights, valves, relays and drives. Small modular PLCs may contain a few dozen integral I/Os, while large rack-mounted modular devices can offer thousands. These large units are often networked to other PLCs and supervisory control and data acquisition (SCADA) systems.

• When connecting with these other systems, PLCs need to communicate with them via a range of ports and communication protocols. Application data recorded by the PLC can be exported to a SCADA system, which is responsible for monitoring multiple connected devices. 

• Users need an HMI (Human Machine Interface) to interact in real time with the PLC, to input and review information. These go from simple displays, with a keypad and text readout, to much bigger touchscreen panels with more complex functions. 

What Are The Different Types Of PLC? 

There are three different types of PLC commonly in use: 

• Fixed integrated PLC

• Distributed PLC

• Modular PLC

Of these, the Fixed and Modular are the most commonly used, based on their hardware setup. An additional option is the Soft PLC, where the PLC is run as a program in a general-purpose computer. This can be secure and efficient if you have the right setup to incorporate it, but requires greater software implementation and maintenance skills. Not all applications support this.

Fixed Integrated Or Fixed I/O PLC

This type, commonly called a fixed I/O PLC, houses both the controller and the I/O modules within a single compact unit. This means that the memory and the number & form of I/O options are pre-determined by the manufacturer. They’re good for small-scale applications, as everything is contained in the unit, including the power supply, processor and memory. They cost less, but are disadvantaged by their lack of flexibility and inability to address more complex tasks. Also, you might have to replace the whole system if any component malfunctions, as their pre-assembled nature makes repairs more difficult.

Distributed PLC

In this type of PLC, various controllers and I/O devices are connected via a network. This means you can control the system remotely and get faster responses from the distributed PLC. There’s less wiring and it’s simpler than other systems, but it may cost more to use if not operating continuously.

Modular PLC

Modular PLCs are popular because the CPU, power supply and I/O options are all contained in separate modules within a chassis. They’re usually rack-mounted, where the rack itself houses the communications module. They’re simpler to use and capable of incorporating various expansion modules that provide more functionality, such as additional I/O units. 

Modular PLCs cost more than compact PLCs, but have the advantage of easier maintenance and scaleability without extra downtime. Modular PLCs can handle more complicated procedures, such as identifying problems quickly and fixing them while other processes continue to run. 

Some disadvantages of modular PLCs are their greater configuration complexity and less efficient installation space. Trouble-shooting costs may increase when modules fail and you’ll have to keep more spare inventory on site. They also require enclosures, since they have low IP ratings. Their nature promotes centralised rather than distributed I/O, contrary to the norm in modern automation technology.

How Do PLCs Work?

The PLC works on the basis of information received from its input devices or connected sensors. It processes the received data and uses pre-programmed parameters to trigger selected outputs. Users can configure the I/Os on their PLCs to get exactly the responses they need for their application. Depending on what I/Os are configured, a PLC is able to monitor and record all sorts of run-time data. Operations include automatic start and stop processes, monitoring operating temperature or machine productivity and generating alarms if a machine malfunctions.

PLC programs are usually written separately, then downloaded onto the controller. Some manufacturers supply this with the PLC. Most programming software for PLCs offers a choice of programming languages, either the newer basic “C” or the traditional Ladder Diagram (LD). Each rung on the logic ladder represents a specific action that the PLC controls. 

Robust and Adaptable Solutions

Programmable Logic Controllers are a robust and flexible control solution for industrial automation, adaptable to almost any application. Fixed, compact PLCs are an ideal solution for small-scale operations, while modular and distributed PLCs are capable of handling larger and more complex processes. Many different arrangements of digital and analogue I/O can be incorporated, with modules or units configured for a range of specific environments. These include resistance to vibration and impact, immunity to electrical noise and extended temperature ranges.