Back to the future of the PLC

Programmable logic controllers (PLCs) are not going away any time soon, and improving technologies combined with user demands will continue their evolution as a foundational automation platform.

By Jeff Payne May 2, 2024
Courtesy: AutomationDirect


Learning Objectives

  • Understand how programmable logic controllers (PLCs) have evolved since 2014 and what their role in automation facilities is now.
  • Learn how communication protocol advances and artificial intelligence/machine learning (AI/ML) also are changing PLCs.

Programmable logic controller (PLC) insights

  • Programmable logic controllers (PLCs), now almost senior citizens, evolve amidst rapid technological advancement, incorporating mainstream technologies while enhancing capabilities, connectivity, and real-time control functions.
  • Users seek openness in industrial systems while valuing industrial-grade reliability. As PLC programming languages diversify, classic ladder logic remains prevalent, ensuring simplicity and wide user acceptance.
  • Robotics, particularly collaborative robotics (cobots), and artificial intelligence and machine learning (AI/ML) represent significant growth areas in industrial automation. PLCs are poised to integrate with these technologies, facilitating seamless automation.

Almost 10 years ago, I wrote about the “Future of the PLC” for this publication. Even back then, it was important to mention that programmable logic controller (PLC) technology was mature at nearly 50 years old. A decade later, a fair question is whether today’s PLCs have fully entered senior citizen status, and if future iterations are destined for the grave.

This discussion is especially relevant considering the rapid—and sometimes seemingly exponential—acceleration in computing hardware, software, intelligent measurement, cloud accessibility and communications connectivity. With these and other advances, information technology (IT) has proliferated steadily into the formerly isolated operational technology (OT) realm.

While the article discussed the dramatically increasing importance of communications and connectivity, it did not specifically include the term “industrial Internet of Things” (IIoT), but today IIoT capabilities are essential for nearly any application.

In light of these developments, here are a few thoughts on what the next decade may hold for PLC evolution and industrial automation applications.

Remaining true to the task

The core mission of a PLC remains the same as always: To deliver deterministic control and reliable monitoring of physical field devices, even under challenging operational conditions. This has been achieved using specialized processors, operating systems, and programming environments, built into hardened platforms. Yet economies of scale continue to drive the adoption of mainstream consumer and commercial technologies into the PLC role, wherever it is practical. The “smaller, faster, better” maxim has held true and will continue to do so, but mostly around the faster and better aspects, as the trend to further miniaturization has leveled off over the past decade.

Many benefits of electronic component, processor, and solid-state memory advances—reduced cost, shrinking size, minimized power consumption, and increased capability—have already been incorporated into PLCs and other industrial electronics. While marginal size, cost, and power improvements will continue, the real advances will be around capabilities.

At this point, platform size is largely constrained by the need for physical wiring to interface with PLC input/output (I/O) modules. Traditional wired I/O remains necessary, but in many cases, the connectivity with field devices is shifting to digital networks and distributed remotely using technologies like IO-Link and wireless.

Multi-core processors incorporated PLC designs now enable deterministic control to be supplemented with extensive additional computational and communication function. For over 20 years, the term programmable automation controller (PAC) has been used loosely to describe an industrial controller with greater capabilities than a classic PLC.

While a PAC may have initially seemed like a distinct product compared with a PLC, time has proven that automation engineers are less concerned with the nomenclature and much more interested in performance and available features when specifying industrial automation.

While market offerings range from basic PLCs to complex PACs, the concept of an industrial control platform has largely merged into a continuous spectrum of capabilities. Moving forward, users will be willing to consider most any type of underlying hardware platform or operating system as an automation platform—which may continue to be called a PLC but will actually be so much more—if it can deliver proven real-time control, while providing other required advanced computing capabilities.

Reconciling flexibility with consistency

Although Windows-based systems dominate the consumer and commercial PC world, and are prominent for the industrial visualization realm, this is not the case for real-time control. PLC/PAC platforms typically run a specialized operating system, although there are some Linux-based options. In very general terms, users must balance their desire for openness—which provides great flexibility and low product costs—with the requirement for industrial-grade reliability historically delivered only by proprietary systems. These proprietary systems also provide a high degree of cybersecurity, albeit primarily through obscurity and to an extent unfamiliarity to hackers.

For many years there has been a trend, or at least great interest, towards more open industrial systems, both in terms of hardware platforms and for programming languages. Some end users have applied generic Raspberry Pi and Arduino hardware to implement automation and data handling projects. Others have avoided experimenting in this way with consumer-grade products due to concerns about reliability, but now a few versions of these platforms have been hardened into industrial-grade devices (Figure 1). Users are showing great demand for the ability to combine contemporary programming platforms with proven industrial I/O and hardware.

Figure 1: Now that contemporary open-source processor platforms are available in industrial-grade form factors, such as the AutomationDirect ProductivityOpen, end users now have options for integrating traditional automation methods with more modern IT-based languages.

Figure 1: Now that contemporary open-source processor platforms are available in industrial-grade form factors, such as the AutomationDirect ProductivityOpen, end users now have options for integrating traditional automation methods with more modern IT-based languages. Courtesy: AutomationDirect

With such a range of hardware options, the next hurdle for openness has been homogenizing the programming environment. Classic PLCs used vendor-specific programming that was difficult to port to other brands. The IEC 61131-3 standard introduced ordered PLC programing languages and data types, but vendor-specific implementations still hampered code portability among brands. Eventually, the CODESYS integrated development environment (IDE) offered a more consistent way to create code using the standard languages to deploy it cross-platform on industrial controllers.

However, none of these initiatives addressed the fact that programmers entering the workforce often preferred to code in more modern IT-based languages such as C++ or Python.

Despite all these efforts heading to openness and modern programming languages, it seems safe to say that classic ladder logic is here to stay for the foreseeable future. Ladder logic enjoys a massive installed base, and it remains a simple coding methodology preferred by many electricians, technicians, and even developers. Its graphical style lends itself to basic troubleshooting and typical industrial automation functions, and its widespread familiarity provides other advantages.

Today most hardware platforms support ladder logic—whether proprietary or implemented via another IDE such as CODESYS—and many also allow other types of coding methods, which can be mixed-and-matched as needed. Various coding languages have their own strengths and weaknesses for specific tasks, and most users like to apply their own judgement when choosing the best tool for solving a problem, while balancing flexibility against complexity. An added bonus for users is moving outside of proprietary languages enables them to curate a library of code, which can be deployed on any type of target hardware, minimizing rework.

The main point today and looking ahead is users desire automation platforms offered and backed by trusted and experienced industrial suppliers, with provisions for supporting any type of preferred programming language.

Tying it all together with communications

Some of the greatest industrial automation strides over the last decade are associated with communications improvements, leading to a truly connected factory. As with controller hardware and programming, the story has been one of moving away from proprietary implementations and towards a more open offering.

Traditional OT-centric fieldbuses, such as DeviceNet, had long offered the reliability and installation form factors demanded by users. But now wired, and even wireless, Ethernet variants are dominating, with several leading industrial communications protocols available. Physical form factor improvements, such as washdown-rated and connectorized components and power over Ethernet (PoE) now enable Ethernet installations to be suitable for industrial environments.

Certain OT protocols such as EtherNet/IP, PROFINET, and Modbus-TCP are associated with makes and models of field devices, while others are optimized for types of automation tasks (such EtherCAT for motion control). While EtherCAT is not new, the incorporation of this protocol natively into more capable PLCs now means low- and medium-complexity motion applications are can be integrated natively into an automation platform without requiring separate motion controllers.

Ethernet-APL is an OT-optimized media, which makes it easier to deploy wired Ethernet out to field devices. IO-Link is on the rise as a streamlined fieldbus—even for basic discrete automation devices—with fit-for-purpose communications capabilities and intelligence.

Bridging OT to IT to securely enable IIoT applications and data transfer in support of remote visualization and analytics requires a different class of communication protocols. OPC UA and message queuing telemetry transport (MQTT) are dominant in this role. While some of their capabilities overlap, there are optimal use cases for both protocols, and users can choose to implement them simultaneously. Other supporting tools, such as Node-RED, have become favored as a graphical method for processing and pushing data to the cloud for consumption by other applications.

From sensor to controller, to on-premises server, to cloud-based resources, to browser, what does this all mean? In the “old days,” smaller controllers would have a limited feature set, so larger devices or multiple integration layers were required to achieve complete connectivity. Today and into the future, users will want these options available in even very basic and low-cost automation platforms (Figure 2).

Figure 2: Today, even a low-cost PLC automation platform like the AutomationDirect CLICK PLUS is packed with advanced logic capabilities, essential motion control, wired/wireless connectivity, a range of IT/OT communications protocols, and more.

Figure 2: Today, even a low-cost PLC automation platform like the AutomationDirect CLICK PLUS is packed with advanced logic capabilities, essential motion control, wired/wireless connectivity, a range of IT/OT communications protocols, and more. Courtesy: AutomationDirect

The role of integrated robotics

For many years, robotics has largely existed as a specialty subset of automation, requiring custom integration into upstream and downstream systems. This is morphing as robotics in general, and collaborative robotics (cobots) in particular, look to be among the single largest growth areas throughout all industrial automation over the next 5 to 10 years (Figure 3). In a related development, vision systems have advanced tremendously in the past decade, and many are very compatible with robots, allowing easy integration in a host of applications.

Figure 3: Robotics represents a fast-growing area of industrial design, and the demand for capable automation platforms and associated sensing technologies will increase as users seek to thoroughly integrate robotics into their operations.

Figure 3: Robotics represents a fast-growing area of industrial design, and the demand for capable automation platforms and associated sensing technologies will increase as users seek to thoroughly integrate robotics into their operations. Courtesy: AutomationDirect

Modern automation platforms need to be prepared to keep up with this changing landscape by providing the requisite processing power, programming instructions, and connectivity to seamlessly integrate with robotics and vision. A contemporary PLC with these capabilities located near field-installed robotics offers a distinct advantage as an automation platform.

AI’s role in the PLC’s future

No future-looking industrial automation article written in 2024 could overlook the potential impacts of artificial intelligence (AI) and machine learning (ML). However, much of the current buzz is around using AI/ML in a live “runtime” role to analyze and react to conditions. As an automation platform, PLCs are not currently ideally suited for this task, but some advanced versions may be able to run live AI/ML algorithms in the future.

Instead, PLCs are well placed to act as the field interface for higher-level AI/ML resources, providing users with abundant, responsive and contextualized data. They also can implement actions dictated by algorithms.

On the other hand, generative AI (Gen-AI) is poised to play a bigger role with PLCs in coming years from a code creation standpoint. Development environments with carefully integrated AI support tools could help users—perhaps even relative newcomers to the field—develop useful automation logic based on libraries and proven code. AI, used as a development tool, could help speed development times, improve code reliability and minimize redundant or repetitive labor.

The future PLC is one piece of an automation platform

Over the next decade, PLCs as we know them will certainly not go away, even if they are referred to as PACs, or edge controllers, or automation platforms, or something else. There will be no single controller technology that can fulfill all roles at all price points.

Instead, PLCs will continue to evolve based on available technologies and user demand, just as they have done so for the past five decades. The priority will be delivering real-time control and reliable monitoring, but they will add even better programming and connectivity functions to improve the user experience, and the speed at which projects can be delivered.

Jeff Payne is the director of business development at AutomationDirect. Edited by Chris Vavra, web content manager, CFE Media and Technology,


Keywords: PLC, programmable logic controller


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