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[Movensys Article] The Limitations of PLCs and a New Starting Point for PCs
- 2025.05.20
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- 최고관리자
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In the automation market, PCs and PLCs are frequently compared by professionals. However, the conclusion is not about which is superior, but rather that each has its own direction and strengths. In other words, each has fields where it excels.
Figure 1. Motion Control Trends Driven by Industrial Automation
At one time, innovation in the automation market centered around the emergence of PLCs as hardware. In the late 1960s, PLCs brought about a major transformation by integrating the complex wiring of relay control panels into a single controller that could implement logic. Based on intuitive Ladder Logic and using dedicated loaders for applying and modifying programs, PLCs drastically simplified the previously cumbersome wiring tasks. Since then, PLCs have continued to evolve and gradually expanded their control range.
On the other hand, computers of the same era—or even earlier—primarily served computing functions. In fact, they were mainly tools to support commercial decision-making within enterprises. The concept of the personal computer (PC), which emerged in the 1970s, quickly spread across numerous fields requiring data computation and storage. As such, PCs originated from IT (Information Technology), while PLCs emerged from OT (Operation Technology), each starting in entirely different domains for different purposes.
When first introduced in the late 1960s, PLCs primarily replaced relay panels and performed basic on/off control using logical combinations of inputs and outputs along with timers and counters. Thanks to advancements in microprocessors and power electronics, PLCs expanded their scope to include analog signal processing, communications, temperature control, and motion control. Looking at PLC technology trends, it’s evident that both software and hardware have made significant progress over the past 60 years. From Ladder Logic to various programming paradigms like Structured Text (ST), Sequential Function Chart (SFC), and Function Block (FB), as well as networking and precise motion control, the control formats and range of today’s PLCs have changed dramatically compared to the past.
Looking deeper into the evolution of PLCs, we can see that many of these advancements came from adopting IT technologies. Terms such as User Defined Data Types, Instance, Class, Array, and Alias—which were once unfamiliar to PLC users—are now naturally integrated. Furthermore, IT features like Web Servers, VNC, and Remote Access are also being applied to PLCs. Nowadays, few would dispute the fact that IT technologies have become essential in PLC environments.
Figure 2. Structural Comparison between Hardware-Based and Software-Based Motion Control Systems
Just a little over a decade ago, most PLC manufacturers operated conservatively, relying on their solid installed base and loyal customers. However, relying solely on proprietary networks and programming methods made it difficult to stay competitive in the market. That’s because enhancing performance or improving programming efficiency in a vendor-constrained environment was less feasible compared to the openness of IT technologies. In terms of cost and time, adopting IT technology was far more advantageous than developing everything internally. A prime example of this is Ethernet-based Fieldbus technology. By riding the wave of IT advancements, PLCs were able to secure performance and evolve into future-proof technologies.
Looking at this process more closely, PLCs are becoming increasingly similar to PCs. Object-oriented programming methods, efforts to ensure cyclic determinism, modules that support the C language, direct communication between MES and databases, and even attempts to embed PC modules within PLCs all reflect efforts to integrate or align the two. Excluding the black-box hardware nature that prioritizes stability, many aspects have become similar. Today, the term "Programmable Logic Controller" no longer fully captures the functionality and scope of PLCs. This is why the term "PAC (Programmable Automation Controller)" is increasingly being used to reflect their broader role in automation systems.
In any case, modern PLCs face the challenge of handling higher performance demands and greater volumes of data. While they manage a considerable amount of data, they still fall short when it comes to accumulating and analyzing real-time data. Recently, the demand for data collection and analysis at the equipment level has been growing, and PC usage is also increasing to meet this need. This trend has also been accelerated by the Fourth Industrial Revolution. As a result, it is now common to see factory systems where PLCs and PCs interoperate. In these cases, PLCs handle control, while PCs are responsible for data management.
However, in certain industries, PCs are used not only for large-scale data processing but also for equipment control, just like PLCs. A representative example is semiconductor equipment. Nevertheless, when we look at the entire automation market, PC-based control still occupies a very small share compared to PLCs. This likely stems from factors such as the long history of PLCs being optimized for automation, the relatively shorter history of PCs in the field, high entry barriers (from the perspective of PLC engineers), and skepticism regarding their stability or sustainability. But the key point is that PCs can control industrial equipment as well. Just as PLCs have expanded their control scope, PCs have also broadened their applicability beyond specific industries. Technologies such as Soft PLC and Soft Motion, once handled solely by PLCs, are now being implemented—and often more effectively—on PCs.
Several factors contribute to this trend. These include improved PC CPU performance, expanded Fieldbus coverage (from remote I/O to full motion control), increased IPC reliability, and the availability of RTOS (Real-Time Operating Systems).
Figure 3. Key Differences among PLC, PC, and PAC Control Architectures
We now live in an era where PLCs and PCs are crossing boundaries and expanding into each other’s specialties, creating substantial overlap. From the user's perspective, this might not be easy to notice. That's because, in the field, it's rare to find equipment vendors who compare both sides directly or engineers who master both technologies. Moreover, PLCs and PCs still have their respective specialized areas, and due to industry characteristics, historical inertia, and fixed infrastructures (supply chains, human/material resources), there is still a strong tendency to maintain or improve within the existing approach. In other words, cases where they compete head-to-head in the market are extremely rare.
Nonetheless, their overlap will inevitably continue to grow. In the near future, we must move beyond the distinction between PC and PLC. Ultimately, we should aim for a direction where users can fully embrace diverse IT technologies, embed their consolidated expertise and know-how into systems, protect their proprietary knowledge, and gain the competitive advantage they need in the market.
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