DCS versus PLC: Is the Debate Over?

By Larry O'Brien

Industry Trends

Many of the process industries experiencing high growth today, such as upstream and midstream oil and gas, water and wastewater, and mining, don’t require the extremely sophisticated functions that are normally found in high-end process automation systems installed in the refining and petrochemical industry. This has led many end users to revive the old debate of “DCS versus PLC or PLC vs DCS”.

In the 1990s through the early 2000s, the combination of PLCs with a third-party HMI, tied together with services from a third-party systems integrator became a popular choice for end users looking for a more economical control solution compared to the big, monolithic DCSs that were available at the time. Of course, the tradeoff of a lower installed cost of the PLC-based system meant that future configuration changes might incur more custom programming or services.

PLC vs DCS: Not an Effective Way to Characterize the Market

In ARC Advisory Group’s view, however, “DCS vs. PLC” is no longer an effective way to characterize the market. Solution offerings from the major suppliers have evolved considerably and there are a whole range of products to choose from. Traditional DCSs and PLCs are still out there, but there’s also a wide range of collaborative process automation systems (CPAS) and programmable automation controllers (PACs) that address an entire spectrum of requirements across the full range of manufacturing industries. These new offerings can greatly reduce the custom integration efforts required in the past. For end users, this requires a little more homework when choosing a solution, but the results are well worth it due to reduced project costs, reduced time to startup, and operational benefits.

DCS Becomes Collaborative Process Automation System (CPAS)

Over the past two decades, traditional DCSs have evolved into what ARC calls collaborative process automation systems (CPAS). The landscape of the DCS market shifted dramatically from a technology-centric approach to a business-centric approach around the beginning of the new millennium with the introduction of information technology into process automation. Many users came to realize that their legacy infrastructure presented a barrier to the benefits of information technology and the expected return on investment. Collaborative Process Automation System (CPAS) Vision

As DCSs embraced commercial off the shelf technology (COTS) and moved away from proprietary technology, the suppliers began to focus more on the business value proposition that automation technology could bring to end users. Having the latest technology was no longer enough to justify automation projects.

Suppliers began to offer more scalable systems that could address very small applications all the way up to very large applications. Many of these systems featured new controllers and I/O hardware that were very different than traditional DCSs. On the surface, many of these process controllers seemed more like PLCs than DCS controllers. In fact, many suppliers with a strong presence in the PLC marketplace were using their expertise in manufacturing high-volume control hardware to make process controllers for new DCSs. The swift adoption of IEC 61131-3 programming capabilities also meant that these new systems could handle discrete control applications in addition to continuous and batch control.

At the same time, new process automation systems began to incorporate more integration with what we would traditionally refer to as (ISA) level 3 manufacturing operations management applications. Other changes included an increasing focus on specific applications and industries, a greater emphasis on services, and an overall reduction in the amount of custom integration that was previously required to get different parts of a process automation system to work together.

The end result was a whole new generation of systems. These incorporated open industry standard technology, were extremely scalable for even the smallest process applications to very large applications, and provided increased integration with operations management and other applications. Many of these new systems also incorporated a wide range of industryspecific products, applications, and services. The DCS suppliers could now effectively address a wide range of industries with smaller and simpler applications.

The CPASs of today have continued to embrace and refine these concepts. Automation continues to become more modular and less monolithic. New information technologies, such as virtualization and cloud technology, have been embraced to greatly reduce costs.

Rise of the Programmable Automation Controllers (PACs)

As DCSs transformed into CPASs, traditional PLCs were transforming into programmable automation controllers (PACs). PACs evolved from traditional PLCs and the PAC concept can be thought of as the discrete manufacturing cousin of CPAS. Like CPAS, PACs can provide real-time logic and process control, in addition to embedded HMI and other functions, on a single platform.

With PACs, end users and OEMs can deploy multiple control applications on a single platform, using common flexible and configurable development tools for design and integration. Like CPASs, PACs facilitate a highly distributed automated environment and employ standard network interfaces and programming languages, allowing data exchange across multi-vendor plant networks. PACs facilitate the move to more open modular control architectures and serve as an interface for standards-based device and control networks that connect to business systems with manufacturing operations management (MOM) applications. ISA-95 provides a standard for organizing and controlling the key business process exchange of information between the enterprise and the control systems.

PACs are designed to maximize software and hardware integration. There is one programming and engineering tool, one programming language, and a single database for the complete system. Users have transparent access to all system parameters and functions. Use of intranet, internet, and ITstandards maximize integration with enterpriselevel systems and applications. PACs lower lifecycle costs and total cost of ownership for users by removing barriers to collaboration and integration. PACs also open up proprietary systems to users and integrators for their applications.

So what separates today’s PAC offerings from traditional PLCs and DCSs and why would users want to switch to PACs? ARC believes the answers to these questions lie in the underlying technology platform and the application and performance ranges. The inherent multi-disciplined capability of a PAC requires multiprocessor architecture and other technology advancements. Other controllers simply do not incorporate the backplane design, networking solution, or internal communications bandwidth to ensure that a system can be scaled appropriately for a wide range of multi-disciplined applications while maintaining optimal performance. Add to this the PAC’s higher level of integration with computing systems, and it’s clear that PACs enable users to focus more on performance of the total system with less concern about monitoring and controlling plant-level devices.

Today’s PACs affect operator interface hardware and programming offerings, as users want greater flexibility in their HMI and programming devices and demand increased portability and a single common HMI and programming tool that can span multi-discipline functionality. Users seeking additional value by switching to PACs are requesting more control data to feed their HMIs, necessitating support for higher-bandwidth communications networks. Ideally, operator interface hardware is interfaced either directly to the PAC or via the same Ethernet network. ARC sees increased movement toward thin client/browser-based HMIs and devices with programming packages already installed. Integrating the operator interface functionality into the programming language makes life much easier for the processor and system builder.

Focus on Requirements First, Then Technologies

The rise of CPASs and PACs does not mean that traditional DCSs and PLCs will cease to exist. Suppliers still sell and support traditional DCSs and PLCs. However, traditional DCSs tend to be limited to plant-level process control and information functions and traditional PLCs can be thought of as products that address specific discrete control requirements; but the demands of today’s information-driven industrial environment clearly go beyond either.

Clearly, the mix of products and functionality available in the marketplace has become fairly complex. End users need to “do their homework” to determine which solution fits their requirements. It’s no longer a question of “DCS versus PLC”, or even “CPAS versus PAC.” Users must consider a wide range of choices across a spectrum of control capabilities. The primary guiding principle should be how you can best meet your business objectives using the available technology.

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