Blurring the line between PLC and DCS

If you work at a refinery or chemical plant, chances are its processes are under direct control of a distributed control system. Industries on the fringe of what has traditionally been known as 'hard’ process industries – or those hybrid industries that run their processes in batches or have characteristics of process and discrete manufacturing – are the ones most likely to bu...

By Jack Smith, Managing Editor February 1, 2007

If you work at a refinery or chemical plant, chances are its processes are under direct control of a distributed control system. Industries on the fringe of what has traditionally been known as ‘hard’ process industries — or those hybrid industries that run their processes in batches or have characteristics of process and discrete manufacturing — are the ones most likely to buck the DCS trend.

Continuous processes are by their very nature constantly changing. Variability of raw materials, ambient and environmental conditions and constraints inherent in processing equipment are just some of the physics-related elements that affect process variables in continuous or semi-continuous/batch processes. Then there are non-physics constraints such as regulatory control, production rates, business climate, market conditions, product mix and operator experience level that impact the effectiveness, efficiency and profitability of today’s process industries.

Traditionally, DCSs have been used primarily in process industries such as oil and gas production and transport; refining; chemical processes; food and beverage; power generation; water treatment; pulp and paper production; and mining separation and metals production. PLCs have been used in discrete industries such as automotive; electronic assembly; bottling facilities; machine parts manufacturing; conveyor belt sorting and material movement; and skid mounted process equipment.

However, batch industries such as food and beverage, pharmaceuticals and bottling have used both — or control systems labeled as hybrid. Some of these industries, such as food and beverage, and bottling overlap because they include both process and discrete operations.

Cindy Scott, DeltaV product manager at Emerson Process Management in Austin, TX said both DCS and PLC technologies are traditional in that they have been around for more than 30 years solving different manufacturing problems. But each system has different strengths and weaknesses. “PLC systems are good for manufacturing processes that require coordination of many on/off functions; DCS systems are more typically found in production processes involving the moving of liquids or solids where heating, blending, reacting, cooling, crystallization and other processing steps involving ramping functions are common,” she said.

Traditional DCS systems grew up serving the continuous process industries. The DCS price point, architecture and footprint forced them to be viable only in the highly analog parts of continuous industries, according to Steve Ryan, director of process solutions at GE Fanuc Automation. “However, even companies within those industries tend to use non-traditional process control systems in other applications and processes within their facilities, because DCSs are not multi-disciplined controllers,” he said.

“The PLC and the DCS each have their ‘sweet spots’ where they are optimized to deliver the best possible results,” said Dan Collins, DCS Solution Partner program manager at Siemens Energy & Automation, Spring House, PA. “Many industries are taking a common-sense approach to control: enlisting a PLC for motor control and a DCS for analog control to cover all the upstream and downstream bases in their production process. Others, such as the hybrid industries, are demanding PLC-like functionality (for example speed and ladder-logic programming) in their DCS solutions.”

The distinctions between traditional DCSs and PLCs regarding applications solved by each, as well as what constitutes one versus the other have been blurring. “The hybrid control system — satisfying regulatory, discrete, sequential and batch control — is fairly well established,” said Jason Urso, director of marketing of Honeywell Process Solutions, Phoenix. “The future vision has to do with the information that these systems capture and manipulate, and its transformation into higher-order applications and knowledge.”

Many companies have opted to run both DCS and non-traditional control strategies across their plant depending on what type of process is being controlled. “The ability to employ a single, hybrid control strategy that combines the benefits of both DCS and PLC/HMI control is a huge advantage for a company today, enabling them to run completely different processes from the same controller environment,” Ryan said.

Process control essentials

Process automation systems have similar architectures, which include I/O, controllers, HMI (or some type of visibility into the process), field devices and networks that connect them. Bruce Jensen, manager of systems marketing and sales support at Yokogawa Corp. of America, Sugar Land, TX, said the difference depends how these components fit together.

“The basic difference in DCS vs. PC/PLC architectures involves how redundancy is handled; databases and database structures; integration of components; integration of subsystems and supervisory systems; and relative sophistication of configuration and engineering tools,” Jensen said.

Randy Balentine, DeltaV product manager at Emerson, said PLC systems tend to differ from DCS architecture in that the manufacturers of the controllers don’t tend to manufacture the hardware/software for the operator interface. They publish interfaces that allow operator interfaces from several different companies be used. This ‘publishing’ of the controller interface can make it more susceptible to attacks by hackers,” he said.

“One thing that is very different in PLC vs. DCS systems is the ability of DCS systems to work with intelligent field devices,” Balentine said. “Today the microprocessors in sensors and valves contain significant information about the performance, health and operation of the field device. Digital communication protocols like HART and Foundation fieldbus exist to communicate this information to the automation system at the same time as the process variable being measured or controlled. This valuable information is available to the systems like DeltaV, but not generally used by PLC systems.”

Although control systems have these elements in common, certain features and characteristics differentiate them, including reliability, performance, interoperability, usability, security and ultimately a continuing lower cost of ownership over the life of the installation.

“In any of the process industries, if the control system doesn’t work, the process is down and money is lost,” said Jensen. “Thus, in continuous processes, 24/7/365 operation is paramount. Hardware reliability, software reliability, ability to do on-line upgrades, make on-line changes and live with different versions of software in the architecture are important aspects.”

“Multi-discipline control,” added Ryan. “Companies today have to have a system to meet the multitude of process applications within their facilities. Also, systems must be inherently integrated to the information layer. Islands of automation simply can’t meet the real-time needs of today’s production requirements. Customers want one controller that does it all, and they want greater visibility into their manufacturing and production processes. In today’s competitive environment — particularly in the process industries — manufacturing must be tightly integrated as part of the supply chain, so companies can remain efficient and reduce costs.”

Some end users have a continuum of elements ‘integrated’ — or in some cases, forced — to work together, and somehow accomplish this with varying results. But Rick Dolezal, process market development manager at Rockwell Automation recommends a way around this. “Advanced process control within a unified engineering environment is becoming vital for companies to compete. Being able to leverage the data streaming from the plant floor is increasingly important so manufacturers can tweak the last bit of productivity out of their production cycles and keep up with the ambitious production schedules demanded of them,” he said.

Bringing ideas together

Advances in technology have allowed companies in the process industries to combine the advantages of DCS and PLC/HMI control without the historic limitations tied to each of those options, according to Ryan. “You can now have the global data access, function blocks and inherent redundancy of a DCS system with the speed, scalability and openness of a PLC/HMI solution — all at an extremely competitive price point,” he said.

“Standards always drive technology and industry applications, but methodologies from standards organizations, such as OPC-UA, are clearly making systems more effective,” Ryan continued. “Another example would be at the fieldbus layer with the Foundation fieldbus. Advancement lies in all of the work being done with wireless technologies and ISA SP 100, which offers significant advantages for manufacturers.”

Many end users tend to focus more on initial price instead of life cycle costs. This trend applies to many industries — not just process control. “It is tempting for end users of automation to put much emphasis on the initial cost of an automaton system,” said Scott. “This approach, if followed, can result in a substantially higher overall life-cycle-cost to the end user. Care should be taken to evaluate the costs of ongoing configuration maintenance; costs to engineer changes and expansions; costs to update the system for security; and costs to keep current with required system upgrades.”

Brian Owens, industrial process market segment manager at Schneider Electric’s Raleigh, NC facility said that one of the biggest mistakes that end users make is tying themselves to one horse. “There are many good reasons to stay with a manufacturer or vendor. Obviously, you have extensive training costs involved with changing, not to mention the interruption in production,” he said. “But end users rely a little bit too much on us to tell them the one-and-only way to do something. What they’re not doing enough of is sharing information among themselves to find a better way to do things.

“Organizations like WBF — those guys all kind of get it,” added Owens. “They come together and they share ideas — they drove the development of S88, for example. End users need to talk to each other and understand from each other what works and what doesn’t.”

“End users, systems integrators and magazine editors really don’t understand the issues surrounding fieldbus, such as intrinsic safety, redundancy and short circuits,” said Scott Saunders, vice president of Moore Industries. “They also don’t know what products are available to solve fieldbus problems. In many cases, users agree to buy a solution because the DCS vendor recommends it.

“Because they don’t know that alternatives are available, they spend far too much on older technology because ‘we’ve always done it that way,’” Saunders continued. “In fact, modern fieldbus equipment costs less, is far more capable and provides many more advantages than traditional fieldbus devices.”

According to Stan DeVries, industry marketing director, InFusion industry solutions consultant at Invensys Process Systems, other common control system-related mistakes that end users make include:

Lack of holistic and comprehensive strategies for change management, information structure, integration, visualization and lifecycle management challenges

Avoiding overall system and architecture issues

Poor collaboration among various disciplines

Attempting to base the architecture on the wrong components.

Jensen said some end users fail to understand the skill set needed for the technologies they are deploying. “A higher level of training and understanding is required,” he said. “The capabilities of the technology that is purchased, installed and operating the plant is grossly under-utilized because there is insufficient manpower and insufficient knowledge of the deployed technology to implement and use the features that exist in the system in order to improve the business.”

The Bottom Line…

Whether physics-related or business related, continuous processes change constantly.

Traditionally, chemical, oil and gas refineries and other process industries have used distributed control systems.

PLCs and DCSs have many similarities — and many differences.

The distinction between DCS and PLC is blurred; so is the distinction between the industries they serve.

Advances in technology combine the advantages of DCS and PLC control without the historic limitations tied to them.