01 January 2003
Barriers bite the dust
DCS, PLC, and PC technologies converge in new breed of hybrid controllers.
By Matthew Lamoreaux
When PC-based control systems came onto the industrial automation and process control scene in the 1990s, proponents of these more powerful, open, and affordable systems declared that they would supplant programmable logic controllers (PLCs) and even distributed control systems (DCSs) for many applications.
But many seasoned control system engineers were not so sure. Vibration, shock, corrosion, extreme temperatures, hazardous areas-conditions that are commonly found in industrial control environments-posed a problem for PC hardware designed for desktops. Further , the most common PC operating system, Microsoft Windows, did not necessarily deliver re liable continuous operation or determinism.
So after serious debate among product developers and users, it became clear that both sides of this debate were right.
The PC had much to offer, but it would not supplant proven industrial control platforms-at least not in the form that rests on our desks.
Working on a product development cycle of years rather than the few months common to each new evolution of the office PC, control equipment manufacturers now offer a new breed of eclectic hybrid controllers. These hybrids promise to redraw traditional product boundaries between analog and discrete control.
As one would expect, each product developer approached development of hybrid systems from the perspective of building out from or onto its core product line. Traditionally, the DCS dominated the realm of analog control. Often, DCS systems sat side by side with separate PLC systems for discrete on/off control.
Establishing communication between the two systems was cumbersome and expensive, if attainable at all. Maintenance of two separate systems was also inefficient.
So it is not surprising that DCS manufacturers began to seriously look at enhancing the discrete control capabilities of their systems. But the inflexibility and expense of DCS frustrated users.
Likewise, PLC manufacturers began to look for ways to incorporate analog capabilities such as proportional-integral-derivative (PID) control into their PLCs. But the same proprietary technologies that made traditional DCS and PLC so good at their respective functions also limited them.
The advent of the PC and open industrial data communications standards allowed manufacturers to finally make meaningful progress in combining analog and discrete control into affordable control systems.
NEW PLAYERS EMERGE
At the same time, a new breed of controls manufacturer emerged that made neither DCS nor PLC. These developers offered affordable solutions on PCs that traditional control manufacturers either did not offer at all or offered only in large, expensive systems.
National Instruments, an early developer of PC-based measurement and control software, launched LabView in 1986. As the product name implies, it originally devel- oped this early PC-based control package for test and measurement applications in laboratory environments.
Another PC-based control manufacturer, Ann Arbor, Mich.-based Entivity Inc.-out of the merger of Steeplechase Software and Think & Do Software, both of which developed PC-based control software-came into being in the early 1990s.
Up until very recently, these PC-based control developers focused on software development. Hardware was the PC, and young lions scoffed at traditional industrial systems such as PLCs and DCSs as relics of a bygone era. But recent developments show a turn in favor of combining PC-based software with embedded industrial hardware reminiscent of-you guessed it-the PLC.
PC CONTROL EMBEDDED
In November 2002, National Instruments announced the availability of its latest and greatest product: Compact Fieldpoint, which is the company's effort to move LabView into harsh industrial environments.
"In essence, this new remote I/O product combines the control capabilities of our flagship LabView software package with an embedded, industrially hardened controller," said Todd Walter, industrial control and measurement product manager for National Instruments.
So what makes it different from a PLC? "LabView provides a programming environment and human-machine interface for both discrete I/O and advanced PID and predictive analog control," said Walter. "We have customers already using this system for applications as diverse as separating crude oil from natural gas and embedded machine control."
But National Instruments is not the only PC-based control manufacturer making a move toward coupling its software with embedded hardware. In October 2002, Entivity announced a partnership with Phoenix Contact Inc. to "develop specific drivers and port its flagship products, Entivity VLC and Entivity Studio, to Phoenix Contact's line of industrial PCs and embedded controllers."
PLC MAKERS AIM AT DCS MARKET
At a Lafarge Canada Inc. cement plant that produces 1 million metric tons per year of cement, engineers needed a DCS that could handle PID loops, analog processing control, motor and discrete on/off valve control, temperature, flow, and pressure control, said a recent Schneider Electric white paper.
The chosen control system for the whole project was Schneider's new Modicon Quantum PLC and Concept software.
Likewise, Rockwell Automation touted the replacement of an outdated DCS with its new ControlLogix PLC in a white paper about an installation for Rhodia ChiRex, a contract pharmaceutical manufacturer of active pharmaceutical ingredients and final stage intermediates for the Food and Drug Administration regulated market.
So what are the key features in these new hybrid control systems from traditional PLC manufacturers that make them capable of performing functions traditionally handled only by DCS? In general terms, they combine integrated architectures that share many similarities with DCSs, advanced PC-based programming software, and beefed-up industrial hardware.
Open networking and data communications standards such as Modbus, Modbus/TCP, ControlNet, EtherNet/IP (ControlNet over Ethernet), OLE for process control, ActiveX, and distributed component object model tie all of the components together, allowing for greater scalability and compatibility.
DCS SUPPLIERS GET DISCRETE
Leading the development of hybrid systems in the DCS realm, Emerson Process Management (previously Fisher-Rosemount) introduced its DeltaV hybrid system in 1998.
Emerson compared DeltaV with a Rockwell Automation Allen-Bradley PLC system for a mainly discrete I/O application in a white paper published in January 2002. DeltaV offers nine key capabilities not available in the PLC system at a price 25% cheaper, according to the Emerson paper.
Indeed, Honeywell now offers hybrid controllers that combine loop and logic control in a single platform. The company recommends its HC900 and UMC800 hybrid controllers for unit process applications requiring analog measurements with discrete actions, discrete measurements with analog actions, and discrete measurements with discrete actions.
These systems provide an integrated operator interface with preformatted displays, and they configure with the same object-oriented, PC-based configuration tool utilizing function block techniques common in a Honeywell DCS.
Invensys's Foxboro I/A Series brings continuous, sequential, and discrete control together in one controller. Each I/A Series discrete I/O module is a micro PLC providing 2-millisecond/5-millisecond ladder logic performance for interlocking applications.
Although the question of what really meets the definition of an open system remains debatable, it is clear that control systems have become more open and more versatile due to the influence of PC technology and open data communications standards.
With major controls manufacturers working to provide the ultimate system that meets the needs of both discrete and analog control, traditional barriers are being broken down. This results in more competition among suppliers, which should benefit end users. IT
Behind the byline
Matthew Lamoreaux is a freelance writer covering the instrumentation and automation market. Contact him at firstname.lastname@example.org.