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By Rich Merritt
What’s a distributed control system? We asked end users, systems integrators and vendors and got answers ranging from “a control system with two PLCs connected by a network,” to “DCS doesn’t exist any more. Modern systems are (insert DCS vendor’s euphemism here).”
One reason we can’t precisely define a DCS is because they have changed so much since 1975 when Honeywell and Yokogawa introduced them. In those early days, DCSs used multiloop digital controllers to perform regulatory control.
But much has changed in the past 33 years. What were once simple control systems are now decision-support and decision-automation systems. Users want systems that will put more people and functions—engineers, managers, maintenance, planning, environmental, and the supply chain—in touch with the process. We want to gather more data, hold it longer, and turn it into useful information. We also want to report problems on an exception basis in a timely manner to the right people—not just the operator—and to automate complex decisions, such as real-time process optimization.
A modern sub-$1,000 PLC can perform regulatory control, probably as well as any DCS—but it can’t perform all the high-level functions. For that, it needs a PC supervisor and a lot of software. Enter the HMI/SCADA vendors, who have added all the necessary high-level software and emerged as a formidable competitor to DCSs.
Figure 1. A “typical” DCS system, depending on whom you ask.
The days of proprietary control systems are over—almost. According to Peter Martin, vice president for strategic ventures at Invensys Process Systems (IPS), the trend toward commerical off-the-shelf (COTS) systems began in 1987 when Foxboro introduced the I/A Series control system and with it the concept of using standard hardware, software and communications in a control system. (For a discussion of “Should Control Engineers Do Windows?” scroll to the bottom of page 3 or use the following link www.controlglobal.com/0812DCS.html.)
Todd Stauffer, PCS 7 Marketing Manager at Siemens Energy & Automation, says, “In 1998, the DCS was coming out of the proprietary age. The move toward open systems and commercial off-the-shelf technology was driven by users and suppliers alike. This transition has allowed the pace of technology innovation to increase. We would have never made it to this point if we had stayed proprietary.”
Proprietary networks have been replaced with various versions of Ethernet and COTS routers, switches and wireless devices. Computer hardware is all standard stuff, all based around the same basic hardware and Windows-based software. In fact, some DCSs use conventional controllers and HMI/SCADA software, says Jim Ford, director at system integrator Maverick Technologies. “DCSs are thrown together all the time with PLCs for basic control and software, such as Wonderware used for the HMI.”
Thanks to their conformity with fieldbus, HART and other industry standards, most process sensors and transmitters have nothing proprietary about them. You can use anybody’s hardware in your control system.
The major DCS vendors also use fieldbus because they have the necessary hardware and software. While fieldbus is not proprietary, only the big DCS vendors have made the necessary investment in things like fieldbus protocol stacks and H1 cards. Few, if any, of the competing HMI/SCADA control systems offer fieldbus, but this will change as soon as fieldbus hardware becomes available for PC-based systems.
So what’s still proprietary? The hardware controllers and I/O from most major DCS. vendors.
And software, of course. Or is it? Some DCSs use historians, asset management programs, loop-tuning programs and ERP software from the same suppliers—as do several of the HMI/SCADA-based control systems, such as Wonderware, Citect, Indusoft and Iconics. Both the big DCS and the HMI/SCADA-based system vendors claim to have completely integrated software from the loop controllers to ERP.
From an architecture standpoint, the major difference between the two is that DCS systems use proprietary hardware, while HMI/SCADA systems use conventional hardware, such as PLCs, panel PCs and hybrid controllers.
From a functionality viewpoint, an HMI/SCADA system can offer exactly the same capabilities as a DCS (except for fieldbus). An HMI/SCADA system uses the same field devices, the same Ethernet networks and the same COTS-based HMIs as a DCS. An HMI/SCADA system has the same type of software available as a DCS.
To counter claims that HMI/SCADA can’t handle “big jobs,” Scott Kortier, marketing communications manager at InduSoft, points to Brookhaven National Labs, in Brookhaven, N.Y., with its Relativistic Heavy Ion Collider (RHIC). Indusoft recently upgraded the RHIC’s control system. “This system includes over 10,000 I/O points, a dozen operator stations plus 15 security-enabled, browser-based displays,” says Kortier.
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