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Smart devices and fieldbuses can let you return to the 1940s with single-loop integrity in a truly distributed system
In 1975, when Honeywell introduced the TDC 2000 distributed control system, it was anything but distributed. In those days, process control engineers had a choice of using minicomputer-based, centralized SCADA systems; racks of electronic or pneumatic single-loop controllers with PID technology that originated in the 1940s and 1950s; and combinations thereof.
Although the ads and brochures of the day proclaimed "distributed," DCSs that came out of Honeywell, Fisher Controls, Taylor Instruments, Fischer & Porter, Beckman, Foxboro, and all the other giant instrument companies were actually centralized systems. They brought all the field data into a central control room, ground it up in microcomputers, displayed the results on three-CRT consoles, and sent the appropriate control signals back out to the field actuators and control elements.
Single-loop controllers were replaced by multi-loop controllers, minicomputers were replaced by microcomputers, and field wiring was replaced by redundant data highways. Gone forever, it seemed, was the concept of single-loop integrity; that is, where a failure in a loop's I/O, valve, sensor, controller, or wiring affected only that loop, not an entire process unit.
Today, modern technology has made it possible to return to those thrilling days of yesteryear and the traditional concept of single-loop control. In a way, we've returned to the 1940s, and a new era of truly distributed control (TrDC).
If you want to return, that is.
We're Learning How
If you want single-loop control, the technology is readily available. You need only two basic elements, points out Ed Bullerdiek, control group leader at Marathon Ashland Petroleum, Detroit. "All you need is reasonably fast and robust networking technology and devices that support control at the local level," he says. That hardware and software is readily available from a variety of sources.
Fieldbus is one such technology (Figure 1). "Fieldbus and its capability to have embedded control algorithms physically on the field devices provides the ability to truly localize islands of process control without having to depend on a central controller," says Arturo Medina, systems engineer and automation specialist at R.S. Stover, Marshalltown, Iowa.
Figure 1: Control on the Wire
Fieldbuses and smart devices allow control to shift from the host CPU (top) to the rack or into the field devices themselves (bottom). (Source: ICE-Pros)
Some vendors agree with using fieldbus for TrDC. "There are two key enabling technologies," says Terry Krouth, vice president of PlantWeb Technology at Emerson Process Management (www.emersonprocess.com). "One is Foundation fieldbus, which is designed from the ground up to enable control in the field with function blocks, scheduling, peer-to-peer communications, security, etc. The other technology includes microprocessors and electronics that run with extremely low power consumption and software that can deliver high performance in an environment where power is gold."
Krouth notes that Foundation fieldbus technology is owned by an independent foundation that supports conformance testing and interoperability. "This gives users freedom to choose suppliers based on product performance and functionality."
"The key to highly distributed control is interoperability," adds Dave Appleby, process solutions product manager at Rockwell Automation (www.rockwell.com). "Foundation fieldbus devices are required to be interoperable, providing a user with tools to implement a control system with products from multiple vendors."
Others are not so sure about fieldbus yet. "I am not a big fan of fieldbus," says Michael LaRocca, senior process control engineering specialist at Solutia's W. G. Krummrich Plant, Sauget, Ill. "Until fieldbus has the ability to implement a high-integrity interlock equal to that offered by hardwiring, I don't think there is sufficient wiring cost savings to justify the additional cost and complexities fieldbus introduces."
"With fieldbus, I'm not sure you are at truly distributed control," cautions Bullerdiek. "If you hang several devices on a segment, they are all at risk if you lose power to the segment."
Richard McCormick, process control engineer at the Raffinerie Jean Gaulin refinery in Levis, Quebec, agrees. "One major restriction is the number of devices per wire loop, especially in intrinsically safe areas," he says. "Also, the current level of redundancy for these controls on the wire' is not fully addressed yet. Losing a loop can mean losing many controls."
Other engineers say fieldbus is not necessary, because Ethernet does the same things. "Embedded computing with TCP/IP Ethernet links will work," says Mike Luffey, vice president of engineering at DeSoto County Electric, Horn Lake, Miss.
A reflow furnace control system developed by Yamatake (Figure 2) illustrates how a standalone system can be developed to control a fast, critical process with off-the-shelf components. According to Russell Kirkman, product development manager at Yamatake (www.yamatakeusa.com), the system can be installed directly in a piece of equipment or in a nearby rack on DIN rails.
"The Internet and Ethernet make it possible to do control with small, very fast processors that can be placed with the I/O right where it's required in an application," says Steve Arnold, product marketing specialist at Schneider Electric (www.schneiderautomation.com). "With Foundation fieldbus, the controller is talking over the bus, so it's not truly distributed control."
Figure 2: Fast Field-Based Furnace Control
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