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The O2 sensor signals the modular controller, which signals the mass flow controller to adjust the nitrogen valve. The controller also controls the heaters. (Source: Yamatake)
If you have a DCS now, you can add TrDC to it. "We are using a wide variety of controllers now, connected via OPC drivers to our HMI system," explains Eric Kettunen, process control supervisor at Bowater Newsprint, Calhoun, Tenn. "We use an appropriate device for each system. For example, we use Siemens PLCs for some safety systems and Modicon Quantums for inexpensive remote controls. We only add I/O to existing legacy DCS systems. We don't purchase new legacy DCS controllers. PLCs and PLC I/O and the ease of networking various manufacturers together has made process control quite simple."
Arnold agrees. "Based on the number of systems we sell that use Modicon Quantums, we think TrDC is growing. In one application, for example, a systems integrator working with a utility was able to exchange expensive turbine governors for a Quantum system. This saved them hundreds of dollars on each installation."
Kettunen is also happy with Ethernet. "Ethernet communication is robust and cost-effective. We stopped relying on one or two suppliers a few years ago. This is not exactly TrDC, but we are no longer bound to proprietary process control networks or specific controllers."
Kettunen is, of course, talking about the joys of working with open systems that actually work together. Ian Verhappen, director of ICE-Pros, a systems integrator in Fort McMurray, Alberta, says it's not always that way. "Open standards and adherence to same without adding enhancements are the keys to success," he notes. "Manufacturers adding enhancements to a standard means it is no longer a standard, and once again they are able to keep everyone else out. This is why independent testing is needed to ensure interoperability and adherence to the standard."
Nevertheless, in spite of a few shortcomings, TrDC is here. You can buy it in fieldbus systems or assemble it yourself with components using Ethernet, Internet, embedded computers, PLCs, wireless networks, and/or cell phones.
But Do You Really Want It?
As we noted, TrDC is somewhat of a return to the 1940s, when process control loops consisted of a control valve, flow or level meter, and a pneumatic single-loop controller, all located in the field. In some cases, the pneumatic controllers were installed in a nearby control room, mostly for protection from the elements. When single-loop electronic controllers entered the picture in the 1950s, the controllers were moved to centralized control rooms, and wires linked them to the field devices. It was the same principle as the pneumatics, but the wires let the controllers be a bit farther away.
Control rooms consisted of racks of single-loop controllers with faceplate analog readouts. Operators could glance down the rack and make sure all the faceplate indicators were centered, which meant the plant was "lined out" and operating perfectly.
With today's devices, we can return to the 1940s and put everything back in the field, including the controllers.
Since 1975, we've heard control engineers repeatedly longing for a return to those days when control was truly distributed. Now, some engineers aren't so sure.
"The operator interface is no longer an inherent part of a field control device," says Bullerdiek. "I'm not sure that having loops still operating buys you much if you can't see what is going on because the network is down. Loss of view is a serious problem, and most facilities will shut down if view cannot be restored in a short period of time."
"I used to be an ardent fan of true distributed control 15-20 years ago," says LaRocca. "I've changed my colors, though. I now believe the low cost, flexibility, and ease of doing advanced control in multi-loop devices far outweighs the integrity and reliability offered by TrDC. This has become even more relevant with the high mean time between failure values available with today's controllers and the availability of fault-tolerant devices for critical loops."
Bullerdiek agrees. "We are seeing much more complicated and interactive control strategies these days. Fragmenting the control hardware may be counterproductive relative to the desire to use more coordinated or multivariable control schemes."
Some process plants may not want single-loop control. Rob Burgman, senior automation engineer at Sun Chemical's pigment division, Muskegon, Mich., works in a batch plant, and he says single-loop controls aren't best there. "Our batch processes are better suited to centralized control, where one has access to all control functionality," he explains. "We operate loops differently, depending on what phase of the S88 batch is executing. Interlocks change depending on what phase is running and what steps are being executed. Our loops only execute during specific parts of the batch process. TrDC would complicate, rather than simplify, our operation."
On the other hand, we haven't had the capability to do TrDC for nearly 30 years, so maybe some engineers don't realize what's possible. Back in the 1940s and 1950s, feedforward, cascade, and multivariable control schemes using single-loop pneumatic and electronic controllers could be incredibly complex. That's all changed.
"One limitation of the original single-loop controllers was they had no capability to network and upload their information to a central data collector," says Jim Loar, engineering group leader at Ciba Specialty Chemicals, Newport, Del. "Data collection was a strip-chart recorder. With field-based controls this network is provided, so I don't see any problem in where the control takes place, as long as it provides the control you need."
"By having control at the site, you're not relying on one centralized processor to control all aspects of an application," Arnold reminds us. "Distributed control means you have quicker response to individual parts of the application, which can be crucial. When an input changes, the output needs to respond accordingly. Distributed control gives you finer control over a system. It doesn't get as far off track as you would with centralized control."
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