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Desborough continues with a litany of problems: "What happens if one loop is tuned too tightly, causing it to oscillate, and this oscillation process propagates to another loop? Is the first loop smart enough to detect the fact that it's influencing the behavior of the other? Is the second smart enough to not cry wolf' and send an alert to the operator that it needs attention? Is either loop smart enough to realize it is not critical to the process and should not be alerting the operator as to its performance?"
He goes on, listing several problems that are easily solved with networks, centralized control, and advanced supervisory software.
Blending in Single-Loop Control
No one is saying we should revert completely to the good ol' days of single-loop controllers operating completely alone. Putting control in the field is a good idea for several reasons, but no one has ever advocated getting rid of the centralized supervisory control systems, HMIs, expert tuning systems, and all the other support software such as asset management, ERP, MES, scheduling, and so on.
"A modern digital architecture combines the reliability of single-loop controllers with the flexibility to do advanced control in the field or in a controller," says Krouth. "You can combine regulatory and a great deal of advanced control in the field devices, and use the advanced strategy and optimization in the central control system to generate setpoints, constraint limits, change gains, and so on."
Some of those who have been working with fieldbus understand TrDC reasonably well. For example, Luis Trejo, a product manager at Emerson Process Management in Mexico, likes TrDC. "In my local office, we have integrated more than 50 systems with fieldbuses, control in the field, and communications to corporate networks," says Trejo. Applications include paper mills, industrial and power boilers, chemical plants, food and beverage, and oil production. "We are getting pretty close to TrDC, and I have always wanted to get there."
"Integrating single-loop entities through a centralized database for optimization gets benefits from both worlds," says Verhappen, who installs fieldbus systems. "TrDC and its single-loop integrity lets you minimize the impact on the control system of a single point of failure, while providing the capability for optimizing across multiple loops in a centralized environment.
"I think we have to be clear on the difference between distributed control and distributed I/O," continues Verhappen. "Many of the TrDC solutions are really only distributing I/O by putting the PLC or smarts in a hardened box in the field. My take is that vendors are doing this to circumvent the wire savings attributed to fieldbus systems by shortening the field wire run to keep market share. Mind you, all of them are doing it, so it must be working."
"There will always be a tension between distribution vs. centralization," says Bullerdiek. "The correct answer will depend on the process or system to be controlled and what the user wants to achieve with the system. There is not, and never was, one correct answer. It is nice to see that the technology is finally getting to the point where we can pick solutions to fit our needs instead of having to force fit our needs into the available solutions. It makes the up-front designer job more difficult, but the improved performance/cost ratio will pay off."
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