April 2011 - Control Global

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C o n t r o l E x c l u s i v e FDT and OPC Collaborate on FDT 2 Standard The joint FDT and OPC Foundation FDT OPC UA Working Group will develop an OPC UA information model for the major upgrade to the current FDT standard that is now in preparation. The new version, described by Glenn B. Schulz, the managing director of the FDT Group, as a “major update” to the current Version 1.2.1, will be released by the end of this year. The FDT Group is an open, independent, not-for-profit association of international companies dedicated to establishing the FDT Technology as an international standard with broad acceptance in the automation industry. FDT standardizes the communication and configuration interface between all field devices and host systems and provides a common environment for accessing the devices’ most sophisticated features. The OPC Foundation defines standards for online data exchange between automation systems. The OPC Unified Architecture (OPC UA) unifies the existing standards and brings them to state-of-the-art technology using service-oriented architecture (SOA). OPC UA can run on Windows-based PC systems., embedded systems and Linux/ UNIX-based enterprise systems. “This is a unique opportunity to take the best-of-breed data and information models for the applications and devices supported by the FDT architecture and leverage the OPC Unified Architecture (UA) information modeling and corresponding services for complete application-to-device integration,” says Thomas J. Burke, president and executive director of the OPC Foundation. “This exemplifies the device-to-cloud computing strategy that supports configuration, communication, run-time and historical data access, and alarming and event services for existing and upcoming devices supported through FDT Group technology.” Schulz adds, “We were looking for a good way to bring devices together to get information throughout the enterprise. Now we have two great platforms that enable getting data across the enterprise, and we are making them interoperable. We are seamlessly integrating FDT and OPC.” The implicit and explicit information model specified by FDT will be defined in a UA companion specification using OPC UA constructs for the purpose of sending FDT data to external applications. Further enhancements will be supportable due to modular construction. Ready by Year’s End The Working Group wants to get the technology out as quickly as possible. They plan to have a deliverable product ready by the end of the year. “Technology changes very quickly,” says Burke. “We want to take every step to foster the rapid and wide adoption of OPC UA/FDT as a standard.” Another goal is to provide for incremental adoption. The cooperation between FDT and OPC will simplify the technical challenges for end users. A clear migration path will exist for legacy equipment or that which conforms to earlier standards. “We anticipate many new applications and places not currently served by OPC or UA also will be able to take advantage of this unique collaboration. This strategy of incremental adoption facilitates easy and simplified deployment into real products and solutions,” says Burke. The group will also proved for backward compatibility and easy customization. The standard’s extensibility makes possible the creation of value-added, machine- or installation-specific software and tools without jeopardizing compatibility with other equipment or software. Finally, the Working Group is committed to maintaining the non-proprietary nature of the standard, keeping it unencumbered by intellectual property restrictions on licensing. “We each dominate our specific markets—FDT in multinetwork intelligent device confirguration and OPC in data interchange,” says Schulz. “For us to be able to bring out an out-of-the-box solution marrying the two will be a huge win for end users.” A draft version of the updated FDT 2.0 standard was on display at Hannover Fair, Hannover, Germany, earlier this month. For more information on the OPC UA or FDT, go to www. fdtgroup.org or www.opcfoundation.org. 58 www.controlglobal.com A p r i l / 2 0 1 1
C O N T R O L T A L K Ultimate Limits to Performance Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments and problems. Write to them at controltalk@putman.net. Stan: Sometimes we get so caught up in sophisticated control strategies and control system technologies that we forget the ultimate limits to what we can achieve. Whether you are talking about advanced PID control or model-predictive control, the dead time, noise, speed and resolutionsensitivity of measurement and the final control element determine how well you can achieve your control objectives. In this column, we are going to talk about some of these ultimate limits. We are grouping resolution with sensitivity, which set, respectively, the quantization and threshold for detection, communication and actuation. For speed, we are focusing on the rate of change of the process and the sample rate of the control system. Greg: Dead time is the deadliest of the limitations. Without dead time, I would be out of a job. During the loop dead time, the controller does not see or does not affect the process. The ultimate limit for peak error for rise time is proportional to the loop dead time. The ultimate limit for integrated error is proportional to the dead time squared. The sources of dead time are insidious. The measurement and final control element sensitivity divided by the signal rate of change and one half of a sample or scan time are additional sources of deadtime not commonly recognized. The phase shift and ultimate period are proportionally increased. Sensitivity and sample time are particularly important in silicon wafer/circuit manufacturing. nTrolus Inc. (http://ntrolus.com/ Default.aspx) is a leader in model-based control for semiconductor production and green energy systems. We talked to its president, Mark Ekblad, who understands the significance of these limits. Mark: Most of the applications involve temperature control. In general, we can only turn off heat. There are no brakes. We have only ambient cooling or sometimes a fixed static cooling load provided by cooling jackets. Consequently, the speed of temperature change for heating and cooling is drastically different. The process often has a near-integrating response where the temperature essentially ramps. Setpoint overshoot must be minimized because of the sensitivity of the process and the long settling time. The aggressive performance of the PID with conventional Ziegler-Nichols tuning relying on feedback action leads to quality problems. Fuzzy logic and gain scheduling is sometimes used, but the special tuning requirements complicate the implementation. Greg: Bioreactor temperature control has similar requirements and problems, particularly in singleuse bioreactors with disposable liners, bench-top and pilot plant units, and the small production- Greg Mcmillan Stan weiner, pe controltalk@putman.net Stan: What are manufacturers predominantly controlling in semiconductor production? A p r i l / 2 0 1 1 www.controlglobal.com 59
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April 2011 - Control Global

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