The best of the best, Part 5

ControlGlobal.com

Keywords: process automation and Control Talk

Control Talk columnists McMillan and Weiner continue their multipart series of talks with some of the Great Minds in Process Control. This month it’s Sheldon Lloyd, past VP of technology for Fisher Controls.

By Greg McMillan and Stan Weiner, PE

Stan: We continue our multipart series of talks with some of the Great Minds in Process Control. This month it’s Sheldon Lloyd, past vice president of technology for Fisher Controls.

Greg: What are the two applications you worked on with biggest benefits?

Sheldon: I was responsible for the development of the Type 546 I/P transducer that for many years was the industry standard for I/Ps. Many of the I/P products on the market at the time were sensitive to vibration. In the development of the 546, I made a special effort to address this issue in the product design. It is rather gratifying that from the introduction of the 546, it has been relatively trouble-free in the field. Even today, there are millions of these units in service.

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As a research engineer for Fisher Controls, I developed the first positioner application guidelines. These guidelines were developed as a result of control-loop stability problems leading to positioners on bypass and not used when needed. Also, I developed Fisher Controls’ guidelines for two-stage pressure reduction to reduce the incidence of instability due to loop interaction.

	Sheldon Lloyd holds an ME and  worked at Fisher Controls and Monsanto. He co-founded Fieldbus Inc. and was its president for 10 years. He holds five patents, is a former senior member of ISA, a licensed engineer and a past member of the Visiting Committee for the Department of Electronics and Computer Engineering, University of Texas. Now retired, he rides elephants and does other fun stuff.

A lot has changed since that time in the design and speed of response positioners can achieve. The new digital positioners in particular represent a major advancement in positioner technology. The introduction of digital process controllers has had a significant impact on the application of digital positioners. In general, the older analog controllers have a much wider bandwidth in their frequency response than their digital equivalents. In many distributed control systems, the PID may be configured for a slow period of execution. Consequently, the rules in applying valve positioners have dramatically changed. The original guidelines advocated application of positioners on selected applications; whereas, the appropriate guideline today is to always use a positioner unless very special conditions exist. This universal use rule is especially true when the positioner settling time is faster than the control module execution time.

Stan: Thus when upgrading a control system that has been in place for many years, it is always good to review what valves do not have positioners and add positioners as part of the project. Often the improvement in performance is significant and can make a big difference in reducing process variability.

Greg: A positioner also can make a big difference when tuning a control loop by reducing the stick-slip and dead- band as seen by the controller.

Stan: What were your greatest technical achievements?

Sheldon: To help better communicate the importance of process control understanding, Gerald Anderson and I wrote the book, Industrial Process Control. This text was used in the control classes offered by Fisher Controls for many years.

I retired from Fisher Controls just about the time Fisher, Rosemount, Yokogawa and Siemens were starting on what would eventually evolve into the Fieldbus Foundation Specification. One of the greatest hurdles was the fact that many of the traditional instrumentation manufacturers were not strong in software development. Thus, soon after the release of the first draft of the Fieldbus Foundation specification, I cofounded Fieldbus Inc. The goal was to make it easier for instrument and valve manufacturers to make the transition to fieldbus technology. I feel this initiative was one of the keys to the wide success that fieldbus has achieved.

Greg: What are the two greatest opportunities in process control today?

Sheldon: There is a significant challenge ahead for our industry in training new engineers in basic process control. With the big reduction in technical resources within many plants, there is less support around to help a new engineer learn the things he or she needs to know. This challenge offers new opportunities to find better ways to transfer process knowledge within engineering organizations. The increasing computing power available in field devices also offers a significant opportunity for manufacturers to provide features that prevent abnormal situations and improve control use.