Advances in PID LoopTuning Produce Benefits

Except, that is, “when you consider the aging workforce and the institutional knowledge of processes and instrumentation that has been slipping away.”

Thus, advances in proportional-integral-derivative (PID) loop tuning should be welcome, says Nash, whose company supplies PID loop tuning software. Why? Because badly and/or too-infrequently tuned PID loops can obstruct, if not drain, profitability. “The environment in which process companies operate is naturally dynamic. There are [also] any number of interacting [control] loops,” Nash observes, estimating hundreds, even thousands, at some facilities.

But, as he notes, loop tuning has typically been a “controlled experiment or disturbance.” That means you “bump” the process and adjust accordingly, Nash clarifies. End-users isolate controller response. Then, with that information, end-users can model the relationship of how the controller reacts (output) to the input (measured process variable), he explains.

However, modeling that typically requires “everything else to be stable” is uneconomical in most process environments, Nash notes. Or, as he says, the “quiet” of steady state may be overcome by “oscillation,” which he suggests means higher degree of variation, even lack of control. That imposes “bad production.”

Minimize bad production

At least two causes exist for such production, Nash relates. First, “achieving steady state typically requires other interacting loops to be held in check, which can affect their effectiveness.” Second, “bump tests can impact production quality.” How so? End-users doing multiple bump tests to get the tuning parameters right can hinder production with significant time delays, Nash asserts. That equals “off-spec production during each bump.”

To work around or inhibit these tuning challenges, as well as ease the loss of institutional experience and knowledge, Control Station and Tokyo-headquartered automation supplier Yokogawa Electric Corp. (www.yokogawa.com) partnered two existing PID-optimization technologies. The new product’s launch occurred in May.

Yokogawa, which distributes the product as the CS3000 distributed control system (DCS), contributed its existing Centum CS3000. Control Station supplied its patent-pending Non-Steady State (NSS) Modeling Innovation, called csTuner, which is a part of its Loop-Pro Product Suite Version 5. “End-users no longer have to achieve steady-state operations before they can perform the bump test,” Nash asserts. The NSS Modeling Innovation doesn’t focus on any specific or average data, but looks dynamically at the data set across the operating range, he explains.

The synergy of the two companies’ integrated technologies appears to present optimistic economics. For example, at one of its facilities, a German-based specialty chemicals manufacturer, Evoniks, has found that use of the technology reduced production cycle time by approximately 9.3 percent for two batch reactors, Nash relates. That decrease equals a 2.5-hour reduction in the cycle time, he adds.

Yokogawa’s Gary Hodgson, business development manager for services, says that the economics of PID optimization also reflect end-users’ concerns about more than just improved throughput. For example, at a plant where he recently observed a furnace’s operation, Hodgson recalls an operator stating that the “chief performance indicator” for the unit’s operation was “reduction in energy consumption.” Overall, by properly controlling loops, “we’re finding clients reducing energy costs by up to 15 percent,” Hodgson adds.

Calling the integrated CS3000-csTuner combination “a tool for the operator class,” Nash and Hodgson note that U.S. manufacturers have historically been reluctant to give control-room operators the responsibility of tuning loops. But, as Nash also remarks, “The aging workforce, the effect of the current recession and other factors are forcing a change in that cultural viewpoint.” And user-friendly, time-reducing advances in PID loop tuning help optimize that transformation.

C. Kenna Amos, [email protected], is an Automation World Contributing Editor.