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MAY-JUN 2019

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By Jim Ford, PhD 42 INTECH MAY/JUNE 2019 WWW.ISA.ORG Single-loop control—Still the mainstay of advanced process control J ust over a decade ago, an article titled, "Ad- vanced Control Strategies Move into the Field" (Control, October 2008), highlighted three evolving trends in the process control world that would "make our dependence on single-loop con - trol part of history." The three trends cited were the: ■ movement of APC controllers to the field con- trol devices ■ increased availability of inexpensive sensors ■ increased use of fieldbus and wireless in the control network. Ten years later, how much of this prediction has come true? Predictably, not much. And, why? Single-loop control (i.e., a control loop with one input and one output) as implemented in various versions of the proportional, integral, derivative control algorithm in modern distributed control systems has two primary functions: ■ servo-control—reacting to changes in the set point (SP) to move the process variable (PV) to its new target ■ feedback control—reacting to changes in the PV to return the PV to SP Servo-control is easy. Choose the right amount of integral action (in combination with the gain action, and with or without gain action on SP changes), and the control algorithm will adjust its output (OP) to move the PV to its new target at the chosen rate with or without overshoot, as desired. Feedback control is much more important and more complex. It is the only mechanism in the entire process control world that can react satis- factorily to changes in unmeasured disturbance variables, such as changes in ambient tempera- ture, rainfall, stream composition, pump load, exchanger fouling, field operator moves, board operator moves, and many others. APC technological advances Advanced process control (APC) got its start and made its mark in the 1960s primarily by reacting to measured disturbance variables. This "reaction" was coined feedforward control. If a disturbance variable to a unit operation can be measured, then the key control variable (CV) for that operation can be kept close to its SP by adjusting the associated manipulated variable (MV) using a simple "model." For example, consider a catalytic reactor where the key CV is the reactor inlet temperature, which is controlled at the basic level on feedback by ad- justing the fuel flow to a fired heater. If the reactor feed rate changes, then a feedforward controller can adjust the fuel flow (the MV) using a dynamic model between the feed rate and the fuel flow. The intended result is little or no change in the reac- tor inlet temperature (the CV)—the primary ben- efit of all APC—a reduction in process variance. Initially, feedforward control was implemented using one input – one output relationships. As the technology developed in the 1970s and 1980s, more complex approaches and strategies devel - oped, involving multiple inputs and outputs with more complex models. Eventually, multivariable, model-predictive control replaced simpler APC approaches and algorithms for both feedforward and feedback control of complex process control applications. But, throughout the past 50 years of APC tech- nology advances, those unmeasured disturbances have not gone away. The APC controllers can now be implemented in field control devices (trend number 1); there are cheaper sensors (trend num- ber 2); and fieldbus and wireless (trend number 3) are now realities. But none of these technologi- cal achievements have been able to mitigate the process instabilities created by unmeasured distur- bances. Their presence still confounds the most experienced control system engineers. That is why single-loop control is still the mainstay of process control, and why those trends discussed earlier (or any others) will never result in APC being imple- mented successfully in its absence.

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