Basic and Advanced Regulatory Control – System Design and Application

INTRODUCTION

The term process control implies that there isa process for which there is a desired behavior and that there issome controlling function that acts to elicit that desiredbehavior. This broad concept can embrace everything from societalprocesses governed by some regulatory control authority toautomated manufacturing processes. In practically all cases,however, a common thread is that some measure of the actual processbehavior is compared with the desired process behavior. Thisfeedback action then generates a control policy that acts tominimize or eliminate the deviation between desired and actualbehavior.

We are concerned in this book with a particular segment ofautomated process control—that which is applied to chemical,refining, pulp and paper, power generation, and similar types ofprocesses. Even within this limited scope of applications, we willlimit the discussion primarily to processes that are operatedcontinuously for long periods of time and within a narrow region ofthe operating variables. In other words, we exclude such importantoperating modes as batch processing, start-ups, and grade changes.Many of the control techniques to be presented here, however, canbe adapted to these other modes of operation.

For the processes we focus on in this book, the process’sbehavior is often characterized by measured values of such processvariables as temperatures, flow rates, pressures, and the like. Thedesired behavior, then, is stated to be the set points of thoseprocess variables. Until fairly recent times, most applications ofindustrial process control used simple feedback controllers thatregulated the flows, temperatures, and pressures. These controllersrequired a form of adjustment called tuning to match theircontrolling action to the unique requirements of individualprocesses. Occasionally, more advanced forms of control, such asratio and cascade, could be found; even more rarely one might finda feedforward control loop. As long as most of the control systemswere implemented with analog hardware, applications were limited tosimple regulatory control. This was due to the cost of additionalcomponents, the additional interconnections more advanced controlrequired, the burden of maintenance, and the vulnerability tofailure of many devices in the control loop. With the advent ofdigital control systems, however, more sophisticated loops becamefeasible. Advanced regulatory control—which includes the previouslymentioned ratio, cascade, and feedforward control as well asadditional forms such as constraint (selector) control anddecoupling—could readily be implemented simply by configuringsoftware function blocks.

With this additional capability, however, a need developed for asystematic approach toward using it. This is called controlstrategy design. In order to design a technically successful andeconomically viable control strategy, the control system engineermust be well grounded in the techniques of feedback control as wellas the tools of advanced regulatory control. The requisiteknowledge includes both how to implement and how to tune. Evenbefore that, however, the control system engineer must be adept atrecognizing when to use (and conversely, when not to use) certaincontrol methods as well as in projecting the expected benefits.

Using advanced regulatory control provides many benefits. One ofthe most important is simply closer control of the process. It willbecome very clear later in this book that with basic regulatory(i.e., feedback) control, there must be a deviation from set pointbefore control action can occur. We will call this the “feedbackpenalty.” The objective of advanced regulatory control is for thecontrol action to be taken by incurring only a minimal feedbackpenalty. The reduction in feedback penalty may be stated in avariety of ways, such as a reduction of the maximum deviation fromset point, as a reduction of the standard deviation, or simply as areduction in the amount of off-spec product produced. Thisreduction in feedback penalty can provide several forms of economicbenefit, such as improvement in product quality, energy savings,increased throughput, or longer equipment life.

Edition:

2

Published:

01/01/2004

Number of Pages:

388