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Applied Process Control by Michael Mulholland

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8Stability and Quality of Control

8.1 Introduction

Control theory has until recently been taught in a way that did not distinguish much between the fields of application, whether those be missiles, aircraft, power systems or chemical processes. The advent of the digital computer has however opened up a vast range of new applications, which have perceptibly moved the emphasis away from the types of things that the classical analysis methods handled. This is particularly the case in the processing industries where the control engineer has a new focus on advanced process control (APC), model predictive control (MPC) and real time optimisation (RTO). Of course, issues of loop stability and controller performance are still important, but in the process industries these are seldom investigated by classical methods such as frequency response and root locus. The new computer-based work is exclusively in the time domain. In comparison with other fields, the frequencies to be dealt with are very low, often arising from the control loops themselves! ‘Good’ initial settings for controllers may be estimated theoretically before start-up, but the fine-tuning tends to be based on trial-and-error experience. The signs of instability are early detected as undesirable process fluctuations, and tuned out. In this largely ‘regulation’ mode, no one is pushing the limits at the edge of instability. One reason for this approach is that the processes are nonlinear and variable, and seldom match a ...

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