Abstract
The regulation of a disturbed output can be improved when several manipulated inputs are available. A popular choice in these cases is the series control scheme, characterized by (1) a sequential intervention of loops and (2) faster loops being reset by slower loops, to keep their control action around convenient values. This paper tackles the problem from the frequency-domain perspective. First, the working frequencies for each loop are determined and closed-loop specifications are defined. Then, Quantitative Feedback Theory (QFT) bounds are computed for each loop, and a sequential loop-shaping of controllers takes place. The obtained controllers are placed in a new series architecture, which unlike the classical series architecture only requires one controller with integral action. The benefits of the method are greater as the number of control inputs grow. A continuous stirred tank reactor (CSTR) is presented as an application example.
Highlights
In process control, it is common to find several manipulated variables regulating a single measurable output (MISO: Multiple Input Single Output control)
The architectures for MISO control can be grouped in two: the parallel [15, 16] and the series disposition of controllers. The latter originally appeared in the habituating control by Henson et al [15] and the midranging control by Allison and Isaksson [17]. It can be seen as a generalization of the valve position control (VPC) for the process industry presented by Shinskey [1] and Luyben [18]
The usefulness of the proposed methodology is being illustrated through the control of a Chemical Stirred Tank Reactor (CSTR), which is a recurrent benchmark in the process control literature because of its unquestionable importance in the chemical and materials industry [21]
Summary
It is common to find several manipulated variables regulating a single measurable output (MISO: Multiple Input Single Output control). Most contributions in the literature solve practical examples with ad-hoc decisions and tailor-made design methods, some general design methods are described in [1, 15, 22,23,24,25] Their common approach is firstly designing the fastest loop (i.e. c2) to achieve certain performance in the regulation of y, and designing the slowest loop (i.e. c1) that takes u2 to the setpoint ru. No method has been reported on how to distribute the performance amongst more than two branches as far as the authors are aware Under these premises, the present work proposes a new series structure of n controllers allowing to exploit the benefits of using n manipulated inputs while avoiding the inconveniences of having integral action in all controllers.
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