Design and Comparison of Plasma H∞ Loop Shaping and RGA-H∞ Double Decoupling Multivariable Cascade Magnetic Control Systems for a Spherical Tokamak
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The article aims to present an approach to design and compare cascade Hinf loop shaping and essentially new cascade RGA-Hinf double decoupling magnetic control systems for a multivariable dynamical plant, specifically plasma in a vertically elongated tokamak. Identification of the present control closed-loops containing a plasma linear model of a relatively high order for the spherical Globus-M tokamak (Ioffe Institute, St Petersburg, Russia) to derive a low order linear model (without the application of reduction algorithms) as a plant under control is undertaken. A robust H¥ loop shaping method was applied to the identified model to design a plasma position, current, and shape (6 gaps between the first wall and plasma separatrix) multivariable controller. A structural analysis was done to get the most effective structure of the square plant with the 3rd gap eliminated in the feedback and a separate loop for the plasma current control. The methodology of the relative gain array (RGA) was applied to this structure to choose the proper correspondences between inputs and outputs (pairing), which brought the plant model closer to a decoupling plant (first decoupling in the open plant model). Further, the Hinf adjustment of the control system with the pairing plant and an additional feedback decoupling matrix (second decoupling of the plant model in the feedback) and PI controllers in the feedback gave increased control system accuracy while tracking references. Comparison of the two control systems designed has shown that double decoupling gives higher performance accuracy and a less robust stability margin, while the robust loop shaping method allows the stability margin to be increased but gave less accurate control of the gaps.