# H-infinity Methods in Control Theory

H-infinity Methods In Control Theory

H (i.e. "H-infinity") methods are used in control theory to synthesize controllers achieving robust performance or stabilization. To use H methods, a control designer expresses the control problem as a mathematical optimization problem and then finds the controller that solves this. H techniques have the advantage over classical control techniques in that they are readily applicable to problems involving multivariable systems with cross-coupling between channels; disadvantages of H techniques include the level of mathematical understanding needed to apply them successfully and the need for a reasonably good model of the system to be controlled. Problem formulation is important, since any controller synthesized will only be 'optimal' in the formulated sense: optimizing the wrong thing often makes things worse rather than better. Also, non-linear constraints such as saturation are generally not well-handled. These methods were introduced into control theory in the late 1970's-early 1980's by George Zames (sensitivity minimization), J. William Helton (broadband matching), and Allen Tannenbaum (gain margin opimization).

The term H comes from the name of the mathematical space over which the optimization takes place: H is the space of matrix-valued functions that are analytic and bounded in the open right-half of the complex plane defined by Re(s) > 0; the H norm is the maximum singular value of the function over that space. (This can be interpreted as a maximum gain in any direction and at any frequency; for SISO systems, this is effectively the maximum magnitude of the frequency response.) H techniques can be used to minimize the closed loop impact of a perturbation: depending on the problem formulation, the impact will either be measured in terms of stabilization or performance.

Simultaneously optimizing robust performance and robust stabilization is difficult. One method that comes close to achieving this is H loop-shaping, which allows the control designer to apply classical loop-shaping concepts to the multivariable frequency response to get good robust performance, and then optimizes the response near the system bandwidth to achieve good robust stabilization.

Commercial software is available to support H controller synthesis.

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