An engineering perspective on nonlinear H∞ control

Linear H_∞ control theory is a popular controller design tool in engineering practice. The notion of the H_∞ norm of a linear I/O-system can be extended to the nonlinear case, allowing the formulation of the nonlinear analog of the H_∞ standard problem. The intention of the paper is threefold: the authors want to give a brief tutorial introduction to the basics of nonlinear H_∞ control theory; they want to critically discuss the applicability and usefulness of nonlinear H_∞ theory for practical nonlinear controller design; and they want to show how nonlinear H_∞ theory can be applied to the problem of synthesizing approximately I/O-linearising controllers. The focus is on the question whether meaningful practical control problems for nonlinear systems can be expressed and solved in the nonlinear H_∞ framework. Approximate I/O-linearisation of a realistic chemical reactor, that cannot be I/O-linearized exactly, is given