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Ivancevic_Applied-Diff-Geom

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<strong>Applied</strong> Bundle <strong>Geom</strong>etry 545and ω 0 be a characteristic 1−form for (4.47). The following statements areequivalent:(1) Equation (4.47) is feedback linearizable in a neighborhood of the originin M;(2) D is involutive in a neighborhood of the origin in M; and(3) ω 0 is integrable in a neighborhood of the origin in M.As is well known, a generic nonlinear system is not feedback linearizablefor n > 2. However, in some cases, it may make sense to considerapproximate feedback linearization.Namely, if one can find a feedback linearizable system close to (4.47),there is hope that a control designed for the feedback linearizable systemand applied to (4.47) will give satisfactory performance if the feedbacklinearizable system is close enough to (4.47). The first attempt in thisdirection goes back to [Krener (1984)], where it was proposed to apply to(4.47) a change of variables and feedback that yield a system of the formż = Az + B u new + O(z, u new ),where the term O(z, u new ) contains higher–order terms. The aim was tomake O(z, u new ) of as high order as possible. Then we can say that thesystem (4.47) is approximately feedback linearized in a small neighborhoodof the origin. Later [Hunt and Turi (1993)] introduced a new algorithm toachieve the same goal with fewer steps.Another idea has been investigated in [Hauser et al. (1992)]. Roughlyspeaking, the idea was to neglect nonlinearities in (4.47) responsible forthe failure of the involutivity condition in above Theorem. This approachhappened to be successful in the ball–and–beam system, whenneglect of centrifugal force acting on ball yielded a feedback linearizablesystem. Application of a control scheme designed for the system with centrifugalforce neglected to the original system gave much better resultsthan applying a control scheme based on classical Jacobian linearization.This approach has been further investigated in [Xu and Hauser (1994);Xu and Hauser (1995)] for the purpose of approximate feedback linearizationabout the manifold of constant operating points. However, a generalapproach to deciding which nonlinearities should be neglected to get thebest approximation has not been set forth.All of the above–mentioned work dealt with applying a change of coordinatesand a preliminary feedback so that the resulting system looks like

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