Abstract
In this chapter, sufficient conditions on a nonlinear system are given to ensure the existence of a transformation into one of the state-affine normal forms. This includes the linearization by output injection and the nonlinear Luenberger design. The former consists in transforming the system into linear dynamics (possibly depending on the input/output), and such that the output is a linear function of the new state. On the other hand, the latter aims at transforming the system into linear dynamics with a stationary Hurwitz linear part but where the output can be any nonlinear function of the new state. In particular, it is shown that under a rather weak backward distinguishability property, any nonlinear system can be transformed into a Hurwitz linear form in an injective way, but through a time-varying transformation.
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- 1.
\(T:\mathbb {R}^{d_x}\rightarrow \mathbb {R}^{d_\xi }\) is an immersion if the rank of \(\frac{\partial T}{\partial x}\) is \({d_x}\). Contrary to a diffeomorphism, this allows to take \({d_\xi }\ge {d_x}\).
- 2.
- 3.
See Definition 1.2.
- 4.
Dynamics with u equal to a constant.
- 5.
- 6.
Any solution exiting \(\mathscr {X}\) in finite time must cross the boundary of \(\mathscr {X}\). See [2, Definition 1].
- 7.
Separating the real/imaginary parts, the observer is thus of dimension \(2({d_x}+1){d_y}\) on \(\mathbb {R}\). See Remark 6.1.
- 8.
This notion is similar to the distinguishability defined in Definition 1.2 but in negative time and with the constraint that \(\overline{t}\) occurs when both solutions are still in \(\mathscr {S}\).
- 9.
See Definition 5.2 in the autonomous case.
- 10.
- 11.
We could have considered a more general Hurwitz form \(\dot{\xi } = A \, \xi + B(y)\) with B any nonlinear function, but taking B linear is sufficient to obtain satisfactory results.
- 12.
The function T depends on u in \(\mathscr {U}\), and we should write \(T_u\) as in Theorem 1.1. But we drop this too heavy notation in this chapter to ease the comprehension. What is important is that the target Hurwitz form, namely \({d_\xi }\), A, and B, is the same for all u in \(\mathscr {U}\).
- 13.
This property is named completeness within \(\mathscr {X}'\) in [2].
- 14.
\(H(\cdot ,\overline{\nu }_m)\) is injective on \(\mathscr {S}\), and \(\frac{\partial H}{\partial x}(x,\overline{\nu }_m)\) is full-rank for any x in \(\mathscr {S}\).
- 15.
This set depends on u, and unfortunately, there is no guarantee that \(\displaystyle \bigcup _{u\in \mathscr {U}}\mathscr {R}_u\) is also of zero Lebesgue measure.
- 16.
Uniformly instantaneously observable, see Definition 1.2.
- 17.
- 18.
In the case where \(A(\theta )\) is in companion form, an explicit inversion algorithm is available in [1].
- 19.
\(\eta = \varPhi \omega \left( \begin{array}{c} -\sin \theta \\ \cos \theta \end{array}\right) \), with \(\theta \) the electrical angle, and \(\omega =\dot{\theta }\).
- 20.
P is symmetric so it is sufficient to compute \(\frac{{d_\xi }({d_\xi }+1)}{2}\) components.
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Bernard, P. (2019). Transformations into State-Affine Normal Forms. In: Observer Design for Nonlinear Systems. Lecture Notes in Control and Information Sciences, vol 479. Springer, Cham. https://doi.org/10.1007/978-3-030-11146-5_6
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