Abstract
In this paper, we consider minimal time problems governed by control-affine-systems in the plane, and we focus on the synthesis problem in presence of a singular locus that involves a saturation point for the singular control. After giving sufficient conditions on the data ensuring occurrence of a prior-saturation point and a switching curve, we show that the bridge (i.e., the optimal bang arc issued from the singular locus at this point) is tangent to the switching curve at the prior-saturation point. This property is proved using the Pontryagin Maximum Principle that also provides a set of non-linear equations that can be used to compute the prior-saturation point. These issues are illustrated on a fed-batch model in bioprocesses and on a Magnetic Resonance Imaging (MRI) model for which minimal time syntheses for the point-to-point problem are discussed.
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Notes
Here, \(N_{\mathcal{T}}(x)\) stands for the (Mordukovitch) limiting normal cone to \(\mathcal{T}\) at point \(x\in \mathcal{T}\), see [28]. It coincides with the normal cone in the sense of convex analysis when \(\mathcal{T}\) is convex.
By optimally invariant subset, we mean a subset \(\Omega \subset \mathbb{R}^{2}\) such that for any initial condition \(x_{0}\in \Omega \), an optimal trajectory stays in \(\Omega \).
Following for instance [24, Sect. 2.8.4], chattering occurs for singular arcs of higher order (at least 2) for which the Legendre-Clebsch condition is not fulfilled.
Since \(\mathcal{T} :=\{x_{f}\}\) is a point, there is no transversality condition at the terminal time.
In contrast with the previous sections in which state variables are \((x_{1},x_{2})\), we chose to adopt the notation \((s,v)\) that is commonly used in the bioprocesses literature for fed-batch operations.
Micro-organism concentration \(X>0\) can be expressed as a simple function of \((s,v)\), namely \(X=M/v+s_{\mathit{in}}-s\), thus (5.1) is enough to describe a bioreactor operated in fed-batch mode.
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Acknowledgements
We are very grateful to E. Trélat for helpful discussions and suggestions about the tangency property at the prior-saturation point.
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Bayen, T., Cots, O. Tangency Property and Prior-Saturation Points in Minimal Time Problems in the Plane. Acta Appl Math 170, 515–537 (2020). https://doi.org/10.1007/s10440-020-00344-8
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DOI: https://doi.org/10.1007/s10440-020-00344-8