Synthesis and analysis of robust control compensators for Space descent & landing

Abstract

Summary In this article, a complete modelling, synthesis, and analysis methodology of control compensators for descent and landing (D&L) on small planetary bodies is presented. These missions are scientifically very rewarding but technically extremely challenging due to the complex and poorly known environment around those bodies, which calls for the ability to manage competing robustness and performance requirements. While this issue is typically addressed via the redefinition of D&L guidance strategies, here, it is tackled through the augmentation with a simple yet robust control compensator. This compensator is designed using linear fractional transformation modelling to capture the interplay with uncertain gravity fields and the recently developed structured H∞ optimisation framework, which has been proved particularly suitable for industry‐oriented applications. The proposed approach is completely generic but uses the scenario of a landing on the Martian moon Phobos as an illustrative example. Different compensators are then verified and compared analytically via the structured singular value μ and through high‐fidelity Monte Carlo simulation.

Systematic performance-oriented guidance tuning for descent & landing on small planetary bodies

“…In fact, in the ideal case, this module has no effect on the system since there are no deviations. For further details on this type of compensation, the reader is referred to (Simplício et al, 2018).…”

“…Representative LFT models of G i SDK (s) have been built through a modelling procedure that relies on the interpolation of dispersed samples of J i f . Details on this process can be encountered in (Simplício et al, 2018). In the end, a set of N = 10 models has been generated, covering 10 design points with uniform closing speed intervals.…”

“…This tool has been successfully employed to design control compensators that effectively enable the use of simpler guidance schemes (Simplício et al, 2018).…”

Review of guidance techniques for landing on small bodies

“…This is however outside the scope of this survey and thus the remainder of the paper focuses on Descent Guidance techniques and their implementation. But as an example of control compensator design, the reader is referred to (Simplício et al, 2018a), where an application to the scenario of a landing on Phobos is presented. In this reference, deviations between reference and measured states were modelled as linear orbital perturbations (Battin, 1987) and control synthesis was performed using robust control techniques.…”

“…As previously shown, these uncertainties can be modelled as time-invariant LFTs, but this formalism also allows to indirectly capture time-varying effects via linear parameter-varying (LPV) models (details on LPV modelling can be found in (Marcos & Bennani, 2009) and references therein). Furthermore, and also favoured by the use of the robust control framework, recent automated tuning tools (Apkarian & Noll, 2006;Gahinet & Apkarian, 2011) have appeared that will be able to exploit the parameterisation of the guidance laws proposed in (58) and (59) -or facilitate the synthesis of control compensators that minimise orbital perturbations with respect to a reference trajectory (Simplício et al, 2018a).…”