Discussion on: "Adaptive Variable Structure Maneuvering Control and Vibration Reduction of Three-axis Stabilized Flexible Spacecraft"

with ∆A, ∆B uncertainty matrices, A, B, C the nominal matrices, Φ(u) a nonlinear continuous function, x, u, y the state, input and output of the system, and f ∈ R a disturbance. The design is based on the adaption of the bounds on the uncertainty matrices and the disturbance, through the application of Barbalat lemma, as common in adaptive control. The theoretical results are then applied to the attitude control of flexible spacecraft subject to external disturbances. The subject of the paper is of interest in attitude control of flexible structures. Many contributions can be found in the literature. In fact, in the last decades the research effort has been focused on the nonlinear nature of the spacecraft model, and various works are available either in for rigid [10, 17, 24, 2] or flexible spacecraft [18, 20, 21, 22, 1, 12, 14], also in the digital setting [19, 9]. The robustness of these nonlinear controllers has been one of the main issues addressed [23, 11, 13]. In this perspective, the present paper raises some perplexities, both for the theoretical and the application contributions. In the remaining of this discussion some observations, which can be considered as issues for further research activities, are illustrated. The authors assume that the uncertainties ∆A, ∆B and the disturbance f are matched by the input