Authors' reply to "Comment on 'Attitude synchronization control for a group of flexible spacecraft"' [Automatica 50 (2014) 646-651]

We thank Rezaee and Abdollahi (2016) for their concern about the proof of Lemma 2 in our paper (Du & Li, 2014). We agree that the analysis in Du and Li (2014) is flawed, and that the proposed control action does not guarantee attitude synchronization. The proposed method/result in our paper (Du & Li, 2014) is still available if an addition assumption is imposed as that in Theorem1 of Igarashi, Fujita and Spong (2009). That is: Assume that the scale part of quaternion for each spacecraft is always positive, i.e., qi,0(t) ≥ 0,∀i ∈ Γ , for all time t ≥ 0. About the reasonability of this assumption, first the experiment results in [3] have been given to support it. Second, the assumption is also emphasized in Abdessameud and Tayebi (2009) (see the above of Remark 1 in Abdessameud and Tayebi (2009)). That is, from a practical point of view, this assumption can be satisfied if we restrict the rotation angleΦi between the [−π, π] due to qi,0 = cos Φi 2 .