Comments on "Direct learning of control efforts for trajectories with different time scales" [with reply]

In the original paper by Xu (ibid., vol.43, p.1027-30, 1998), a problem of learning motion trajectories with different time scales based on the learned control inputs was considered. It is suggested that the author was apparently unaware that such an idea was previously reported by Kawamura et al. (1987, 1995) based on the time-scale property of robot dynamics. In this note, we briefly review the literature and point out the difference between Xu's work and that of Kawamura et al. We show that the proof developed in the latter can also be applied to derive the time-scale learning controller for the class of nonlinear system considered in Xu. In reply, Xu acknowledges that he overlooked the work of Kawamura et al. He remarks that his original article was in the event published as 2 articles. He also mentions his work currently in progress.