Comments on "Comments on \´Calibration of wrist-mounted robotic sensors by solving homogeneous transform equations of the form AX = XB\´" [with reply]

In the above-named work (ibid., vol.7, p.877-8, (Dec. 1991)), H. Zhuang and Z. S. Roth point out that a particular solution can be simplified by using quaternions to represent rotations. While it is true that this approach gives rise to an algorithm more efficient than the one considered, the commenter argues that it is incorrect to claim that a unique solution for R/sub X/ exists if and only if the axes of rotation of R/sub A1/ and R/sub A2/ are nonzero (theorem 1 of the original work). A counterexample is presented to prove this point. In replying, Zhuang and Roth note the error in the original work and provide an analysis leading to the revision of their original theorem 1.<>