Comments on "A revisit to the gain and phase margins of linear quadratic regulators" [with reply]

In the above paper by Zhang and Fu (ibid. vol.41 (1996)), an example is given showing that the LQ controller gives an arbitrary small gain margin with respect to variations of the open-loop plant. As a remedy, a dynamic-state feedback is proposed which is claimed to give an arbitrary large gain margin. This is incorrect. In fact, the proposed dynamic state feedback controller does not even stabilize the nominal system. In reply, Zhang and Fu show how the error can easily be fixed by modifying the controller concerned, and point out out that the modified controller has the same features as the original one.