• Damping

To assess the robustness of the *H*∞ controller, two different damping ratios, 0.1 and 1.5 times the initial one, respectively, were considered for the PS. The time histories of the change ratio under the step response are given in Figure 16. It is found that the change ratios were negligible compared to those in Figure 15. With time, the change ratio tended to zero, which means that the steady-state error was zero. This indicates that the damping characteristics had limited influences on the step responses, and the *H*∞ controller exhibited a strong robustness.

**Figure 16.** Change ratio of step response for different stiffnesses. (**a**) *CE* = 0.1*CE*0; (**b**) *CE* = 1.5*CE*0.
