Purpose <p>The underactuated dynamics in the tuned mass damper (TMD) can be decoupled by using the output redefined active TMD (ATMD) to give better performance in vibration mitigation applications. However, the robustness of the output redefinition design to the variation of primal system parameters has not been addressed. In this paper, robustness comparison of TMD and output redefined ATMD is performed, and an adaptive version of the output redefined ATMD is proposed to show their further robustness evaluations.</p> Methods <p>The root locus technique is utilized to compare the robustness of the TMD and the output redefined ATMD with respect to the variations of the inertia and the stiffness in the primal system. Since the proposed adaptive output redefined ATMD contains nonlinear dynamics, the root locus method is no longer valid. Both time-domain and frequency-domain simulations of perfect tuning and detuning scenarios are performed to give further robustness comparisons.</p> Results <p>The passive TMD is sensitive to the parameter variations in the primal system. Both the output redefined ATMD and the proposed adaptive version are equally robust to various parameter variations.</p> Conclusion <p>The two output redefined ATMD designs may give satisfactory performance and robustness to parameter variations. However, the proposed adaptive version may be realized without the knowledge of the nominal system parameters.</p>