This paper presents analysis and experiment results on regulation of a two-link robot with a constantly revolving arm. The rotating member imposes a harmonic excitation on the system. A control algorithm inspired by passive dynamic vibration absorber is devised to turn the base link to a target position asymptotically. The displacement of the base link is the only state variable required in the feedback loop. Asymptotic stability is proved using a tool based on the center manifold theory. Real-time experiments are conducted to verify the performance of the proposed method.