We present the internal model principle applied to time-invariant nonlinear passive systems that are zero-state detectable. We solve a disturbance rejection problem (with reference signal zero), where the disturbance can be decomposed into a finite superposition of sine waves of arbitrary but known frequencies and an L2 signal. The proposed LTI controller assures that the state trajectories of the plant (and hence also the error signal) converge to zero. We derive and use a technical result concerning the behavior of a strictly output passive system with an L2 input, which is of independent interest.
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