  This paper proposes a tracking controller based on the concept of flat inputs
and a dynamic compensator. Flat inputs represent a dual approach to flat
outputs. In contrast to conventional flatness-based control design, the
regulated output may be a non-flat output, or the system may be non-flat. The
method is applicable to observable systems with stable internal dynamics. The
performance of the new design is demonstrated on the variable-length pendulum,
a non-flat nonlinear system with a singularity in the relative degree.
