  In this paper we address the problem of tracking control of nonlinear systems
via contraction analysis. The necessary conditions of the systems which can
achieve universal asymptotic tracking are studied under several different
cases. We show the links to the well developed control contraction metric, as
well as its invariance under dynamic extension. In terms of these conditions,
we identify a differentially detectable output, based on which a simple
differential controller for trajectory tracking is designed via damping
injection. As illustration we apply to electrostatic microactuators.
