Practical output tracking of nonlinear systems with applications to underactuated mechanical systems

Shows that for a significant class of nonlinear systems with uncontrollable unstable linearization, global practical output tracking is achievable by smooth feedback, although asymptotic output tracking is usually not possible, even locally. Smooth tracking controllers are explicitly constructed via a modified adding a power integrator approach. This design method also leads to solutions to various open control problems, including practical output tracking of an underactuated unstable two degrees of freedom mechanical system