Almost-global tracking of simple mechanical systems on a general class of Lie Groups

We present a general intrinsic tracking controller design for fully-actuated simple mechanical systems, when the configuration space is one of a general class of Lie groups. We first express a state-feedback controller in terms of a function-the "error function"-satisfying certain regularity conditions. If an error function can be found, then a general smooth and bounded reference trajectory may be tracked asymptotically from almost every initial condition, with locally exponential convergence. Asymptotic convergence from almost every initial condition is referred to as "almost-global" asymptotic stability. Error functions may be shown to exist on any compact Lie group, or any Lie group diffeomorphic to the product of a compact Lie group and R/sup n/. This covers many cases of practical interest, such as SO(n), SE(n), their subgroups, and direct products. We show here that for compact Lie groups the dynamic configuration-feedback controller obtained by composing the full state-feedback law with an exponentially convergent velocity observer is also almost-globally asymptotically stable with respect to the tracking error. We emphasize that no invariance is needed for these results. However, for the special case where the kinetic energy is left-invariant, we show that the explicit expression of these controllers does not require coordinates on the Lie group. The controller constructions are demonstrated on SO(3), and simulated for the axi-symmetric top. Results show excellent performance.     

Loss of structurally stable regulation implies loss of stability

We refer to systems violating the conditions for structurally stable regulation as critical. At least one of the following must fail at a critical system: asymptotic rejection of the exogenous system, stabilization of the closed-loop system, or structural stability. This has not been explicitly addressed. We show that any structurally stable regulator is necessarily destabilizing when applied to a critical system, that destabilization is linked to structural stability, and we characterize the resulting unstable poles.