  This paper studies virtual constraints on Lie groups. Virtual constraints are
invariant relations imposed on a control system via feedback. In this work, we
introduce the notion of \textit{virtual constraints on Lie groups}, in
particular, \textit{virtual nonholonomic constraints on Lie groups}, in a
geometric framework. More precisely, this object is a controlled invariant
subspace associated with an affine connection mechanical control system on the
Lie algebra associated with the Lie group which is the configuration space of
the system. We demonstrate the existence and uniqueness of a control law
defining a virtual nonholonomic constraint and we characterize the trajectories
of the closed-loop system as solutions of a mechanical system associated with
an induced constrained connection. Moreover, we characterize the reduced
dynamics for nonholonomic systems in terms of virtual nonholonomic constraints,
i.e., we characterize when can we obtain reduced nonholonomic dynamics from
virtual nonholonomic constraints. We illustrate the theory with some examples
and present simulation results for an application.
