  The goal and the main result of the paper is to provide a complete
description of the field of rational differential invariants of one class of
second order ordinary differential equations with scalar control parameter with
respect to Lie pseudo-group of local feedback transformations. In particular,
considered class describes behavior of conservative mechanical systems. We
construct the class of rational differential invariants that separate regular
orbits. It is well known that differential invariants form algebra with respect
to the operation of addition and multiplication. In our case, constructed
rational differential operators form a field (in algebraic sense).
