  The normal form and zero dynamics are powerful tools useful in analysis and
control of both linear and nonlinear systems. There are no simple closed form
solutions to the general zero dynamics problem for nonlinear systems. A few
algorithms exist for determining the zero dynamics, but none is straightforward
and all are difficult to apply to large dimensional problems. A Closed form
solution to the zero dynamics problem would motivate more usage of this
powerful technique. The author presents here a simple algebraic methodology for
the normal form and zero dynamics calculation of a class of nonlinear systems,
mostly found in dynamical mechanical systems. The solution is in closed form so
that application of the theorem presented is straight forward. As an
illustration, the zero dynamics calculations for the complex dynamics of a
flexible spacecraft is presented to demonstrate the simplicity and usefulness
of the proposed closed form solution.
  Keywords: Control, Differential Geometry, Normal Form, Zero Dynamics,
Nonlinear Systems, Feedback Linearization, Attitude Dynamics, Flexible
Spacecraft.
