  A double pendulum subject to external torques is used as a model to study the
stability of a planar manipulator with two links and two rotational driven
joints. The hamiltonian equations of motion and the fixed points (stationary
solutions) in phase space are determined. Under suitable conditions, the
presence of constant torques does not change the number of fixed points, and
preserves the topology of orbits in their linear neighborhoods; two equivalent
invariant manifolds are observed, each corresponding to a saddle-center fixed
point.
