  The paper presents a methodology for the enhanced stiffness analysis of
parallel manipulators with internal preloading in passive joints. It also takes
into account influence of the external loading and allows computing both the
non-linear "load-deflection" relation and the stiffness matrices for any given
location of the end-platform or actuating drives. Using this methodology, it is
proposed the kinetostatic control algorithm that allows to improve accuracy of
the classical kinematic control and to compensate position errors caused by
elastic deformations in links/joints due to the external/internal loading. The
results are illustrated by an example that deals with a parallel manipulator of
the Orthoglide family where the internal preloading allows to eliminate the
undesired buckling phenomena and to improve the stiffness in the neighborhood
of its kinematic singularities.
