This paper introduces an approach for solving the
in-hand manipulation problem, that is, the change of a grasped
object pose from an initial configuration to a final one without
breaking contact, with robot fingers having out-of-plane motion,
redundancy, and equipped with soft fingertips. The proposed
technique is based on keeping the initial grasp equilibrium
condition as a constraint to resolve finger redundancy. Two important aspects of in-hand manipulation with soft fingertips are
then studied; namely: i) the modeling of soft fingertip contacts
between fingers and 3D objects, and ii) an efficient method
for computing joint angles to move a grasped object between
different poses without losing grasp equilibrium. Numerical
and empirical experiments using soft fingertips with different
shore hardnesses with a two-fingered robot hand, composed
of four-degree-of-freedom fingers with out-of-plane motion, are
conducted. Results successfully validate the introduced strategy
and its components.