2.3 Manipulation control
a virtual position of the hand. The object's pose is determined considering the
forces acting on the virtual object. Dampers are also allowed in these connections,
and both the cases of a serial and a parallel connection have been considered. A
virtual grasp matrix is introduced for a better understanding of some proprieties of
the grasp, both virtual and real. The dynamics of the system is expressed towards
the virtual object an the proposed impedance control law has been applied.
In order to achieve the desired motion of the manipulated object, the fingers should
operate in a coordinated fashion. In the absence of physical interaction between
the fingers and the object, simple motion synchronization shall be ensured. On the
other hand, the execution of object grasping or manipulation requires controlling
also the interaction to ensure the stability of the grasp [72, 82].
From a purely kinematic point of view, an object manipulation task can be
assigned in terms of the motion of the fingertips and/or in terms of the desired
motion of the manipulated object. The work of the planner is to map the desired
task into the corresponding joint trajectories for the fingers and it always requires
the solution of an inverse kinematics problem.
Moreover, if the robotic system is equipped with high-gain motion controller
at low level, the so called resolved-velocity control can be performed, in which
the effects of dynamics or disturbances are neglected. In this case, the system
is considered as an ideal positioning device and the high controller can act at a
velocity level. Since only the kinematics is exploited to derive such control law,
often this approach is called kinematic motion control, and it is well known in the
robotic literature [53, 89], and used also in some manipulation tasks .
It is worth noticing that the manipulation system can be redundant also if
the single fingers are not: this is due to the presence of the additional degrees of
freedom (DOFs) provided by the contact variables. These redundant DOFs can be
suitably exploited to satisfy a certain number of secondary tasks, aimed at ensuring
grasp stability and manipulation dexterity, besides the main task corresponding
to the desired motion of the object. But the kinematic redundancy resolution has
not been investigated deeply throughout the literature, since the focus of previous
papers seems to be on constrained kinematic control [38, 66], or manipulability