Chapter 2. Prehension

Chapter 6 - During Contact 273
internal representations of the object‘s properties based on previous
manipulative experiences. For passive, predictable objects, they
suggested somatosensory afferent signals intervene only
intermittently, according to an ‘event driven’ control policy. In
contrast, for active objects, control of grip and other reaction forces
rely more regularly on somatosensory input, due to the unpredictable
and erratic physical characteristics of the object. Grip forces appeared
to be automatically regulated according to variations in the amplitude
and rate of imposed load forces. Further experimentation is needed to
define the effects of shearing forces, other aspects of object
unpredictability, and to extend the research which has focussed
primarily on pad opposition.
6.6.2 Manipulation by robots
As noted in considering the functional demands on a hand
posture, applying forces to stably grasp the object is separate from
imparting motion to an object for manipulation or transportation. In
robotics, various algorithms have been developed for modelling the
grasping forces (the internal forces described in Section 6.3.1) from
the manipulation forces (the forces that impart motion to the object).
For example, Yoshikawa and Nagai (1990) identify internal forces
using geometric characteristics for a three-fingered grasp with
frictional, point contacts. The grasping force is defined as an internal
force which satisfies the static friction constraint. It consists of the
unit vectors directed between the contacts. Using these unit vectors,
grasp modes are specified for grasping arbitrarily shaped objects. The
manipulating force is then defined as a fingertip force which satisfies
the following 3 conditions:
(1) it produces the specified resultant force;
(2) it is not in the inverse direction of the grasping force; and
(3) it does not contain any grasping force component.
The method of manipulation they analyze is fixed contacts, since point
contacts with friction do not allow rolling or sliding at the contacts.
They present an algorithm for decomposing a given fingertip force into
manipulating and grasping forces (note that the analytic solution may
not be unique because of the existence of multiple grasp modes).
Li and Sastry (1990) proposed a control algorithm for
manipulation in two modes: fixed contacts and rolling contacts. Tasks
are modelled as ellipoids in wrench space and twist space, with the