Grasp recognition is accomplished using the
grasp taxonomy, which is based on the
While the non-volar grasp can be recognized directly from its branch of
taxonomy, volar grasp recognition is not as easy,
due to the high degree of contact
between the hand and the object. It turns out that using higher-level
descriptions of the grasp facilitates the recognition of a volar grasp.
The grasp taxonomy is augmented with higher-level grasp abstraction concepts
such as the
virtual finger and
This is done for the following reasons:
- They enable the operator to easily check the correctness of the system
recognition of the human grasp.
- They enable the volar grasp to be further identified.
- They provide an intermediate abstract representation which facilitate the
mapping from the human hand to that of the manipulator.
In order to recognize the volar grasp, we have to first map the
real fingers that are touching the object (i.e., "active") to virtual fingers.
Mapping real fingers to virtual fingers
Associated with each contact in the
is its location and the object normal at the contact. The object normal is
used as an approximation of the force acting at that contact.
The mapping from real fingers to virtual fingers is done by comparing
the normals between different fingers. For each virtual finger, the comparison
yields an index called the cohesive index (between 0 and 1)
which indicate the degree to
which all the real fingers in that virtual finger act in a similar manner.
If all the fingers act exactly in the same direction, this index would be 1.
The grasp cohesive index is a global measure of how all the
real fingers act in the similar manner in
each of their respective virtual fingers.
It is in essence the geometric mean of all the cohesive indices of the virtual
fingers. The mapping is automatically done by exhausive consider all possible
configurations and choosing one that results in the maximum grasp cohesive
The conceptual picture of mapping of real fingers to virtual fingers
is shown below.
By performing the mapping of real fingers to virtual fingers, we can then
grasp abstraction hierarchy
that involves, at the lowest level, positional
information of finger segments, to the highest level, namely the identity of
the grasp itself.
Volar grasp recognition
It turns out, based on empirical results, that the grasp cohesive index
(obtained from the real finger to virtual finger mapping)
can be used to discriminate between different types of volar grasps.
While the cylindrical and type 2 "coal-hammer" grasps cannot be differentiated
based on this index, they can be identified based on the degree of thumb
Note: The cylindrical grasp is one in which the thumb is adducted
(i.e., the thumb is lying in the plane of the palm).
A "coal-hammer" grasp is identified by the high
degree of abduction (i.e., the thumb is positioned almost perpendicularly
to the plane of the palm). The labels of types 1 and 2 associated with
the "coal-hammer" grasp are used to indicate contact of the thumb with the
object (with type 1 associated with the non-contact case).
- S.B. Kang and K. Ikeuchi, "Grasp recognition using the contact web,"
Proc. IEEE/RSJ Int'l Conf. on Intelligent Robots and Systems,
Raleigh, NC, July 1992.
- S.B. Kang and K. Ikeuchi, A framework for recognizing grasps,
Tech. Rep. CMU-RI-TR-91-24, Carnegie Mellon University, Nov. 1991.
- S.B. Kang and K. Ikeuchi, "Toward automatic robot instruction from
perception: Recognizing a grasp from observation," IEEE Int'l
Journal of Robotics and Automation, vol. 9, no. 4, Aug. 1993.
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