Chapter 2. Prehension

416 A pp e n dices
between the thumb (VF1) and middle finger (VF2). Having the extra
contact point at the radial side of the palm (VF3) gives the hand the
ability to use forks, knives and pens. For somewhat large objects, pad
opposition occurred between the thumb (VF1) and the index and
middle fingers (VF2). The ring and little fingers, formed by the glove,
could slightly assist VF2. In addition, a hook grip could be formed
using the active fingers as VF3. Interestingly, a ‘powder grip’
provided automatic adaptation to the shape of the gripped object. This
was achieved using a non-stretch skin of cloth-reinforced PVC
partially filled with glass beads, which deformed in a way to rigidly
pack the beads, thus providing additional stability and padding.
Following experiments with sensor-based devices, the
Southampton University hand (Chappell & Kyberd, 1991) was
developed to create a prosthesis that has greater function and is hand-
like in appearance and action. It has five functioning digits and four
degrees of freedom. The index finger acts independently from the
other three fingers, which move in tandem. The other two degrees of
freedom are in the thumb. Three types of sensors are supported: force
sensors tell the location of contact with the object; slip transducers
indicate if the object is slipping from the grasp; and potentiometers in
the proximal joints measure degree of flexion. Pad opposition can
occur in two ways: first, between the thumb (VF1) and the index
(VF2), and secondly, between the thumb (VF1) and all four fingers
(VF2), although in both cases the thumb opposes the index and middle
fingers. Palm opposition can also be selected. Here, the fingers (VF2)
oppose the palm (VF1). If the object is small enough, the thumb will
move around to the radial side of the index (thus combining side
opposition with palm opposition). Otherwise, it will aid the fingers in
opposing the palm. On contact, the controller applies a small load on
the fingers. If the object begins to slip, tension is increased. This
automatic response can be overriden by the user. Kyberd et al. (1987)
have combined the use of tactile and slip sensory information with
intelligent thumb positioning to simplify the control sites problem, and
thus have created a powerful prosthetic hand with five different
postures.
Meeks and LeBlanc (1988) studied three prototypes as possible
designs for new prosthetic terminal devices. Prehensor A, developed
from a study of human hands, uses a three-jaw chuck grasp and has
many curvatures for multi-point contacts with objects. Providing pad
opposition between two jaws, it has a cutout in the larger jaw that can
act as a VF3 for grasping pens. Prehensor B was based on aesthetic