Chapter 2. Prehension

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D.3.3 Belgrade/USC hand
The Belgrade/USC hand (see Figure D.3), with its five fingers,
was built by Rajko Tomovic and colleagues (Bekey, Tomovic, &
Zeljkovic, 1990). It has four independent degrees of freedom, and
uses reflex control. The motions of the three joints are not
individually controllable; they are connected by linkages in order to
move similarly to human fingers during grasping as the fingers flex (a
virtual ). The thumb is articulated with two coupled joints, and capable
of rotation about an axis normal to the palm, to bring it into opposition
with any of the other s. A unique feature of the hand is its autonomous
shape adaptation. Four motors are mounted in the wrist structure to
provide the external degrees of freedom. Two motors move the
thumb, while the others move two fingers each as a virtual finger. The
virtual finger drive is applied to each pair of fingers through a lever
structure, such that if the motion of one real finger is inhibited, the
second can continue to move, thus achieving shape adaptation without
external control (Bekey et al., 1990; Tomovic, Bekey, Karplus,
1987). The consequence of this design is that the hand is well suited
to autonomous grasping of objects of arbitrary shape; it is capable of
preshaping; and is simple to control, since all the motors are located in
the wrist structure. Touch sensors are located on the finger tips and on
the palm, position sensors are embedded within the fingers. The hand
is mounted on a Puma 560 robot.
The idea is to reduce the enormous number of possible degrees of
freedom of multied hands to a small number of ‘standard’
configurations for grasping tasks. A set of eight grasp modes was
developed (Liu and Bekey, 1986), based on the typical use of robot
hands in industrial settings. These postures, as was seen in Table 2.1,
include the lateral pinch, power grasp, span, precision pinch, etc. The
underlying control philosophy for grasp execution is based on a shape
adaptive principle. Finger positions adapt to the shape of the object
and grasp force is appropriate to its weight and surface friction
characteristics. In contrast with the approach taken in the Utah/MIT
hand, Tomovic and Bekey believe that this aspect of control should be
built into the hand itself, using contact, pressure and slip sensors. An
earlier version of such a shape adaptive hand was built some 25 years
ago for prosthetic purposes (Tomovic & Boni, 1962).