Page 28 - Electric Drives and Electromechanical Systems

20 Electric Drives and Electromechanical Systems

FIG. 1.10 The Whole Arm Manipulator’s anthropomorphic hand. The manipulator was designed to access a
glovebox through the same rubber glove as used by human operators.
serving manipulator and use Teflon-coated cables in flexible sleeves to transmit
forces to the finger joints. To reduce coupling and to make the finger systems
modular, the designers used four cables for each three degree of freedom finger
making each finger identical and independently controllable (Salisbury, 1985).
The Utah-MIT Dexterous hand (Jacobsen et al., 1986), is an example of an
advanced dexterous system. The hand comprises three fingers and an opposed
thumb. Each finger consists of a base roll joint, and three consecutive pitch joints.
The thumb and fingers have the same basic arrangement, except the thumb has a
lower yaw joint in place of the roll joint. The hand is tendon driven from external
actuators.
The Robonaut Hand (Ambrose et al., 2000), is one of the first systems being specif-
ically developed for use in outer space: its size and capability is close to that of a
suited astronaut’s hand. Each Robonaut Hand has a total of fourteen degrees of
freedom, consisting of a forearm which houses the motors and drive electronics, a
two degree of freedom wrist, and a five finger, twelve degree of freedom hand.

The design of the fingers and their operation is the key to the satisfactory operation of
a dexterous hand. It is clear that two constraints exist. The work by Salisbury (1985)
indicated that the individual fingers should be multi-jointed, with a minimum of three
joints and segments per finger. In addition, a power grasp takes place in the lower part of
the finger, while during a precision grasp it is the position and forces applied at the
fingertip that is of the prime importance. It is normal practice for the precision and
power grasp not to occur at the same time.
In the design of robotic dexterous end effectors, the main limitation is the actuation
technology: it is recognised that an under-actuated approach may be required, where the
number of actuators used is less than the actual number of degrees of freedom in the
hand. Under-actuation is achieved by linking one or more finger segments or fingers
together: this approach was used in Southampton’s Whole Arm Manipulator, Fig. 1.10.
As discussed by Birglen et al. (2008) approximately 50% of all robotic hands are of