Page 99 - Human Inspired Dexterity in Robotic Manipulation

Approaching Human Hand Dexterity Through Highly Biomimetic Design 97

Fig. 6.6 Schematic drawing showing the bulging process of the tendon sheaths (the
pulleys) during finger flexion. Left: Different insertion sites of the tendon sheaths.
Right: Mechanical analogy of the bending finger showing the increase in moment
arms under the effects of elastic pulleys.

during rock climbing. We have briefly mentioned the function of tendon
sheaths when introducing the flexor tendons in the previous section. Based
on the distance to the MCP joint, each section of the tendon sheaths has
been named after a numbered annular pulley in nomenclatures of hand
anatomy due to their important functions. Mechanical engineers design
different pulley systems to apply forces and transmit power through cables.
The tendon sheaths in the human hand work as a series of elastic pulleys to
help transmit forces from the muscles to the joints, as the tendon sheaths
can flatten down when the finger straightens and bulge out when the finger
bends.
This is another important biomechanical feature that we are interested in
mimicking. As illustrated in Fig. 6.6, when the finger straightens, no matter
how hard the flexion muscles contract, reduced flexion torque is transmitted
to the finger due to decreased moment arms at the fingers. In our daily lives,
this won’t happen to our hand frequently, unless co-contraction of the hand
(contracting both the extensor and flexor muscles simultaneously) is really
required. Therefore, the elastic pulley system can effectively deliver large
torques to the finger joints when a firm grip of an object is needed, but keep
the torques at the finger joints small when the hand is at rest.
Together with the gliding mechanism of the extensor hood, the elastic
pulleys of the fingers are the essential building blocks that greatly affect the
dexterity of the human hand. It is of our interest to incorporate these salient
features into the design of our biomimetic robotic hand.