Page 106 - Human Inspired Dexterity in Robotic Manipulation

104 Human Inspired Dexterity in Robotic Manipulation

Before each finger was mounted onto the wrist of the robotic hand,
another important mechanism was implemented to mimic the function of
tendon sheaths on the palmar side of the fingers. We named them as the elas-
tic pulley system, which consists of 14 patches of laser-cut rubber sheets. The
rubber sheets were slightly prestretched before being attached onto the fin-
ger so that they could closely conform to the contour of the bone. Then
flexor tendons made of high strength Spectra strings (200 N yield strength)
were routed through the rubber tendon sheaths via several rivet reinforced
ports (see Fig. 6.14). One end of the flexor was to be inserted into the distal
joints of the hand, and the other end was directly connected to the electric
servo. Once the servo starts rotating, each robotic finger bends with the
retraction of the strings during which the bulging process of the elastic pul-
leys occurs simultaneously. The design of the rubber tendon sheaths closely
mimics that of the human counterpart (see Fig. 6.6) by providing mechanical
advantages that allow torques at the finger joints to gradually increase as long
as the servos keep pulling the strings.
The final step was to mount the thumb and fingers onto the wrist of the
robotic hand. The wrist of our proposed robotic hand has 1-DOF at the base
of the ring and little fingers. As we addressed at the beginning of this chapter,
the biomimetic design of the wrist is left for future works; the current ver-
sion only serves as a static base for testing the fingers. But the strings’ routing
paths at the wrist are closely mimicking the carpal tunnel of the human hand.
In total, ten Dynamixel servos [17] (nine MX-12W and one AX-12A) are
used to actuate the robotic hand (as shown in Fig. 6.1). Two servos are used
to control the flexion and extension of the ring and little fingers through a
differential pulley transmission. The index and middle fingers are separately
controlled by two pairs of servos so that each of them can bend and

Fig. 6.14 The elastic pulley system implemented on the palmar side of the biomimetic
hand. Left: The flexor tendons of the robotic hand are running underneath the laser-cut,
elastic tendon sheaths. Right: Snapshots of a separate finger showing the bulging
process of the elastic pulleys during finger flexion.