Page 100 - Human Inspired Dexterity in Robotic Manipulation

98 Human Inspired Dexterity in Robotic Manipulation

6.3.5 Biological Joints
A biological joint is defined based on the contact of two adjacent bones.
Their commonly shared contact surfaces decide the possible motions of
the joint. Different types of joints facilitate a different set of finger motion,
known as the ROM. As shown in Fig. 6.7, during the bending motion, the
three finger joints work as mechanical hinges. However, the MCP joints
have one extra set of active ROM that allows the finger to move from side
to side, which are known as the abduction and adduction (ad/b) motions. In
addition, the MCP joints also have one passive ROM that permits a twisting
motion around the axis of the finger phalanges. Thus, in our following dis-
cussion, we are only going to focus on explaining the mechanism of the
MCP joint because the 1-DOF hinge joint can be included as a special case.
When it comes to replicating the MCP joint, robotics researchers often
have the following three options: a 3-DOF ball-socket joint, a 2-DOF
universal joint, and a 3-DOF elastic joint. The universal joint is good at
transmitting rotary motion in shafts, but lacks the 1-DOF that allows the
finger to passively twist with respect to the axial direction at the MCP joint
(see Fig. 6.8). Although the elastic joint can realize all three DOFs, it cannot
support a large load along the axial direction due to the natural compliance
of the elastic component. Although not suitable for replacing the MCP joint,
successful application of the elastic joints to the design of hinge joints have
been widely acknowledged. Compared to these two designs, the mechanism

Fig. 6.7 Illustrations showing the regular range of motion (ROM) of the fingers during
finger flexion. Different from the MCP joint, the flexion motions of DIP and PIP joints are
usually coupled.