Page 140 - Human Inspired Dexterity in Robotic Manipulation

Hand Design—Hybrid Soft and Hard Structures 137

fingers [10], soft elastomer quadrupedal robots [17], safe interaction gripper
[15], inflatable rubber pockets based gripper [9], puncture resistant soft grip-
per [16], modular soft robotic grippers [13], and jamming gripper [18,19].
The presence of both types of softness facilitates grasping of many kinds
of objects.
Even if a robotic hand does not have sufficient DOFs required for
manipulating an object, it can complete the manipulation by making (inten-
tional) contact with the environment and modifying the orientation of the
object. Contact with the environment can potentially extend the manipu-
lation capability of robotic hands. Recent studies of motion planning based
on this concept where motion planning is presented in more detail are
[39–42]. Note that motion planning is beyond the scope of this study.
A pioneering study of robot hand design that considers contact with the
environment is the study of a gripper based on a remote center of compli-
ance [43]. Contact with the environment was effectively utilized to realize
grasping with robotic hands possessing softness [11,12,36]. There are few
studies in which contact with the environment is intentionally utilized to
facilitate the realization of a versatile robotic hand.
An additional concept is to utilize the function of the parallel gripper.
The parallel gripper is a powerful tool to handle many kinds of objects,
although it is difficult for it to pick up a thin or small object on a table. Thus,
the developed gripper must possess the capabilities of the parallel gripper as
well as other gripping modes. Based on these concepts, the underactuated
soft gripper was developed.
As illustrated in Fig. 7.25, the rigid layer within the fluid of the fingertip
was constructed as two separated cuboids to improve grasping ability. If an
object is thinner than the distance between the two cuboids, the cuboids
block the motion of the grasped object. However, if an object is larger,
the cuboids provide a wide contact area. In both cases, a large grasping force
can be applied. The distance between the fingers is controlled by a servo-
motor (Dynamixel XM430-W210) through a screw feed mechanism.
The function of scooping is critical to pick up an object on a table, especially
when the object is small or thin. Thus, the fingernail and joint were embed-
ded to realize scooping.
As illustrated in Fig. 7.26A, if the fingertip is pressed against a table or
another object, the underactuated joint rotates. To fix the rotation and real-
ize the pinching and enveloping grasps, a ratchet mechanism and a torsion
spring were installed at the underactuated joint. The fixation of the ratchet
is released by fully opening the gripper (Fig. 7.26B). When the gripper