Page 147 - Human Inspired Dexterity in Robotic Manipulation

Hand Design—Hybrid Soft and Hard Structures 143

grasped are a paper cup filled with liquid, soft tofu, and daifuku (Japanese
sweet). The fluid fingertips were effective in grasping these objects. Exam-
ples of hard and fragile objects that were grasped are a toy model, a glass bot-
tle, an electric bulb, and a thin plate. If the object has a complex shape, the
fluid fingertip adapts to the shape. If the object is thin, the scooping function
is effectively carried out. Next, deformable objects, such as packaging bags
and cords, were grasped by adopting the enveloping mode. Under this mode
the required force for grasping is reduced. Therefore, the mode functioned
effectively while grasping deformable objects. Thin and deformable objects,
such as a business card made of paper and a PET sheet, were grasped at the
tips. Either the scooping or the hyperextending function was utilized. Long
objects having a length of >80 mm, such as a balloon, a PET bottle, and a
juice box, were grasped by adopting the parallel gripper mode.
A representative example of small objects is an M3 washer having a width
of 7 mm. For this object, the parallel gripper mode could not be adopted
because of the space under the fluid fingertips and as illustrated in
Fig. 7.30A-2, a width of 8 mm is required to grasp an object with the parallel
gripper mode. For such an object, although the scooping function is effec-
tive, and the pinching mode was selected. Fig. 7.30 illustrates the allowable
size of objects for every grasping mode, namely, the graspable size of the
developed gripper. This is the primary limitation of the developed gripper.
Moreover, grasping failed frequently when the object size was close to the
size limitation (see Table 7.6). The other limitations are that the grasping
mode cannot change on a supporting surface where the fingernails cannot
slide (another method should be adapted) and a sufficiently large reaction
force (2.5 N) to rotate the underactuated joints cannot be obtained.
Fig. 7.32 presents the recommended procedure for selecting the grasping

Fig. 7.32 Recommended procedure for selecting grasping mode.