Page 119 - Human Inspired Dexterity in Robotic Manipulation

Hand Design—Hybrid Soft and Hard Structures 117

Outer rubber Fluid

Pressure sensor
Silicon layer
Fluid layer
(A) (B)
Fig. 7.2 Basic structure of fluid fingertip. (A) Cross sectional view of the fluid fingertip.
(B) Manufactured fluid fingertip.

of the fingertip, gel (Schick SWS-CG180) was the filling fluid. Inside the
gel, a silicon layer (Shinetsu Silicon KE-1308) was also embedded to deal
with rigid objects. The fluid layer and silicon layer correspond to the pulp
and bone in the human fingertip, respectively. To measure fluid pressure, a
pressure sensor was installed at the back side of the inside plate. Another
pressure sensor was located at the base of the silicon layer so that the force
applied to it can be measured. When surface deformation is small, there is no
contact pressure exerted on the silicon layer, and the fluid layer is utilized for
grasping. If the deformation is large, contact pressure is exerted on the silicon
layer and the applicable force correspondingly becomes large. The following
is the reason for putting an incompressible fluid inside the fingertip. An arbi-
trary pressure applied anywhere is transmitted equally at anyplace in the con-
tainer according to Pascal’s law. Therefore, fluid pressure can be controlled
at a location away from the fingertip, and yet, contact pressure is uniform.
Thus, the pressure control unit was installed separately from, but connected
to the fingertip through a tube, as shown in Fig. 7.3. A slide screw mech-
anism was utilized and was connected along with a DC motor (Maxon
Amax16 1.2 W) for controlling the position of the silicon piston. The vol-
ume of the fluid is controlled by the piston and the fluid pressure changes
according to the volume control. Note that fluid pressure can also change
due to fingertip deformation resulting from its contact with objects.
Note further that fluid pressure corresponds to contact pressure. Thus, a
single control unit can regulate multiple fingertips through multiple tube
connections.
It should be noted that BioTac [4–7] also developed a fingertip that has
similar outer deformable surface structure, incompressible fluid, and rigid
layers. However, its main difference with the fluid fingertip shown in
Fig. 7.2 lies in the function of the fluid layer. Besides being thin, the fluid