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Hand Design—Hybrid Soft and Hard Structures 129

under each condition. The other experimental conditions were the same as
that shown in Fig. 7.15. The compression test results are shown in Fig. 7.18
where mean values are displayed for convenience. At the initial fluid pres-
sures of 1.5, 2.7, and 4.0 kPa, the curve converged to one line. When the
initial fluid pressure was 6.0 kPa, a linear behavior was observed, but it was
different from those observed under the other conditions. In all cases, a linear
behavior was observed, but at different starting points of increase. Recalling
that the basic concept for fracture avoidance is to detect a linear compression
behavior and to stop compressing (or stop compressing before the linear
behavior is over), then, all that is needed is to develop a method for the
detection of a linear compression behavior. An overview of the method
follows:
Step 1. Detect the point where fluid pressure starts increasing.
Step 2. Detect the points where compression behavior is linear.
More details about each step are described hereafter.

7.3.2.2 Step 1: Detect the Point Where Fluid Pressure Starts Increasing
As it can be seen from Fig. 7.18, fluid pressure starts increasing after the tofu
is compressed by some distance. This starting point corresponds to the state
where the contact pressure of the object (tofu) becomes equal to the fluid
pressure of the fluid fingertip [3], which is a phenomenon that occurs when
the stiffness of the fluid fingertip is larger than that of the object (tofu). This
phenomenon is considered with a simple model shown in Fig. 7.19 where

Phase change point Fracture point
Elastic / linear behavior starting point
8 1.5 kPa
7 6 2.7 kPa
Fluid pressure P [kPa] 5 4 3 6.0 kPa
4.0 kPa

1 2
0
0 5 10 15 20
Pushing distance x [mm]
Fig. 7.18 Fluid fingertip compression test results. Initial fluid pressures when there is no
contact were 1.5, 2.7, 4.0, and 6.0 kPa [3].