Page 130 - Human Inspired Dexterity in Robotic Manipulation

128 Human Inspired Dexterity in Robotic Manipulation

grasping force at the instance compression is stopped, there are two possible
options. One is to give up grasping the object, whereas the other is to
increase the grasping force (compression) along with the risk of fracture.
The option of immediately stopping compression upon detection of a linear
behavior is a little conservative. On the other hand, if compression is allowed
to continue until the detected linear behavior is over, such a choice is some-
how dangerous because the end of the linear behavior is close to the fracture
point. Nevertheless, it must be mentioned that this is not about a grasping
condition, but a fracture avoidance condition. Frictional and equilibrium
conditions (balancing of object weight) are additionally required for realiz-
ing grasping [24]. It must also be remarked that the conditions just described
apply to materials possessing the same compression behavior as that shown in
Fig. 7.16, which can generally pertain to gel-like objects.

7.3.2 Methodology for Detecting Linear Compression
Behavior for Fracture Avoidance [3]
7.3.2.1 Compression Test With a Fluid Fingertip
To avoid fracture, the compression behavior of a fluid fingertip utilized as an
indenter must be understood. The compression test using a fluid fingertip
(instead of the indenter in Fig. 7.15) was conducted following the experi-
mental setup shown in Fig. 7.17. The fluid fingertip is the same as that shown
in Fig. 7.3 except for the pressure sensor and the control unit. A high pre-
cision pressure sensor (KEYENCE, AP-12s) was utilized to observe the fluid
pressure values more accurately. Additionally, a simple control unit was uti-
lized because its purpose was only to control initial fluid pressures, which
were set to 1.5, 2.7, 4.0, and 6.0 kPa when there was no contact (i.e., before
the actual compression test). The experiments were conducted three times

Pump
Fluid fingertip
x
Load cell
f com

Target object Automatic positioning stage

0.0 kPa
Fig. 7.17 Experimental setup for the compression test using a fluid fingertip on soft tofu.