Page 129 - Human Inspired Dexterity in Robotic Manipulation

Hand Design—Hybrid Soft and Hard Structures 127

x
Indenter
Load cell
f com

Automatic positioning stage
Target object
Fig. 7.15 Experimental setup for compression test of soft (Kinugoshi) tofu [3].

Elastic
Curved behavior behavior
Fracture
3 Yielding
Compression force f com [N] 1.5 2 1
2.5

0.5
0
0 5 10 15

Pushing distance x [mm]
Fig. 7.16 Results of the compression test [3].

Fig. 7.16 shows the results of the compression test. After an initial curved
behavior, a linear behavior was observed with the increase in the pushing
distance, followed by yielding, then fracture. The curved behavior is
believed to be due to the increase in density which occurs if it is assumed
that the distances between the tofu particles decrease with compression.
When particle concentration occurs, both stiffness and density increase.
When concentration stops and density increase is saturated, elastic behavior
appears. It was confirmed that this profile can be observed in other objects
similar to gel, such as agar, gelatin, and a banana. This profile was also
reported in the experimental analysis of the mechanical properties of colla-
gen gel [23].
Because linear behavior appeared before yielding, the observation of a
linear behavior can be regarded as signal for fracture. Therefore, a basic strat-
egy for fracture avoidance is to stop the compression if the linear behavior is
detected. However, it must be noted that stopping compression upon detec-
tion of a linear behavior does not guarantee success in grasping, rather, frac-
ture avoidance. If a target object cannot be grasped with the corresponding