IEEE Robotics & Automation Magazine - September 2013 - 80

Distal (exp.)
Medial (exp.)
Proximal (exp.)

Contact Area

Normal Load (N)

5

Internal Layer

Figure 11. A triangular grid.

contact force (i.e., overall pad compliance), which is an integral (rather than a local) property of the pad. In addition, we
suppose that N elements are involved in the contact simultaneously and the contact area is displacement independent.
To replicate the compression law of the human pulps (Figure 13) while reducing pad thickness, the TE internal layer
might be designed using a series of microbeams inclined
with respect to the orthogonal to the external surface (normal axis, Figure 12), thus transforming normal loads acting
on the contact into bending actions applied on each beam.
The microbeams are placed on the edge of the TE, as shown
in Figure 12. The TE dimensions t, h, k, and j in Figure 12
are found by trial and error with the aid of the finite element
method (FEM) and aim
at replicating the human
finger experimental LD
Even defining the optimal
curves shown in Figure
functionalities of a robotic 13. Note that the FEM
simulations are carried
out by imposing a gradual
end-effector is quite a
displacement to the
indenter along its normal
challenging task.
axis and by measuring the
related reaction force's
variation. Similarly to the human pulp, this peculiar TE
geometry presents a quasilinear LD curve characterized by a
very low stiffness for small displacements, the load rapidly
increasing once the microbeams collapse (Figure 12) on the
outer skin. In such a situation, the TE behaves similarly to a
pad made of a uniform soft material. In the actual implementation, the pad thickness is set to 3 mm (i.e., half of the

Normal Axis
h

1

4
3
2
1
0

0

0.2

Internal
Layer
External
Layer
(b)

(c)

Figure 12. The model mesh and deformed TE (collapsed beam): (a) detailed view of the
TE, (b) TE model mesh, and (c) deformed TE.

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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september 2013

0.6 0.8
1
1.2
Displacement (mm)

1.4

1.6

thickness of previously published solutions [4]), whereas the
TE final design (Figure 11) is characterized by t = 0.5 mm ,
h = 2 mm , k = 1 mm , and i = 45c. The triangle facet, l, is
chosen on the basis of (1), and the TE material is Tango Plus
Fullcure 930 (hardness 27, Shore A), a polymeric resin used
for stereolithographic prototyping processes. For instance,
considering the distal phalanx, the contact area is a
20 # 15 mm 2 rectangle meshed by means of 36 TEs. The
resulting surface area of a single TE is 8.3 mm 2 . Figure 13
shows the numerical (FEM) or experimental (exp.) relationship between the normal load (N) and the resulting displacement (mm) for 1) the structured pad prototype
depicted in Figure 11, 2) a uniform PAD of the same thickness (3 mm) made of a softer material, and 3) the human
finger. It can be seen that a 3-mm-thick structured pad represents a substantial step forward in human finger mimicry
in terms of stiffness, when compared to previously published solutions where different materials and higher pad
thicknesses are used. Similar procedures but different contact areas and numbers of TEs are adopted for the design of
the medial and proximal pads, whose characteristics are also
reported in Figure 13. The potentialities of the DLD concept
are experimentally evaluated on hemispherical soft pads
shaped over a rigid core [28]. The
pads to be mounted on the hand
prototype are shaped as in Figure 10,
Indenter Imposed
and their physical implementation is
Displacement
shown in Figure 1.

k

(a)

0.4

Figure 13. The displacement (mm) versus normal load (N) for a
DLD pad and a human fingertip.

i

t

DLD Pad (FEM)
Unform Pad (exp.)

Conclusions and Perspectives
In this article, an overview of the
design solutions and enabling technologies along with some possible
directions for improving the design
of the robot hands are discussed. In
particular, considering the numerous
projects developed in the past and
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013
