IEEE Robotics & Automation Magazine - September 2013 - 79

Rigid Core
Soft Pad

Indenter (External Surface)
Figure 10. The DLD concept.

Sensorial Apparatus
The introduction of the compliant structures and the transmission systems based on the tendons, together with the use
of innovative actuation systems like the twisted-strings actuation, requires the use of appropriate sensory equipment and
the adoption of suitable control strategies. These strategies
become necessary for dealing with the intrinsic system compliance and for compensating for the side effects, such as nonlinearities and friction, which result as a consequence of the
aforementioned design choices. The underlying philosophy is
basically to attempt shifting the system complexity from the
time-consuming domain of mechanical design to the easily
reprogrammable domain of control. Generally speaking, a
robotic hand can be equipped with position/velocity sensors
for the measurement of the joints' and/or actuators' configuration, with force sensors for measuring the force applied on the
wrist, the palm, the phalanges, and/or the joints and tactile
sensors for reconstructing the pressure map during the contact (usually restricted to the fingertip) by means of an array
of sensitive elements. In some limited cases, proximity and
single pressure sensors are also part of the robot hand sensory
equipment. For many reasons, to improve the reliability and
to simplify the sensory subsystem, it is preferable to use sensitive elements with intrinsic high immunity to electromagnetic
disturbances and with limited requirements in terms of both
conditioning electronics and amplification. This is the main
reason why the use of optoelectronic components is studied
for all the sensors needed in the robot hand. In particular, an
LED-photodetector couple with a wide angle-of-view is
adopted for the implementation of the joint position sensors
[24] depicted in Figure 9(a) [resolution of about (1/100)°],
whereas components with a narrow angle-of-view are used
for the force sensors [25] shown in Figure 9(b) and (c) (resolution of 0.05 N). Many different principles can be used for
the implementation of the tactile sensors, but the problem is
still the integration of all the electronics and acquisition system within the limited space of the fingertip. To solve this
issue, a tactile sensor based on discrete surface-mount device
optoelectronic components is developed [26], and in Figure
9(d), the grid of 4 # 4 taxels of the sensors prototype is
shown. This sensor allows direct data acquisition (without any
amplification circuit) about the deformation of the soft pads
mounted above the sensor grid. For example, if a plane is

pushed against the soft cover, the tactile sensor can detect the
overall reaction force with a resolution of 0.2 N and the plane
position and orientation (with respect to the sensor center)
with a resolution of 100 µm and 0.2°, respectively [27].
Design Solutions for the Hand Soft Cover
The soft covers are introduced as they improve functionality,
safety, and acceptance by the users. The majority of soft pads
studied so far were made by viscoelastic polymers homogeneously shaped over an internal rigid core mimicking the
bone or the robotic finger
inner structure. In such a
case, for a given external
The main design goal
geometry, the parameters
that contribute mainly to
is to search for the
the pad compliance are the
material hardness and the
maximum achievable
layer thickness [28]. A
higher thickness implies
integration between the
higher compliance, which
is beneficial in terms of
various components.
safety and grasp stability/
sustainability. On the other
hand, high pad thickness
signifies an undesired increase of limb dimension. In parallel,
a higher material hardness, which is beneficial in terms of the
surface reliability, signifies an undesired lower compliance. In
practice, thickness reduction for a given compliance becomes
a significant design goal as long as, usually, the adopted solutions trade off between the need of slender robotic limbs and
good material properties.
Looking for alternative solutions to homogeneous soft
covers, the concept of differentiated layer design (DLD)
(which allows increasing the pad's compliance while minimizing its thickness) is proposed by the authors [28]. This
concept consists of the adoption of a single material dividing
the overall thickness of the pad into layers with different
structural designs (i.e., an external continuous skin layer coupled with an internal layer with voids). Figure 10 shows a
DLD soft pad, whereas Figures 11 and 12 show the pad's
internal and external layers.
In particular, the pad quasistatic relation between the
applied normal load, F, and the contact deformation, d
[load-deformation (LD) curve] can be described as a nonlinear function: F = f (d). Given the pad thickness and material,
the overall contact area can then be split into finite elementary triangular sub-regions or triangular elements (TE)
defined by tiling the plane regularly with equilateral triangles
as in Figure 11. Once the element LD curve is designed to
obtain the desired nonlinear characteristic, the number of
elements, N, can be chosen such that
F = N $ ft (d),

(1)

where ft (d) is the element LD curve. Naturally, the proposed procedure concerns the definition of an overall pad
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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013