IEEE Robotics & Automation Magazine - September 2013 - 74

employing easy-to-control compliant actuators that are sufficiently compact to be hosted in a human-sized forearm
(see the section "Actuation")
● employing an appropriate but simple sensory apparatus to
enhance the hand functionality and to compensate for the
side effects raised by the mechanical design choices (see the
section "Sensorial Apparatus")
● exhibiting surface compliance through a purposely
designed soft cover that closely mimics human skin (see
the section "Design Solutions for the Hand Soft Cover").
These design approaches, described in the following text,
are adopted for implementation in the UB Hand III [see Figure 2(a)] and are now exploited within the DEXMART project (http://www.dexmart.eu/). The last outcome of the project
is a novel robotic hand, the UB Hand IV, whose prototype is
shown in Figure 1.
●

Design Solutions for the Finger Structure
Many solutions presented in the past [e.g., UB Hand II [7],
Figure 2(b)] were inspired by exoskeletal models, the load-carrying frame being a hollow structure characterized by closed
cross sections with good stiffness/weight ratios. Nonetheless,
this traditional design approach leads to a poor exploitation of
the available space inside the finger, which is mainly used to
host the articulations and the transmissions. A different, bioinspired concept is based on the endoskeletal model and
shows a functional distinction between an inner, stiff framework (the bones) and an outer, compliant layer (the flesh).
This design solution allows space to be saved for hosting the
sensors, the related electronics, and the pads, which are now
placed around the articulated structure rather than inside it.

(a)

(b)

Figure 3. The UB Hand IV finger prototypes: (a) integrated CJs
and (b) pin joints integrated into the phalanx. (Photo courtesy of
Gianluca Palli at the Laboratory of Automation and Robotics of
the University of Bologna.)

z

z

y

y
O

O
l

l

x
(a)

x
(b)

Figure 4. The large-displacement rotational CJs: (a) SPIR CJ and
(b) HEL CJ.

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As for the finger joints, the main design goal is to search
for the maximum achievable integration between the various
components, in the perspective of structural simplification,
potentially leading to one-step monolithic manufacturing and
consequent reduction of the assembly complexity. In particular, the use of compliant joints (CJs) or sliding kinematic pairs
has been investigated by the authors in the last few years [4],
[8] as possible alternatives to the classical rotational joints
based on bearings and similar hardware. As examples of the
above concepts, several solutions are explored along with the
development of different UB hand prototypes:
● modular fingers composed of plastic phalanges (obtained
by injection moulding) and CJs shaped as close-wound
alloy springs [Figure 2(c)]
● monolithic fingers manufactured in Teflon by computer
numerical control (CNC) machining, with CJs shaped as
notch hinges [also, the tendons are integrated into the
structure; see Figure 2(d)]
● monolithic fingers with integral CJs made of the same
material (Fullcure 720) as the phalanx structure [Figure 3(a)]
● fingers with pin joints integrated into the phalanx body
simply consisting of a plastic shaft that slides on a cylindrical surface [Figure 3(b)].
Concerning the integrated pin joints, despite the sliding
contacts, the joints can withstand an indefinitely large number
of opening-closing cycles at a tendon tension of about 80 N.
On the other hand, stiction and dynamic friction deteriorate
the open-loop position control of the finger and can lead to
the mechanism locking as the contact pressure between the
shaft and the hub increases (due to increased tendon traction). Nonetheless, the adoption of a sensory system and suitable control strategies [9] can compensate for such side effects.
Concerning the CJs, their benefits when compared to traditional kinematic pairs include the absence of wear, backlash,
and friction, while still ensuring size and weight reduction. On
the other hand, critical issues of the CJs are possible fatigue
failure and undesired displacements. In fact, restricting the
analysis to single degree-of-freedom (DoF) rotational CJs,
these devices are conceived to allow a principal displacement
along a desired reference direction (called compliant, see, e.g.,
[10]) when subjected to a principal load (torque or force) acting along the same direction. The ratio between the principal
displacement and the principal load is called the principal
compliance. The secondary or parasitic displacements along
the other reference directions may occur in real applications
both for the presence of the secondary or parasitic loads acting
along those directions and for the presence of a compliant axis
drift. As for the axis-drift, even in the absence of secondary
loads, the point that models the center of rotation of the CJs
can be subjected to a spatial motion during joint deformation.
In any case, both fatigue failures and secondary displacements
are heavily dependent on the inseparable binomial material
morphology, the achievable joint shapes being directly connected to the manufacturing technology. For instance, the CJs
with relatively simple geometries, such as the beam-like
http://www.dexmart.eu/
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