IEEE Robotics & Automation Magazine - September 2013 - 75

flexures depicted in Figure 2(d), can withstand infinite load
cycles and can be used to produce the monolithic fingers. In
the same way, the close-wound spring CJs depicted in
Figure 2(c), also withstanding infinite load cycles (at a tendon
tension of about 80 N), are less sensitive to parasitic motions
but unsuitable for the production of one-piece compliant
structures. Recalling that the main design goal aims at reducing the assembly complexity, at present, different technologies
and a wide range of materials (including the lightweight metal
alloys) can be used to produce the articulated finger structure
in a single production step (fully integral finger). Such technologies include the aforementioned CNC machining, plastic
molding (such as shape deposition manufacturing [11]), selective laser sintering, fused deposition modeling, stereolithography, and electron beam melting. Nevertheless, recent advances
in plastic material technology suggest that the use of polymers
might be well suited for the production of artificial hands
when a lightweight, relatively economical solution is sought.
In practice, a proper choice of materials and construction processes may lead to the production of CJs directly integrated
into the rigid links. In this case, one can obtain articulated
structures that are compact, light, safe, robust to impact, inexpensive, and slender (such that the external sensors and the
compliant covers are easily integrated).
Hence, the real design challenge is to determine the best
CJ morphology that allows the desired principal displacement
while minimizing parasitic effects. In particular, aiming at a
compact CJ design and looking at the minimization of the
parasitic effects, two large-displacement CJ morphologies are
investigated by the authors [10]. Both these types of joints, as
depicted in Figure 4, are characterized by the same specifications in terms of range of motion, dimensions, principal compliance, material, and production technology. More details
about comparison metrics among different CJ morphologies
are reported in "CJs Comparison Criteria." In particular, these
joints are employed in the realization of the compliant finger
shown in Figure 3(a), which is made of FullCure 430 Durus
White, a photosensitive resin with mechanical properties similar to polypropylene. The reliability and fatigue life are still an
issue. Nonetheless, further testing is in progress concerning
fingers made with high-performance thermoplastic materials
(such as Stratasys Ultem 9085).
Design Solutions for the Sensorimotor System
Tendons and Tendon Net
The current technology does not allow the arrangement of 20
or more actuators in a human-sized robot hand while meeting speed and force requirements. For this reason, it is often
necessary to remotely place the actuators. In particular, as previously discussed, the tendon-based transmission systems
represent one promising solution toward the implementation
of dexterous anthropomorphic robotic hands. Besides this
consideration, it should be noted that the tendons are also
present within the human hand, with the purpose of connecting bones and transmitting forces. Due to the relatively com-

plicated human tendon network, rather than directly
imitating the biological model, many different simplified solutions are proposed in the literature. In fact, the optimization of
the transmission system, in terms of friction reduction and
decoupling among hand
movements, requires the
tendons to traverse the
Many anthropomorphic
endoskeleton or to be
routed close to the center
robot hands have already
of rotation of the joints by
means of suitable canals
been designed, often trying
(whereas in the biological
models, the tendons slide
to replicate or enhance
around the bones). The
well-known and effective
the specific features of the
configurations are [12]:
● 2N configuration: Each
human hand.
joint is actuated by two
independent tendons.
This configuration allows the independent control of the
joint torques and the regulation of the internal forces but
requires 2N actuators.
● N configuration: Each joint is actuated by a tendon connected in a loop to the actuator. This configuration allows
the independent control of each joint, but it requires a
mechanism for the tendon pretension.
● N + 1 configuration: The joints are actuated by a tendon
net composed by a number of tendons equal to the number of joints plus one. This configuration allows the use of
the minimum number of actuators and, simultaneously,
allows the avoidance of pretension mechanisms.
● Unilateral tendon actuation: This configuration exploits the
energy stored in the CJs during the closing phase to perform the opening phase without requiring further energy
from the actuation [4], [11].
In tendon actuation systems, different movements can be easily coupled by simply connecting two or more tendons to the
same actuator, thus obtaining a defective actuation, avoiding
additional mechanisms, and reducing both the costs and the
complexity.
Another important problem in the tendon-based actuation is the routing of the tendons from the motors to the
joints. Usually, the tendons are routed by means of pulleys,
sheaths, or sliding surfaces, and pulleys reduce the friction
forces at a minimum level by acting along the tendon. This
approach implies a more complicated mechanical design,
due to the presence of bearings and similar hardware partially reducing the advantages introduced by the use of tendons. The use of sheaths is a convenient solution due to its
simplicity, but it introduces distributed friction along the tendon, which means hysteresis and dead zones in the transmission system characteristics [9]. The selection of the tendon
material plays a crucial role. Usually, very thin steel ropes are
used. This solution allows for the linear force-elongation
behavior of the tendon, but it introduces some design and
assembly constraints because of the limited curvature radius
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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013