IEEE Robotics & Automation Magazine - September 2013 - 78
(a)
(b)
(c)
Figure 7. The finger prototype actuated by a twisted-string
actuator: (a) detailed view of the robotic finger, (b) detailed view
of the motors of the twisted string actuators, and (c) the robotic
finger and its actuation module. (Photo courtesy of Gianluca Palli
at the Laboratory of Automation and Robotics of the University of
Bologna.)
Twisted String (Red)
+
Tendon Network (Green)
24 Twisted-String
Actuators
Figure 8. The UB Hand IV virtual prototype including 24 twistedstring actuators. (Photo courtesy of Gianluca Palli by means of
CAD.)
LED
Photodiode
(a)
(b)
8,969 mm
11,308 mm
(c)
(d)
Figure 9. (a)-(d) The preliminary prototypes of the UB Hand IV
sensors based on optoelectronic components. (Photo courtesy
of Gianluca Palli at the Laboratory of Automation and Robotics of
the University of Bologna.)
78
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IEEE ROBOTICS & AUTOMATION MAGAZINE
*
september 2013
polymers [17], and piezoelectric actuators [18], but these
technologies are still far from implementation in anthropomorphic robot hands.
The actuation of the commercial robotic hands developed
so far is based essentially on rotative electric motors or linear
pneumatic actuators, usually McKibben motors. The use of
pneumatic actuators (besides the higher power density with
respect to electric motors) presents some difficulties from the
control point of view and also conflicts with the integration
requirements. In fact, the valves and the compressor cannot
be included in the hand structure because of their dimensions
and weight. Concerning robotic hands with electric actuators,
different types of transmission systems are adopted: the
motors (usually dc or brushless motors) can be directly placed
within the fingers and the palm and connected to the joint by
means of a harmonic drive [6] or spur and worm gears [19],
[20]; the motors can be placed in the forearm, transmitting
the mechanical power by means of flexible shafts [21] or
leverages or, as previously stated, the tendons routed by
means of pulleys [7] or sheaths [4], [22].
Considering the problems related to integration and size,
rotative electric motors are now the best technological solution for actuation. Hence, conversion from rotative to linear
motion is needed in the case of tendon-based transmission.
Usually, this problem is solved by means of pulleys or ball
screws connected to the motor gearbox. These solutions, even
if reliable and effective, can cause some problems regarding
the dimensions and the costs. Moreover, commercial miniaturized electric motors present the best efficiency at very high
speed and very limited torque. Hence, the adoption of gearboxes with high reduction ratio and low friction is needed,
thus increasing the complexity, dimensions, and costs of the
actuation unit. To solve these problems, the so-called twistedstring actuation system [23] is developed. With respect to
conventional solutions, the main advantages of this actuation
system are: 1) the direct connection between the motor and
the tendon without intermediate mechanisms (such as gearboxes, pulleys, or ball screws), 2) the direct transformation
from rotative to linear motion, 3) the extremely reduced friction (only an axial bearing is needed), 4) the very high reduction ratio, 5) its intrinsic compliance, and 6) the possibility of
using very small high-speed motors. The basic concept of this
actuation system is depicted in Figure 6: the overall length of
the transmission is reduced by twisting the tendons at one
end by means of the motor, resulting in a linear motion of the
other tendon end. Figure 6 also depicts the scheme of a
twisted-string transmission system actuating a single finger,
whereas Figure 7 shows a prototype of the proposed device.
In particular, the twisted-string transmission is tested over
more than 10,000 working cycles at a tendon tension of 40 N,
showing no significant variation on the behavior of the system [23]. This actuation modality allows us to obtain a very
compact and lightweight actuation module as can be seen in
the computer-aided design (CAD) of the hand prototype
shown in Figure 8 and composed of 24 actuators easily
located in the forearm.
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013
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