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bilical cable that guarantees the communication robustness.
5 Locomotion module
5.1 Actuation
The mechatronic architecture of this worm-like robotic system has been
defined paying attention to the possible use and advantages of smart mate-
rials, such as magneto-rheologic fluids (MR), electro active polymers
(EAP) and SMA (Shape Memory Alloys). Compared to traditional actua-
tion systems, smart materials join actuation and structural functionalities,
with advantages in terms of mass, dimensions and simplicity. This choice
allows to build a light and high force-density device. The working princi-
ple of SMAs is well known: they exploit a solid-state transition between
two crystal lattices forms: austenite, stable at high temperatures, and mart-
ensite, stable at low temperatures. Martensitic variant of the crystal shows
a plastic-like behaviour due to twinning of the lattice, while austenite is
characterized by a linear elastic stress-strain slope [12]. During the transi-
tion between these states, caused by a change in temperature, the material
can recover a previously memorized shape thanks to de-twinning typical of
martensite-austenite transformation. Memorization of the shape occurs upon a thermal process similar to tempering, characterized by a long pe-
riod at high temperature and sudden cooling. In proposed solution Nitinol
traction springs are used to increase actuation displacement, that for a
straight wire is of the magnitude order of 8% of its own length.
5.2 Architecture
To obtain the required behaviour for the single locomotion module, three
SMA springs and a steel bias spring are used. The actuators are placed
with a 120° symmetry. When Nitinol springs are cool, thus deformable, the
bias spring lengthen them and the module elongates. When traction springs
become austenitic (heated), they regain their memorized shortened shape
and contract segment. Heating of one spring causes a 30° bending of ther
module in that direction. A Pro/E virtual mock-up of three segments is
shown in Fig. 1 together with the peristaltic motion sequence. Fig. 1. Pro/E model of one module of the robotic worm actuation module (a), bel-
low (b) and actuation sequence for a three segments robotic worm (c); actuation
sequence for a three segmented body (d)
The external covering has to accomplish two different tasks: providing
insulation from external environment and working as thickening/clamping
element when segment shortens. Different solutions were considered. The
best solution from all points of view is the use of a bellow, providing grip
and granting sealing, Fig. 1 (b). The presence of a covering brings prob-
lems too, dealing with the increased reaction force and dealing with heat
exchange.
5.3 Tests and fabrication
A session of tests has been done on the SMA springs to obtain a more pre-
cise knowledge of their behaviour, their characteristics have been intro-
duced in the dynamic model to dimension the locomotion module. Follow-ing, we did another set of tests, to evaluate the right duty cycle for actua-
tors, in order to avoid superheating and find possible speed of the SMA ac-
tuated device.
The characteristics of the SMA springs are the following: length: from
16mm to 140mm; diameter: 6mm; activation temperature: 45-50°C; reac-
tion speed: some 0,1 s in heating, cooling time is much more complicated,
depending from ambient condition, reached temperature during heating
phase.
We built a test bench equipped with load cells Burster type 8435 to
measure force. LabView has been used to acquire data and for a first rough
filtering of noise.
As first tests actuation forces versus displacement were measured. For
every length the registered trend was the same as in the square in Fig. 2