IEEE Robotics & Automation Magazine - September 2013 - 28

Device Overview
A transformer-like system that can transfer energy from a docking station to a robot is described here. The system is a kind of
transformer with a split core: 1) the primary coil (the source)
and 2) the secondary coil (the receiver). Constraints due to the
bioinspired approach during the artifact development are on the
basis of the design of the device.
Low weight and low encumbrance are the most important
features to be taken into account to host the secondary coil
within the robot. The main criterion in the design is the maximization of the ratios, transferred power versus weight, and
size. Efﬁciency is of secondary importance for our purposes
and is just considered for heating issues. The novelty of the
Artifact
Head

Segments

Tail

Secondary
Winding Control
Electronics
Figure 3. LAMPETRA artifact. The ﬁrst segment is the headlike section. This hosts the charging system and the control
electronics. The total length of the robot is around 80 cm
depending on the number of segments and on the tail length.
Every segment implements magnetic muscle-like actuators [7]
and local control electronics.
Usable Room
Case I

Case II

Robot Skin
Figure 4. Comparison between the designs of the secondary
magnetic core section. The room available and the coupling in
Case 2 are better than in Case 1.

Primary Core Section
Coupling Interface
Secondary Area for
Coil Windings
Secondary Core Section
Magnetic Flux

Figure 5. Longitudinal section of the device shows the
architecture of the transformer. The secondary magnetic core
section is coupled with the primary, which is divided into six
sections mounted around the secondary to maximize the
coupling area.

IEEE ROBOTICS & AUTOMATION MAGAZINE

september 2013

system is the hollow shape of the secondary magnetic core
section in which the electronics can be hosted.
The inductive power transfer was chosen after considering
the advantages and drawbacks of the systems in the literature.
Model
The secondary coil was modeled ﬁrst to ﬁt the constraints
imposed by the size and shape of the robot, which is eel
shaped and composed of many segments. The diameter is
around 50 mm, and the length of the robot depends on
the number of segments (Figure 3). The charging system
is supposed to be placed in the head segment with a
length of 70 mm.
For the secondary coil magnetic core section, two different designs were considered to reduce the bulk and the
weight and to maximize the transferred power. Figure 4
presents the two possible approaches. The two represented metal cross sections have the same cross area, but
Case 1 has a uniform metal core, whereas Case 2 has a
hollow shape. Case 1 is sometimes used for underwater
vehicles [8], [9]. However, this solution does not ﬁt our
case because of the constraints related to bioinspiration.
For example, with the increase in the air gap, the magnetic
inductance would be reduced too much, and the electronic components would have to be placed around this
structure, which would result in a difﬁcult design.
On the other hand, Case 2 has more free room available for
the electronics board, and the magnetic core is attached to the
head shell, thus minimizing the gap with the external inductor.
The resulting longitudinal section envisaged for the device
is shown in Figure 5.
The secondary magnetic core section is separated from the
primary section by a coupling interface, which is the air/water
gap. The primary metal core section, consisting of six elements, surrounds the secondary.
Design constraints are the overall dimensions and mass,
turns, magnetic ﬂux, and desired power. After a parametric
study reported in the following, we chose the optimum
working point for our application in terms of maximum
delivered power to weight ratio. The primary coil was
dimensioned after the secondary, but in this case, dimensions were not a constraint because coupling geometry
(interface with the secondary magnetic core section) and
electromagnetic ﬂux generation were the only two requirements. Again, we chose the best compromise between power,
dimensions, and number of turns.
Equations and Dimensioning
Figure 6 shows dimensioning parameters of the cross section
of the secondary core shape.
To design the secondary coil, we started with the secondary coil core design. Equations (1) and (2) describe the
core geometry in relation to the length and diameter,
respectively. The magnetic ﬂux ﬂows from the primary coil
to the secondary coil in the magnetic core, crossing the little air gap between the two (Figure 5). In the cross section

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013