IEEE Robotics & Automation Magazine - March 2016 - 45

Biologically Inspired Swimming Snake Robots
The use of robotic underwater vehicles has rapidly increased
during the last several decades due to technological innovations that enable these mechanisms to operate in deep and
harsh subsea environments. Today autonomous underwater
vehicles (AUVs) and remotely operated vehicles (ROVs) are
widely used subsea for different challenging tasks [1], such as
inspection, surveillance, maintenance, repair, and construction, and they are extensively used in the subsea oil and gas
industry and by the science community. Swimming snake robots represent an interesting alternative to conventional
ROVs and AUVs.
For centuries, engineers and scientists have gained inspiration from the natural world in their search for solutions to
technical problems, a process termed biomimetics. To this
end, inspired by biological swimming creatures, underwater
snake robots carry the potential of meeting the growing need
for robotic mobility in underwater environments. These
mechanisms have a long, slender, and flexible body, enabling
them to reach and operate in locations not accessible by larger
and more conventional underwater vehicles. At the same
time, a swimming snake robot carries manipulation capabilities as an inherent part of its body since it is essentially a mobile manipulator arm. Underwater snake robots thus have the
potential to improve the efficiency and maneuverability of
modern-day underwater vehicles. A particularly relevant application concerns inspection and maintenance of subsea oil
and gas installations, where the ability to reach tight locations
in between pipe structures is important. Moreover, for the biological community and marine archeology, snake robots that
can swim smoothly with limited noise and navigate in difficult environments such as shipwrecks are very interesting [2].
To realize operational snake robots for such underwater applications, a number of different control design challenges
must first be solved. An important control problem concerns
the ability to follow given reference paths, and this is the topic
of this article.
Work on biologically inspired snake robots was at first
largely restricted to land-based studies, for which reviews on
modeling, implementation, and control of snake robots have
been presented in [3]-[5]. Empirical and analytic studies of
snake locomotion were reported by [6], while the work of [7]
is among the first approaches to develop a snake robot prototype. Several land-based snake robots [8]-[10] and biologically inspired swimming robots [11]-[19] have been
constructed since then. Due to the complex dynamics of
swimming snake robots, several different modeling approaches have been carried out in the literature [2], [14],
[20]-[28]. Several results have been reported in the related
field of design, modeling, and control of underwater robots
that mimic the movement of fish [18], [19], [29]-[32]. In addition, sandfish lizard locomotion has been studied as the inspiration for a robot design in [33]. A comparison of these
approaches is presented in [34].
Most modeling approaches for underwater snake robots
omit the fluid moments (fluid torques) by considering that

their effect on the motion of the robot is negligible [22], [26],
[35]. However, including the impact of the fluid torques on the
power consumption of the system (see, e.g., [25]), will improve
the accuracy of the model from a hydrodynamic and energyefficiency point of view. The works in [23], [25], and [36] propose the modeling of fluid torques, with the drag force and
torque evaluated numerically. These approaches lack a closedform solution, which is a drawback since a hydrodynamic
model in closed form is advantageous for model-based analysis and control design. The works in [2], and [37] present a
closed-form hydrodynamic model, where hydrodynamic forces and torques are considered and where there is no need for
algorithmic computations of drag effects. Furthermore, in this
approach, both linear and nonlinear drag forces (resistive fluid
forces), the added mass effect (reactive fluid forces), the fluid
moments, and current effects are considered. The resulting
closed-form model is well suited for model-based control design schemes. In this article, the adopted control design will be
based on the model presented in [2] and [37].
Previous control approaches for underwater snake robots
proposed in the literature have mainly been concerned with
forward and turning locomotion [14], [38]. Thus, the next
step would be path-following control. To this end, [22], [39],
and [18] propose controllers for tracking straight and curved
trajectories based on synthesizing gaits for translational and
rotational motion of various fish-like mechanisms. The evolution from fish to amphibian using the same concept is presented in [13] by employing central pattern generators.
Moreover, [22] and [40] propose controllers for tracking
straight and curved trajectories for eel-like motion. In [41],
the path to be followed by the underwater snake robot is defined by straight lines of interconnected points, combining an
artificial potential field-based path planner with a new waypoint guidance strategy. A different waypoint guidance strategy is described in [42] for a carangiform swimmer, having the
waypoints defined a priori.
Several previous works consider control schemes for eellike robot locomotion. In particular, [43] develops a feedback
control scheme for three-dimensional (3-D) movement of the
robot's continuous model presented in [23]. In [44], motion
control of a 3-D eel-like robot without pectoral fins is described, while in [45] a multivariable constrained feedback
control scheme is proposed, considering a reduced model of
an eel robot. A methodology for path following of eel-like robots is presented in [46] based on autonomous gait generation
extracted from the controlled local system curvature. Openloop motion planning for eel-like robots is presented in [14],
[22], and [38], including the experimental evaluation of the
adopted techniques. Furthermore, in [22], experiments for
closed-loop straight-line tracking using image-based position
feedback are implemented with disturbance rejection in the
plane. Nevertheless, these preliminary experiments were not
satisfactory, as mentioned in [22], although they prove the
general concept. In [34], an underwater snake robot is commanded to track a straight-line path in the presence of ocean
currents of unknown direction and magnitude, controlled by
march 2016

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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