Stabilization process in marine surface robotics inspired by swimming snakes – SSSNAEQ

To achieve these objectives, the SSNAEQ approach is based on three work packages (WP), WP0 being dedicated to project management

- WP1: Analysis and modelling of the stabilisation mechanisms through theoretical, numerical and experimental studies of the interactions between a serpentine body and a
numerical and experimental studies of the interactions between a serpentine body and a free surface.

- WP2: Design and characterisation of an instrumented robotic platform developed to study the surface stability mechanism.

- WP3: Development and validation of surface stability control laws based on the physical models of WP1 and taking advantage of the intelligence embodied (action+perception) in the morphology of the robot designed in WP2


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The SSSNAEQ project aims to design a marine robot inspired by snake-like locomotion on the water surface. The design (WP2) and control (WP3) of the robot are underpinned by a real understanding of the behaviour of semi-aquatic snakes and the physical constraints of the environment (WP1)

Therefore, the first task of WP1 (WP1/T1) focuses on a preliminary study of the snake stabilisation mechanisms in order to transfer this knowledge to the robot design. We conducted a review of the biology of aquatic and semi-aquatic snakes including morphological aspects, buoyancy control and their perception of their environment.
We are working in WP1 to develop concise models of hydrodynamic forces adapted to surface swimming validated by physical measurements (WP1/T2-T3). In this context, we are adapting wake generation models, comparing them with Boundary Element Method (BEM) simulations and validating them with measurements in a channel. These different steps are ongoing.

The objective of WP2 is to develop a robot capable of stabilising itself on the surface. We have developed a poly-articulated robot named NATRIX (WP2/T1), which is now operational with six modules including the ARIM system and a head-couple.

The control part of WP3 also progressed with the development of a posture control algorithm for a geometrically accurate model of the snake. After establishing a static stability model (WP1-T2), the control principle is based on the decomposition of a movement as a sequence of static equilibria driven by bending and torsion.

In view of the identified objectives, the project has validated all the steps foreseen in the Gantt chart for the first three semesters. The main milestone is the development of the robot. Two papers are published, two submitted and 4 conferences are planned.

During the year 2020, we were able to achieve our goals, although the health crisis did not allow us to be ahead of schedule. This robot is the first snake robot designed to stabilise itself on the surface. We therefore have an extraordinary opportunity to achieve unprecedented results in 2021.

Fundamental results in Riemannian physics and geometry produced during the project pave the way for a better understanding of the stabilisation processes of snake-like bodies at the surface.


We have also established several results on the study of semi-aquatic snakes, which allows us to place our study in a truly bio-inspired approach. Indeed, the emergence of a bio-inspired technology must go hand in hand with our knowledge of the model animal: the snake. As we all know, this animal suffers from a poor image. We would like to contribute to a better understanding of these animals, which are highly threatened by human activities.

There are still a number of projects to be completed in the areas of control law and instrumentation. We are relatively confident because these areas are at the very heart of the research team's expertise.

Furthermore, we have started to communicate about this experimental platform and I am already receiving proposals for collaboration from the physics community (E. Eloy and F. Candelier, Univ. Marseille) and biologists (M. Segall, American Museum of Natural History, New York).

Our results will be presented at several international (on-line) conferences: ICTAM 2020+1 (August 2021), dynamics-day (August 2021) and IROS (October 2021).

In parallel, I have started writing an ERC (mentioned in the ANR SSSNAEQ project), for a submission in 2022 (and a start in 2023, at the end of the ANR)

Article accepté

-> Herault, J. (2020). A geometrically exact approach for floating slender bodies with finite deformations. Applied Ocean Research, 101, 102220.

-> Herault, J., Clement, \'E., Brossillon, J., LaGrange, S., Lebastard, V., \& Boyer, F. (2020, July). Standing on the Water: Stability Mechanisms of Snakes on Free Surface. In Conference on Biomimetic and Biohybrid Systems (pp. 165-175). Springer, Cham.


Article Soumis

- Herault J., Xiao, X., Clement, \'E., Boyer, F., ., Lebastard, V, , Stability mechanism of aquatic snakes on water: from the biological aspects to the bio-inspired robot. For a special issue on «Living Machines: From Biological Role Models to Soft Machines« in IOP Bioinspiration \& Biomimetics, soumis mars 2021.


-> Xiao, X., Clement, \'E., Boyer, F., ., Lebastard, V, Herault J., Quasi-static motion of a new serial snake-like robot on a water surface: a geometrical approach. For The IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)}, soumis février 2021.

Article en cours de rédaction

J. Herault, V. Lebastard, and F. Boyer, L. Paez, K. Melo, and A. Ijspeert, R. Thandiackal, Gait transition induced by hydrodynamic sensory feedback and central pattern
generators in an anguilliform swimming robot. Potentially submitted to Physical Review Letter.

A new generation of robots inspired by aquatic snake swimming has recently emerged in academic contexts. These robots are more compact, manoeuvrable and energy efficient than autonomous surface vessels. Thus, thanks to their exceptional manoeuvrability, they could intervene in complex situations (capsizing, surface debris, oil spill) thanks to their exceptional manoeuvrability. However, despite this potential, they suffer from too precarious surface stability in extreme conditions (swell, wind), as buoyancy forces associated with certain body shapes can overturn the robot. The SSSNAEQ project aims to remove the scientific barriers to surface instability by taking inspiration from snakes, using their entire body as a stabilizing organ to dynamically control the distribution of buoyancy moments. Also, SSSNAEQ will be the first theoretical and experimental study of the static and dynamic equilibrium processes on the surface of self-propelled hyper-redundant bodies. SSSNAEQ's progress will be based on the following questions for robots and snakes. What are the static stability conditions of a hyper-redundant body floating on the surface? What are the hydrodynamic forces acting on a self-propelled body that can affect its surface stability? How to use the physics of these interactions and serpentine morphology to control the stability of a hyper-redundant robot? Thus, SSSNAEQ will pursue the three main objectives of analyzing the surface stabilization mechanisms of hyper-redundant bodies, designing an innovative prototype of a serpentine bio-inspired robot, and developing control laws on both a hydrodynamic model and sensory reflexes, based fully on the morphology of the robot. This bio-inspired approach will benefit from a collaboration with the Natural History Museum of Nantes. The scientific challenge here will be to develop new models of fluid-structure interactions on the surface of a fluid, concise enough to be exploited by controlling the stabilization of the robot. To this end, the project will continue a theoretical and experimental study of fluid-structure interactions at the fluid surface. In a second step, a robotic platform will be developed to improve our understanding of the stabilization process that will be implemented in the control system. The originality of tour project relies in the development of two new technologies: a new type of actuation: the ARIM system, based on a control of the robot's morphology, and a new type of perception, the E-sense of touch, allowing the robot to perceive its level of immersion through the electrical sense. The synthesis of these concepts and technologies will be carried out by a control law exploiting action/perception synergies, in order to control locally the de/stabilizing effects on the surface in order to in-fine control global stability. Breaking with current technologies, the embodied intelligence in the robot body will relieve the control part by integrating the main principles of bio-inspiration: redundancy, agility and resilience. SSSNAEQ is a unique opportunity to provide an innovative and bio-inspired technological solution to environmental and societal problems.