Integrated Bistable Modules for Frugal Discrete Miniature Robots

Context and Motivations
Miniature robotics at the meso scale opens new perspectives for applications in constrained environments, such as intra- or extra tubular inspection, minimally invasive medical robotics, or the exploration of complex structures. In this dimensional context, conventional approaches inspired by rigid robotics rapidly reach their limits, particularly in terms of energy consumption, integration complexity, and interaction safety.
Discrete (multi stable) robots, built from the assembly of bistable modules, constitute a promising alternative. These systems rely on mechanical elements featuring multiple stable equilibrium states, enabling discrete motions through structural deformation (rotation, translation, etc.) with low energy consumption, since energy is only required during state transitions, thus promoting system frugality. System robustness is ensured thanks to modular redundancy, which allows compensation for certain failures or uncertainties without requiring heavy instrumentation.
However, the systematic design of such robots at a miniature scale, while simultaneously considering fabrication constraints, materials, actuation, sensing, dynamics, and assembly, remains a largely open research domain.
 

Objectives of the PhD
The main objective of this PhD is to rethink bistable module concepts for miniature robotics and to develop a comprehensive methodology for the design, modeling, control, and experimental validation of frugal multi stable robots, from the elementary module to the assembled robot. The work will address both theoretical foundations and their experimental validation, including module fabrication, characterization, and integration, as well as the implementation of discrete control strategies adapted to multi stability.
 

Research Program
The scientific developments will consider two complementary levels. The module level will focus on the design, modeling, actuation, and experimental characterization of various bistable structures. The robot level will address the assembly, dynamic analysis, and control of multi stable systems obtained by combining these modules.
The first research axis will focus on the conceptual design and fabrication of elementary bistable modules in 2D or 2½D, specifically tailored for miniaturization (meso scale). The work will aim to:
- Build a library of parameterized bistable modules (characteristic dimensions, materials, geometry, etc.);
- Develop mechanical models, either classical or using Artificial Intelligence, accounting for dynamic effects and nonlinear interactions;
- Experimentally characterize their behavior, including stable equilibrium states, associated resisting forces, state switching forces, dynamic aspects, and repeatability under loading conditions.
An optimization effort will be conducted at the module level, with a strong emphasis on structure–actuator coupling, in order to improve energy efficiency (high energy density, low consumption). The use of micro actuators (e.g. Shape Memory Alloys) will be favored. Finally, the integration of sensors will be investigated to estimate the internal state of each module and to measure interaction forces between modules and with the environment.
The second research axis will address the design and control of multi stable robots, obtained from the serial assembly of bistable modules. The work will include :
- Defining the optimal assembly, by determining the number of modules, their type (selected from the library), and their dimensional range;
- Studying dynamic effects and their influence on the overall robot behavior;
- Implementing discrete control strategies adapted to system multi stability, based on a quasi open loop control, using only modules’ state and internal/external interaction forces information.
 

Expected Outcomes
At the end of the PhD, the expected outcomes are:
- A library of frugal bistable modules, manufactured, characterized, and validated;
- A synthesis methodology enabling the design and assembly of multi stable robots adapted to different applications;
- A discrete robot demonstrator, such as a mobile robot for intra- or extra tubular inspection, or a miniaturized manipulator arm, for industrial or medical applications.

References

[1] Benjamin Calme, Lennart Rubbert, Yassine Haddab. Towards a Discrete Snake-like Robot Based on SMA-actuated Tristable Modules For Follow The Leader Control Strategy. IEEE Robotics and Automation Letters, 2023, 8 (1), pp.384-391.

[2] Abdenbi Mohand Ousaid, Ismaïl Bouhadda, Gilles Bourbon, Patrice Le Moal, Yassine Haddab, et al.. Compact Digital Microrobot Based on Multistable Modules. IEEE Robotics and Automation Letters, 2021, 6 (2), pp.1926-1933.

[3] Mouna Ben Salem, Guillaume Aiche, Lennart Rubbert, Pierre Renaud, Yassine Haddab. Design of a Microbiota Sampling Capsule using 3D-Printed Bistable Mechanism. EMBC 2018 - 40th International Conference of the IEEE Engineering in Medicine and Biology Society, Jul 2018, Honolulu, United States. pp.4868-4871.

[4] C. Libersa, M. Arsicault, J.-P. Gazeau & J.-P. Lallemand, A peculiar flip-flop actuator for an in-pipe microrobot, Robotica, Cambridge University Press, Vol. 22, No 5, 2004, pp. 547-561.

[5] Guimin Chen, Quentin T. Aten, Shannon Zirbel, Brian D. Jensen and Larry L. Howell, A Tristable Mechanism Configuration Employing Orthogonal Compliant Mechanisms. Journal of Mechanisms and Robotics. 2010. DOI: 10.1115/1.4000529.

[6] Larry L. Howell, Compliant Mechanisms. Wiley, 2001, ISBN-13: 978-0471384786

The PhD research will be mainly conducted at LIRMM in Montpellier (south of France) and will include periods of work at CEA LIST in Palaiseau (Paris suburbs, France).

Experimental Facilities
The PhD student will benefit from additive manufacturing platforms, including at LIRMM e.g. a Stratasys 3D printer, silicone fabrication processes (Fab 3D), but also access to the Pro3D fabrication center of the University of Montpellier.

Applicants should hold a Master’s degree or Engineering degree in robotics, control, mechanics, mechatronics, mechanical engineering, or related fields. Strong backgrounds in mechanical modeling completed by knowledge on control are required, along with a strong interest in innovation in miniature robotics. Skills in scientific programming (e.g. Python, MATLAB, or equivalent) and numerical simulation are expected. An interest in experimental work, including design, prototyping (especially via 3D printing), and robotic or mechatronic system characterization, will be highly appreciated. Autonomy, analytical skills, critical thinking, and an interest in multidisciplinary research are essential to successfully carry out this work.