Project context:
The RPC-JaM project aims at advancing fundamentally the development of parallel continuum robots, a class of deformable robots obtained by assembling slender elastic legs in parallel. Their natural compliance enables them, to a certain degree, to adapt safely to their environment in case of contact, making them particularly interesting in tasks with sensitive environments such as medical interventions and cobotics. Despite all their advantages observed and demonstrated in the litterature, developing these robots comes with challenges such as the presence of singularities and elastic stability deficiency, models with high computation time, and a complex design. To solve these issues, we propose to follow an original approach where the robot is seen as the assembly of modular limbs, each one having its own sensors, actuators and processing units. The objective of this project is first of all to tackle the previous scientific challenges, eventually simplified as they are considered as the limb level and not the entire structure, with an additional work on how the assembled legs cooperate. Our goal is also to federate french research on this strongly emerging topic, and to disseminate the results to the public. Indeed, instead of buiding demonstrators for specific applications, we propose to valorize these results through the development of interactive art with one or several parallel continuum robots, which will be exposed, and to observe and analyse the people’s reactions. The RPC-JaM project is composed of 4 partners (Centre Inria de l’Université de Lille, LS2N in Nantes, FEMTO-ST in Besançon, ENSADLab in Paris) in relation with le Lieu Unique in Nantes).
PhD Context:
Similar to their rigid counterpart, Continuum Parallel Robots (CPR) can reach singular configurations in their workspace. They can encounter type 1 singularities, which correspond to the workspace boundaries and actuation limits, and type 2 singularities where the robot loses stiffness and stability, and buckles suddenly. Type 2 singularities result in dynamic snapping motions which are difficult to control and stabilize due to the high stiffness of the structure relative to its low inertia. The current approaches to manage them is either to avoid them, in cases where the robot shape must be controlled at every moment such as surgery through natural orifices, or to release them to create additional dynamic forces and compensate for others such as needle penetration or gravity [Nyloid art project, Cod.Act 2013]. In every case, the first step is to identify these singularities in the workspace offline and online, which is a challenge due to the computational cost of existing approaches. The existing detection criteria target changes of elastic stability, which can be computed either by using the optimal control theory or analyzing the Hessian matrix of the robot potential energy. Both approaches require knowing the CPR's full state, which is classically obtained with dynamic models that can be demanding in terms of computational resources. Indeed, the slender legs are typically modeled with non-linear rod models such as Cosserat rods and assembled using geometrical constraints. The resulting system of equations to solve, in its strong form, is a set of non linear partial differential equations with mixed boundary conditions that often require specific integration schemes and root-finding algorithms to solve. This problem of computational cost makes it challenging 1) to analyse efficiently and offline the workspace ans singularities of a given design of CPR, which is of great interest for design, planning and control, and 2) to detect online the appearance of singularities. When considering a single rod subject to known forces at their ends, however, it is possible in certain cases to obtain analytical criteria that are very fast to compute, such as the well-known equations of critical buckling force for clamped beams.
Environment:
The PhD will take place in the team Defrost of the Centre Inria de l’Universté de Lille, in collaboration with the ARMEN team of LS2N in Nantes. It will be co-supervised by Quentin Peyron (export in continuum and soft robotics modeling, design and singularity analysis), and Sebastien Briot (expert in parallel rigid and continuum robotics modeling and singularity analysis).
