Quentin Boehler

Definition and Computation of Tensegrity Mechanism Workspace

Tensegrity mechanisms using linear springs as tensioned elements constitute an interesting class of mechanisms. When considered as manipulators, their workspace remains however to be defined in a generic way. In this article, we introduce a workspace definition and at the same time a computation method, based on the estimation of the workspace boundaries. The method is implemented using a continuation method. As an example, the workspace assessment of a two degrees of freedom (DOF) planar tensegrity mechanism is presented. [DOI: 10.1115/1.4029809]

Interventional MR elastography for MRI‐guided percutaneous procedures

MRI‐guided thermal ablations require reliable monitoring methods to ensure complete destruction of the diseased tissue while avoiding damage to the surrounding healthy tissue. Based on the fact that thermal ablations result in substantial changes in biomechanical properties, interventional MR elastography (MRE) dedicated to the monitoring of MR‐guided thermal therapies is proposed here.

Design and evaluation of a novel variable stiffness spherical joint with application to MR-compatible robot design

In this paper, the design of a new variable stiffness spherical joint for MR-compatible robotics is presented. It is based on the use of prestressed cable-driven mechanisms in singular configurations to provide large stiffness variation ranges, including zero stiffness configuration as required by the medical context. An original implementation is proposed, with a prestress adjustment system using pneumatic energy and taking advantage of multimaterial additive manufacturing. The proposed component combines compactness, MR-compatibility and is lightweight. The system is evaluated on a dedicated experimental setup with validation of the expected behavior, with in particular a very large achievable range of stiffnesses. The approach is effective for the design of such device and constitutes a novel solution for the design of variable stiffness devices with complex motions.

Parallel Singularities for the Design of Softening Springs Using Compliant Mechanisms

In this paper, the design of nonlinear softening springs using compliant mechanisms is investigated. The use of compliant structures is of great interest, because of the resulting absence of backlash and friction. We demonstrate that the existence of parallel singularities is a necessary condition for the architecture of a compliant softening spring. From this result, two original arrangements of softening springs are derived, with the introduction of traction and torsion softening springs. A synthesis is performed and the traction spring is numerically and experimentally assessed. As nonlinearity can also be obtained from material properties, the interest of using additive manufacturing with multi-material capability is investigated. Rubber-like materials exhibit a hyper-elastic behavior. Their integration in the proposed compliant architecture is shown to be of interest to customize the geometry of a softening spring according to the designer requirements.Copyright

Towards the Control of Tensegrity Mechanisms for Variable Stiffness Applications: A Case Study

Cable-driven tensegrity mechanisms can be considered to control at the same time position and stiffness. Adequate control solutions have however not been proposed yet. This paper focuses on the development of an original control method using a tension distribution algorithm adapted from related work on cable-driven mechanisms. The algorithm is being used to modify the mechanism configuration together with its stiffness through the level of prestress in the system, which constitutes a step towards the exploitation of such mechanisms for variable stiffness applications. Simulations show encouraging results on the stiffness variation capacity of the presented mechanism.

Influence of Spring Characteristics on the Behavior of Tensegrity Mechanisms

There is today a growing interest for tensegrity mechanisms. Their analysis is however challenging because of their self-stress state. The most popular tensegrity mechanisms use linear springs as tensioned elements. Their synthesis for given user requirements is an open issue. In this article, we propose as a first step to better understand the influence of the spring characteristics, that constitute important design parameters. The influence of spring free length is in particular assessed, considering two planar tensegrity mechanisms. Impact of the spring selection on the workspace, the stiffness and the actuation requirements is observed. The simulation results outline that using nonzero free length springs can be of interest, and conclusions are given on further steps towards a synthesis method.