Etienne Frédéric Gabriel Thalmann

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A load cell with adjustable stiffness and zero offset tuning dedicated to electrical micro- and nanoprobing

Charles Baur, Simon Nessim Henein, Michal Stanislaw Smreczak, Etienne Frédéric Gabriel Thalmann, Loïc Benoît Tissot-Daguette

The tasks of electrical micro- and nanoprobing require precision that goes beyond human perception and therefore, depend on sensors providing real-time feedback. Efforts towards automation in micro- and nanoprobing and emerging areas such as branched nanowire networks, has resulted in a growing number of applications where electrical probing without simultaneous force control is insufficient. This article presents the design of a novel mesoscale flexure-based load cell dedicated to micro- and nanoprobing. By integrating systems for stiffness adjustment and a zero offset tuning, the force-displacement characteristic of the device can be adapted to suit a wide range of applications, from the measurement of large forces up to 60 mN to a resolution as high as 10.1 nN, and even negative stiffness (bistable) behavior. By controlling the tuning during measurements, a virtual dynamic range of 6.38 ⋅ 106 is achievable, which is one order of magnitude greater than existing commercial products. We validated the results experimentally on a titanium alloy prototype fabricated by electrical discharge machining (EDM). Experimental and finite element results were also used to validate the analytical model of the load-cell. Additionally, the device allows for probe tip replacement, which is a significant advantage compared to existing MEMS load cells, and comprises a gravity compensation system to accommodate a wide range of commercially available probe tips.

2022

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Buckled beams have become key building blocks in compliant mechanisms to achieve nonlinear behaviors, such as bistability, constant-force and stiffness tuning. However, designing and modeling such components is challenging due to their highly nonlinear characteristics and existing models typically lack accuracy or rely on numerical computations. This paper aims at giving closed-form formulas to efficiently characterize the snap through behavior and facilitate the design process of rotationally actuated pinned–pinned and fixed–pinned bistable buckled beams. A new generic analytical model for precompressed beams based on Euler–Bernoulli beam theory is first established and then applied to these two configurations. Finite element and experimental validations are performed, showing excellent agreement with the model. The results show that the angular input at the pinned beam extremity is significantly decoupled from the angular or moment output at the other extremity, until snap-through. Additionally, we show that the stable output values can be controlled by adjusting the precompression displacement of the buckled beam. Finally, we demonstrate the practicality of the studied building blocks and their modeling on a novel bistable gripper design.

2022

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Triple Crossed Flexure Pivot Based on a Zero Parasitic Center Shift Kinematic Design

Simon Nessim Henein, Etienne Frédéric Gabriel Thalmann

Thanks to their absence of play, absence of contact friction and possible monolithic fabrication, flexure pivots offer advantages over traditional bearings in small-scale, high accuracy applications and environments where lubrication and wear debris are proscribed. However, they typically present a parasitic center shift that deteriorates their rotational guidance accuracy. Existing solutions addressing this issue have the drawbacks of reducing angular stroke, prohibiting planar design, or introducing overconstraints or underconstraints. This article presents a new triple crossed flexure pivot we have named TRIVOT whose kinematics theoretically nullify its parasitic center shift without overconstraints nor internal mobility. In the physical implementation, the center shift is non-zero but we show using the finite element method (FEM) that it is reduced by one order of magnitude in comparison to the widely used crossed flexure pivot (CFP). This allows to choose a crossing ratio of the flexures that either maximizes the angular stroke limit for given flexures or results in a compact planar design with the possibility of a remote center of compliance (RCC). Based on a pseudo-rigid-body model (PRBM), formulas for the rotational stiffness and angular stroke limit of the TRIVOT are derived, which are then validated by FEM. Finally, we show that a high support stiffness can be achieved based on a preliminary study for a mechanical watch time base application. We expect this new pivot to become a competitive alternative to the standard CFP for applications where high accuracy and compactness are required.

2022

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