Lennart Rubbert

Using Singularities of Parallel Manipulators to Enhance the Rigid-Body Replacement Design Method of Compliant Mechanisms

The rigid-body replacement method is often used when designing a compliant mechanism. The stiffness of the compliant mechanism, one of its main properties, is then highly dependent on the initial choice of a rigid-body architecture. In this paper, we propose to enhance the efficiency of the synthesis method by focusing on the architecture selection. This selection is done by considering the required mobilities and parallel manipulators in singularity to achieve them. Kinematic singularities of parallel structures are indeed advantageously used to propose compliant mechanisms with interesting stiffness properties. The approach is first illustrated by an example, the design of a one degree of freedom compliant architecture. Then, the method is used to design a medical device where a compliant mechanism with three degrees of freedom is needed. The interest of the approach is outlined after application of the method.

Design and Optimization for a Cardiac Active Stabilizer Based on Planar Parallel Compliant Mechanisms

L. Rubbert, P. Renaud and J. GangloffLSIITUniversity of Strasbourg - CNRS67000 Strasbourg, FranceEmail: rubbert@unistra.frpierre.renaud@unistra.frjacques.gangloff@unistra.frABSTRACTGiving assistance to surgeons during beating heart proce-dures is currently a great challenge in medical robotics: a highlevel of safety is required while the beating heart yields highforces and dynamics. In this article, we investigate the designof an active cardiac stabilizer that will provide a motionless areaof interest during the surgery. A device architecture is introducedthat is based on planar parallel mechanisms. Such mechanismsare particularly interesting for their manufacturing simplicityand compactness. With the considered architecture, sphericalcompliantjoints basedon a planar structure need to be designed.Here we present the use of a 3-RRR spherical parallel mecha-nism. Its kinematic and stiffness analysis are performed usingpseudo-rigid body modeling. An optimization of the mechanismis then achieved, using a modified ant colony optimization tech-nique. The achievable performance of this type of compliantspherical joint is then discussed before concluding on the deviceadequacy with respect to the surgical requirements.1 INTRODUCTIONThe medical contextIn the field of cardiac surgery, coronary artery bypasssurgery (CABG) is a widespread technique that consists in su-turing grafts on the heart surface to improve the myocardiumoxygenation. Up to now, the surgeon operates on a motionlesssurface, thanks to the use of an artificial heart-lungmachine. Thedynamics of a beating heart, assessed in [1], are indeed too highto consider that the surgeon can follow the heart surface [2]. Theuse of the heart-lung machine has been proven to cause harmfuleffects in some patients [3,4]. Off-pump CABG,i.e. surgery ona beating heart, is thus of great interest.Passive mechanical stabilizers have been proposed to con-strain the heart motion around the area of interest during thegrafting, and allow the surgeon to perform off-pump CABG. Ina minimally invasive context [5], i.e. when the tools are insertedthrough small incisions called trocars, their performance is how-ever reported to be limited [6,7]. It has been confirmed by ex-perimental evaluations [8]. Residual motion of the heart surfaceis in the order of a few mm, which is not compatible with themedical task when compared to the diameter of the grafts, inthe order of 2 mm. Roboticists have therefore proposed roboticassistance to ease the development of off-pump CABG. Severalworks are relying on the use of a teleoperation scheme [9,10].The surgeon would control with a master console the surgicaltools, held by a slave system. The motion of the tools wouldbe synchronized with the beating heart motion, so that the sur-geon only performs the surgical task. This constitutes a smartapproach, without any mechanical constraint on the heart. How-ever, safety remains an issue since any control error of the slavesystem may lead to the heart injury. Furthermore, it is still to-day a very challenging approach from a mechanical engineeringProceedings of the ASME 2012 11th Biennial Conference on Engineering Systems Design and Analysis ESDA2012 July 2-4, 2012, Nantes, France 1 Copyright

A Planar Compliant Mechanism with RRP Mobilities Based on the Singularity Analysis of a 3-US Parallel Mechanism

A new design method for parallel compliant mechanisms based on the singularity analysis of parallel mechanisms is presented in this paper. Here a 3-US parallel mechanism is introduced and its singular configurations are analyzed with Grassmann–Cayley algebra for the design of a compliant mechanism with RRP mobilities. A novel architecture of compliant mechanism, based on a 3-UU parallel mechanism, is presented and finally its stiffness properties are analyzed with a finite element method.

Gravity-Insensitive Flexure Pivot Oscillators

Classical mechanical watch plain bearing pivots have frictional losses limiting the quality factor of the hairspring-balance wheel oscillator. Replacement by flexure pivots leads to a drastic reduction in friction and an order of magnitude increase in quality factor. However, flexure pivots have drawbacks including gravity sensitivity, nonlinearity, and limited stroke. This paper analyzes these issues in the case of the cross-spring flexure pivot (CSFP) and presents an improved version addressing them. We first show that the cross-spring pivot cannot be simultaneously linear, insensitive to gravity, and have a long stroke: the 10 ppm accuracy required for mechanical watches holds independently of orientation with respect to gravity only when the leaf springs cross at 12.7% of their length. But in this case, the pivot is nonlinear and the stroke is only 30% of the symmetrical (50% crossing) crossspring pivot’s stroke. The symmetrical pivot is also unsatisfactory as its gravity sensitivity is of order 10 ppm. This paper introduces the codifferential concept which we show is gravity-insensitive. It is used to construct a gravity-insensitive flexure pivot (GIFP) consisting of a main rigid body, two codifferentials, and a torsional beam. We show that this novel pivot achieves linearity or the maximum stroke of symmetrical pivots while retaining gravity insensitivity. [DOI: 10.1115/1.4039887]

A Design Method Based on Ant Colony Optimization for Compliant Mechanisms: Introduction and Application to a Surgical Tool

This paper presents a design method dedicated to compliant mechanisms, with emphasis on the use of ant colony optimization to determine the optimal geometry of a mechanism. Ant colony optimization is of particular interest because it does not need any fine tuning of its internal parameters. This robustness and the efficiency of the design method are assessed in the context of the design of a surgical tool. The method is then used to propose a new architecture for an active cardiac stabilizer with integrated actuation, contrary to existing architectures.Copyright