Synthesis and singularity analysis of N-UU parallel wrists: A symmetric space approach

We report some recent advances in kinematics and singularity analysis of the mirrorsymmetric N-UU parallel wrists using symmetric space theory. We show that both the finite displacement and infinitesimal singularity kinematics of a N-UU wrist are governed by the mirror symmetry property and half-angle property of the underlying motion manifold, which is a symmetric submanifold of the special Euclidean group SE(3). Our result is stronger than and may be considered a closure of Hunt's argument for instantaneous mirror symmetry in his pioneering exposition of constant velocity shaft couplings. Moreover, we show that the wrist can, to some extent, be treated as a spherical mechanism, even though dependent translation exists, and the singularity-free workspace of a N-UU wrist may be analytically derived. This leads to a straightforward optimal design for maximal singularity-free workspace

Singularity Analysis of Zero-Torsion Parallel Mechanisms

International audienceThis paper presents the singularity analysis of four 3-DOF symmetric zero-torsion parallel mechanisms. These mechanisms are composed of three identical legs ending with a spherical joint that is constrained to move in one of three equally spaced plane intersecting at one line. The computation of the singularity loci is based on the degeneracy of the system of screws applied on the platform by the legs. The whole study is based on the use of a special orientation representation, previously introduced under the name of Tilt-and-Torsion angles. This representation is briefly introduced. Then the interdependence between the Cartesian coordinates of the general class of parallel mechanisms is derived. Finally, the singularity loci are derived and the size of the workspace taking into account all singular configurations is shown

Kinematic Characterisation of Hexapods for Industry

International audiencePurpose-The aim of this paper is to propose two simple tools for the kinematic characterization of hexapods. The paper also aims to share the authors' experience with converting a popular commercial motion base (Stewart-Gough platform, hex-apod) to an industrial robot for use in heavy duty aerospace manufacturing processes. Design/methodology/approach-The complete workspace of a hexapod is a six-dimensional entity that is impossible to visualize. Thus, nearly all hexapod manufacturers simply state the extrema of each of the six dimensions, which is very misleading. As a compromise, we propose a special three-dimensional subset of the complete workspace, an approximation of which can be readily obtained using a CAD/CAM software suite, such as CATIA. While calibration techniques for serial robots are readily available, there is still no generally-agreed procedure for calibrating hexapods. We propose a simple calibration method that relies on the use of a laser tracker and requires no programming at all. Instead, the design parameters of the hexapod are directly and individually measured and the few computations involved are performed in a CAD/CAM software such as CATIA. Findings-The conventional octahedral hexapod design has a very limited workspace, though free of singularities. There are important deviations between the actual and the specified kinematic model in a commercial motion base. Practical implications-A commercial motion base can be used as a precision positioning device with its controller retrofit-ted with state-of-the-art motion control technology with accurate workspace and geometric characteristics. Originality/value-A novel geometric approach for obtaining meaningful measures of the workspace is proposed. A novel, systematic procedure for the calibration of a hexapod is outlined. Finally, experimental results are presented and discussed

The effect of swingarm stiffness on motorcycle stability: Experimental measurements and numerical simulations

This paper focuses on the effect of swingarm deformability on motorcycle stability and in particular on the weave mode. Multibody models for the analysis of stability and handling of single track vehicles require a lumped element representation of the deformability of the critical structural elements of the vehicle. The twist axis method is used to identify lumped stiffness and damping elements able to represent bending and torsion deformability of the swingarm. Experimental tests and identification results dealing with two different swingarms are presented. The identified lumped stiffness and damping elements are implemented in a multibody code and some numerical stability analyses are carried out. Calculated results show that swingarm deformability has a small effect on the stability of super sport motorcycles, whereas the stability of the weave mode of enduro motorcycles is affected by swingarm deformability in a specific range of speeds

Parallel Robots with Homokinetic Joints:The Zero-Torsion Case

A two degree-of-freedom (DOF) homokinetic joint provides the freedom of spatially pointing directions without spinning (zero torsion). In this paper, we investigate structural synthesis of several classes of zero-torsion parallel robots using homokinetic joints

Kinematics and workspace analysis of a 3ppps parallel robot with u-shaped base

This paper presents the kinematic analysis of the 3-PPPS parallel robot with an equilateral mobile platform and a U-shape base. The proposed design and appropriate selection of parameters allow to formulate simpler direct and inverse kinematics for the manipulator under study. The parallel singularities associated with the manipulator depend only on the orientation of the end-effector, and thus depend only on the orientation of the end effector. The quaternion parameters are used to represent the aspects, i.e. the singularity free regions of the workspace. A cylindrical algebraic decomposition is used to characterize the workspace and joint space with a low number of cells. The dis-criminant variety is obtained to describe the boundaries of each cell. With these simplifications, the 3-PPPS parallel robot with proposed design can be claimed as the simplest 6 DOF robot, which further makes it useful for the industrial applications

Redesigning a flexural joint for metal-based additive manufacturing

Traditional rigid mechanisms exhibit problems such as assembly difficulties, friction and lubrification. Flexure-based compliant mechanisms, instead, are monolithic and gain their mobility thanks to proper design and materialdeflection. Designing and producing a compliant mechanism accurately and conveniently iscrucial. Thanks to its capabilities, additive manufacturing (AM) approach can provide optimal design and production and open the way to new, unexploited performances. This study investigates the redesign of a traditional cantilevered pivot. The redesign considers the performance improvements by exploiting the advantages of the AM based on laser powder bed fusion (L-PBF). The maximum tensileand compressive loads of the redesigned joint were identified. The structure was optimised by considering the most critical geometricalparameters in terms of mechanical performance. The geometricalfactorscomply with the design rules for L-PBF process, to maximise the dimensional and surface accuracies.The new approach to the flexural joint design presented in this paper provided higher mobility if compared with the traditional approach. Therefore, this study makes a major contribution to research on the production of precision alignment mechanisms and scientific instruments