Kinetostatic Performances Research Articles

Design optimization of a flexure spring used in small-sized ultra-precise optical instrument

Small-sized ultra-precise optical devices require compact compliant ortho-planar springs (COPS) aka. flexure springs, for precise, frictionless linear motion which depends highly on the design. A self-developed arm-hinge-linked design, named “Panto-style” flexure spring was optimized by selecting 5 design parameters (thickness: t, hinge width: W, arm length 1 and 2: L1 and L2, arm angle: Ө) and constructing sets of design of experiments (DOEs).Signal-to-noise ratio (SNR) and response surface model (RSM) regression were obtained in terms of axial deformation. The highest response from the main effects plot was the thickness (t), followed by hinge width (W). The angle of the arm (Ө), was considered a non-relevant parameter.The parameters optimization was implemented with constraint input and output. Kinetostatic performances (axial/radial deformations, and stress) were predicted, validated, and compared (RSM, KNN, FE simulations, and experiments) using the optimized design.The average value of KNN and RSM (KNN + RSM) increased the accuracy of axial deformation value compared to RSM alone. To conclude, RSM design parameter optimization followed by KNN + RSM has successfully predicted the output results (axial/radial deformation and stress) confirmed both numerically and experimentally.

Instantaneous Kinematics and Free-from-Singularity Workspace of 3-XXRRU Parallel Manipulators

3-XXRRU parallel manipulators (PMs) constitute a family of six-degrees-of-freedom (DOF) PMs with three limbs of type XXRRU, where R and U stand for revolute pair and universal joint, respectively, and XX indicates any actuated two-DOF mechanism that moves the axis of the first R-pair. The members of this family share the fact that they all become particular 3-RRU structures when the actuators are locked. By exploiting this feature, the present paper proposes a general approach, which holds for all the members of this family, to analyze the instantaneous kinematics, workspace, and kinetostatic performances of any 3-XXRRU PM. The results of this study include the identification of singularity conditions without reference to a specific actuation system, the proposal of two specific dimensionless performance indices ranging from 0 to 1, the determination of the optimal actuation system, and the demonstration that 3-XXRRU PMs, when appropriately sized and actuated, possess a broad singularity-free workspace that is also fully isotropic. These findings hold significance in the context of the dimensional synthesis and control of 3-XXRRU PMs. Moreover, when combined with the closed-form solutions for their positional analysis, as demonstrated in a previous publication by the same authors, 3-XXRRU PMs emerge as intriguing alternatives to other six-DOF PMs. The efficacy of the proposed approach is further illustrated through a case study.

Collision-free workspace and kinetostatic performances of a 4-DOF delta parallel robot

In this paper, the mechanical interference of the 4-DOF delta parallel robot, which is designated to be used as pick-and-place robot, within its practical workspace is investigated. A new geometric algorithm is introduced which enables to identify mechanical collisions, namely the collisions of links with each other and the collision of links with the installed conveyor at the bottom of the robots workspace. Obtaining the close-to-reality workspace, upon eliminating the parts which lead to collision, is an essential and undeniable process in the workspace analysis of a pick-and-place robot. Moreover, the most common kinetostatic performance indicators including sensitivity, dexterity, and manipulability for the first time are presented and investigated for the 4-DOF delta parallel robot within the workspace.

Conceptual design and kinetostatic analysis of a modular parallel kinematic machine-based hybrid machine tool for large aeronautic components

Abstract Precision manufacturing and intelligent assembly in the aeronautic industry pose new challenges for the design of Flexible Manufacturing Systems (FMSs). To address these issues, a modular Parallel Kinematic Machine (PKM)-based hybrid machine tool is proposed. The 5-axis hybrid machine tool is conceptually designed by integrating 3-Degree-Of-Freedom (DOF) over-constrained PKMs with a 2-DOF serial mechanism including a unique double-circumferential guideway. By integrating accessory devices, the proposed machine provides a promising alternative solution to building a novel FMS for the flexible manufacturing requirements of larger arc-shaped aeronautic components. A kinematic model is established to conduct an inverse kinematic analysis to predict the reachable workspace by using a ‘sliced partition’ searching algorithm. To evaluate the kinetostatic performance of the hybrid machine tool, a modified kinetostatic model is established by including the compliances of all joints and limb structures. The prediction accuracy of the proposed kinetostatic model is validated by numerical simulations. Two kinetostatic performance indexes, the average reaction index and the average displacement index, are formulated and applied to reveal the gravity-caused joint reactions and elastic displacement of the hybrid machine tool, respectively. The analyses show that the kinetostatic performances of the machine are heavily affected by gravity; thus, gravity must be considered during the early design stage. Notably, with minor revisions, the proposed kinetostatic model and performance indexes can be applied to other hybrid mechanical systems to efficiently evaluate their kinetostatic performances.

Performance-Based Design of the CRS-RRC Schoenflies-Motion Generator

Rigid-body displacements obtained by combining spatial translations and rotations around axes whose direction is fixed in the space are named Shoenflies’ motions. They constitute a 4-dimensional (4-D) subgroup, named Shoenflies’ subgroup, of the 6-D displacement group. Since the set of rotation-axis’ directions is a bi-dimensional space, the set of Shoenflies’ subgroups is a bi-dimensional space, too. Many industrial manipulations (e.g., pick-and-place on a conveyor belt) require displacements that belong to only one Schoenflies’ subgroup and can be accomplished by particular 4-degrees-of-freedom (4-DOF) manipulators (Shoenflies-motion generators (SMGs)). The first author has recently proposed a novel parallel SMG of type CRS-RRC1. Such SMG features a single-loop architecture with actuators on the base and a simple decoupled kinematics. Here, firstly, an organic review of the previous results on this SMG is presented; then, its design is addressed by considering its kinetostatic performances. The adopted design procedure optimizes two objective functions, one (global conditioning index (GCI)) that measures the global performance and the other (CImin) that evaluates the worst local performance in the useful workspace. The results of this optimization procedure are the geometric parameters’ values that make the studied SMG have performances comparable with those of commercial SMGs. In addition, a realistic 3D model that solves all the manufacturing doubts with simple and cheap solutions is presented.

An Optimization Approach Toward a Robust Design of Six Degrees of Freedom Haptic Devices

This work presents an optimization approach for the robust design of six degrees of freedom (DOF) haptic devices. Our objective is to find the optimal values for a set of design parameters that maximize the kinematic, dynamic, and kinetostatic performances of a 6-DOF haptic device while minimizing its sensitivity to variations in manufacturing tolerances. Because performance indices differ in magnitude, the formulation of an objective function for multicriteria performance requirements is complex. A new approach based on Monte Carlo simulation (MCS) was used to find the extreme values (minimum and maximum) of the performance indices to enable normalization of these indices. The optimization approach presented here is formulated as a methodology in which a hybrid design-optimization approach, combining genetic algorithm (GA) and MCS, is first used. This new approach can find the numerical values of the design parameters that are both optimal and robust (i.e., less sensitive to variation and thus to uncertainties in the design parameters). In the following step, with design optimization, a set of optimum tolerances is determined that minimizes manufacturing cost and also satisfies the allowed variations in the performance indices. The presented approach can thus enable the designer to evaluate trade-offs between allowed performance variations and tolerances cost.

Towards an underactuated finger exoskeleton: An optimization process of a two-phalange device based on kinetostatic analysis

The current paper deals with the kinetostatic analysis and consequent optimization of an underactuated mechanism, whose aim is to constitute an exoskeleton device, designed to enhance the force of the human finger with whom it is coupled. Given an exoskeleton architecture, optimal from the point of view of the simplicity, the geometric parameters are further optimized, to let the transmitted forces be as similar as possible to the specific force profile, set as a reference for the device. In particular, extra-vehicular activity (EVA) for astronauts is recognized as a possible field for hand exoskeletons, since EVA gloves are very demanding in terms of strength and fatigue. Thus, the force profile for a specific EVA glove is provided as an example, and different optimized devices are obtained from the stochastic optimization process. Their kinetostatic performances are hence analyzed and discussed, thanks to a properly defined performance index. Finally, a verification of the actual encumbrance of the synthesized structure is performed, with respect to EVA-specific constraints.

Multiobjective Optimization of 6-dof UPS Parallel Manipulators

The optimal design of parallel kinematic machines goes through two fundamental stages. The first one concerns the structural synthesis. It enables, a priori, to determine the choice of families of the most adapted architectures according to the desired applications such as flight simulators, machine-tools, medical applications, etc. This can be done by applying several techniques such as: screw theory, Lie groups, graph theory, finite element method, etc. The second one concerns the dimensional synthesis and aims to determine the dimensions of the architecture that has been selected during the structural synthesis. This stage remains a major task because the criteria of performance of a given architecture are strongly dependent on its sizing. In this paper, we present a dimensioning methodology of the architectural parameters of the 6-dof UPS (U: universal joint, P: prismatic joint, and S: ball-and-socket joint) parallel manipulators (the positions of the attachment points of the actuators on the base and mobile plate as well as the radius of the base and the mobile plate). The problem will be formulated as a multiobjective optimization problem (MOOP) by taking into account simultaneously several criteria of performance such as the workspace, kinetostatic performances, stiffness, and dynamic dexterity. The SPEA-II genetic algorithm is adopted to solve this type of MOOP.

The “point of isotropy” and other properties of serial and parallel manipulators

As well known, the kinetostatic performances (repeatability, stiffness, maximum force or velocity) of serial and parallel manipulators depend strongly on the kinematic structure and on the manipulator configuration inside its working space. The manipulator performances are often analyzed using the manipulability ellipsoids which depend on the manipulator Jacobian. This paper investigates the significance of the classical definition of manipulability ellipsoid highlighting its lack of significance in some circumstances and proposes a new extended definition which takes into account the different performances of each actuator. This definition is particularly useful for manipulators with different kinematic chains and various types of actuated joints (e.g. revolute and prismatic). Then the paper reviews the concept of isotropy and its properties: a manipulator exhibits an isotropic behaviour when it has the same performances along all the directions of the working-space. Then, the authors introduce the new concept of Point of Isotropy both for serial and parallel manipulators, showing how in some circumstances a non-isotropic manipulator may be transformed into an isotropic one simply changing the location of its TCP (Tool Center Point). This concept may be used to design new manipulators or to make isotropic already existing manipulators just modifying the shape or dimension of the last link. The theoretical investigation of this new concept is supported by its application to the design of an isotropic Stewart–Gough platform.

Architecture optimization of a 3-DOF translational parallel mechanism for machining applications, the orthoglide

This paper addresses the architecture optimization of a three-degree-of-freedom translational parallel mechanism designed for machining applications. The design optimization is conducted on the basis of a prescribed Cartesian workspace with prescribed kinetostatic performances. The resulting machine, the Orthoglide, features three fixed parallel linear joints which are mounted orthogonally, and a mobile platform which moves in the Cartesian x-y-z space with fixed orientation. The interesting features of the Orthoglide are a regular Cartesian workspace shape, uniform performances in all directions, and good compactness. A small-scale prototype of the Orthoglide under development is presented at the end of this paper.