Tian Huang

Stiffness estimation of a tripod-based parallel kinematic machine

This paper presents a simple yet comprehensive approach that enables the stiffness of a tripod-based parallel kinematic machine to be quickly estimated. The approach can be implemented by two steps. In the first step, the machine structure is decomposed into two substructures associated with the machine frame and the parallel mechanism. The stiffness model of each substructure is formulated by assuming that the components in the other substructure are rigid. This is followed by the second step that enables the stiffness model of the machine structure to be achieved by linear superposition. A 3D representation of the stiffness distributions within the usable workspace are depicted with comparison to those obtained through the finite element analysis.

The Local Dexterity, Optimal Architecture and Design Criteria of Parallel Machine Tools

Abstract In this paper, the analytical expressions of the local dexterity loci of parallel machine tools described by the singular values of the Jacobian matrix are formulated. It has been proved that the different kinematic performance indexes to evaluate the dexterity of the machine tools are inherently identical. The parametric relationship to obtain isotropy configurations is derived. By introducing the concept of relative dexterity, the design criteria in conjunction with the detailed procedure are proposed according to the specific requirements. It has been concluded via examples that if the dimensional parameters are designed in such a way that the local kinematic performance is satisfactory, so is the global performance provided that the reachable orientation of the mobile platform is limited due to the constraints of the passive joints and actuated variables.

Closed form solution to workspace of hexapod-based virtual axis machine tools

A novel methodology is presented in this paper for the workspace analysis of virtual axis machine tools. The workspace is defined which enables to describe in a unified framework both the position and orientation capabilities of the mobile platform. Given a range of the orientation of the mobile platform, the piecewise closed solution to the workspace boundary is formulated. It is indicated for the first time that the workspace boundary in fact is the cap of twelve envelope surfaces. Two examples are given to illustrate the effectiveness of this approach.

Reusability Assessment for Manufacturing Systems

Manufacturing firms have begun reconfiguring and reusing production systems to save resources consumed on system changes that were brought about by the high product variety and frequent model change. As a result, assessing manufacturing system reusability has become an important issue. This paper introduces intrinsic and effective reusability of manufacturing systems and proposes quantitative metrics to assess them. Using an automotive assembly system as example, this paper also explores how system configurations and task allocation affect reusability. Insights from such a study will help improve manufacturing system reuse for families as well as generations of products.

Finite Element Analysis and Comparison of Two Hybrid Robots-the Tricept and the TriVariant

This paper deals with the finite element analysis (FEA) of two 5-DOF reconfigurable hybrid robots, the Tricept and the TriVariant, by means of ANSYS parametric design language (APDL). The research interests are focused on: (1) precise FEA formulation of different types of joints by setting of contact elements, and (2) the development of an effective modeling strategy for the rapid stiffness and natural frequencies estimation associated with different configurations. The computational results show that two robots are of similar static and dynamic performances provided that they have identical geometrical dimensions, elastical and inertial parameters

A Simple yet Effective Approach for Error Compensation of a Tripod-Based Parallel Kinematic Machine

Abstract This paper presents a simple yet effective approach for the error compensation of a tripod-based parallel kinematic machine. In this approach, the forward kinematics is formulated in such a way that the coordinate of the tool tip in z direction can be explicitly represented by the dimensional parameters and actuated variables. This enables one to implement the linear least square method to estimate the kinematic parameter errors. This approach has been successfully used for the error compensation of a prototype. The measurements of a test workpiece show that once the error in the z direction is properly compensated, the errors in x and y directions can be automatically compensated.

Kinematic Synthesis of Hexapods with Specified Orientation Capability and Well-Conditioned Dexterity

Abstract This paper systematically investigates issues relevant to the kinematic synthesis of hexapod-based machine tools. Based on a full understanding of the geometrical characteristics of workspace boundary, the concept of a prescribed workspace having a given orientation capability is defined. Two performance measures are presented—the local dexterity and workspace radius ratio. The closed-form solution to these measures is formulated by means of singular value decomposition and differential geometry. The influences of the fundamental design parameters on the performance indices are discussed, resulting in an optimal design having no strut interference. An example of application to the kinematic design of a prototype hexapod is given to illustrate the effectiveness of this approach.

Determination of closed form solution to the 2-D orientation workspace of Gough-Stewart parallel manipulators

A novel methodology to formulate the closed form solution to the orientation workspace of Gough-Stewart parallel manipulators is presented by using a fictitious four-bar spatial linkage model. The possible mechanical constraints are considered which include the strut length and the passive joint limitations. Several examples are given to illustrate the effectiveness of this approach.

Closed form solution to the position workspace of Stewart parallel manipulators

A novel methodology is presented for the workspace analysis of Stewart parallel manipulators. The workspace is defined that enables a description to be made in a unified framework of both the position and orientation capabilities of the mobile platform. Given the orientation capability of the mobile platform, the piecewise closed solution to the workspace boundary is formulated. It is indicated for the first time that the workspace boundary is in fact the cap of twelve envelope surfaces, each of which is generated by a family of spherical surfaces having the movable center located on a two-branch closed spherical curve. Two examples are given to illustrate the effectiveness of this approach.

Interpolation Error Prediction of a Three-Degree Parallel Kinematic Machine

In this paper, an NC interpolation algorithm for a tripod-based parallel kinematic machine is investigated. The algorithm can be implemented in two steps, the rough interpolation in the Cartesian space and the precise interpolation in the actuator space. The upper bound of the theoretical interpolation error due to the interpolation algorithm in the precise interpolation and nonlinear mapping is analyzed. The representation of the interpolation error distribution within the Cartesian space is depicted in terms of the variations of the interpolation period and the programming velocity. It was concluded that this error is sufficiently small and may be neglected.