Fengfeng Xi

Modeling and predicting surface roughness of the grinding process

A new method for predicting the surface roughness of the workpiece for the grinding process is developed in this paper. The conventional method determines the surface roughness based on the model using the mean value of the grain protrusion heights, which leads to a formula. However, the analytical value based on the formula is substantially smaller than measurement. To overcome this problem, the proposed method takes into consideration the random distribution of the grain protrusion heights. As such, there is no formula, and a numerical solution is developed. To solve this problem, first the intersecting points of any two grains with different heights are determined. To determine the final profile considering thousands of the grains, a search method is developed to systematically solve the workpiece profile, starting with the highest protruded grain in a descending order of the grain protrusion heights. Furthermore, a truncated Gaussian distribution model is developed to relate the wheel volume wear to the change in the mean value of the protrusion heights. Simulation shows that the proposed method yields the results that are consistent with measurement, thereby proving the effectiveness of the method.

CAD-based path planning for 3-D line laser scanning

The problem of path planning for 3-D line laser scanning is addressed in this article. The path under study is for a line, as opposed to a point in conventional CNC and CMM toolpath planning. While the objective of the latter case is to drive a point along a curve, that of the former case is to drive a line across a surface. To solve this problem, a path planning method is developed. First, a slicing approach is put forward to divide a part CAD model into a number of sections, since the laser scanning line is limited by the scanning width and a single scanning pass cannot cover the whole surface. Second, for each section, a scanning pass is generated to cover the section surface. To ensure a maximum coverage for each section, the surface profile is determined, and the top of the scanning window is set along the upper boundary of the surface profile. Third, combination of scanning all the sections forms a complete scan of a surface. It is shown that with the proposed method a 3-D line laser scanning process can be effectively automated.

Bearing Diagnostics Based on Pattern Recognition of Statistical Parameters

In this paper, a new bearing defect diagnostic and classification method is proposed based on pattern recognition of statistical parameters. Such a pattern recognition problem can be described as transformation from the pattern space to the feature space and then to the classification space. Based on trend analysis of six commonly used statistical parameters, four parameters, namely, RMS, Kurtosis, Crest Factor, and Impulse Factor, are selected to form a pattern space. A 2-D feature space is formulated by a nonlinear transformation. An intraclass transformation is used to cluster the data of different bearing defects into different regions in the feature space. The classification space is constructed by piecewise linear discriminant functions. Training the classification space is performed, in this paper, by using data of bearings with seeded defects. Diagnosis of the defected bearings in the classification space then becomes straightforward. Numerical experiments show that the proposed method is effective in indicating both the location and the severity of bearing defects.

Optimal Module Selection for Preliminary Design of Reconfigurable Machine Tools

Presented in this paper is a feature-based method for selecting an optimal (minimum yet sufficient) set of modules necessary to form a reconfigurable machine tool for producing a part family. This method consists of two parts. In the first part, a feature-module database is created to form a selection space, where the machinable geometric features identified in STEP are defined as functional requirements (FRs) and the structural component modules derived from the conventional machine tools as design parameters (DPs). An inner FR-to-DP mapping mechanism within the database is based on the Membership Grade Matrix, which defines metrics to quantify the degree of association between a FR and a DP Within the confines of the selection space built upon this FR-DP database, the second part of the method involves a two-step procedure for module selection. The first step is to select the modules from this space to construct all the required individual configurations of the reconfigurable machine tool. The second step is to maximize the number of common modules among the originally selected modules through re-selection. A case study on designing a reconfigurable machine tool dedicated to a given family of die molds is conducted and discussed.

Development of a sliding-leg tripod as an add-on device for manufacturing

This paper presents the work on developing a sliding-leg tripod as a programmable add-on device for manufacturing. The purpose is to enhance the capabilities of any machine by providing it with a more flexible range of motion. This device can be used as a toolhead for CNC machine tools and robots, or as a work stage for coordinate measuring machines and laser scanning systems. In this paper, system modelling, analysis and control of this device is presented. System modeling includes mobility study, kinematic model and inverse kinematics. System analysis includes workspace analysis, transmission ratio and stiffness analysis. System control includes path planning, joint space control and Cartesian space prediction. It is shown that the proposed device can provide flexibility and dexterity to machines.

Optimum design of parallel kinematic toolheads with genetic algorithms

In this paper, the optimum design of parallel kinematic toolheads is implemented using genetic algorithms with the consideration of the global stiffness and workspace volume of the toolheads. First, a complete kinetostatic model is developed which includes three types of compliance, namely, actuator compliance, leg bending compliance and leg axial compliance. Second, based on this model, two kinetostatic performance indices are introduced to provide a new means of measuring compliance over the workspace. These two kinetostatic performance indices are the mean value and the standard deviation of the trace of the generalized compliance matrix. The mean value represents the average compliance of the Parallel Kinematic Machines over the workspace, while the standard deviation indicates the compliance fluctuation relative to the mean value. Third, design optimization is implemented for global stiffness and working volume based on kinetostatic performance indices. Additionally, some compliance comparisons between Tripod toolhead and other two principal Tripod-based Parallel Kinematic Machines are conducted.

Integrating Java 3D model and sensor data for remote monitoring and control

Abstract This paper presents a novel approach and a framework for web-based systems that can be used in distributed manufacturing environments. A prototype is developed to demonstrate its application to remote monitoring and control of a Tripod—one type of parallel kinematic machine. It utilizes the latest Java technologies (Java 3D and Java Servlets) as enabling technologies for system implementation. Instead of using a camera for monitoring, the Tripod is modeled using Java 3D with behavioral control nodes embedded. Once downloaded from its server, the 3D model behaves in the same way of its counterpart at client side. It remains alive by connecting with the Tripod through message passing, e.g., sensor signals and control commands transmissions. The goal of this research is to eliminate network traffic with Java 3D models, while still providing users with intuitive environments. In the near future, open-architecture devices will be web-ready having Java virtual machines embedded. This will make the approach more effective for web-based device monitoring and control.

A comparative study on tripod units for machine tools

Abstract Since most machining operations require a maximum of five axes, the tripod design is becoming popular for development of parallel kinematic machines. Combination of a tripod with a gantry system forms a five-axis machine with both large workspace and dexterity. In this paper, a comparative study is carried out on various tripod units proposed in the literature including Tricept, Z 3 Head, Georg V and Parawrist. For this purpose, a parametric model is developed that applies to a class of tripod units. Based on this model, four different types of tripods are compared, and their differences are analyzed.

Coupling Effect of A Flexible Link and A Flexible Joint

In this article the coupling effect of a flexible link and a flex ible joint is investigated. The system considered is a flexible link driven by an actuator through a flexible joint. First the dynamic equations of a flexible link-joint system are derived. Solutions of the system dynamic equations show that the fre quencies and mode shapes of the flexible link-joint system are parameterized in two ratios. One is the ratio of the moment of inertia of the link to that of the rotor of the actuator. The other is the ratio of the bending stiffness of the link to the torsion stiffness of the joint. Two important phenomena are discovered. The first is that in terms of the two ratios, there exist three regions of roots of the frequency equation—stiffening, softening, and mixed—and the coupling effect can be characterized by these three regions. The second is that the coupling effect can be reduced by appropriately choosing the combination of the two ratios. The results presented in the article are useful for design and control of flexible link-joint systems.

Tripod Dynamics and Its Inertia Effect

In this paper the tripod dynamics and its inertia effect is studied. The tripod design is becoming popular for the development of parallel kinematic machines (PKMs). The combination of a tripod with a gantry system forms a hybrid machine that offers the advantages from both serial and parallel structures. The tripod dynamics under study includes the mass of the moving platform as well as those of the legs. The natural orthogonal complement method is applied to derive the dynamic equations. The inertia effect of the moving platform and the legs is investigated in terms of two parameters, namely, the ratio of the total leg mass to the mass of the moving platform, and the velocity of the moving platform. The dynamic equations are separated by three terms, inertia, coupling, and gravity. Quantitative studies are carried out by simulation to examine how the two parameters affect the three respective terms. Based on the simulation results, the dynamic equations can be simplified by retaining the dominant terms while neglecting less significant ones. The simplified dynamic equations provide an efficient model for design and control of tripods.