Reconfiguration and tool path planning of hexapod machine tools

Hexapod machine tools have the potential to achieve increased accuracy, speed, acceleration and rigidity over conventional machines, and are regarded by many researchers as the machine tools of the next generation. However, their small and complex workspace often limits the range of tasks they can perform, and their parallel structure raises many new issues preventing the direct use of conventional tool path planning methods. This dissertation presents an investigation of new reconfiguration and tool path planning methods for enhancing the ability of hexapods to adapt to workspace changes and assisting them in being integrated into the current manufacturing environments. A reconfiguration method which includes the consideration of foot-placement space (FPS) determination and placement parameter identification has been developed. Based on the desired workspace of a hexapod and the motion range of its leg modules, the FPS of a hexapod machine is defined and a construction method of the FPS is presented. An implementation algorithm for the construction method is developed. The equations for identifying the position and orientation of the base joints for the hexapod at a new location are formulated. For the position identification problem, an algorithm based on Dialytic Elimination is derived. Through examples, it is shown that the FPS determination method can provide feasible locations for the feet of the legs to realize the required workspace. It is also shown that these identification equations can be solved through a numerical approach or through Dialytic Elimination using symbolic manipulation. Three dissimilarities between hexapods and five-axis machines are identified and studied to enhance the basic understanding of tool path planning for hexapods. The first significant difference is the existence of an extra degree of freedom (γ angle). The second dissimilarity is that a hexapod has a widely varying inverse Jacobian over the workspace. This leads to the result that a hexapod usually has a nonlinear path when following a straight-line segment over two sampled poses. These factors indicate that the traditional path planning methods should not be used for hexapods without modification. A kinematics-based tool path planning method for hexapod machine tools is proposed to guide the part placement and the determination of γ angle. The algorithms to search for the feasible part locations and γ sets are presented. Three local planning methods for the γ angle are described. It is demonstrated that the method is feasible and is effective in enhancing the performance of the hexapod machine. As the nonlinear error is computationally expensive to evaluate in real time, the measurement of total leg length error is proposed. This measure is proved to be effective in controlling the nonlinear error

Nonlinear Friction-Induced Vibration of a Slider-Belt System

A mass–spring–damper slider excited into vibration in a plane by a moving rigid belt through friction is a major paradigm of friction-induced vibration. This paradigm has two aspects that can be improved: (1) the contact stiffness at the slider–belt interface is often assumed to be linear and (2) this contact is usually assumed to be maintained during vibration (even when the vibration becomes unbounded at certain conditions). In this paper, a cubic contact spring is included; loss of contact (separation) at the slider–belt interface is allowed and importantly reattachment of the slider to the belt after separation is also considered. These two features make a more realistic model of friction-induced vibration and are shown to lead to very rich dynamic behavior even though a simple Coulomb friction law is used. Both complex eigenvalue analyses of the linearized system and transient analysis of the full nonlinear system are conducted. Eigenvalue analysis indicates that the nonlinear system can become unstable at increasing levels of the preload and the nonlinear stiffness, even if the corresponding linear part of the system is stable. However, they at a high enough level become stabilizing factors. Transient analysis shows that separation and reattachment could happen. Vibration can grow with the preload and vertical nonlinear stiffness when separation is considered, while this trend is different when separation is ignored. Finally, it is found that the vibration magnitudes of the model with separation are greater than the corresponding model without considering separation in certain conditions. Thus, ignoring the separation is unsafe.</jats:p

Platform for supplementing mathematics tuition at secondary school level

E-Learning is an integral part of the teaching and learning methodology nowadays. With the advanced development of the e-learning system, online learning platforms with customizing features and functions are generated and quickly adapted to meet the requirements of the growing users. These help to save the time and money for users and it is a convenient and popular way to learn in recent years. Web-Based Learning Management Systems(LMS) software products are widely developed for academic/education. There are quite a few LMSs in the market. The efficiency and more user-friendly platform is the main concern that been kept in mind throughout the developing this project. Compared to product reviews and features, Moodle (Module Object-Oriented Dynamic Learning Environment) is the choice chosen for this project. This project is to develop an efficient and automated online learning platform for supplementing mathematics tuition at the secondary school level to help the student learn smarter and achieve better academic performance in school. It is to provide a simple and easier way for students to continue learning math with a better experience base in their self-directed and self-paced environment. This report describes the entire cycle of the platform development. It starts with the introduction of the project. It includes the motivation, objective, and the research of the relevant literature review in this report. System implementation including the Moodle installation steps and related Moodle features developed are listed. This platform will include those learning resource, such as course material, recorded lecture and live lecturing and provide related topic exercises, quizzes, assignments, revisions to catering for secondary school students. After that, this report also shows the system testing methodologies: functional and non-functional testing. Finally, at the end of this report, conclusion, and recommendation will be discussed for future works.Bachelor of Engineerin

Self-Loosening Failure Analysis of Bolt Joints under Vibration considering the Tightening Process

By considering the tightening process, a three-dimensional elastic finite element analysis is conducted to explore the mechanism of bolt self-loosening under transverse cyclic loading. According to the geometrical features of the thread, a hexahedral meshing is implemented by modifying the node coordinates based on cylinder meshes and an ABAQUS plug-in is made for parametric modeling. The accuracy of the finite element model is verified and validated by comparison with the analytical and experimental results on torque-tension relationship. And, then, the fastening states acquired by different means are compared. The results show that the tightening process cannot be replaced by a simplified method because its fastening state is different from the real process. With combining the tightening and self-loosening processes, this paper utilizes the relative rotation angles and velocities to investigate the slip states on contact surfaces instead of the Coulomb friction coefficient method, which is used in most previous researches. By contrast, this method can describe the slip states in greater detail. In addition, the simulation result reveals that there exists a creep slip phenomenon at contact surface, which causes the bolt self-loosening to occur even when some contact facets are stuck

Investigation on the drag coefficient of supercritical water flow past sphere-particle at low reynolds numbers

Supercritical water fluidized bed is novel reactor for the efficient gasification of coal to produce hydrogen. The Euler-Euler and Euler-Lagrange methods can be used to simulate the flow behaviors supercritical water fluidized bed. The accuracy of the simulated results with the two methods has a great dependence on the drag coefficient model, and there is little work focused on the study on particle’s drag force in supercritical water. In this work, the drag coefficients of supercritical water flow past a single particle and particle cluster. The simulated results show that the flow field and drag coefficient of single particle at supercritical condition have no difference to that at ambient conditions when the Reynolds number is same. For the two-particles model, a simplification of particle cluster, the drag coefficients of the two particles are identical at different conditions for the same Reynolds number. The variation characteristics with the Reynolds number and particles’ positions are also same

INFLUENCE OF OVER-SPEED PRELOADING ON THE FATIGUE PERFORMANCE OF IMPELLER

Over-speed preloading is one of the important parts in the process of impeller manufacturing,it is a method of checking the impeller’s quality and a subtle strengthen treatment. To explain the mechanics principle of over-speed preloading,finite element models of the impeller are established and then we simulate the process of the over-speed preloading. Firstly,an elastic-plastic analysis is carried out on the semi-open impeller,and the results show that there will be residual stress generated in the high stress regions after preloading,which can reduce the maximal stress produced in the later normal loading. Secondly,analysis of the fatigue life of the semi-open impeller at two different conditions of the starting and stopping and the speed rotating is accomplished through FE-SAFE. By contrast with the results of processing with and without over-speed preloading,we can see that the treatment of over-speed preloading can improve the fatigue life of impeller in a great extent. Moreover,over-speed preloading can also avoid the occurrence of fatigue damage in some conditions of impeller’s speed rotating. Finally,fatigue analysis has been done for closed impeller in a variety of different loading conditions. The results show that the impeller treated with over-speed preloading can resist larger aerodynamic load and avoid fatigue damage