Parallel Machine Tool Research Articles

Program Suite for Conceptual Designing of Parallel Mechanism-Based Robots and Machine Tools

In the development of robots and machine tools, in addition to conventional and serial structures, parallel mechanism-based kinematic structures have been used over a longer period of time. Aside from a number of advantages, the irregular shape and relatively small dimensions of the workspace formed by parallel mechanisms rank among the major weaknesses of their application. Accordingly, this fact has to be taken into consideration in the process of designing parallel mechanism-based robots or machine tools. This paper describes the categorization of criteria for the conceptual design of parallel mechanism-based robots or machine tools, resulting from workspace analysis as well as the procedure of their defining. Furthermore, it also presents the designing methodology that was implemented into the program for the creation of a robot or machine tool space model and the optimization of the resulting solution. For verification of the criteria and the programme suite, three common (conceptually different) mechanisms with a similar mechanical structure and kinematic characteristics were used.

Study on Error Analysis of Parallel Machine Tool Based on Monte-Carlo Method

Aiming at the complexity of the parallel machine tool error assessment, the distance between the ball joints on the execution platform is considered as the constraint condition to establish the forward kinematic equation, and a mathematic model of space position and attitude errors is established. The paper proposes a method to the error analysis of the parallel machine tool based on Monte-Carlo simulation according to the structure characteristic, and gives a universal computation program. The feasibility of the method is verified by comparing with the traditional motion vector equation differential method in final.

Fuzzy Adaptive PID Control for Parallel Machine Tool

The hydraulic control system, which is important composition of parallel machine tool, is a high order, nonlinear, parameter uncertain system, which seriously affect dynamic performance of machine tool, So it is very difficult to gain good performance with traditional control. By making a single-channel control of parallel machine tool as the research object, a fuzzy adaptive PID controller is proposed based on the traditional PID control and fuzzy control theory. Using the Fuzzy PID controller, PID parameters can real-time change and the system has good adaptive ability and robustness. System dynamic simulation is made by the MATLAB. Simulation results reveal that the performance of this adaptive fuzzy PID controller is much better than the conventional PID controller, even in the external disturbance and system conditions changing, this controller has the the characteristics of fast response and good robustness.

Optimal design and development of a 2-DOF PKM-based machine tool

This paper presents multiobjective optimization of a typical 2-degree-of-freedom (DOF) parallel kinematic machine (PKM) tool that has only single DOF joints. Nondimensional indices, namely global stiffness index (GSI), global conditioning index (GCI), and workspace index, are considered as the objectives for optimization. The indices GSI and GCI depict the variation of stiffness and dexterity of PKM within the workspace. The leg length and distance between two rails on which actuators slide are treated as design variables as these greatly influence the characteristics of PKM. A multiobjective genetic algorithm (MOGA) approach is implemented in MATLAB to find an efficient solution to this complex optimization problem. Fitness function includes inverse kinematics equations, Jacobian and stiffness matrices to compute and optimize the nondimensional indices. First, the optimal value of each index is obtained by single-objective GA. To further improve the results, a hybrid function PATTERNSEARCH is used. This helps to select appropriate boundary conditions for MOGA. To obtain the optimal values of all the three indices, a multiobjective GA is carried out. The results are compared with a conventional exhaustive search method of optimization. The obtained results show that the use of MOGA enhances the quality of the optimization outcome. Secondly, a prototype has been designed and developed with the optimal dimensions. The actual workspace of the PKM and influence of leg collision on the workspace are studied. Finally, a preliminary experimentation was carried out. A comparison between PKM and the three-axis serial kinematic machine tool is presented.

Stiffness enhancement of a redundantly actuated parallel machine tool by dual support rims

A novel kinematic structure called as the Eclipse-IA is suggested for an application to the parallel machine tools. The stiffness enhancement design based on the Eclipse-IA structure is also introduced with its stiffness analysis for redundantly actuated parallel mechanisms. Relationship between the new structure and the stiffness properties including determinant and isotropy of the corresponding stiffness matrix is examined. Moreover, the design parameters are optimized to maximize the stiffness properties for the application of the Eclipse-IA as a multi-functional machine tool. By various simulation cases, the suggested parallel mechanism and the redundant actuation are verified to increase the stiffness properties against arbitrary external forces.

Optimal Kinematic Calibration of Parallel Manipulators With Pseudoerror Theory and Cooperative Coevolutionary Network

Accuracy is one of the most crucial factors which affects the profound laboratory research and extensive industrial application of parallel robotic manipulators. Kinematic calibration is a necessary approach to make the nominal value approximately equivalent to the actual value for the pose of end-effector under different input of actuation variables. Since the error source of parallel manipulator is strong coupling, highly nonlinear, and uncontrollable, the pseudoerror theory is proposed by considering multiple errors, including manufacturing and assembly error, thermal error, and nonlinear stiffness error, as a single hypothetical error source, which only causes the deflection of joint variables. A novel cooperative coevolutionary neural network (CCNN) is designed to establish the complex nonlinear relationship between joint variables and the related deviation with respect to the measured pose of the end-effector. With CCNN, the pseudoerror in arbitrary joint configuration can be obtained, and thus, the control parameters can be adjusted accordingly. The results are validated through the case studies about a parallelogram-based 3-DOF parallel manipulator and a parallel robotic machine tool. This approach is generic and feasible for all types of robotic system.

Eccentric universal joints for parallel kinematic machine tools: variants and kinematic transformations

In the 1990’s enhanced capabilities of parallel kinematic machine tools (PKM) regarding stiffness, motion accuracy and dynamics have been announced generously. Extensive research and development has been done in this area to date. Regarding this, the application range of PKMs is still small. Looking for reasons often yields inadequate motion accuracy as well as insufficient workspace. In either case the passive joints of the kinematic chains pose essential weak points. This article is about eccentric universal joints as an alternative approach that can avoid drawbacks of conventional universal joints and finally enhance the application range of PKMs. However, one of the consequences is a more complex and no longer unique kinematic transformation. This is significant in a control context, where iterative calculations should be avoided when possible due to real-time demands and the uniqueness of the solution has to be guaranteed in any case. Though the general inverse kinematic problem of industrial robots is identical, no generic closed-form (non-iterative) solution is known so far. But often closed-form as well as unique solutions are possible yet by considering particular features of the kinematic chain. In this paper after an introduction types of joint eccentricity are discussed and corresponding inverse kinematic transformations are derived with control purposes in mind.

Dynamic model of an overconstrained PKM with compliances: The Tripteor X7

This paper deals with dynamic modeling of a parallel kinematic machine tool: the Tripteor X7. This machine has a hybrid kinematic, so it presents a serial wrist attached to an overconstrained parallel unit. Two dynamic models are proposed in this paper. The first one is built with only rigid bodies. It aims at identifying the inertial parameters and those of the movement transformation devices. In the second model, joints and legs compliances are introduced. This model is then used to study the impact of elements deformations on the generated surfaces. All this simulations are compared to experimental measurements realized directly on the Tripteor. Highlights? Overconstraint Parallel Kinematic Machine tool. ? Dynamic model of PKM for machining simulation. ? Tripteor X7 Parallel Kinematic Machine tool (PKM) for High Speed Machining (HSM).

Adaptronical compensation of geometrical machine errors

Static, quasi-static and dynamic displacements influence the accuracy of machine tool results. The low-frequency parts of these displacements can, on the one hand, be traced back to static stresses resulting from gravity as well as process loads and, on the other hand, to geometrical machine errors and the faulty positioning in the working area resulting from this. Dynamic loads, however, are characterised by the distribution of masses and stiffnesses. This paper aims to present an approach to adaptronically compensate for static and quasi-static displacements while, at the same time, showing how a component can fulfill the functions of a sensor and an actuator. In order to achieve this, an intelligent adaptronical strut was designed for which the piezoelectric transducer can fulfill actuated as well as sensoric tasks at the same time. Based on the principle of vibrating strings, a vibrating string is used to induce vibrations which allow for the static, quasi-static and dynamic machine displacements to be recorded using the developed integrated sensors and actuators. A first prototype was integrated into a machine tool to verify the concept. Static and dynamic measurements endorse the functionality of this approach. Machining trial runs show the effectiveness of this approach in a parallel kinematic machine tool with regards to adaptronically compensating for geometrical machine errors.

Elastic Deformation Error Model for Calibration and Compensation of Parallel Kinematic Mechanism Machine Tool

In the motion error of parallel kinematic mechanism machine tools, the elastic deformation caused by external forces, such as gravity, as well as the kinematic parameter error, is a significant factor. Internal forces generated by the rotational resistance of passive joints also deform mechanical components. In this paper, an elastic deformation error model that can deal with external and internal forces is presented. A compensation system and a calibration method of kinematic parameters employing this model are also presented. It is experimentally verified that this calibration method and compensation system improve the motion accuracy of a parallel kinematic mechanism machine tool.

Sensitivity Analysis of an Overconstrained Parallel Structure Machine Tool, the Tripteor X7

A main limitation of parallel kinematics machine tools (PKM) use for machining tasks is their accuracy low level mainly due to geometrical transformation errors. Indeed, such machine tools are geometrically controlled with an inverse kinematics model (IKM). For a large number of PKM, this IKM is defined with an analytic method and by introducing geometrical parameters. Thus, the influence of each geometrical parameter on the tool pose accuracy should be estimated via a sensitivity analysis. In the case of an overconstrained PKM, the IKM is generally computed with a numerical method. The sensitivity analysis leads to complex computation. The aim of this paper is to present a sensitivity analysis on the Tripteor X7, an overconstrained machine tool. This study is also a first step to define the geometrical identification method on this overconstrained mechanism.

A comparison study of three degree-of-freedom parallel robotic machine tools with/without actuation redundancy

This article presents a comparison study of two unique parallel robotic machine tools (PRMTs): a three degrees of freedom (DOF) PRMT and a 3-DOF PRMT with actuation redundancy. The 3-DOF PRMT has three driving units with linear motion, while the 3-DOF PRMT with actuator redundancy has four of these units. For both designs, the linear motion driving units are identical and both machines have the same passive constraining link in the middle of the structure. The manipulator with actuator redundancy is designed to prevent singularity and to improve stiffness. The inverse kinematics and Jacobian matrix of these two PRMTs are analysed. The stiffness properties are determined and global stiffness is optimised using genetic algorithm. The finite-element analysis is conducted to find the maximum and minimum stress points of each link and the moving platform. The dynamic simulations are implemented to analyse the velocity and acceleration of both structures. A comparison of the global stiffness indicates that the 3-DOF PRMT with actuator redundancy outperforms the one without actuator redundancy.

Research on Error Compensation by Semi-Closed Loop Control for a 3-UPS Parallel Machine Tool

This paper took a 3-UPS Parallel Machine Tool (PMT) as the object of research; it mainly introduced the process of establishing the compensation strategy for this PMT. Firstly the kinematics equations on driving chain and constraint chain was established on the basis of kinematics analysis. Then according to the structural characteristics and the results of kinematics analysis, the error compensation strategy of feedback correction type with the semi-closed loop control mode was used in the error compensation for this PMT by the method of installing respectively the encoders on the each joint of parallelogram mechanism, namely the compensation way of “parallel driving and series feedback” was adopted. Finally this paper has also deduced the theoretical model of error compensation. The research results in this paper provided a theoretical basis for realizing error compensation of this PMT, and had important practical significance for improving machining precision of PMT

Analysis on Influence of the Position Error of Joint Points in the Fixed Platform of 3-UPS Parallel Machine Tool

This paper took one kind of 3-TPT Parallel Machine Tool (PMT) as the object of research. It mainly analyzed the influences of the position error of joint points in the fixed platform to the position error of tool of this PMT. Firstly, on the basis of introducing the structure of PMT, the kinematics of this machine was analyzed, and the inverse kinematics equation of this PMT was obtained. Then the influence situation of the position error of joint points in the fixed platform to the position error of tool was studied based on the kinematics analysis results and the error independent function principle, and the corresponding mathematical expression of error influence was established. Thus the theoretical foundation was laid for the comprehensive establishment of error model and the error compensation of this PMT

The Measurement Method to Position Coordinates of Movable Platform for a Parallel Machine Tool

This paper took a kind of 3-TPT Parallel Machine Tool (PMT) as the object of research, and it mainly researched the measurement method to position coordinates of movable platform in PMT. First the paper introduced the structure of 3-TPT PMT. Then illustrated the device structure and the basic measurement principle for measuring the position coordinates of movable platform. Finally carried on the theoretical analysis to the specific measurement method and established the corresponding mathematics relationship. The measurement method that was presented in this paper had characteristics of simple structure, convenient and flexible operation, economical and so on, and simultaneously the research results of this paper laid a theoretical foundation for improving the movement precision of this PMT and had important practical significance

Forward Kinematics Analysis of 6-UPS Parallel Machine Tool with Cross Rod

6-UPS Cross rod parallel machine tool is an improvement of the Stewart parallel mechanism.As two layers in the moving platform destroy the symmetry of generally Stewart platform,it is difficult to calculate the analytical solutions of the forward kinematics problem efficiently.Lodrigues parameters are used to describe the rotation matrix.Through the analysis of the coupling relationships among the position and orientation variables of the moving platform,and eliminated position vector parameters in the basic motion equations,the 4th order equations which have three parameters are obtained.Thus,with the use of Newton iterative method,the iterated function that solved the equations is constructed.The method is proved correct and effective through the analysis of the actual parallel machine tools.

Analysis on Influence of the Position Error of Joint Points in the Fixed Platform of 3-UPS Parallel Machine Tool

Analysis on Influence of the Position Error of Joint Points in the Fixed Platform of 3-UPS Parallel Machine Tool

The Measurement Method to Position Coordinates of Movable Platform for 3-TPT Parallel Machine Tool

This paper took a kind of 3-TPT Parallel Machine Tool (PMT) as the object of research, and it mainly researched the measurement method to position coordinates of movable platform in PMT. First the paper introduced the structure of 3-TPT PMT. Then illustrated the device structure and the basic measurement principle for measuring the position coordinates of movable platform. Finally carried on the theoretical analysis to the specific measurement method and established the corresponding mathematics relationship. The measuring device that was presented in this paper had characteristics of direct-viewing measurement relationship, simple structure and low cost etc., and simultaneously the research results of this paper laid a theoretical foundation for improving the movement precision of this PMT and had important practical significance.

Research on the Singularity and Stationarity of 3-TPT Parallel Machine Tool

This paper takes a new-type 3-TPT 3-DOF parallel machine tool as the object of study, established the positive and negative solution of kinematic equations of the machine, and derived the Jacobian matrix and Jacobian inverse matrix of the machine tool. On this basis, the singularity and stationarity of the machine tool were researched by using Matlab and got the expression of the absolute value of Jacobian matrix determinant of the machine tool. The velocity distribution curves of telescopic driving rods of the machine tool were simulated, too. The results show that the machine tool has advantages such as good stationarity and no singularity and so on. The study provides certain theoretical basis for the structural optimum design and performance analysis of parallel machine tool.

Error Analysis and Measurement of 6PUS-UPS Parallel Machine Tool

Firstly, structure component of the 6PUS-UPS PMT (Parallel Machine Tool) was introduced. Secondly, errors during the machine tool debugging were analyzed and measured, and those determined by the 6PUS-UPS PMT structure character were focused on, including the clearance error between the slider and the guide way, the vibration error and the verticality error and so on. Lastly, the verticality error was measured using the digital display micrometer and the frame level, which has great influence on the 6PUS-UPS PMT precision