NC codes optimization for geometric error compensation of five-axis machine tools with one novel mathematical model

The International Journal of Advanced Manufacturing Technology - This paper presents the optimization compensation based on the mathematical expressions of geometric error model for the accuracy...     

基于球杆仪的五轴机床RTCP误差检测及补偿

引言RTCP(rotational tool center point)功能是五轴机床的一个重要功能,字面意思是“旋转刀具中心”,业内往往会稍加转义为“围绕刀具中心转”,也有一些人直译为“旋转刀具中心编程”。其实质为保持刀具中心点不变实现刀具的转动。RTCP功能的加入有效地提高了数控机床的加工效率,因此,RTCP精度是五轴联动数控机床的重要精度指标。     

Thermal error analysis,modeling and compensation of five-axis machine tools

The role of five-axis CNC machine tools (FAMT) in the manufacturing industry is becoming more and more important, but due to the large number of heat sources of FAMT, the thermal error caused by them will be more complicated. To simplify the complicated thermal error model, this paper presents a new modelling method for compensation of the thermal errors on a cradle-type FAMT. This method uses artificial neural network (ANN) and shark smell optimization (SSO) algorithm to evaluate the performance of FAMT, and developing the thermal error compensation system, the compensation model is verified by machining experiments. Generally, the thermal sensitive point screening is performed by a method in which a large number of temperature sensors are arranged randomly, it increases the workload and may cause omission of the heat sensitive point. In this paper, the thermal imager is used to screen out the temperature sensitive points of the machine tool (MT), then the temperature sensor is placed at the position of the heat sensitive point of the FAMT, and the collected thermal characteristic data is used for thermal error modeling. The C-axis heating test, spindle heating test, and the combined movement test are applied in this work, and the results show that the shark smell optimization artificial neural network (SSO-ANN) model was compared to the other two models and verified better performance than back propagation artificial neural network (BP-ANN) model and particle swarm optimization neural network (PSO) model with the same training samples. Finally, a compensation experiment is carried out. The compensation values, which was calculated by the SSO-ANN model are sent to the real-time error compensation controller. The compensation effect of the model is then tested by machining the ‘S’-shaped test piece. Test results show that the 32 % reduction in machining error is achieved after compensation, which means this method improves the accuracy and robustness of the thermal error compensation system.

     

五轴数控机床几何误差建模方法研究

五轴数控机床是实现工件复杂表面精密加工的重要设备,而机床本身精度是保证加工精度的重要前提。以一台大型五轴数控加工机床为研究对象,分析各项误差,应用多体系统运动学理论,建立移动轴与旋转轴的几何误差数学模型,推导出刀具相对工件坐标系的位置与姿态误差表达式,为误差补偿提供精确数学模型,提高机床加工精度。     

A new high-efficiency error compensation system for CNC multi-axis machine tools

To enhance the accuracy of CNC machines for the request of modern industry, an effective static/quasi-static error compensation system composed of an element-free interpolation algorithm based on the Galerkin method for error prediction, a recursive software compensation procedure, and an NC-code converting software, is developed. Through automatically analyzing the machining path, the new error prediction method takes into consideration the fact that the machine structure is non-rigid, and can efficiently determine the position errors of the cutter for compensation without computing a complex error model on-line. The predicted errors are then compensated based on a recursive compensation algorithm. Finally, a compensated NC program will be automatically generated by the NC-code converting software for the precision machining process. Because of the advantage of the element-free theory, the error prediction method can flexibly and irregularly distribute nodal points for accurate error prediction for a machine with complex error distribution characteristics throughout the workspace. To verify the algorithm and the developed system, cutting experiments were conducted in this study, and the results have shown the success of the proposed error compensation system.     

摇篮式五轴机床空间误差的简化建模方法

针对摇篮式五轴机床进行空间误差建模研究。根据机床结构,综合考虑运动误差和主轴热误差的影响,提出了一种新的机床坐标系设置方法,从而降低了误差模型的复杂性,建立了简化的空间误差模型。该模型数学表达式简单,物理意义明确,为进一步进行误差辨识和误差补偿奠定了理论基础。     

A general methodology for machine tool accuracy enhancement by error compensation

A general methodology for increasing the accuracy of machine tools by compensating for the inherent systematic errors is described. This methodology in     

五轴数控机床几何误差建模与分析

基于多体系统基本理论推导出相邻体理想坐标变换以及误差变换矩阵并通过拓扑方法拓展到任一体理想坐标及误差变化公式。进而应用到五轴机床对应的零部件进行机床几何误差建模。最后推导出刀具形成点与工件被加工点的空间位置误差模型。并结合实验探究五轴数控机床37项误差参数对实际运动中的刀具形成点的位置误差影响,为之后的误差补偿和机床精度预测奠定理论基础。     

数控机床误差检测技术新进展

介绍了现有的各种典型的检测方法和技术特点,重点介绍了国内外近二十年、特别是最近十年来数控机床误差检测领域的研究新进展,具体包括DBB法、激光干涉仪法应用研究的最新成果以及平面光栅和R-test等新型检测仪器和方法;分析了当前数控机床误差检测研究中存在的问题,并对今后的研究趋势做了进一步展望。     

大型数控龙门铣床几何误差补偿方法研究

针对大型数控龙门铣床几何误差的问题,建立了大型数控龙门铣床的几何误差模型,分析了大型数控龙门铣床的几何误差源;利用API(T3)激光跟踪仪高精度大尺寸的测量特点及数据处理能力,提出了X、Y、Z轴线位移误差、角位移误差及各轴间垂直度误差的辨识算法,通过激光测量与计算准确地辨识了大型数控龙门铣床的几何误差;建立了大型数控龙门铣床加工空间几何误差数学模型,采用基于对象的事件驱动机制的程序设计语言Visual Basic开发了几何误差补偿软件,实现了几何误差补偿;现场检测了大型数控龙门铣床空行程平面运动轨迹及工件的平面度。研究结果表明,该方法使平面加工精度提高了50.77%,并验证了几何误差模型的正确性及几何误差补偿方法的有效性。     

一种新的数控机床热误差实时补偿方法

根据数控机床加工热误差,首先,利用粗集理论方法,分析各变量与热误差之间的相关性,选择出机床热误差补偿的重要特征参数。然后提出一种动态反馈网络建立数控机床加工热误差补偿模型,新的数控机床热误差实时补偿方法具有补偿误差精度高,网络学习收敛速度快,误差补偿实时性好等特点。仿真实验结果证明了该方法的可行性和有效性。     

A methodology for error characterization and quantification in rotary joints of multi-axis machine tools

A novel methodology was developed and implemented for error characterization, evaluation, and its quantification in a rotary joint of multi-axis machine tools by using a calibrated double ball bar (DBB) system as a working standard. This methodology greatly simplified the measurement setup and accelerated the error quantification. Moreover, the methodology is observed to be highly economical by reducing the tooling and instrumentation. However, considerable time and little more efforts are required in measuring and computing the errors. The developed methodology is capable of evaluating the five degrees of freedom (DOF) errors including two radial errors, one axial error, and two tilt errors out of six DOF error components of rotary joints. A DBB system with point accessory was used for direct measurement by change of length at multi-points and various locations, whereas mathematical modeling technique was implemented, and equation solvers were exercised for error evaluation and its quantification. Validation and authentication of the methodology and results obtained were carried out by simulation process prior to implementing the methodology on rotary joints of a five-axis turbine blade grinding machine. The results obtained from “A” and “B” rotary joints of the turbine blade grinding machine were presented by applying cubic spline technique for error modeling and its further utilization. The methodology was found extremely pragmatic, quite simple, efficient, and easy to use for error characterization and its quantification in rotary joints of multi-axis machine tools.     

五轴石材切板机床刀具补偿算法的研究

针对五轴石材切板机床的圆形锯片安装中心相对于机床控制点存在偏置以及锯片本身尺寸导致过切的问题,对切削点相对于控制点的空间位姿关系进行了分析,对加工图元为直线时的各种工况进行了详尽研究,提出了一种基于美国3S开放式系统(Servo Works S-140M)的偏置补偿算法,建立了刀位轨迹与数控轨迹之间的联系。通过系统仿真功能验证了该算法的正确性,并通过实际加工进行了测试。研究结果表明,所加工出的零件能满足加工工艺和精度要求,提高了加工效率和加工精度,降低了生产成本。     

Product of exponential model for geometric error integration of multi-axis machine tools

This paper proposes a product of exponential (POE) model to integrate the geometric errors of multi-axis machine tools. Firstly, three twists are established to represent the six basic error components of each axis in an original way according to the geometric definition of the errors and twists. The three twists represent the basic errors in x, y, and z directions, respectively. One error POE model is established to integrate the three twists. This error POE formula is homogeneous and can express the geometric meaning of the basic errors, which is precise enough to improve the accuracy of the geometric error model. Secondly, squareness errors are taken into account using POE method to make the POE model of geometric errors more systematic. Two methods are proposed to obtain the POE models of squareness errors according to their geometric properties: The first method bases on the geometric definition of errors to obtain the twists directly; the other method uses the adjoint matrix through coordinate system transformation. Moreover, the topological structure of the machine tools is introduced into the POE method to make the POE model more reasonable and accurate. It can organize the obtained 14 twists and eight POE models of the three-axis machine tools. According to the order of these POE models multiplications, the integrated POE model of geometric errors is established. Finally, the experiments have been conducted on an MV-5A three-axis vertical machining center to verify the model. The results show that the integrated POE model is effective and precise enough. The error field of machine tool is obtained according to the error model, which is significant for the error prediction and compensation.     

A new conceptual approach for systematic error correction in CNC machine tools minimizing worst case prediction error

A new artifact-based method to identify the systematic errors in multi-axis CNC machine tools minimizing the worst case prediction error is presented. The closed loop volumetric error is identified by simultaneously moving the axes of the machine tool. The physical artifact is manufactured on the machine tool and later measured on a coordinate measuring machine. The artifact consists of a set of holes in the machine tool workspace at locations that minimize the worst case prediction error for a given bounded measurement error. The number of holes to be drilled depends on the degree of the polynomials used to model the systematic error and the number of axes of the machine tool. The prediction error is also function of the number and location of the holes. The feasibility of the method is first investigated for a two-axis machine to find the best experimental setting. Finally based on the two-axis case study, we extend the results to machine tools with any number of axes. The obtained results are very promising and require only a short time to produce the artifact     

Applying bidirectional kinematics to assembly error analysis for five-axis machine tools with general orthogonal configuration

Since a five-axis machine tool has two more rotary axes and two more degrees of freedom than a three-axis machine tool, it can manufacture a complex surface more efficiently. However, there are more error terms due to the extra axes. Error sources for machine tools include structural error, dynamic error, and static error. The static error, which includes thermal and geometric errors, is the main source of machining inaccuracy in machine tools. Although a large number of studies have been made on geometric errors, the influence of individual error term on volumetric error is seldom discussed. This paper analyzes assembly error that belongs to the category of static error, and the analytic method can be applied to general orthogonal configurations. By adopting the machine tool form-shaping function, the effect of assembly errors on volumetric errors has been investigated. And the error terms that cannot be compensated by driving single control axis have been recognized and explored for general orthogonal configurations.     

Dynamic and geometric error assessment of an XYC axis subset on five-axis high-speed machine tools using programmed end point constraint measurements

This paper presents a technique for assessing the volumetric errors on a five-axis machine tool for motion involving two linear axes and one rotary axis at selected feed rates using data from two sources. The first source of data is obtained through a programmed end point constraint procedure with measurement of the 3D volumetric positioning errors between a point on the tool holder and another fixed to the machine table reference frame. The tests involve maintaining the nominal coincidence of these two points whilst exercising the three axes. The second source of data is the position feedback signal from the encoder provided by the machine controller. Tests were carried out at low and high feed rates to evaluate the effect of geometric and dynamic errors. Polynomial functions are used to represent and then predict the geometric errors. The predicted geometric errors are then added to the dynamic errors provided by the servo errors from position feedback signals and propagated to the tool centre point and are compared with the measured volumetric errors. It shows that the influence of the geometric errors are dominant at low feed, whereas the effects of the servo errors of the linear axes become dominant as the feed increases, reaching 80% of the total error at a feed of 10,000 mm/min.