An improved feedrate scheduling method for NURBS interpolation in five-axis machining

Five-axis machining plays an important role in manufacturing by dint of its high efficiency and accuracy. While two rotation axes benefit the flexibility of machining, it also brings limitations and challenges. In order to further balance machining precision and efficiency, an improved feedrate scheduling method is presented considering geometric error and kinematic constraints for the Non Uniform Rational B-Spline (NURBS) interpolation in five-axis machining. A simplification method is proposed to calculate the geometric error which describes the deviation between the ideal tool path and the real tool path induced by the non-linear movement. A linear relation between geometric error and feedrate is built to limit the feedrate. The constraints determined by single axis kinematic performance and tangential kinematic performance are also considered. Under these constraints, a constrained feedrate profile is determined. Aiming to get more constant feedrate in the difficult-to-machine areas, this work proposes a scheduling method which combines morphological filtering and S-shape acceleration/deceleration (acc/dec) mode. Simulations and experiments are performed to compare the proposed feedrate scheduling method with two previous feedrate scheduling method and the results prove that the proposed feedrate scheduling method is reliable and effective.     

Adaptive feedrate scheduling for NC machining along curvilinear paths with improved kinematic and geometric properties

In precision machining of free-form surfaces at high speed, it is very important to ensure part accuracy and machining properties. To achieve this, this paper develops a guide spline-based feedrate scheduling method for machining along curvilinear paths with simultaneous constraints of chord errors and ac/deceleration. Based on two metrics about feedrate scheduling from the view of geometric and kinematic properties, the nonlinear relationships between the arc-length parameter and path parameter are subsequently established with a parameter correction spline. Then a guide spline associated with the modified curvature radius of paths is constructed to the schedule feedrate. It is shown that determining the largest safe feedrate with constant chord error can be reduced to the issue of changing the feedrate proportionally with the square root of path curvature radii and ensuring the safe feed acceleration can be converted into the issue of presetting the largest slope of guide spline. The simulation results prove that the proposed feedrate scheme has potential applications in the field of finish machining.     

具有电磁阀微喷功能的3D打印机构建及应用

3D打印技术已经在生物医学、传感器制造等领域得到广泛应用。然而,传统3D打印机由于自身工作方式的不同,适用范围不同,难以同时兼容生物制备和传感器制备。因此,以电磁阀作为3D打印机的喷头,自主设计并构建具有独特工作方式的3D打印机,将高精度液滴分配方式应用于3D结构打印。研究料筒气压、电磁阀开关时间、喷头高度对打印液滴直径的影响,并对点、线、网、面图案的打印质量进行评估;利用所构建的打印机制备具有真皮乳头结构的组织工程皮肤和阵列式柔性压力传感器,测试组织工程皮肤的细胞活性及压力传感器的压-阻转换特性。结果表明,打印液滴直径尺寸与料筒气压、喷头开启时间、喷头高度均成正相关;3D打印机对点、线、网、面图案的打印效果较为理想;可成功应用于具有真皮乳头结构的组织工程皮肤和柔性压力传感器的制备。所构建的3D打印机融合喷墨式打印机和挤出式打印机的优势,对打印墨水有较好的兼容性,具有打印方式简单、打印速度快、成型质量高的优点,对组织工程皮肤及柔性压力传感器的研究具有重要意义,为实现具有感知功能仿生皮肤的制备奠定基础。     

Design of a real-time adaptive interpolator with parameter compensation

The interpolator is the key part of a CNC system, which has strong effect on machining accuracy, tool motion smoothness, and machining efficiency. In this paper, a real-time adaptive interpolator is developed for non-uniform rational B-spline curves (NURBS) interpolation while considering the maximum acceleration/deceleration of the machine tool. In this proposed interpolator, both constant feedrate and high accuracy are achieved while the inconsistency of feedrate is dramatically reduced as well. In order to deal with the acceleration/deceleration around the feedrate-sensitive sharp corners, a look-ahead function is introduced to detect and adjust the feedrate adaptively. Furthermore, a parameter compensation scheme is proposed to eliminate the parametric truncation error which has been analyzed by several researchers but still not incorporated into any real-time interpolator so far. A case study was conducted to evaluate the feasibility of the developed interpolator.     

Adaptive feedrate interpolation with multiconstraints for five-axis parametric toolpath

A good adaptive feedrate will be helpful for improving machining accuracy and efficiency, as well as avoiding the excess of the machine’s physical capabilities and feed fluctuations during machining. Therefore, it is highly desirable to consider the constraints of geometric error, cutting performance, and drive constraints in the feedrate scheduling of the parametric curve interpolator for five-axis computer numerical control machining. In this paper, a novel multiconstraints feedrate scheduling method is proposed for the parametric curve interpolator in five-axis machining. In the method, the feed optimization model is first built with the constraints of geometric error, the maximum feedrate and acceleration of cutter tip, and the maximum feedrate and acceleration of five-drive axes. Then, the relations between each constraint and the cutter tip feedrate are derived by means of near arc length parameterization. After that, a linear programming algorithm is applied to obtain the optimal feed profile on the sampling positions of the given tool path. Finally, illustrated examples are given to validate the feasibility and applicability of the proposed feedrate scheduling method. The comparison results show that the proposed method has an ability of the simultaneous guarantees of geometric accuracy, cutting performance, and drive characters of machine tools.     

An approach to design and fabrication of a piezo-actuated microdroplet generator

In this research, a piezo-actuated microdroplet printing device for drop on demand (DOD) is studied. Microdroplet devices are used in applications such as inkjet printing, rapid prototyping, and production of metal powder. An experimental device is designed and manufactured, in which the fluctuation of a flexible diaphragm–by a piezoelectric element–pushes the liquid out of the nozzle and produces droplets. The deflection of diaphragm due to different voltages is investigated by analytical and experimental study. In experiments, beside deflection voltage, the effect of suction and compression time and nozzle diameter on droplet size, droplet velocity, satellite droplets, and cutoff length is also investigated. High-speed camera is used to take photo of the formation of droplets. In order to calculate droplet diameter and velocity, outputs of high-speed camera are processed by MATLAB R2008a. Results obtained by analytical and experimental are in good agreement with each other and could be used to control droplet properties. It is shown that the device is able to produce droplet of diameter from 450 to 1,000 μm. Velocity of droplet can be also controlled in a range of about 0.2 to 1.4 m/s. The repeatability is investigated by ink printing on a paper attached to a rotary table.     

超声振动干式车削尺寸精度实验研究

采用超声振动干式车削方法对钨基合金材料进行了车削加工实验,研究了切削参数、振动参数对零件加工尺寸精度的影响。结果表明,在切削深度一定的前提下,对于加工圆度影响的主次关系为:进给量>振动振幅>切削速度;而对锥度影响的主次关系为:振动振幅>切削速度>进给量。在低速低进给加工方式下,振动车削的抑振效果不明显,但随着切削速度、进给量的增大,振动切削抑振效果显著。本实验条件下,当振动振幅为3μm时,对工艺系统具有较佳的抑振效果。     

A look-ahead and adaptive speed control algorithm for high-speed CNC equipment

A novel look-ahead and adaptive speed control algorithm is proposed. The algorithm improves the efficiency of rapid linking of feedrate for high-speed machining and avoids impact caused by acceleration gust. Firstly, discrete S-curve speed control algorithm is presented according to the principle of S-curve acceleration and deceleration. Secondly, constraints of linked feedrates are derived from several limits, including the axis feedrate and feed acceleration limits, the circular arc radius error limit, and the machining segment length limit. With these constraints, the optimal linked feedrate is sought to achieve the maximum feedrate by using look-ahead method. Since the actual ending velocity of machining segment equals to the corresponding optimal linked feedrate, speed control of each segment can be executed. Finally, the proposed algorithm is implemented in a pipe cutting CNC system, and experimental results show that the proposed algorithm achieves a high-speed and smooth linking feedrate and improvements in productivity and stationarity.     

An adaptive feedrate scheduling method of dual NURBS curve interpolator for precision five-axis CNC machining

Non-uniform rational b-spline (NURBS) tool path is becoming more and more important due to the increasing requirement for machining geometrically complex parts. However, NURBS interpolators, particularly related to five-axis machining, are quite limited and still keep challenging. In this paper, an adaptive feedrate scheduling method of dual NURBS curve interpolator with geometric and kinematic constraints is proposed for precision five-axis machining. A surface expressed by dual NURBS curves, which can continuously and accurately describe cutter tip position and cutter axis orientation, is first used to define five-axis tool path. For the given machine configuration, the calculation formulas of angular feedrate and geometric error aroused by interpolation are given, and then, the adaptive feedrate along the tool path is scheduled with confined nonlinear geometric error and angular feedrate. Combined with the analytical relations of feed acceleration with respect to the arc length parameter and feedrate, the feed profiles of linear and angular feed acceleration sensitive regions are readjusted with corresponding formulas and bi-directional scan algorithm, respectively. Simulations are performed to validate the feasibility of the proposed feed scheduling method of dual NURBS curve interpolator. It shows that the proposed method is able to ensure the geometric accuracy and good machining performances in five-axis machining especially in flank machining.     

A note on spark bubble drop-on-demand droplet generation: simulation and experiment

The fluid dynamics of the spark bubble-generated droplet is studied both experimentally and numerically. The emphasis is especially on the droplet behavior after pinch-off. Commercial inkjet printers often produce satellite droplets along with parent droplets which are not desirable from the viewpoint of printing efficiency. Furthermore, standard drop-on-demand droplet generators are normally restricted to the generation of droplets with the same size as the nozzle diameter. In the spark bubble droplet generation method, a spark-generated bubble induces droplet formation through a hole in a solid surface separating the liquid and air interfaces. Immediately after ignition occurs, a bubble forms and creates pressure waves as it expands and contracts in a nonsymmetrical fashion. These pressure waves, depending on the geometries of the bubble location, plate, and hole may cause a single droplet smaller than the plate aperture to form and break up. In this article, a combined numerical and experimental study has been conducted to investigate the droplet behavior created in this manner. A high-speed camera is utilized to capture the droplet formation process. The numerical simulations have been carried out using the boundary integral spatial solution coupled with the time integration, i.e., a mixed Eulerian–Lagrangian approach. There is reasonable agreement between the simulations and experiment.     

High accurate interpolation of NURBS tool path for CNC machine tools

Feedrate fluctuation caused by approximation errors of interpolation methods has great effects on machining quality in NURBS interpolation, but few methods can efficiently eliminate or reduce it to a satisfying level without sacrificing the computing efficiency at present. In order to solve this problem, a high accurate interpolation method for NURBS tool path is proposed. The proposed method can efficiently reduce the feedrate fluctuation by forming a quartic equation with respect to the curve parameter increment, which can be efficiently solved by analytic methods in real-time. Theoretically, the proposed method can totally eliminate the feedrate fluctuation for any 2nd degree NURBS curves and can interpolate 3rd degree NURBS curves with minimal feedrate fluctuation. Moreover, a smooth feedrate planning algorithm is also proposed to generate smooth tool motion with considering multiple constraints and scheduling errors by an efficient planning strategy. Experiments are conducted to verify the feasibility and applicability of the proposed method. This research presents a novel NURBS interpolation method with not only high accuracy but also satisfying computing efficiency.     

Study on droplet formation with surface tension for electrohydrodynamic inkjet nozzle

Droplet ejection from an electrohydrodynamic (EHD) inkjet nozzle depends on many factors such as an onset voltage, liquid conductivity, surface tension, etc. Since the surface tension has an influence on the contact angle between the nozzle surface and the liquid droplet, the surface tension change should be investigated for the control of the droplet ejection. In this study, surfactant, which can weaken the surface tension force, was used to analyze the effect of the surface tension. Furthermore, hydrophobic coating of the nozzle surface was considered as another factor for the droplet ejection. Also, a flow visualization technique was developed to observe the droplet formation and ejection from the EHD inkjet nozzle by various surface tension values.     

CNC系统中三次B-样条曲线的高速插补方法研究

基于参数方程的矢量表示方法,导出CNC系统中三次B－样条曲线的一种高速插补算法。该算法不仅理论上可使所有插补点落在曲线上,而且由于实时插补过程中只有加法运算,因而插补速度极高,基本上适用于任何硬件环境。误差分析表明,只要合理选择参数增量,总能保证插补的弓高误差满足加工精度要求,总能控制机床的实际运动速度满足程编要求。     

Simulation of droplet generation through electrostatic forces

This paper represents the multiphysics simulation of droplet generation of ink containing conductive nano-particles through electrostatic forces on substrate. The main focus is to investigate the phenomena by generating the drops through a nozzle with the help of electrostatic forces. The electrostatic based deposition system has vast application in printed electronics and biotechnology. In electrostatic deposition mechanism for droplet generation, a strong electric potential is applied at the tip of the nozzle; due this electric potential, the liquid containing the nano-particles experience strong electrostatic static forces at the interface with the air (at the tip of the nozzle). When these electrostatic forces exceed the internal (viscous forces) and external forces (surface tension), a deformation takes place which results in the flow of the liquid in the form of droplets. The size of the droplet is dependent on different parameters like applied voltage, properties of the ink, dimension of the nozzle. To have better understanding of this, a numerical simulation was performed based on multi-physics approach. Multiple simulations were performed by changing the position of electrode in nozzle and varying the applied voltage. Droplet size with respect to applied voltage was evaluated; electric field with respect to applied voltage and time for the droplet generation was also evaluated through these simulations. This study will help in better understanding the parameters of droplet generation phenomena and optimal design of the nozzle for the electrostatic inkjet system.     

具有限定的弦差和加减速度的插补器

作者提出了一种数控机床在插补过程中通过调整进给速度从而将弦差限定在一定的误差范围内的插补算法。提出的插补器是一种变进给速度插补器,在保证弦差位于一定误差范围内的同时,增加了数控机床的效率。本文给出了仿真结果以证明该插补器的可行性。     

液态金属电子墨水与印刷基底之间的撞击作用机理研究

液态金属印刷电子学是正在兴起的学科前沿,其利用室温液态金属代替传统电子墨水在基底上直接写出电子电路,具有独特的快捷、低成本优势。本文首次研究了制约此类电子印刷质量的一个关键问题,即液态金属墨水与印刷基底材料之间的撞击作用机理。通过实验评估了氧化效应对液态金属液滴对几类典型基底界面(打印纸、硅胶板和橡胶板)的碰撞特性的影响,对比研究了基底材料、液滴碰撞速度、液滴尺寸等的影响规律,并通过理论模型解释了液态金属与不同基底之间的粘附性差异机理。研究结果对于今后筛选理想的印刷材料及提升液态金属电子打印质量有重要意义。     

Adaptive dimensional control based on in-cycle geometry monitoring and programming for CNC turning center

This paper presents a method to compensate the dimensional deviation, irrespective of the sources for its components, and to integrate the dimensional verification and dimensional control processes. Nowadays, approach in compensation of dimensional deviation is based on decomposing the deviation. The decomposing criterion is the error source such as positioning errors, thermal deformation, mechanical loads, tool wear, kinematical errors, dynamic force, and motion control. Then, one or even more components are modeled and compensated. On contrary, the proposed method is based on the decomposing of the tool path and consideration of the batch samples. The decomposition criteria ignores the error sources as: (1) speed of variation in space of the total deviation value for the tool path decomposition and (2) the speed of variation in time of the deviation model parameters values for batch samples decomposition. The data from the geometry holistic monitoring are used for both modeling and compensation of systematic component of the total error, also for checking the compliance with technical requirements. Two algorithms for processing of the data provided by geometry monitoring, namely the adaptive?Cpredictive algorithm and adaptive?Coptimal algorithm, are presented. Nine experimental batches were machined to verify the efficiency of the proposed method using various model structures and processing algorithms. The results of method application have shown a reduction of deterministic and even nondeterministic part of the total error in what concern accuracy and precision. For the entire batch, the level of remanent error is less than 5% for deterministic part, and less than 75% for nondeterministic part. These results are clearly better than the other results reported; moreover, they refer to the whole processing error and entire batch.     

机器人工具坐标系自动校准方法

摘 要：为了实现工业现场中快速高效的对机器人工具坐标系进行标定,同时提升坐标标定的精度,提出了一种机器人工具坐标系快速标定方法,搭建了标定传感器系统和实验平台。对标定系统的标定测量速度和标定精度进行了测量和研究。该标定方法使机器人按照预定圆周和直线的轨迹进行运动,运动时统计该系统在光电标定传感器中出现的时刻,得到工具坐标系偏斜的位置。本文分析了由于光电标定传感器可能由于装配的原因坐标系不能形成直角坐标系而造成的误差,同时根据坐标系转换的关系,从而排除此项误差。实验结果表明：机器人工具坐标系的标定精度为±0.5mm,标定及恢复时间为15s。该结果满足汽车生产线的精度和效率的要求,能够快速有效的对机器人工具坐标系进行标定。     

<Emphasis Type="Bold">Accurate Machining of Freeform Surfaces by Restraining Cutter Contouring Errors</Emphasis>

Machined parts having sculptured surfaces pose challenges in the field of CAD/CAM. Sculptured surfaces are essential in the manufacture of components with curved geometry, which are demanded mostly in the aerospace and die and mould industries. This paper presents an adaptive cutter path restraining method for freeform surface machining and its implementation in milling. The ultimate goal is to achieve high contouring accuracy for sculptured parts machining which is a principal index for the performance evaluation of CNC machines. The proposed method is robust in achieving the desired surface cutting with the capability of satisfying pre-specified tolerance requirements using certain adaptive laws. The given tolerance is measured as the angular deviation by which the generated cutter path differs from the desired path. Since the feedrate is considered to be the most significant cutting parameter, only feedrate variations from 5 mm s^-1 to 30 mm s ^-1 are applied in this system. The tool paths generated with and without the adaptive mechanism are compared. The proposed methodology has been tested on a CNC milling system with an open-architecture controller. The experimental results demonstrate that the proposed tolerance feedback mechanism is very effective for producing parts with sculptured surfaces.     

基于冗余误差控制的非均匀有理B样条曲线插补算法研究

提出了一种能有效控制冗余误差的非均匀有理B样条曲线插补算法.该算法综合了等弓高误差插补算法和恒定进给速度插补算法的优点,小曲率情形时在保证加工精度的前提下,通过引入进给倍率因子,增大进给速度以改善误差过度冗余;同时在大曲率情形下,可控制弓高误差在限定的误差范围以保证轮廓精度.这样既可保证轮廓精度,又可提高加工效率.仿真实例证实了该插补算法的有效性和可行性.