A Piezo Tool Actuator for Precision Turning of Hardened Shafts

A piezo actuator based fast tool servo is presented for precision turning of cylindrical shafts. The monolithic actuator housing transmits motion to the tool assembly using solid flexures. The actuator has a stroke of 36 μm, 3200 Hz natural frequency, and 370 N/μm stiffness. Two sets of additional piezo actuators are placed in the transverse direction to clamp the tool for increased stiffness during hard turning operations. A sliding mode controller rejects cutting forces and compensates the piezo stack nonlinearities. The tool position is controlled to within 10 nm during finish hard turning on a conventional CNC lathe.     

Sliding mode cutting force regulator for turning processes

Continuous sliding mode control is applied to turning processes for cutting force regulation. The motivation of the use of the slide mode control scheme is to solve the nonlinearity problem caused by the feedrate override command element in the commercial CNC machine tool. When the adaptive control algorithm is applied to the commercial CNC machine tool, it is one of the practical methods that the programmed feedrate is overridden after the control algorithm is carried out. However, most CNC lathe manufacturers offer limited number of data bits for feedrate override, thus resulting in nonlinear behavior of the machine tools. Such nonlinearity brings ‘quantized' or discrete effect so that the optimal feedrate is rounded off before being fed into the CNC system. To compensate for this problem, continuous sliding mode control is applied. Simulation and experimental results are presented in comparison with those obtained from applying adaptive control which is a widely used approach in cutting force regulation. Adaptive control loses its effectiveness in the presence of nonlinearity since it generally requires linear parametrization of the control law or the system dynamics. Experiments are conducted under various machining conditions, subject to changes in spindle speed, material of work-piece, and type of machining process. The suggested slide mode controller shows smoother cutting force fluctuation, which cannot be achieved by the conventional adaptive controller. The experimental set-up reflects the emphasis on the practicality of the sliding mode controller. In order to avoid the use of a dynamometer in the course of measuring the cutting force, the indirect cutting force measuring system is used by means of feed drive servo-motor current sensing.     

基于ASGSO的数控机床进给伺服系统分数阶控制

为满足数控机床对加工精度的要求,针对交流永磁同步电机驱动的进给伺服系统在不同工况下存在的扰动、摩擦、变负载、惯性力矩等非线性问题,设计一种自适应步长的萤火虫算法(ASGSO)优化的分数阶PID控制器(ASGSO-FOPID)。FOPID控制器相比传统控制器动态性能突出,能够对非线性环节进行更好的控制。利用ASGSO算法全局搜索能力,获得最优数控机床进给伺服系统分数阶PID控制器参数。建立数控机床进给伺服系统模型,分别采用PSO-FOPID、状态转移STA-FOPID、ASGSO-FOPID控制进给伺服系统进行对比。仿真和实验结果表明：提出的ASGSO-FOPID控制器具有跟踪速度快、抗干扰能力强、精度高等优点,该方法能够有效地提高数控机床的定位精度和加工精度。     

UMAC时基控制原理及其在非轴对称微结构表面切削中的应用

针对非轴对称微结构表面加工的实时性要求,采用压电陶瓷驱动的快速伺服刀架(FTS)作为微进给机构,实现刀具沿z向高频响、短行程的快速精密进刀运动,并构建了基于UMAC多轴运动控制器的微结构车削数控系统。应用UMAC的时基控制功能实现快速伺服刀架(FTS)进给与主轴转角θ的同步,来完成非轴对称微结构表面的加工。     

Dynamic variable depth of cut machining using piezoelectric actuators

This paper describes the design of a piezoelectric actuated cutting tool and implementation of digital servo controls for machining surfaces with dynamically varying depth of cut. Through a flexure hinge, the tool holder could generate 50 μm travel at the tip of the cutting insert. Tool motion errors of less than 0.5 μm were achieved in tracking cyclic waveforms by employing a digital repetitive servo control. When applied to turning aluminum and steel workpieces with variable depth of cut using carbide tools, less than 5 μm machined surface errors were measured.     

高精度数控机床进给伺服系统的模糊自适应PID控制

数控机床进给伺服系统是一个复杂的机电耦合系统。由于具有较强的时变参数特性、负载扰动和电机的非线性,很难给出控制系统的精确模型。基于数控机床进给伺服系统的数学模型,提出一种模糊自适应PID控制器的设计方法。将该控制器应用于数控机床进给伺服系统控制,可获得良好的控制性能。仿真结果表明：该方法不仅具有无静态失真,而且响应速度快、超调小。这种模糊自适应PID控制器具有较高的稳定性和精度。     

基于数控车削加工的刀尖圆弧半径补偿误差研究

针对数控车削复杂圆弧面零件时产生的形状误差问题,从单一象限圆弧面的刀尖圆弧半径补偿原理出发,分析出假想刀尖方位代码不一致是导致复杂圆弧车削时出现形状误差的原因,并在此基础上提出3种解决方法。     

Real-time cutting force induced error compensation on a turning center

A real-time error compensation system has been developed to reduce the cutting force induced planar error of a two-axis turning center by using sensing, metrology, modeling, and computer control techniques. Ten error components are formulated as a two-dimensional error field. A piezoelectric force sensor mounted in the pocket under the tool turret is used to characterize the cutting forces. A compensation controller based on an IBM/PC has been linked with the existing computer numerical controller (CNC) to correct machine errors in real time. Three different types of cutting tests were performed and the results showed that the maximum diameter error in the workpiece was reduced by 67–85% using this compensation system.     

Tracking control and active disturbance rejection with application to noncircular machining

Control systems are usually required to track reference signals while operating under the influence of disturbances. A fast tool servo system for noncircular machining application works under such conditions, resulting in large control efforts. This paper presents a linear active disturbance rejection controller design for a voice coil motor-driven fast tool servo system for noncircular machining application. The controller is designed through an extended state observer to estimate and compensate the variant dynamics of the system, nonlinearly variable cutting load, and other uncertainties. Then, a simple proportional derivative controller produces the control law. To improve the tracking performance of the fast tool servo, the tracking error from the trial-cutting workpiece is added to the reference input and used as feed-forward error compensation. In such a combined control arrangement, the active disturbance rejection controller provides active disturbance rejection ability for the controller, and the feed-forward error compensation controller improves the tracking precision. Both the tracking control and disturbance rejection performances are thus enhanced. In real-time control and implementation, the effects of finite word length, position feedback resolution, and short sampling period are analyzed and addressed. Machining experiments are conducted, and the results illustrate the control system synthesis procedures and a substantial improvement over the tracking error generated by the linear active disturbance rejection controller alone.     

Micromachining of ceramic surfaces: Hydroxyapatite and zirconia

Computerised Numerical Control (CNC) precision machining can be employed as a fast and reproducible method for surface micropatterning. For biomedical applications an efficient and reproducible micropatterning of zirconia and calcium phosphate based materials is highly sought in order to guide implant interactions with surrounding biological tissues for a better osseointegration. Therefore, CNC precision machining of zirconia and hydroxyapatite substrates is investigated in this study and optimised process parameters are reported. By microgrinding and micromilling microgrooves with a minimum width of 100 μm were obtained and process parameters such as cutting tool diameter and feed velocity discussed. As all samples were sintered prior to the micropatterning process, the influence of sintering temperature on the pattern quality, size and hardness of the obtained samples are studied. Vickers hardness of the different sintered ceramic surfaces was measured to correlate the possible wear impact on the tip of the cutting tools. The stiffness and the hardness of the used cutting tools were measured and their effect on the cutting results was discussed. The pattern quality and the average roughness in the machined microgrooves were analysed by 3D-profilometry and imaged by SEM. Comparison of the two machining techniques yielded more defined and less fractured micropatterns for microgrinding. The process efficiency for both methods was limited by the economic life time of the tool tips. For CNC grinding the life time was downsized due to more pronounced abrasive wear. For both materials the hardness was the crucial process parameter, which was adjusted by the sintering temperature. For milling of zirconia the sintering should not exceed a temperature of 1100 °C to minimize tool wear. A temperature of lower 1200 °C is suggested for the milling of HA. For sintering temperatures higher than 1200 °C the machining of both ceramic surfaces was hardly possible. The feed velocity was found not having a significant influence on the obtained micropattern width. The preset line pitch of 100 μm was excellently reached for both applied machining processes. It was found that lower feed velocities and smaller tool diameters caused deeper micropatterns.     

基于SERCOS-Ⅱ的机床数控系统实时通信与同步控制研究

现代数控机床和加工中心对实时接收数据和同步运动控制提出了更高的要求.论文对脉冲式数控系统进行简要述评的基础上,提出了基于SERCOSⅡ串行实时总线通信的机床数控系统架构:以运动控制器为主站,进给伺服驱动器为从站的主从式环形网络,并以通信周期为基准协调该多CPU结构.论文将提出的系统用于XKHL650型立式加工中心数字化控制,并提出了各进给轴伺服驱动电流环同步控制方法.该方法使各进给轴伺服驱动电流指令达到1μs的同步精度,从而保证了机床各进给轴动作的一致性.实验和加工运行表明基于SERCOSII总线通信的数控系统满足高实时性高精度的加工要求.     

电子齿轮在数控机床中的应用

机床进给伺服驱动系统是数控机床的重要组成部分,控制精度是非常重要的指标.本文介绍了电子齿轮在数控机床中的应用情况,介绍了电子齿轮的概念,以配置了安川伺服电机和驱动器的数控进给系统为例,介绍了电子齿轮比的计算过程.     

数控机床切削过程中的动态特性测量方法北大核心

针对高速高精度切削加工过程中机床动态特性分析较为困难的问题,提出一种快速扫频正弦切削法来测量数控机床切削过程中的动态特性。该方法采用车削过程中的切削力作为激振力进行试验模态分析,主要通过以恒定的进给量切削正弦轮廓的圆柱形工件,并线性增加主轴转速,从而将切削力转化为数控机床频率响应函数的激励输入。结果表明:与传统的冲击测试方法对比,快速扫频正弦切削测试的共振峰柔度和频率均有所降低,分别降低了约6.7%和16.7%。此外,通过希尔伯特变换分析,观察到冲击测试和快速扫频正弦切削测试之间有很大的区别。在快速扫频正弦切削测试中,可以直接识别出切削过程中非线性的影响,为准确测量机床切削过程中的动态特性提供了理论依据。     

高速切削CNC系统的研究

比较了高速切削加工与传统数控加工的区别 ,指出了高速切削对CNC系统的特殊要求 ,基于这些要求分析了目前高速切削CNC系统存在的四个问题 :①CNC体系结构封闭 ;②与CAM系统集成不够 ;③插补器和进给伺服控制器存在局限 ;④CNC缺乏对已知信息的综合考虑。针对这些问题的解决 ,提出基于PC的开放式CNC系统是解决问题的可行方案 ,应用面向对象的方法建立了这种方案的层次化参考模型 ,最后 ,给出了其实现的硬件拓扑结构     

Simulation of the main cutting force in Crankshaft turn broaching

The measurement of the cutting forces of a turn-broaching machine is very complex due to the relative movement between workpiece and tool. In this work the cutting forces were simulated through the modeling of the process kinematics and by applying the Kienzle equation. A new experimental approach was proposed to determine the cutting forces using a conventional CNC turning machine tool. Through a series of experiments, the model has been calibrated. A comparison between the numerical and experimental results showed a similar trend. The effect of maximum cutting depth, workpiece diameter, cutting edge inclination angle, and feed rate on the main cutting force has been studied.     

精密硬态切削过程金属软化效应与表面塑性侧流的研究

由于普通CNC车床上实施精密硬态车削仍存在被加工表面机械性能不稳定的因素，从而导致该工艺的实施仍面临挑战且不易实现良好的加工表面质量。本文通过实验研究了精密硬态车削淬硬GCr15轴承钢，讨论了PCBN刀具后刀面磨损对已加工表面材料塑性侧流的作用。实验结果表明，由于刀具后刀面磨损量增大导致高的切削温度从而使金属软化是决定材料塑性侧流形式的决定性因素，当后刀面磨损量超过0．2mm时，发生严重塑性流动，已加工表面与原始表面微观组织无明显变化，而显微硬度有所提高。     

Feed-forward control of fast tool servo for real-time correction of spindle error in diamond turning of flat surfaces

A fast tool servo is designed and tested to obtain sub-micrometre form accuracy in diamond turning of flat surfaces. The thermal growth spindle error is compensated for real time using a fast tool servo driven by a piezoelectric actuator along with a capacitive displacement sensor. To overcome the inherent non-linearity of the piezoelectric actuator, Proportional Integral (PI) feedback control with a notch filter is implemented. Besides, feed-forward control based on a simple feed-forward predictor is added to achieve better tracking performance. Actual machining data are discussed in detail to prove that the proposed fast tool servo is capable of fabricating flat aluminum specimens of 100 mm in diameter to a form accuracy of 0.10 μm in peak-to-valley error value.     

基于倍福系统伺服优化的研究

激光切割机是高速、高精度的数控机床,对机床的动态特性及跟随误差要求很高.本文对配置倍福系统的激光切割机进行伺服优化分析,并通过海德汉二维光栅进行圆度测试.测试结果表明影响机床精度的主要因素有伺服参数、减速机反向间隙、机床自身的震动等.     

精密直线伺服装置数字预见控制器设计

直线伺服装置是实现非圆截面类零件数控车削加工的关键,针对其加工目标轨迹已知的特点,本文基于二次型最优控制理论设计了直线伺服装置数字预见控制器.该控制器由一个具有PI-PD形式的最优状态反馈控制器和预见前馈补偿器构成.为了进一步克服外部干扰、模型误差和参数变化对预见控制的影响,提出了一个离散时域扰动观测器.仿真结果表明,该控制方法能够满足高速高精度条件下对非圆截面零件的车削加工要求.     

汽车齿轮车削和滚齿复合机床研究

根据汽车行业对高精度齿轮的需求,通过自主创新的途径,重点攻克了车削和滚齿复合机床总体结构、机床数控系统、高速干式滚切工艺系统温度场控制、硬质合金高速干切滚刀等关键技术难题,研发一种汽车齿轮高精度高速车削和滚齿复合机床。机床采用基于运动控制卡的开放式数控系统,采用3种方法减少滚齿加工机床热误差变形,采用三合一组合齿轮加工刀具。将齿轮加工的全部工艺集成在一台机床内,能通过一次装夹完成车削、滚齿和去毛刺加工。由于节省了上下料时间,生产效益得到了很大的提高,实现了齿轮滚切高效、高精度、节能环保加工。