Theory and application of a combined feedback–feedforward control and disturbance observer in linear motor drive wire-EDM machines

This paper presents a combined two-degree-of-freedom controller and disturbance observer design for a direct drive motion control system actuated by permanent-magnet linear synchronous motors (PMLSM). A feedback controller based on pole-placement design method is proposed to achieve desired tracking performance as well as stabilize the closed-loop system. A newly designed feedforward controller is proposed to reduce tracking errors based on an inverse model of the direct drive system. A digital disturbance observer is implemented to be included in the proposed feedback–feedforward control structure to compensate for nonlinear friction, cogging effects, and external load disturbance. Furthermore, the proposed control scheme has been verified as being internally stable. Experimental results indicate that the proposed controller can achieve a high contouring accuracy of ±0.3 μm as well as provide disturbance rejection and robustness. The maximum contour error of circular trajectory was reduced from 8.5 to 3.2 μm in comparison with proportional-integral-derivative (PID) controller.     

Experimental study on a hybrid-driven servo press using iterative learning control

Servo presses provide flexible punch motions, which satisfy different production needs. To achieve this merit, the control system must maintain the punch motion accurately despite versatile desired trajectories or varied loadings. Against this backdrop, the current paper proposes an iterative learning control (ILC) scheme for a hybrid-driven servo press. A proportional derivative (PD) type ILC controller that contains a closed-loop feedback controller is adopted. The sensitivity Jacobian is introduced into the controlling algorithm as the proportional gain in order to smoothen and increase the error convergence rate. The proposed ILC controller is then developed and verified on a servo press prototype. Experimental validations of a cup-shaped drawing are also carried out. The results show that the proposed ILC scheme effectively made the punch position root-mean-square (RMS) errors converge to less than 0.2 mm within five iterations. The precision was also improved to less than 50 μm that was equivalent to 35–40% of the original level without the ILC.     

Accurate tracking controller design for high-speed drives

This paper presents a motion control strategy that utilizes a combination of vibration avoidance, sliding mode control, and torque ripple and friction compensation techniques to facilitate high bandwidth–high accuracy tracking in ball screw drives. Torsional vibrations are modeled with finite element analysis and experimentally identified. Excitation of the first and second mode is avoided by designing appropriate notch filters. Following the attenuation of structural vibrations, rigid body motion is controlled using adaptive sliding mode control, where a positioning bandwidth of 223 Hz is achieved. Nonlinear friction and motor/mechanical torque ripples are modeled and compensated in feedforward. With the proposed control strategy, an experimental tracking accuracy of 0.95 μm has been achieved while traversing the axis at a feedrate of 1000 mm/s with 1g acceleration.     

Rapid identification technique for virtual CNC drives

This paper presents a technique for rapid identification of machine tool drives by conducting a short G-code test. The proposed strategy uses commanded and measured axis profiles and requires minimal intervention to the servo control loop. The drive system is identified as a whole, including the feed mechanism, motor, amplifier, and the control law. The methodology is fairly general and applicable to linear or ball screw drives, controlled with commonly used controllers like P, PI, PID, P–PI Cascade or Adaptive Sliding Mode Control; with or without feedforward dynamic or friction compensation. In order to guarantee the stability of identified dynamics, bounds are imposed on the pole locations. The identified models can be used in a Virtual CNC system for predicting the contouring and tracking errors to different part programs. Simulation and experimental case studies are presented, where tracking and contour errors are successfully predicted using drive models identified with the proposed strategy.     

Integrated real time compensation system for errors introduced by measurement probe and machine geometry in commercial CMMs

In this paper, an integrated real time compensation system has been proposed for the errors introduced by the probe and machine geometry in commercial CMMS. For efficient probe error mapping, a compact notation (ε, θ) for probing direction and corresponding concentric diagram is proposed. A set of integrated volumetric error equations are also proposed for the volumetric error map integrating the probe error and the machine geometric error. For the real time error compensation scheme, an interface box and an appropriate software driver have been designed and implemented between the CMM and machine controller. The data flow is intercepted and corrected according to the integrated volumetric error map without the controller being noticed. Thus the real time error compensation system has been successfully implemented for the machine with minimum interference to the existing coordinate measurement system. The developed system has been applied to a commercial CMM on the shop floor, and the measurement accuracy has been improved remarkably.     

A new method for monitoring micro-electric discharge machining processes

Micro-electric discharge machining (μ-EDM) is a very complex phenomenon in terms of its material removal characteristics since it is affected by many complications such as adhesion, short-circuiting and cavitations. This paper presents a new method for monitoring μ-EDM processes by counting discharge pulses and it presents a fundamental study of a prognosis approach for calculating the total energy of discharge pulses. For different machining types (shape-up and flat-head) and machining conditions (mandrel rotation and tool electrode vibration), the results obtained using this new monitoring method with the prognosis approach show good agreement between the discharge pulses number and the total energy of discharge pulses to the material removal and tool electrode wear characteristic in μ-EDM processes. On applying tool electrode vibration, the machining time becomes shorter, because it removes adhesion. The effect of tool electrode vibration in order to remove adhesion can be monitored with good results. In order to achieve high accuracy, the tool wear compensation factor has been successfully calculated, since the amount of tool electrode wear is different in each machining type and condition. Consequently, a deeper understanding of the μ-EDM process has been achieved.     

Adaptive compensation of quasi-static errors for an intrinsic machine

An adaptive compensation strategy for quasi-static error correction in intrinsic machines is proposed and tested. The proposed methodology consists of systematic modelling of the machine forward kinematics, including quasi-static errors, as well as direct modelling of the inverse kinematics using nonlinear regression analysis. The result is a model which is a hybrid of physical modelling and regression analysis modelling. In addition, the methodology includes a compensation strategy of the machine contouring errors using the state observer technique for on-line adaptive compensation. A CMM is chosen as a test bed for validation of the proposed methodology. Systematic modelling is carried out in two stages for the forward and inverse kinematics. Regression based models are verified using two different tests. The statistical analysis of variance technique (ANOVA) is used to select the best model in addition to model testing using an independent set equal to approximately 10% of the fitting data. The obtained models are then employed in two compensation strategies; one for the measurement error correction, and another one for the contouring error correction by motion command modification in the forward control path. For contouring tests, the CMM behavior at different thermal states is estimated using experimentally obtained Effective Coefficient of Thermal Expansion (ECTE). Simulations of the machine in contouring selected trajectories are carried out over a range of thermal states. Results obtained show an improvement in the CMM performance to a level close to the machine resolution. The CMM performance is tested using the standard ASME B.89.1.12M-1990 evaluation test, as well as a novel modified version of the test accounting for a thermally varying environment. Machine errors are significantly reduced using the proposed methodology.     

Fabrication direction optimization to minimize post-machining in layered manufacturing

Layered manufacturing (LM) can fabricate any complex 3D physical model without geometric limitations by depositing 2D layers in a given direction. However, the surface of the LM processed part is excessively rough in comparison with that of general NC machined part due to the LM process itself. Hence to improve the surface quality, additional post-machining such as coating and grinding process is required. This post-machining, however, is also detrimental to the original geometry of the part and is time consuming. To minimize the required post-machining region (RPMR) in the LM, an intelligent methodology to determine the optimal fabrication direction is proposed in this paper. An expression of the distribution of surface roughness is presented and the relation between the RPMR and fabrication direction is investigated taking real LM environments into consideration. The objective is to minimize the RPMR for general LM technology. A genetic algorithm is applied to obtain reliable solution for a complex geometry CAD model. Validity and effectiveness of the proposed methodology are tested by various experiments and applications.     

Friction characterization and compensation of a linear-motor rolling-guide stage

In this article, characterization and compensation for the friction at the micro (plastic module and nonlinear spring) and macro modes (nonlinear Coulomb and Stribeck) of a linear-motor rolling-guide stage are reported. Because the system dynamics under the micro and macro modes are very different, the paradox to determine the optimal servo gains for both modes was solved by a two-stage fuzzy PID controller. The fuzzy PID controller possesses a switching mechanism that continuously tunes the servo gains according to the system states. For the precision reversal path tracking at low speed, the system dynamics is dominated by the complicated and nonlinear friction characteristics. Due to the nonlinear spring characteristics, the friction was found unsymmetrical at the reversal points of the low speed path tracking. A simplified unsymmetrical friction model was proposed and proved to improved motion precision well. From the simulations and experiments, the proposed controller shows good tracking and positioning accuracy after friction compensation.     

插装式二维电液伺服阀的动态特性研究

插装式电液伺服阀较传统的电液伺服阀进一步提高了伺服阀的功重比,并具有优良的动态特性。提出一款插装式二维电液伺服阀并重点研究其动态特性。介绍插装式二维电液伺服阀的结构及工作原理。建立插装式二维阀的数学模型并进行仿真分析,尤其对阀芯旋转黏性阻尼系数进行推导。仿真结果表明：开环模式下插装式二维阀的阶跃响应时间为10 ms,幅频宽为40 Hz;闭环模式下阶跃响应时间为4 ms,幅频宽为100 Hz。通过搭建试验平台对插装式二维阀进行动态特性测试,试验结果表明：开环状态下阀的阶跃响应时间为7 ms,幅频宽为38 Hz,闭环状态下两项数据分别为6 ms和117 Hz。试验结果与仿真结果基本吻合,表明插装式二维电液伺服阀具有优良的动态特性。     

Surface roughness prediction using measured data and interpolation in layered manufacturing

Layered manufacturing (LM) technology can efficiently fabricate 3D physical models without the restriction of geometric complexities. However, because of the LM process itself, the surface quality of processed parts is often unsatisfactory compared to that of machined parts made using traditional numerically controlled manufacturing technology. Hence, the surface roughness has become a matter of utmost concern despite the many potential advantages of LM. In the initial step of the LM process, reasonable process planning can be achieved by predicting the surface roughness in advance. Therefore, we propose an elaborate methodology to predict the surface roughness of LM processed parts. Theoretical and real characteristics of surface roughness distributions are investigated to reflect actual roughness distributions in the predictions, and a roughness distribution expression that can obtain surface roughness values for all surface angles is introduced using measured roughness data and interpolation. A prediction application is presented, and the validity and effectiveness of the proposed approach are demonstrated through several application examples.     

移动机器人神经网络补偿计算力矩控制

针对存在动力学不确定建模项、建模误差及外界干扰的移动机器人,设计RBF神经网络补偿计算力矩控制算法。基于反步法设计运动学辅助速度控制率。根据动力学理想名义模型,基于计算力矩法设计一般的力矩控制器。在此基础上,建立具有不确定建模项、建模误差及外界干扰的移动机器人动力学模型,基于计算力矩法设计带有RBF神经网络补偿的力矩控制器,神经网络的权值由自适应律给出。最后,利用Lyapunov理论证明了系统的稳定性。仿真结果表明：神经网络对系统不确定项具有良好的逼近性能,相比于一般的计算力矩控制,所提出的神经网络补偿计算力矩控制算法具有更好的跟踪性能,控制系统具有更好的鲁棒性。     

Robustification of CNC Controllers for Machine Tools Motor Drives

This paper presents a methodology for enhancing the robustness of a CNC controlled system by convex optimisation of the Youla parameter. The methodology requires as a first step the design of an initial polynomial controller. This controller is then robustifled considering temporal and frequency constraints, which are formulated by means of the Youla parameterisation within a convex optimisation framework. The optimal Youla parameter is finally obtained by solving this optimisation problem. In this way, a compromise between robustness and closed loop behaviour can be easily managed. An application to the position control of an induction motor drive is presented, where the robustness of different controllers (PID or GPC) regarding model uncertainties in high frequency is enhanced while respecting a temporal template for the disturbance rejection.     

Performance robustness and stiffness analysis on a machine tool servo design

The machine tool servo design is very different from the traditional high performance servo systems. Traditional servo design depends heavily on a precise system model so that frequency domain or time domain compensation techniques can be applied, and many synthesis tools such as LQG, H_∞, or LMI, … are available based on this concept. While these synthesis tools are becoming very useful in many situations, they are not used in the machine tool servo design. Most high performance machine tool systems still rely on the more primitive PID or PDF controllers in conjunction with complicated friction and temperature compensation algorithms. This paper investigates the different approaches based on a house-designed servo control board. With the ability to change the servo control algorithms on a test machine tool, the performance robustness and servo stiffness were tested both numerically and experimentally. The numerical tests conducted with PID, PDF, and LQG controllers showed that the PID controller is the easiest to achieve good performance, but the PDF controller contains the best performance robust margin. Experimental results also indicated that the PDF controller exhibits far superior robustness properties over the other two controllers.     

一种用于液压伺服系统位置跟踪的混合模型预测控制器设计

为了提高液压伺服系统对目标位置的跟踪准确度,设计一种用于液压伺服系统位置跟踪的混合模型预测控制器。首先,通过对液压伺服系统建模,分析其组成结构,建立液压缸与伺服阀的运动学模型。然后,对传统模型预测控制器的工作状态进行分析,获取其对应的线性时不变模型。并在传统模型预测控制器的基础上,利用通过布谷鸟搜索算法改进的PID控制器,设计了混合模型预测控制器。最后,利用所设计的混合模型预测控制器,对阶跃、方波以及不规则信号产生的目标位置轨迹进行了跟踪测试。测试结果显示:所设计的混合模型预测控制器不仅能够跟踪多种信号产生的目标位置轨迹,而且跟踪准确度较高、波动性较小;混合模型预测控制器在跟踪阶跃、方波以及不规则信号产生的目标位置轨迹时,相比传统模型预测控制器的跟踪结果,最大偏离度分别减小了6.77%、17.39%、19.64%,说明所设计的混合模型预测控制器能够控制液压伺服系统对目标位置进行良好的跟踪。     

Homogenization treatment of high Nb containing TiAl alloys with as-cast and as-forged microstructures

The effect of heat treatment on the microstructure evolution of a high Nb containing TiAl alloy has been studied. The results indicate that β-segregation, β-segregation and S-segregation in the as-cast and as-forged alloys can be effectively eliminated at the temperature above Tα （1350-1400℃） for long holding time （12-24 h） and the full lamellar （FL） microstructure is gained. For the two alloys, the lamellar colony sizes are 120 μm and 2000 μm, respectively after heat treatment at 1400℃ for 12 h. Meanwhile, the sizes are 210 μm and 3000 μm, respectively at 1350℃ for 24 h. To get a fine homogenous microstructure, the primary as-cast alloy is first subjected to preheat treatment for eliminating the segregations. After the preheat treatment, the ailoy is processed by the multi-step canned forging to attain the microstructure with fine grain size.     

High speed CNC system design. Part II: modeling and identification of feed drives

Accurate modeling and identification of the feed drives' dynamics is an important step in designing a high performance CNC. This paper presents a method for identifying the dynamic parameters, as well as the friction characteristics of machine tool drives. The inertia and viscous friction are estimated through an unbiased least squares scheme. The friction model is developed by observing the disturbance torque through a Kalman filter, while jogging the axes under closed loop control at various speeds. The overall axis model is used in designing a high speed feed drive control system, which has been presented in Part III of this paper. As verification of the identified friction model, contouring test results without and with friction compensation are also presented.     

A thermo-electric model of material removal during electric discharge machining

In Electro Discharge Machining (EDM), melting is the main process for metal removal. However, for short pulses (discharge duration <5 μs), melting does not account for the results as shown by the experiments reported. For short pulses, metal does not get enough time to get adequately heated and almost no melting takes place. The electrostatic force acting on the surface is a very important factor in the removal of metal for short pulses. For long pulses (discharge duration>100 μs), this electrostatic force becomes very small and does not play a significant role in the removal of metal. In the model proposed, the electrostatic force acting on the metal surface and the stress distribution inside the metal due to this electrostatic force have been estimated. The variation of the yield strength with depth inside the metal has also been found out and finally the ‘crater depth' due to this electrostatic force has been calculated.     

Comparison between X-ray micro-computed tomography and optical scanning tomography for full 3D strain measurement by digital volume correlation

In this study, we present three-dimensional (3D) measurements by digital volume correlation (DVC) with images from X-ray micro-computed tomography (XμCT) and optical scanning tomography (OST). In this article, we compare both techniques and we define their field of application by evaluating their measurement uncertainty in the case of rigid body translations, homogeneous strain tests and localised compression tests. For displacement, measurement uncertainty is around 0.037 voxel for OST and 0.049 voxel for XμCT which correspond to a measured strain about 0.1%. For larger strain, uncertainty increases with strain intensity but relative error remains constant about 10%. To resume, measurement uncertainties given by DVC are similar to the ones which we obtain generally for a two-dimensional (2D) study with DIC.     

基于神经网络的直流无刷电机控制系统

提出了一种直流无刷电动机的N-PI转速调节器的设计方法.在直流无刷电动机的高性能速度跟踪中,若仅采用传统的PI调节器,则难以克服系统超调和短时振荡问题.采用复合N-PI的控制方法,利用神经网络的自学习自适应功能在线调整PI控制参数.文中提出了一种模型参考自适应与神经网络相结合的控制策略,利用在线辨识技术,对参数变化实时补偿,及时修正神经网络权值的计算.最后,在Matlab/Simulink下进行了仿真,结果表明,运用这种设计方法很好地抑制了超调和振荡.