Control of linear motor machine tool feed drives for end milling: Robust MIMO approach

Linear motors are getting promising for use as high speed, high accuracy machine tool feed drives. The cutting force in the machining process are directly reflected to the linear motor due to no gearing mechanism. To achieve high accuracy machining, the controller for the linear motor system should be designed to compensate for the cutting force.

In this work, a MIMO H_∞ controller for the linear motor machine tool feed drives has been designed to reduce tracking errors induced by cutting forces for end milling. The controller is designed using normalized coprime factorization method for the dynamic model of the linear motor system. The model includes constant in-line and cross coupling force gain, since the feedback cutting force can be considered as the product of the constant gain and the moving velocity of each axis.

Analysis of the structured singular value shows that the designed controller has good robust performance despite wide variations of the cutting force and physical parameters. It is directly implemented on a linear motor X–Y table which is mounted on a milling machine to have cutting experiments via a DSP board. Experimental results verified effectiveness of the proposed scheme to suppress the effects of the cutting force in the high feed rate.     

Recording of real cutting forces along the milling of complex parts

In this paper, a data acquisition system that simultaneously allows the recording of cutting forces and cutting tool position (coordinates X, Y and Z) is presented. Thus, the geometry of the surface being machined and the values of the cutting forces generated during the surface milling can be correlated. In this manner, two usual problems in milling experimentation are solved. First, those derive from the continuous changes in the feed rate value due to the special look ahead functions (being on-line applied by the numerical control in all high-speed milling centres), which affects the correlation between part geometry and forces. Second, those originate by the presence of unexpected stocks coming from previous semi-finishing operations, which changes the value of cutting forces respect to those calculated taking into account the theoretical tool engagement into the part.The objective of this work is the development of a diagnostics tool for allowing the detection of potential milling problems by researchers, making more profitable the performance of machining test on real complex parts. This tool can be used in the optimization of the milling of test-parts, machining real geometries that incorporate problems different from those observed in the linear tests at constant linear feeds (in end-milling conditions).The final goal of the system is the generation of cutting forces maps as a function of the geometry of real workpieces, which is an advance respect the usual forces recording in function of time or tool rotation typical of linear machining tests. In order to do that, the system incorporates a dynamometric plate for the measurement of the three components of the cutting force, as well as an acquisition card connected to the analog output of the position control loops. After machining, the file containing position and force data is post-processed and chromatic and vector maps are generated.The force analysis utility has been applied to three cases, in order to assess its feasibility in machining research projects. The first example shows a reduction of the number of tests and therefore machining time (even by 18 times) in the validation of a mechanistic cutting force model. Second example is focused on the detection of unexpected tool engagement conditions in complex parts. And the last one addresses to the milling of thin walls, for investigation of static and dynamic milling problems.     

A new method based on Fiber Bragg grating sensor for the milling force measurement

Milling force is an important parameter to describe the mechanical processing chip removal process, and it has a direct influence on generation of heat, tool wear or failure, quality of machined surface and accuracy of the work piece. Its accurate measurement is a significant basis for judging process state and improving the reliability of machining system. In this study, through analyzing the variation rule of ring diameter, a new method that using Fiber Bragg grating sensors and variation rule of ring diameter to measure the milling force has been proposed, and the basic structure of annulus has also been designed. A dynamometer has also been constructed, and the preliminary verification test was done. Through the analysis of experimental data, the dynamometer based on annulus elastic body can be used in milling force test, and it owns high sensitivity.     

Hyperworks在床身动静态分析及优化中的应用

床身刚度是影响高速机床加工精度的一个重要因素。针对床身刚度是否满足加工精度要求的问题,以双直线电机高速进给的铣床床身为研究对象,采用有限元方法建立床身的动力学模型,并利用有限元软件Hyperworks对床身进行动静态分析,找出该床身的动静态性能薄弱部位,利用Hyperworks的Optistruct拓扑模块对床身进行结构优化。优化后的结果表明,3个方向静刚度平均提高了1 000 N·μm-1,固有频率提高了约30 Hz,x和z向频率响应的振幅约减小50%,为同类型结构设计提供一定的参考。     

Simulation of circular silicon pressure sensors with a center bossfor very low pressure measurement

A characteristics simulation method of circular silicon-diaphragm piezoresistive pressure sensors was developed to obtain accurate sensors for very-low-pressure measurement. The anisotropic stress-strain relationship of a silicon single-crystal plate and the nonlinear characteristics of silicon piezoresistive gauges were considered. Nonlinear deflection and strain formulas of circular silicon diaphragm sensors with a center boss and sensors with a center boss and ribs were derived by taking the effects of the large deflection and the support stiffness of the diaphragms into account. Based on these considerations, the characteristics of the sensors were simulated. The simulated results show good agreement with the observed results and indicate that output voltage can be greatly increased while maintaining low nonlinearity even in the low-pressure range by narrowing the rib width and thinning the diaphragm thickness of sensors with a center boss and ribs. This is because the rib strain that produces output voltage is increased while maintaining small deflection by using this type of sensor     

Fuzzy logic for constant force control of end milling

The machining condition usually has significant variation resulting from the change of cutting depth and the intrinsic property of the workpiece. In order to maintain the performance of a classical proportional integral derivative control system, the tool life and machining quality, conservative feedrate, and cutting depth change are prespecified as the limitations of computer numerically controlled operators. Therefore, constant cutting force control is proposed as a useful approach for increasing the metal removal rate and the tool life. However, the model-based controller cannot handle the nonlinearity of a force control system due to cutting condition variations. Here, a fuzzy controller with learning ability was employed to improve both the system performance and the adaptability. This control approach vias implemented on a retrofit old-fashioned milling machine for the end milling process. The experimental results show that this control strategy has smooth feedrate and good cutting force dynamic responses     

回转激光位移传感器逼近式孔心定位方法

对于提高大型工件轴孔加工的精度和效率,孔心的精确定位至关重要。提出了一种基于激光位移传感器的非接触式回转逼近孔心定位方法。将传感器固定于机床的执行切削的旋转主轴上,并将主轴旋转中心置于理想圆心附近。在机床旋转的同时,分别沿与主轴轴线正交且相互垂直的两个方向上移动主轴,直到测量变化量达到最小(理论上可以为零),此时可以认为轴孔中心达到重合。利用逼近式孔心定位方法,可以快速精确定位与机床关联的圆心的相对坐标,从而指导后续加工。由于采用非接触测量方法,保障了测量的安全性。实验结果表明,此方法能够提高孔心的定位精度和效率。     

Sensors for Applications in Magnetic Resonance Environments

This paper analyzes sensing methods compatible with magnetic resonance imaging (MRI) and functional MRI (fMRI) reported in the literature, and presents the three generations of MR-compatible force/torque sensors we have developed for robotic systems to interact with human motion. Conventional sensors such as camera-based measurement systems, strain gauges or commercial force/torque sensors, and optical encoders may be used, if placed sufficiently away from the imaging region and equipped with adequate shielding and filtering in order to minimize electromagnetic interference caused by electric cables, the transducer, and electronics of surrounding equipment. However, electromagnetic interference can be avoided by using light transmission over optical fibers, in which case sensitive and noisy electronic components can be placed outside the MR room, and the MR compatibility issue is restricted to the used materials. Good performance can be obtained with sensing elements made from materials adapted to the location of use, combined with reflected or differential light intensity measurement over optical fibers. We have developed various force and position sensors based on this principle, ranging from MR Safe (for a definition and discussion of the terms MR Safe and MR Conditional, see Gassert , IEEE Eng. Med. Biol. Mag., pp. 12--14, May/Jun. 2008) milled polymer probes to MR Conditional assemblies combining beryllium copper blades with a polymer body, as well as smaller aluminum probes realized through a combination of milling and electric discharge machining. It appears that, in contrast to actuators, good performance is not in tradeoff with MR compatibility.     

Decentralized structure-integrated spatial force measurement in machine tools

New manufacturing processes and extended movability of modern machine tools, such as five-axis kinematics or hexapods, increase the demand for in-process measurement of spatial forces and torques in up to 6 degrees of freedom (DoF). The approach proposed in this paper is based on the idea of integrating 6 single-axis force sensors into the machine’s structure and converting these sensor forces to spatial forces and torques at the tool centre point (TCP) using a measurement model. This concept is advantageous to costs, ruggedness and available workspace when compared to state-of-the-art 6 DoF force/torque transducers. At the same time, the achievable measuring accuracy is similar and also significantly better than the accuracy of drive current based force evaluation. On the other hand, structure and machine influences have to be addressed by suitable measurement models. This article presents design, parametrization, verification, and characterisation of these measurement models on the example of four integration concepts, two in rigid bar frameworks and two in bar kinematics. Further, experimental results are shown which are classified in comparison to a 6 DoF F/T sensor and drive current based force measurement. Finally, other influences, such as structural design and deformations, as well as the integration of sensors and models into the machine’s control software are discussed.     

Improvement of form accuracy in hybrid machining of microstructures

Micromachining is gaining popularity due to recent advancements in Micro Electro Mechanical Systems. Using conventional micromachining, it is relatively difficult to produce moving components in the order of microns. In this study, an attempt is made to fabricate microstructures using a combination of turning and electrodischarge machining (EDM). Several sets of experiments have been performed to study the characteristics of the hybrid machining process. From the experiments, it has been observed that a higher form of accuracy could be obtained by integrating the on-machine fabrication of the tool and by subsequently using the same tool for EDM. The main cause of the form error is due to the deflection of the shaft during turning. Hence, an attempt is made to observe the deflection of the shaft using a deflection sensor. The influence of micro-EDM parameters such as feed rate, discharge circuits, and gap control parameters on material removal rate and tool wear is also discussed in this study.     

含金属芯压电陶瓷纤维的传感特性研究

压电陶瓷纤维(MPFs)是一种新型的智能材料。该文简述了其几何形状、制备工艺,从理论上分析了其应变传感原理。实验研究了MPFs的准静态、动态应变传感特性。研究结果表明,MPFs传感器灵敏度很高,静态响应的线性度很好,动态响应则有非线性和迟滞现象。最后探讨了MPFs的几何尺寸对传感性能的影响,分析了迟滞现象产生的原因。     

Design and Fabrication of a Four-Arm-Structure MEMS Gripper

This paper is focused on the design and fabrication of a four-arm-structure microelectromechanical systems gripper integrated with sidewall piezoresistive force sensors. Surface and bulk micromachining technologies are employed to fabricate the microgripper from a single-crystal silicon wafer (i.e., no silicon-on-insulator wafer is used). A vertical sidewall surface piezoresistor etching technique is used to form the side direction force sensors. The end effector of this gripper is a four-arm structure: two fixed cantilever arms integrated with piezoresistive sensors are designed to sense the gripping force. The resolution of the force sensor is in the micronewton range and, therefore, provides feedback of the forces that dominate the micromanipulation processes. An electrostatically driven microactuator is designed to provide the force to operate the other two movable arms. In this way, it creates a deflection of 25 mum at the arm tip, and the range of the operation is 30-130 mum. Experimental results show that it can successfully provide force sensing and play a main role in preventing the damage of microparts in micromanipulation and microassembly tasks.     

A smart boring tool for process control

A mechatronic metal cutting tool has been developed to improve the accuracy and flexibility of line boring machining in the automotive industry. Laser position sensors and piezoelectric actuator were integrated into the rotating body of the boring tool. To compensate the boring bar droop and effects of cutting forces, a fast tool servo utilizing feedback control of the boring tool insert position was designed and embedded in the rotating tool assembly. In addition to position control, a self-monitoring algorithm that utilizes disturbance estimator has been put together in the controller. Experimental results demonstrated that the developed cutting process controller improves the accuracy of the boring tool as well as reliably detects the process failures, such as tool tip breakage, without additional monitoring equipment.     

Force Sensing Resistor and Its Application to Robotic Control^①

The force sensing resistor(FSR) and its construction and characteristic are described.By using the optimal electronic interface,the end result which is a direct proportionality between force and voltage is obtained.The circuits of application for force and position measurements in the robotic control are given.The experiment that FSRs are placed on the fingers of BH-1 dexterous hand as tactile sensors to measure the contacting forces shows FSR‘s force sensitivity is optimized for use in the control of robot contacting with environment.     

一种简单的电容式三维力柔性触觉传感器设计

随着机器人智能化程度不断提高,机器触觉和机器视觉技术均得到了很大发展,但是目前大部分触觉传感器均不能满足触觉感知应用的要求。因此,提出了一种新型的具有滑觉检测功能的柔性三维力触觉传感器。该传感器能够将力量变化转化为气隙变化,进而引起电容发生变化。然后将接触力建模为实测电容的多项式函数,对传感器输出进行校正进而实现法向力测量和滑动检测。测试结果显示,提出的传感器能够以较高的精确度测量法向力,重复性好,并且能够检测滑动现象,在0-100mN测量范围内的法向灵敏度约为0.4 pF/N,均方根误差为1%。此外提出的传感器结构简单、成本较低,易于大规模生产和应用。     

拖曳法测量微粒光阱力和光阱刚度的实验研究

采用聚焦激光束拖曳扩散悬浮在液体中透明的电介质小球,对3种半径范围从0.5μm到22.0μm的微粒进行了最大横向光阱力的测量,得到了PN量级的光阱力。对于其中尺寸较大的聚甲基丙烯酸甲脂(PM-MA)小球,测量了不同拖曳速度下球心偏离光阱中心的位移Δx,计算了该条件下的光阱刚度。对光阱外缘区也进行了光阱力测量,并得出光阱力随Δx的变化关系以及光阱力的作用范围。对同一实验条件下不同半径酵母菌的最大光阱力及光阱刚度的比较测量发现,随着酵母菌半径变大,在液体中匀速运动的临界速度变小,受到的最大光阱力变大,但光阱刚度降低。     

Fuzzy estimation of feed-cutting force from current measurement-a case study on intelligent tool wear condition monitoring

It is very important to use a reliable and inexpensive sensor to obtain useful information about manufacturing processing, such as cutting force for monitoring automated machining. In this paper, the feed-cutting force is estimated using inexpensive current sensors installed on the ac servomotor of a computerized numerical control (CNC) turning center, with the results applied to the intelligent tool wear monitoring system. The mathematical model is used to disclose the implicit dependency of feed-cutting force on feed-motor current and feed speed. Afterwards, a neuro-fuzzy network is used to identify the cutting force with current measurement only. This hybrid math-fuzzy approach will reduce the modeling uncertainty and measurement cost. Finally, the estimated cutting force is applied in the tool-wear monitoring process. Successful experiments demonstrate robustness and effectiveness of the suggested method in the wide range of tool-wear monitoring applications.     

Force Sensing Resistor and Its Applition to RobotiC Control

Abstract: The force sensing resistor (FSR) and its con'struction and characteristic are described. By using the optimal electronic interface, the end result which is a direct proportionality between force and voltage is obtained. The circuits of application for force and position measurements in the robotic control are given. The experiment that FSRs are placed on the fingers of BH - 1 dexterous hand as tactile sensors to measure the contacting forces shows FSR's force sensitivity is optimized for use in the control of robot contacting with environment.     

Extrinsic Fabry-Perot interferometer for measuring the stiffness of ciliary bundles on hair cells

We have developed an extrinsic Fabry-Perot interferometer (EFPI) to measure displacements of microscopic, living organelles in the inner ear. The EFPI is an optical phase-shifted instrument that can be used to measure nanometer displacements. The instrument transmits a coherent light signal to the end of a single glass optical fiber where the measurement is made. As the coherent light reaches the end of the fiber, part of this incident signal is reflected off the internal face of the fiber end (reference reflection) and part is transmitted through the end of the fiber. This transmitted light travels a short distance and is reflected off the surface whose displacement is to be measured (the target). This sensing reflection then reenters the fiber where it interferes with the reference reflection. The resulting interference signal then travels up the same optical fiber to a detector, where it is converted into a voltage that can be read from an oscilloscope. When the target moves, the phase relation between reference and sensing reflections changes, and the detector receives a modulated signal proportional to the target movement. Reflections of as little as 1% at both the sensor tip and target surfaces produce good results with this system. We use the EFPI in conjunction with fine glass whiskers to measure the stiffness (force per unit deflection) of stereociliary bundles on hair cells of the inner ear. The forces generated are in the tenths of picoNewton range and the displacements are tens of nanometers. Here we describe the EFPI and its development as a method for measuring displacements of microscopic organelles in a fluid medium. We also report experiments to validate the accuracy of the EFPI output and preliminary measurements of ciliary bundle stiffness in the posterior semicircular canal.     

Intelligent Diagnosis and Prognosis of Tool Wear Using Dominant Feature Identification

Identification and prediction of a lifetime of industrial cutting tools using minimal sensors is crucial to reduce production costs and downtime in engineering systems. In this paper, we provide a formal decision software tool to extract the dominant features enabling tool wear prediction. This decision tool is based on a formal mathematical approach that selects dominant features using the singular value decomposition of real-time measurements from the sensors of an industrial cutting tool. Selection of dominant features is important, as retaining only essential features allows reduced signal processing or even reduction in the number of required sensors, which cuts costs. It is shown that the proposed method of dominant feature selection is optimal in the sense that it minimizes the least-squares estimation error. The identified dominant features are used with the recursive least squares (RLS) algorithm to identify parameters in forecasting the time series of cutting tool wear. Experimental results on an industrial high-speed milling machine show the effectiveness in predicting the tool wear using only the dominant features.