A laser sensor with multiple detectors for freeform surface digitization

An integrated laser scanning sensor for freeform surface digitization is presented. The sensor consists of a diode laser light source and four position-sensitive device (PSD) detectors. The stand-off distance of the sensor is 180 mm and the measurable range is 90 mm. The Lambert model is applied to calculate the displacement between the sensor and the measured point on the object surface along the optical axis, under the assumption of a diffusive surface. The inclination angle of the measured point from the vertical plane of the laser beam is calculated by mathematical inference. Those data are used in error compensation to improve system precision. The new design of multiple detectors could increase the measurable angle and could solve the dead space problem in single point laser of triangulation measurement. The computer simulation and actual measurements show that the displacement resolution reaches 50 μm, and the system performs well in regards to stability and repeatability. The sensor system could be mounted on the NC machine orX-Y platform for freeform surface digitization.     

Intermittent Process Measurement of a Freeform Surface Profile with a Circular Triangulation Laser Probe on a Machining Centre

This paper describes the development of a non-contact type system for measuring a freeform surface on a machine tool. A laser probe, model OTM-3A20 made by Wolf & Back Co., was integrated into a CNC machining centre as a non-contact sensor. An adjustment device for the laser probe was designed to minimise the cosine error caused by assembly inaccuracy. An alignment test of the measuring laser beam was carried out using a calibrated specimen. The systematic accuracy of the circular triangulation laser probe and a standard triangulation laser probe, with respect to the surface roughness, surface slope, and coating colour of the workpiece, was investigated by using an HP5529A laser interferometer system. The measuring system, which consists of a personal computer, a CNC controller of a machining centre, a Renishaw MP10 touch-trigger probe system, and the controller of the laser probe system, was integrated information-technically. Automatic measuring software was developed for the purpose of measuring path simulation, generation of NC codes, and error analysis of the measured data. The profile error of the tested object, measured by the laser probe and the coordinate measuring machine respectively was found to be within 45 μm. In this case, the tolerance of the designed part is about 50 μm, so the developed system can be applied to the inspection of mould production in bakelite according to the experimental results. ID="A1"Correspondance and offprint requests to: Dr Fang-Jung Shiou, Department of Mechanical Engineering, National Taiwan University of Science and Technology, No. 43, Section 4, Keelung Rd. 106 Taipei, Taiwan. E-mail: shiou@mail.ntust.edu.tw     

激光位移传感器在自由曲面测量中的应用

以点激光位移传感器(HL-C211BE)为对象,研究它在自由曲面测量中的应用。针对激光位移传感器因测点倾角代入的测量误差,提出了一个可以量化的倾角误差模型。基于直射式点激光三角法原理,分析了激光光路的几何关系,从会聚光斑光能质心发生的偏移推导出倾角误差模型。随后,用高精度激光干涉仪和正弦规对激光位移传感器进行校对实验,并用误差模型对测量结果进行补偿。结果显示,补偿后激光位移传感器的测量精度得到明显提高。对一非球面凸透镜进行了实验测量,得到了自由曲面测点倾角的计算方法,并用倾角误差模型修正了测量数据。实验结果表明,量化的倾角误差模型可以将激光位移传感器的测量误差控制到小于10μm,满足激光位移传感器在自由曲面测量中应用的要求。     

点激光测头激光束方向标定

为了使点激光测头能在任意方向上实现测量功能,提出一种逆向工程中标定激光束方向的方法,设计了一种标定面方向可调的标定块配合标定。标定过程中,让激光测头在标定面上分别沿X、Y、Z3个轴方向做等间距运动,根据进给步长与激光束长度变化量之间的关系确定激光束的方向。以三坐标测量机为平台,给出了以任意方向安置点激光测头时,测量值从传感器坐标系到基准坐标系的转换过程,并对标定算法及整个标定过程进行了详细描述。最后,通过与接触式测量进行对比实验,验证标定后点激光测头的测量效果。实验结果表明,用该方法标定的点激光测头在3σ范围内沿任意方向的测量误差为(0.0452±0.0168)mm,满足逆向工程的测量要求。     

非接触式缸盖平面度误差检测方法与测量系统研究*

为解决发动机缸盖生产铸造过程中缸盖底面平整度误差检测问题,设计一种激光非接触式发动机缸盖底面平整度在线检测系统,提出一种基于对角中线的平面度误差检测算法。根据现场平面度检测需求,设计利用激光臂纵轴和缸盖横轴传送的交互运动的发动机缸盖表面平面度误差检测系统;通过发动机缸盖表面检测四个顶角特征点对角线中线建立发动机缸盖平面度检测的数学模型,利用回归方程确定最小二乘法平面为理想平面,求出平面度误差;并对激光位移传感器进行精度标定,给出传感器误差标定回归方程,并应用该检测系统完成对不同型号的发动机缸盖检测。结果表明：该系统最大检测面积为400 mm×2 000 mm,测量范围为160~450 mm,测量精度为0.03 mm,而且结构简单,检测速度快,完全能够达到在线检测要求。     

Multi-beam laser probe for measuring position and orientation of freeform surface

This study develops a novel optical non-contact probe that measures the position and orientation (normal vector) of a freeform surface. The probe system comprises a five-laser-beam projector and a charge-coupled device (CCD) camera. The probe is integrated on a three-axis platform. Five designed laser beams project onto a measuring surface, where five light spots are observed. The CCD captures the image of this surface and processes it. The 3D coordinates of the five light spots can be then computed. The normal direction at the central spot on the measuring surface is determined from two crossed curves through the coordinates of these five light spots. Two crossed curves are constructed using the Bezier method. The normal vector is the cross-product of two tangent vectors to the two crossed curves at central spot. A scheme for calibrating and making measurements using this five-laser-beam probe is proposed and verified experimentally. Experimental results demonstrate that this five-laser-beam probe system can measure the position and orientation of a freeform surface. The range of depths that can be measured using this probe is 2.4 mm and the range of angles is 40°. The positional measuring accuracy of the complete system is approximately 30 μm while the orientational accuracy is 1.8°.     

Measurement of multi-degree-of-freedom error motions of a precision linear air-bearing stage

This paper describes the measurement of straightness error motions (vertical straightness and horizontal straightness) and rotational error motions (pitch, yaw and roll) of a commercial precision linear air-bearing stage actuated by a linear motor. Each of the error motions was measured by two different methods for assurance of reliability. The stage was placed in the XY-plane and moved along the X-direction. The pitch error and yaw error, which were measured by an autocollimator and the angle measurement kit of a laser interferometer, were about 8.7 and 1.6 arc-s, respectively, over a travel of 150 mm with a moving speed of 10 mm/s. The roll error was measured by the autocollimator through scanning a flat mirror along the X-direction. The second method for roll error measurement was to scan two capacitance-type displacement probes along the flat surface placed in the XZ-plane. The two probes with their sensing axes in the Y-direction were aligned with a certain spacing along the Z-axis. The roll error can be obtained by dividing the difference of the outputs of the two probes by the spacing between the two probes. The roll error was measured to be approximately 11.8 arc-s over the 150 mm travel. The horizontal straightness error and the vertical straightness error (Y- and Z-straightness errors) were measured by using the straightness measurement kit of the laser interferometer. The second method for straightness measurement was to scan the flat surface with a capacitance-type displacement probe. The horizontal and vertical straightness errors of the stage over the 150 mm travel were measured to be approximately 207 and 660 nm, respectively.     

Automated surface finishing of plastic injection mold steel with spherical grinding and ball burnishing processes

This study investigates the possibilities of automated spherical grinding and ball burnishing surface finishing processes in a freeform surface plastic injection mold steel PDS5 on a CNC machining center. The design and manufacture of a grinding tool holder has been accomplished in this study. The optimal surface grinding parameters were determined using Taguchi’s orthogonal array method for plastic injection molding steel PDS5 on a machining center. The optimal surface grinding parameters for the plastic injection mold steel PDS5 were the combination of an abrasive material of PA Al₂O₃, a grinding speed of 18000 rpm, a grinding depth of 20 μm, and a feed of 50 mm/min. The surface roughness R_a of the specimen can be improved from about 1.60 μm to 0.35 μm by using the optimal parameters for surface grinding. Surface roughness R_a can be further improved from about 0.343 μm to 0.06 μm by using the ball burnishing process with the optimal burnishing parameters. Applying the optimal surface grinding and burnishing parameters sequentially to a fine-milled freeform surface mold insert, the surface roughness R_a of freeform surface region on the tested part can be improved from about 2.15 μm to 0.07 μm.     

飞机强度试验舵面偏角测量方法的研究

为满足飞机结构强度试验中舵面偏角测量的要求,提出了一种基于位移测量的舵面偏角测量方法。该方法通过安装于舵面悬挂支臂上的两个拉线式位移传感器,测量相对于舵面悬挂接头上同一个测量点的位移,利用三角公式进行计算,得到舵面的偏转角度。对比验证试验表明,该方法的测量性能指标满足飞机结构强度试验要求,解决了传统利用倾角传感器无法测量方向舵偏角的难题,提高了试验数据采集的效率。     

基于Freeform的逆向工程数据修复

介绍了基于力反馈技术的Freeform触觉式设计系统在逆向工程测量数据修复方面强大的功能。利用非接触式光栅扫描仪测量飞机模型的表面数据点云,通过Freeform的分割黏土、膨胀、镜像、平滑等功能,成功地恢复了飞机模型完整的表面数据。     

时栅角位移传感器在线自标定系统

为解决时栅角位移传感器在实际应用中的在线标定问题,提出了一种定角平移自标定方法并设计了相应的自标定系统。该方法首先把圆周封闭的自然基准转换成定角基准,在时栅内部建立了自标定基准。然后,根据傅里叶级数的性质,将定角基准平移到傅里叶变换的幅值和相位中,建立了测量值之差与误差之差的函数关系。通过对测量值之差进行傅里叶分析,重构了时栅角位移传感器的误差函数。最后,讨论了影响自标定精度的误差来源,并设计了传感器的零点纠错算法。为了检验自标定效果,利用激光干涉仪实验装置与自标定系统进行了对比试验。结果表明:定角平移自标定精度为1.9″,与理论计算的自标定误差(1.5±0.5)″的结论相符。提出的自标定方法在解决时栅自身标定基准的同时,满足了精密测量领域对时栅精度和可靠性的要求。     

新型自由曲面三维激光扫描系统

从机械结构、控制系统及操作系统结构等几个角度介绍一种三维激光扫描系统设计方案。系统集成了先进的数据处理技术、激光扫描技术、先进控制技术及机器人技术,有效地借助于光机电一体化技术实现了表面检测及三维表面模型重构智能化及自动化。系统由具有电驱动装置的激光测头、C形滑臂、升降旋转台、电控系统及主控计算机组成。通过所设置测量参数,系统能够对不同的被测物体进行扫描路径规划,进而通过激光测头、C形滑臂及转台的全自动协调控制完成无盲点三维表面测量任务。试验结果表明,该三维激光扫描系统的扫描精度达到0.1 mm,速度为10 000点/s,能够应用于小型工件及模型、模具表面数据检测及模型重构。     

Position-oriented process monitoring in freeform abrasive machining

Process monitoring is necessary for the identification and avoidance of process disturbances that could cause poor surface integrity at selected machining parameters. In this paper, a position-oriented process monitoring strategy is introduced which enables determination of process characteristics for freeform abrasive machining. Through correlation of internal machine data of position and power during machining with laser displacement measurement, position-orientated maps of power and specific energy can be generated to enable an evaluation of the machining efficiency of the abrasive machining process. Measurement chains are described, and experimental results reveal that the measurement system provides a significant insight into the process by identifying regions of high power, depth of cut, engagement and specific energy on freeform parts.     

Parameter optimization of non-vertical laser cutting

In some cases, in order to avoid interference during 3D laser cutting of thin metal a laser head could not be kept vertical to the surface of a work piece. In such situations, the cutting quality depends not only on “typical” cutting parameters but also on the slant angle of the laser head. Traditionally, many tests had to be done in order to obtain the best cutting results. In this paper, an experimental design is employed to reduce the number of tests and an artificial neural network (ANN) is set up to describe quantitatively the relationship between cutting quality and cutting parameters in the non-vertical laser cutting situation. A quality point system is used to evaluate the cutting result of the thin sheet quantitatively. Testing of this novel method shows that the calculated “quality point” using ANN is quite closely in accord with the actual cutting result. The ANN is very successful for optimizing parameters, predicting cutting results and deducing new cutting information.     

Large-area profile measurement of sinusoidal freeform surfaces using a new prototype scanning tunneling microscopy

This paper presents large-area profile measurement of ultra-precision diamond turned sinusoidal surfaces by using a specially developed scanning tunneling microscopy (STM). The new prototype of STM system employs a long stroke PZT servo actuator as the Z-directional scanner, an integrated capacitance displacement sensor to accurately measure the Z-directional profile height, a motorized stage with long traveling stroke for carrying out large-area scanning. A simple method for self-calibration of the inevitable sample tilt is proposed in order to achieve large-area measurement without tip-crashing or losing of tip-sample interaction. Several types of ultra-precision machined sinusoidal freeform surfaces with different geometrical parameters are measured by the new STM system over large scanning areas at the scale of millimeters. Specially, a sinusoidal surface with peak-valley amplitude of 22 μm and periodical wavelength of 550 μm is successfully measured and imaged by the STM system. The measurement repeatability error, repeatability standard deviation and measured profile deviation are also evaluated. It is confirmed that the new STM system is capable of carrying out large-area as well as large-amplitude measurement of the ultra-precision machined sinusoidal surfaces.     

A three-dimensional non-contact measurement system

A simple three-dimensional (3D) non-contact measurement system was proposed which consisted of a 1D laser displacement sensor and a servo control coordinate system. The laser sensor was installed on theZ-axis perpendicular to theX, Y-holding table. A filter was designed to remove the noise in the measured data. An extraction technique of characteristic points was proposed to catch the edge data points of the measured objects. Then the Hough-transform technique of image processing was employed to calculate the position and dimension of the edge of measured objects based on the extracted characteristic points. The experimental results show that this simple structure can effectively measure the position and dimension of 3D objects with reasonable accuracy.     

Constrained deformation of freeform surfaces using surface features for interactive design

There is an increasing demand in conceptual design for more intuitive methods for creating and modifying freeform curves and surfaces in CAD modeling systems. The methods should be based not only on the change of the mathematical parameters but also on the user's specified constraints and shapes. This paper presents a new surface representation model for freeform surface deformation representation. The model is a combination of two functions: a displacement function and a function for representing an existing NURBS surface called a parent surface. Based on the surface model, the authors develop several novel deformation methods which are named SingleDef (Single-point constraint based deformation method), MultiDef (Multiple-points constraints based deformation method), CurDef (Curve constraints based deformation method) and FeatDef (Feature constraint based deformation method). The techniques for freeform surface deformation allow conceptual designers to modify a parent surface by directly applying point constraints, curve constraint or a surface constraint to the parent surface. The deformation methods are implemented in an experimental CAD system. The results show that designers can easily and intuitively control the surface shape.     

Laser vision sensing based on adaptive welding for aluminum alloy

A laser vision sensing based on the adaptive tungsten inert gas (TIG) welding system for large-scale aluminum alloy components was established to fit various weld groove conditions. A new type of laser vision sensor was used to precisely measure the weld groove. The joint geometry data, such as the bevel angle, the gap, the area, and the mismatch, etc., aided in assembling large-scale aerospace components before welding. They were also applied for automatic seam tracking, such as automatic torch transverse alignment and torch height adjustment in welding. An adaptive welding process was realized by automatically adjusting the wire feeding speed and the welding current according to the groove conditions. The process results in a good weld formation and high welding quality, which meet the requirements of related standards. Translated from Journal of Beijing University of Technology, 2006, 32(8): 714–718 [译自: 北京工业大学学报]     

Parameter prediction in laser bending of aluminum alloy sheet

Based on the basic platform of BP neural networks, a BP network model is established to predict the bending angle in the laser bending process of an aluminum alloy sheet (1–2 mm in thickness) and to optimize laser bending parameters for bending control. The sample experimental data is used to train the BP network. The nonlinear regularities of sample data are fitted through the trained BP network; the predicted results include laser bending angles and parameters. Experimental results indicate that the prediction allowance is controlled less than 5%–8% and can provide a theoretical and experimental basis for industry purpose. __________ Translated from Optics and Precision Engineering, 2007, 15(6): 915–921 [译自: 光学精密工程     

基于位置敏感探测器的六自由度精密位姿测量系统

位置敏感探测器(Position Sensitive Detector,PSD)是一种高精度的二维位移测量传感器,利用三片二维PSD的组合实现空间六自由度相对运动的位移和角度测量。测量系统主要包括三片PSD传感器(包括PSD光敏面和发光管)、低噪声的信号调理和AD采集电路,采用三片PSD正交布局方案,通过PSD光敏面的光点位置计算相对运动的位移和角度。设计了六自由度的PSD标定测试系统,用于PSD测量系统中心偏移和发光管安装误差的标定测试。测试结果表明,PSD测量系统的测量范围优于位移±10mm、角度±2.5°,标定后PSD测量系统的噪声误差为位移0.1mm、角度0.02°,测量系统的绝对位移误差小于0.5mm、角度误差小于0.14°,满足系统0.5mm和0.5°的指标要求。此外,对PSD传感器的环境适应性进行了评估。PSD测量系统具有量程宽、精度高、线性度好的优点,成功应用于天舟1号货运飞船微重力主动隔振装置的相对运动测量中。