Continuous Scan, a method for performing modal testing using meaningful measurement parameters; Part I

This paper presents the first part of a work about modal testing using meaningful measurement parameters. Scanning Laser Doppler Vibrometer (SLDV) systems are becoming largely used both in industry and university for performing vibration measurements. A reason for the success of SLDV systems can be found in their capability of measuring vibration remotely and under different environmental conditions which, when hostile, can inhibit other transducers to work correctly. Hence, SLDV system can be very practical and useful in many engineering applications. SLDV systems are being used as a contactless transducer measuring vibrations from a discrete number of measurement positions marked on the specimen whenever an optical access to it is available. Hence, the advantage of a modal test carried out using accelerometers and one carried out using a SLDV system can be: (i) the automation of the measurements and (ii) the increase of the spatial resolution of the measured modes. This suggests that SLDV systems can be used as a practical replacement of accelerometers operating the same measurement method. Continuous Scanning method is a novel approach of using contactless transducers for measuring vibrations. The most important difference between a discrete and a continuous approach is the method of measuring a vibration pattern. A discrete method measures the level of vibrations at discrete positions on a structure whereas a continuous method captures the modulation of the vibrations produced by the excited modes. This is possible when a transducer can travel across a vibrating surface. This first part of the work presents a new approach of continuous scanning measurement method using a multi-tonal excitation waveform. The paper starts from a comparison between a step and continuous scan mode to introduce a novel approach of continuous scan and multi-tonal excitation waveform. The objective of this first part of work is to present and understand that measurement parameters, such as measurement positions, and can be carefully chosen to improve the measurement technique. A laboratory test piece and a helicopter tail cone are used as examples for the application of this new measurement method approach.     

BLADE-TIP TIMING MEASUREMENT OF SYNCHRONOUS VIBRATIONS OF ROTATING BLADED ASSEMBLIES

Blade-tip timing (BTT) is a promising method for the detection, measurement and analysis of blade vibrations in rotating bladed assemblies. However, the intricacies of the method when applied to real rotating structures undergoing synchronous (Engine Ordered) vibrations are not yet fully understood. In this paper, a mathematical model is developed to simulate data from typical BTT tests of rotating assemblies. The simulator is then used in order to provide a qualitative analysis of several phenomena that can be associated with the synchronous vibrations of rotating assemblies, including mistuning, coupling, excitation at multiple Engine Orders and simultaneous synchronous and asynchronous responses. It is concluded that none of these phenomena on its own will render the identification of the frequency and amplitude of blade vibrations impossible. However, there is no single BTT data analysis method that is able to deal with all of these phenomena.     

混合单频激励下连续扫描激光多普勒振动测试

提出了一种混合单频激励下激光连续扫描振动测试方法,提高了连续扫描激光多普勒振动测试效率。首先,基于单频激励下连续扫描激光多普勒振动测试的原理,研究了混合单频激励下连续扫描激光多普勒振动测试技术,提出了适合混合单频激励的结构工作变形提取方法;其次,通过仿真测试,从理论上验证了方法的有效性;最后,以悬臂梁结构为例,进行了连续扫描和离散点扫描测试的试验验证。结果表明,混合单频激励下的连续扫描激光振动测试及分析方法可准确获得结构工作变形,具有效率高、空间分辨率高等优点,对进一步的工程应用具有实用价值。     

Mode shape reconstruction of an impulse excited structure using continuous scanning laser Doppler vibrometer and empirical mode decomposition

For vibration testing, discrete types of scanning laser Doppler vibrometer (SLDV) have been developed and have proven to be very useful. For complex structures, however, SLDV takes considerable time to scan the surface of structures and require large amounts of data storage. To overcome these problems, a continuous scan was introduced as an alternative. In this continuous method, the Chebyshev demodulation (or polynomial) technique and the Hilbert transform approach have been used for mode shape reconstruction with harmonic excitation. As an alternative, in this paper, the Hilbert-Huang transform approach is applied to impact excitation cases in terms of a numerical approach, where the vibration of the tested structure is modeled using impulse response functions. In order to verify this technique, a clamped-clamped beam was chosen as the test rig in the numerical simulation and real experiment. This paper shows that with additional innovative steps of using ideal bandpass filters and nodal point determination in the postprocessing, the Hilbert-Huang transformation can be used to create a better mode shape reconstruction even in the impact excitation case.     

Identification of modal parameters and aeroelastic coefficients in bladed disk assemblies

Unknown excitation forces are applied to bladed disk assemblies leading to forced vibration responses and non-contact measurement of such vibrations are obtained using blade tip-timing signals. In the tip-timing data analysis, a set of optical probes is mounted on an engine casing and measures the times of arrival of each blade. These timings are then used to estimate the vibrations of different blades and current research focuses on analysis methods for interpretation of the measured vibration data from the limited number of probes. We present in this paper a methodology to identify the modal properties of mistuned bladed disk from tip-time data. The tip-timing measurements are under-sampled and we propose a technique to obtain a continuous signal. The subspace algorithm is then applied to the reconstituted signal for modal parameter identification. A technique to identify aeroelastic coefficients from output data only is also proposed. The main difficulties that we have to overcome are the unknown excitation forces, the very high modal density (77 eigenfrequencies in a narrow band of 1 Hz) and the signal reconstitution from under-sampled data. Numerical and experimental results are presented.     

Application of computed order tracking, Vold-Kalman filtering and EMD in rotating machine vibration

This paper presents a study on rotating machine vibration signals by using computed order tracking, Vold-Kalman filtering and intrinsic mode functions from the empirical mode decomposition method. Through the sequential use of intrinsic mode function and order tracking methods, both speed synchronous and non-synchronous vibrations that modulate orders in rotating machine vibrations are distinguished, which is difficult when using each of the techniques in isolation alone. Simulation and experimental studies demonstrate the ability of extracting vibrations that modulate order signals through combining the techniques.     

激光扫描追踪人体目标位姿的算法研究

针对人体目标位姿的追踪问题,对二维激光扫描仪的工作原理进行了归纳,对所测取数据的分析算法进行了研究,设计了一种基于激光图像扫描的新型算法,利用C语言与Matlab混合编程,通过对人体目标肩部截面进行椭圆拟合,实时测取了运动人体目标的距离、角度和方位信息,并通过Matlab GUI显示.二维激光扫描仪通过旋转的光学部件发射光脉冲,形成了二维的扫描面,以实现区域扫描及轮廓测量的功能.研究结果表明,利用激光扫描仪测取的人体目标的位置信息与真实的人体目标的位置信息相比基本吻合,距离范围为0m～4m,距离精度优于0.05 m,角度范围为240°,角度跟踪精度优于2°,方位范围为360°,方位精度最高1 °,该算法可用于需要对人体目标进行实时追踪的工程应用中,例如用于对人体目标实时跟随的移动平台或机器人中.     

Rapid and accurate measurement for phase-change optical recording bits

Conducting atomic force microscopy (CAFM) and scanning surface potential microscopy (SSPM) have been used to image the phase-change optical recording bits. Commercially available digital versatile discs (DVD) + rewritable (RW) with initialization process were measured in experiments. Comparing the measurement results of both, the measurement resolution of CAFM is far superior to that of SSPM. With the DVD + RW disc rotating at a linear speed of 3.5 m/s, appropriate writing laser power range, may be precisely identified by CAFM as 10-15 mW. This is sufficient to verify the high-resolution recording bits research method. This new method may also be applied to the development of new types of phase-change recording materials.     

Comparison of FRF measurements and mode shapes determined using optically image based, laser, and accelerometer measurements

Today, accelerometers and laser Doppler vibrometers are widely accepted as valid measurement tools for structural dynamic measurements. However, limitations of these transducers prevent the accurate measurement of some phenomena. For example, accelerometers typically measure motion at a limited number of discrete points and can mass load a structure. Scanning laser vibrometers have a very wide frequency range and can measure many points without mass-loading, but are sensitive to large displacements and can have lengthy acquisition times due to sequential measurements. Image-based stereo-photogrammetry techniques provide additional measurement capabilities that compliment the current array of measurement systems by providing an alternative that favors high-displacement and low-frequency vibrations typically difficult to measure with accelerometers and laser vibrometers. Within this paper, digital image correlation, three-dimensional (3D) point-tracking, 3D laser vibrometry, and accelerometer measurements are all used to measure the dynamics of a structure to compare each of the techniques. Each approach has its benefits and drawbacks, so comparative measurements are made using these approaches to show some of the strengths and weaknesses of each technique. Additionally, the displacements determined using 3D point-tracking are used to calculate frequency response functions, from which mode shapes are extracted. The image-based frequency response functions (FRFs) are compared to those obtained by collocated accelerometers. Extracted mode shapes are then compared to those of a previously validated finite element model (FEM) of the test structure and are shown to have excellent agreement between the FEM and the conventional measurement approaches when compared using the Modal Assurance Criterion (MAC) and Pseudo-Orthogonality Check (POC).     

Novel Method for Spatial Angle Measurement Based on Rotating Planar Laser Beams

Spatial angle measurement,especially the measurement of horizontal and vertical angle,is a basic method used for industrial large-scale coordinate measurement.As main equipments in use,both theodolites and laser trackers can provide very high accuracy for spatial angle measurement.However,their industrial applications are limited by low level of automation and poor parallelism.For the purpose of improving measurement efficiency,a lot of studies have been conducted and several alternative methods have been proposed.Unfortunately,all these means are either low precision or too expensive.In this paper,a novel method of spatial angle measurement based on two rotating planar laser beams is proposed and demonstrated.Photoelectric receivers placed on measured points are used to receive the rotating planner laser signals transmitted by laser transmitters.The scanning time intervals of laser planes were measured,and then measured point's horizontal/vertical angles can be calculated.Laser plane's angle parameters are utilized to establish the abstract geometric model of transmitter.Calculating formulas of receiver's horizontal/vertical angles have been derived.Measurement equations' solvability conditions and judgment method of imaginary solutions are also presented after analyzing.Proposed method for spatial angle measurement is experimentally verified through a platform consisting of one laser transmitter and one optical receiver.The transmitters used in new method are only responsible for providing rotating light plane signals carrying angle information.Receivers automatically measure scanning time of laser planes and upload data to the workstation to calculate horizontal angle and vertical angle.Simultaneous measurement of multiple receivers can be realized since there is no human intervention in measurement process.Spatial angle measurement result indicates that the repeatable accuracy of new method is better than 10".Proposed method can improve measurement's automation degree and speed while ensuring measurement accuracy.     

激光跟踪扫描曲面测量的自适应算法研究

针对自由曲面壳体的测量问题,研究了非接触测量时激光束跟踪测量高精度自由曲面壳体的一种自适应测量算法。从测量速度和精度两方面考虑,提出采用圆弧切线外插算法控制激光束跟踪扫描,实现曲面的自适应跟踪测量。采用该算法在三坐标测量机上用激光束跟踪快速扫描测量电力安全帽实物样件,并对其进行实物原型的数字化处理,完成了电力安全帽反求工程测量任务。测量实验表明采用该算法跟踪和测量精度高,测量速度快。     

Management of the variability of vibration response levels in mistuned bladed discs using robust design concepts. Part 2: Tolerance design

The mistuning pattern on a bladed disc is controlled in Part 2 of the two-part article either by (i) imposing a small maximum allowable mistune according to the small mistuning approach or (ii) incorporating non-identical blades of specific patterns, known as the intentional mistuning approach. These approaches resemble the tolerance design stage of the Taguchi method of robust design. The first-order maximum amplification factor sensitivity in a single-degree-of-freedom (DOF)-per-sector system is derived to support a new definition of the interblade coupling ratio and to illustrate the dependence of the maximum amplification factor sensitivity on design parameters of a bladed disc. It is found that the variability of the forced vibration response levels in flexible bladed discs can be reduced by controlling the degree of mistune within realistic levels. The potential of a “linear” mistuning pattern to become an effective intentional mistuning pattern is evaluated by observing the amplification factors of bladed discs with combined intentional mistuning and additional random mistuning. A tool based on the importance sampling method is used to reduce the computational effort in determining the magnitude of intentional mistuning. Guidelines of designing bladed discs with a lower variability of forced vibration response levels are given according to the findings in casting the blade mistuning problem as a robust design problem.     

Management of the variability of vibration response levels in mistuned bladed discs using robust design concepts. Part 1: Parameter design

Small unavoidable differences (e.g. 5%) between blades on a bladed disc, called mistuning, can lead to a huge variation of forced vibration response levels, and some of them are extremely high (e.g. 500% of the level experienced on every blade is all blades are identical). In this first half of a two-part article, a novel approach of designing a bladed disc with a lower chance of encountering high vibration response levels is evaluated. A robust design concept is applied to manage the variability of the vibration response levels, and the new approach resembles parameter design in Taguchi method of robust design. A “robustness map” is created using simulations results of a 6-degree-of-freedom (6-DOF) system, and such a map is validated by two more complicated models. The robustness map is used to explain the behaviour of bladed discs investigated in previous studies and to give possible methods of delivering more robust bladed disc designs.     

汽轮机叶片——叶轮系统耦合振动特性的理论计算和实验研究

对汽轮机叶片—叶轮系统（轮系）耦合振动特性进行了理论和实验研究。首先介绍了一种简便实用的计算轮系振动特性的数值积分方法。此方法考虑了叶片的剪切变形、转动惯量、叶轮轮孔处支承柔度、叶片和叶轮连接处的叶根柔度及离心力等影响轮系振动的各种主要因素，给出了焊拉筋和松拉筋的处理方法，可供计算各种结构轮系振动的静、动频谱和相应的振型。其次，本文利用锤击法对叶片—叶轮系统模型进行了模态分析和参数识别，得到系统各阶固有频率和振型，理论计算结果与实验值的比较较为吻合。     

Sub-micron position measurement and control on precision machine tools with laser interferometry

The trend towards tighter part tolerances from precision machine tools is placing increased demands on position transducers on these machines. Improved accuracy, repeatability and resolution from the position transducers is required to match this trend.This paper describes a new performance modelling technique for laser interferometer systems. Each error affecting the measurement accuracy and repeatability is described in detail, along with the method to determine these performance measures. Two products from Hewlett-Packard — wavelength tracking compensation and the high resolution interferometer — are also described that will help meet the future measurement requirements for precision machine tools.     

Traceability of torque transducers under rotating and dynamic operating conditions

Torque transducers in industrial applications are usually used under dynamic and rotating operating conditions. The present article provides a survey of state-of-the-art studies which describe methods for providing traceability of torque measurements under such operating conditions.The method of investigating the effects of rotation is a comparison of measurements of torque in a rotating shaft train gained simultaneously with two different methods. The first method is using a rotating torque transducer, the second one is with a cradle-mounted absorption dynamometer. The method for investigating the effects of dynamic torque is based on experiments with periodic torque but only minimal rotational motion. Torque is generated by a rotary exciter which forces a setup with a torque transducer and some auxiliary components into a periodic rotary motion. The torque measurement provided by the torque transducer is compared to a reference torque. The reference torque is gained by accurately measuring the angular acceleration of the rotation in conjunction with a priori knowledge of the mass moment of inertia.The developments on the periodic dynamic torque have been performed by the Physikalisch-Technische Bundesanstalt (PTB), the German National Metrology Institute. The investigations on rotating torque transducers have been performed by Hottinger Baldwin Messtechnik (HBM), Germany, a manufacturer of torque transducers with a wide experience in torque calibration.     

Development of a measuring method for several types of programmed tool paths for NC machine tools using a laser displacement interferometer and a rotary encoder

Many circular motion measuring methods for NC machine tools have been proposed, however, the drawback common to many of these methods is the restriction on the radius size due to the short measuring range of the displacement transducers used. Moreover, most of these measurement tools are specialized, and can only perform circular test path measurements. A circularity test method using a laser displacement interferometer and a rotary encoder has been developed. The measuring method features a much longer range of motion than ordinal circular test methods such as the double ball bar (DBB) method and, therefore, the radius restriction on these measurements is greatly reduced. Moreover, this measuring system can also be used for the evaluation of positioning accuracy and other more complex test paths.

The proposed device consists primarily of a laser displacement interferometer and a rotary encoder. The holders for the interferometer head and the retroreflector are connected with a stainless steel rod. The retroreflector holder has a synthetic resin linear bearing allowing it to move relative to the interferometer head so that both optical components are always facing each other. The laser interferometer measures the change in distance between the interferometer head and the retroreflector, and the rotary encoder measures the rotation angle of the stainless steel rod.

In this paper, the background, measuring principle and apparatus structure are briefly described. The experimental setup is also presented. The apparatus was employed in several measuring experiments, including circularity tests for a vertical machining center. The results from these experiments support the validity of this measurement apparatus.     

Accuracy assessment of vehicles surface area measurement by means of statistical methods

This paper presents an assessment of the different methods and techniques for measuring a vehicle’s surface area, including a close-range photogrammetric system with calibrated digital cameras, a mid-range laser scanning system and an expeditious method based on a specific tape measurement. These methods are compared with respect to their task-specific accuracy related to the area measurement by means of statistical test for the detection of the significant results. An uncertainty evaluation of these techniques is performed using a standard device that was previously calibrated with a coordinate measuring machine. The assessment of the gathered data reveals a potential for reducing the time and cost associated with close-range photogrammetry for vehicle surface area measurement and digitisation.     

Operational modal analysis in the presence of harmonic excitations by the use of transmissibility measurements

Operational modal analysis (OMA) is based on the assumption that the forces on the structure are the result of a stochastic process, so being white noise. In practice, however, structural vibrations observed in operation cannot always be considered as pure white-noise excitation. In many mechanical structures the loading forces are often more complex and even harmonic components can be present in the response. This is especially true, when measuring on mechanical structures containing rotating parts (e.g. cars, turbines, windmills), but also civil engineering structures may have responses superimposed by harmonic components. OMA procedures are, strictly speaking, not applicable in these situations. Current techniques may encounter difficulties to correctly identify the modal parameters, especially for modes with eigenfrequencies close to the harmonic frequencies. In this paper a recently proposed OMA technique based on transmissibility measurements will be applied. This method reduces the risk to wrongly identify the modal parameters due to the presence of harmonics. The unknown operational forces can be arbitrary (coloured noise, swept sine, impact, etc.) as long as they are persistently exciting in the frequency band of interest.