3D digital image correlation methods for full-field vibration measurement

In the area of modal test/analysis/correlation, significant effort has been expended over the past twenty years in order to make reduced models and to expand test data for correlation and eventual updating of the finite element models. This has been restricted by vibration measurements which are traditionally limited to the location of relatively few applied sensors. Advances in computers and digital imaging technology have allowed 3D digital image correlation (DIC) methods to measure the shape and deformation of a vibrating structure. This technique allows for full-field measurement of structural response, thus providing a wealth of simultaneous test data. This paper presents some preliminary results for the test/analysis/correlation of data measured using the DIC approach along with traditional accelerometers and a scanning laser vibrometer for comparison to a finite element model. The results indicate that all three approaches correlated well with the finite element model and provide validation for the DIC approach for full-field vibration measurement. Some of the advantages and limitations of the technique are presented and discussed.     

Accuracy and noise analyses of 3D vibration measurements using laser Doppler vibrometer

The in-plane and out-of-plane vibration components are used for 3D vibration measurements. The latter can be calculated by using three laser scanning vibrometers (LSVs) or by moving a single LSV to three different locations. These vibration components are calculated from the vibration signals measured at each of the three locations and the angles between the local coordinates and the LSV locations. The accuracy of the in-plane and out-of-plane vibration components can be degraded depending on the measurement angle. In addition to accuracy, the noise contained in the LSV can be amplified depending on the measurement angle. Hence, it is necessary to implement an analysis methodology for the angles, which is conducted for 2D vibration measurements first before extended to 3D. Finally, experiments are performed for both 2D and 3D at small and appropriate angles, and the elicited results are compared to those elicited using a 3D accelerometer.     

Sensitive ultrasonic vibrometer for very low frequency applications

Ultrasonic measurement of distance is a well-known low cost method but only a few vibrometers have been developed because sensitivity, spatial resolution, and bandwidth are not high or wide enough for standard laboratory applications. Nevertheless, compared to optical vibrometers, two interesting properties should be considered: very low frequency noise (0.1 Hz to 1 kHz) is reduced and the long wavelength enables rough surfaces to be investigated. Moreover, the ultrasonic probe is a differential sensor, without being a mechanical load for the vibrating structure as usual accelerometers based on contacting transducers are. The main specificity of the presented probe is its ultralow noise electronics including a 3/2 order phase locked loop which extracts the phase modulation related to the amplitude of the detected vibration. This article presents the main useful physical aspects and details of the actual probe. The given application is the measurement of the vibration of an isolated optical bench excited at very low frequency with an electromagnetic transducer.     

三维光电振动测量技术的校准方法研究

为了分析三维光电振动测量仪测量的三维数据准确性,提出了一种利用标准小球做相对校准的简便方法。组成三维光电振动测量仪的5台激光测振仪发出的5束激光同时经过1个透镜,聚焦在位于透镜焦点处且安装在振动源上的标准小球球面上,软件处理小球振动时的三维分量,并与小球3个正交方向放置的3台激光测振仪测得的数据对比。通过微调小球的摆放位置,测量小球不同振动方向的三维振动信息,最后对数据进行误差分析,测量结果验证了该校准方法的可行性和适用性。     

A new multiprobe method of roundness measurements

This paper presents a new multiprobe method for roundness measurements called the mixed method. In this method, displacements at two points on a cylindrical workpiece and an angle at one of the two points are simultaneously monitored by two probes. The differential output of the probes cancels the effect of the spindle error, and deconvolving the differential data yields the correct roundness error. The mixed method is compared to the traditional 3-point method with respect to the transfer function and resolution. Unlike the 3-point method, the mixed method can completely separate the roundness error and the spindle error, and can measure high-frequency components regardless of the probe distance. Resolution can also be improved throughout the entire frequency domain by increasing angular separation of the probes. An optical sensor specifically suited to the mixed method is designed and used to make roundness measurements. A fiber coupler and single-mode fibers are used in the sensor to divide a light beam from a laser diode into two beams, resulting in a compact sensor with good thermal drift characteristics. The displacement meter of the sensor is based on the imaging system principle and has a resolution of 0.1 μm. The angle meter is based on the principle of autocollimation and has a resolution of 0.5 in. A measurement system is constructed to realize measurements of roundness by using the optical sensor. Experimental results confirming the effectiveness of the mixed method for roundness measurements are also presented in this paper.     

A new method for processing impact excited continuous-scan laser Doppler vibrometer measurements

A scanning laser Doppler vibrometer (LDV) can acquire non-contact vibration measurements from a structure with high spatial detail in an automated manner; one only need redirect the laser via computer-controlled mirrors to acquire measurements at additional points. However, since most LDV systems are only capable of measuring one point at a time, conventional scanning vibrometry cannot be effectively employed in some situations, for example when the time record is long at each measurement point or when the structure changes with time. Conventional scanning LDV systems are also difficult to employ with impact excitation because there is considerable variation in the impact location, angle and the character of the impacts, which leads to errors in the mode shapes that are extracted from the measurements. This paper presents a method by which one can determine the mode shapes, natural frequencies and damping ratios of a structure from as little as one response record by sweeping the laser continuously over the vibrating structure as the measurement is acquired. A novel resampling approach is presented that transforms the continuous-scan measurements into pseudo-frequency response functions, so they can be processed using standard identification routines to find the modal parameters of the structure. Specifically, this work employs a standard multi-input–multi-output identification routine and the complex mode indicator function to the continuous-scan laser Doppler vibrometry (CSLDV) measurements. The method makes no assumptions regarding the shape or properties of the surface and only requires that the laser scan periodically and that the structure vibrate freely. The method is demonstrated experimentally on a free–free beam, identifying the first nine mode shapes of the beam at hundreds of points from a few time histories. For this system, this represents a two-order of magnitude reduction in the time needed to acquire measurements with the LDV.     

Analysis of acoustic Doppler current profiler mean velocity measurements in shallow flows

Acoustic Doppler current profilers (ADCPs) are commonly used instruments for measurement of natural streamflow and flow in manmade channels. Velocities measured in a profile by the instrument are used to estimate the discharge in a channel. A Teledyne RD Instruments StreamPro ADCP was used to measure the mean velocity simultaneously with a laser Doppler anemometer (LDA) in a laboratory flume. An average of 3.9% under-prediction of the mean velocity measured by the ADCP occurred when compared to the measurements of the LDA. Moreover, this study shows that the sampling duration of the measurements significantly impacts the mean point velocities measured by up to 50%.     

High-resolution, very broadband force measurements by solid-state laser transducers

Novel high-performance force transducers are discussed. Force measurement is based on the photoelastic effect in solid-state lasers (diode-pumped Nd:YAG) and provides an electrical frequency output. Our experiments demonstrate that the dynamic range of a single monolithic laser transducer covers at least 9 decades in which the output frequency is strictly proportional to the input force magnitude. Within the measurement bandwidth from DC up to at least 100 kHz force modulation frequency, static sensitivity of the transducer equals its dynamic sensitivity. Furthermore, input signals with modulation frequencies up to the MHz range can also be detected. The total measurement range covered by the applied laser technology ranges from nano-newton to mega-newton depending on the size of the laser crystals.     

A versatile stereo photogrammetry based technique for measuring fracture mode displacements in structures

The measurement of fracture mode displacements in structures which are susceptible to cracking such as adhesive joints in composite components – is becoming increasingly important. Such measurements are essential for the understanding of the root causes for specific fracture damage types. Furthermore, they can be used to assess the remaining life span of a structure for its safe operation. An improved version of a previously devised small displacement measurement system (SDMS) is used to measure local relative displacements (LRDs) at the trailing edge of a wind turbine blade. A purpose-made automated image processing software (AIPS) allows a rapid and reliable evaluation of a multitude of subsequently taken measurements at a high-precision level. The SDMS is used to measure the LRDs at three different locations close to the trailing edge of a wind turbine rotor blade. In addition, complementary measurements obtained by linear transducers are compared with the associated LRD component obtained by the SDMS. The 3D LRD measurements showed to be in good agreement with the predictions of non-linear finite element analysis. The paper closes with a brief discussion of the proposed measurement approach and the nature of LRDs as they appear in close vicinity to trailing edge joints.     

Water jet velocity uncertainty in laser Doppler velocimetry measurements

The present work deals with the uncertainty evaluation in water jet velocity measurements carried out by means of a laser Doppler dual-incident-beam velocimeter in reference-beam configuration developed at the WJLab (Water Jet Laboratory of Dipartimento di Meccanica of Politecnico di Milano). The applied experimental procedure makes it possible to calculate the measurement uncertainty through the determination of its various components. Once uncertainty is known, the laser Doppler system is suitable for objective and significant velocity evaluations but also for improvements allowed by the knowledge of the most effective uncertainty sources. Such a subject is typically not considered by the specific water jet literature, but is becoming more and more important due to the evolution of water jet machining towards high precision applications.