Continuous Scanning Laser Doppler Research Articles

An adaptive continuous scanning laser Doppler vibrometry technique for measuring vibrational mode shapes of structures with holes

Abstract Continuous Scanning Laser Doppler Vibrometry (CSLDV) enables fast and full-field vibration measurement in an intact area of a structure. This paper further proposes a novel adaptive CSLDV method that can obtain the Operational Deflection Shapes (ODS) and mode shape of structures with holes. Firstly, an adaptive path planning method is applied to the surface of the tested structure, which can automatically adapt to the size and position of round and square holes, as well as the resolution requirements for the measurement. Secondly, based on the symmetry and other characteristics of the planned path, the time domain signal is processed by delay optimization and demodulation to obtain the ODS of each vibration mode. Finally, a modal test is conducted on a flat structure with a square and circle holes as an example. A validation experiment is also performed using SLDV. The results show that the mode shapes obtained from both methods are consistent and the Modal Assurance Criterion (MAC) between them are all above 0.96. The method can realize CSLDV of flat plate structures with arbitrary square and round holes, and has the advantages of high efficiency and dense measurement points, which enhances its engineering applicability.

Operational Deflection Shape Measurements on Bladed Disks with Continuous Scanning Laser Doppler Vibrometry.

The continuous scanning laser Doppler vibrometry (CSLDV) technique is usually used to evaluate the vibration operational deflection shapes (ODSs) of structures with continuous surfaces. In this paper, an extended CSLDV is demonstrated to measure the non-continuous surface of the bladed disk and to obtain the ODS efficiently. For a bladed disk, the blades are uniformly distributed on a given disk. Although the ODS of each blade can be derived from its response data along the scanning path with CSLDV, the relative vibration direction between different blades cannot be determined from those data. Therefore, it is difficult to reconstruct the complete vibration mode of the whole blade disk. In order to measure the complete ODS of the bladed disk, a method based on ODS frequency response functions (ODS FRFs) has been proposed. While the ODS of each blade is measured by designing the suitable scanning paths in CSLDV, an additional response signal is obtained at a fixed point as the reference signal to identify the relative vibration phase between the blade and the blade of the bladed disk. Finally, a measurement is performed with a simple bladed disk and the results demonstrate the feasibility and effectiveness of the proposed extended CSLDV method.

Application of Time Synchronous Averaging in Mitigating UAV Noise and Signal Loss for Continuous Scanning Laser Doppler Vibrometry

The laser Doppler vibrometer (LDV) has been shown to be effective for a wide application of vibration assessments that are well accepted. One of the new avenues for exploring alternative measurement scenarios, mounting LDVs on unmanned aerial vehicles (UAVs) is emerging as a potential avenue for remote and harsh environment measurements. Such configurations grapple with the challenge of the LDV sensor head being sensitive to UAV vibration during flight and signal loss due to tracking error. This study investigates the effectiveness of several Time Synchronous Averaging (TSA) techniques to circumvent these obstacles. Through comprehensive evaluations, all three TSA techniques under investigation demonstrated significant potential in suppressing UAV-induced noise and minimising the effects of signal dropout. Traditional TSA showcased a remarkable sixfold enhancement in signal quality when analysed via the mean square error. However, the study also highlighted that while TSA and Multi-Cycle Time Synchronous Average (MCTSA) elevated signal clarity, there is a trade-off between noise suppression and signal duration. Additionally, the findings emphasise the importance of synchronisation between scanning and target vibration. To achieve optimal results in Continuous Scanning Laser Doppler Vibrometer measurements, there is a need for advanced algorithms capable of estimating target vibration and synchronising scanning in real-time. As the study was rooted in steady-state vibrations, future research should explore transient vibration scenarios, thereby broadening the application scope of TSA techniques in UAV-mounted LDV systems.

Spatial Vibration and Displacement Measurement Through Single-Point Continuous and Interval Scanning Laser Doppler Vibrometer

The scanning laser Doppler vibrometer (SLDV) is an extremely valuable method for studying the dynamic properties of vibrating objects, such as those in industrial production, entomology research, and biomedical testing. The traditional SLDV method scans samples using an inclined laser, which ignores the three-dimensional (3-D) information of the sample, resulting in a relatively complicated calculation process. In industrial processes and scientific research, the measured target may have a known 3-D regular surface. When a structure with height information moves laterally, it causes a change in longitudinal information, which is of great benefit in the analysis of the 3-D vibration and displacement information of the target. In this article, continuous and interval scanning laser Doppler methods are proposed to expand the measuring capability of SLDV methods, based on the scanning of spherical structures. First, a spherical structure is introduced to simplify the solution to spiral vibrations. The high-precision displacement can also be calculated by fitting the circular scanning curve. Finally, the scanning method for 3-D displacement measurement is experimentally validated, and the theoretical errors are demonstrated. This work provides a new methodology to realize both periodic and nonperiodic displacements in all degrees of freedom (DOF) with high accuracy using a single-point SLDV, which is expected to have a long-term impact on industry vibration analysis and biological motion research. Most importantly, setting a reflector with a spherical surface on the end-effector of the micro–nano robot can realize the 3-D cross-scale repeated localization.

Selection of optimal exciting frequency and Lamb wave mode for detecting wall-thinning in plates using scanning laser Doppler vibrometer

The continuous excitation-based scanning laser Doppler vibrometer (SLDV) has been gaining prominence as a defect-detecting tool because of its superior performance in discovering faults, such as thinning of structures, compared to conventional non-destructive evaluation methods. To date, the optimal test mode and frequency for interdigital transducers have been heuristically selected. Although this approach is effective for problems involving a small number of modes, heuristic algorithms have limitations with respect to problems involving multiple complex modes. Therefore, in this study, the determination of the inspection mode and frequency is formulated as an optimization problem, and the effective wavenumber and bandwidth of the high-pass and bandpass filters in signal processing are also considered. Numerical optimization examples show that the proposed method is effective and robust. An experiment with a thick plate with shallow defects was performed, and the quantitative images obtained showed the correct location of the defects with the proposed numerical optimal solution.

The CSLDV Technique for Vibration Measurement of a Plate Structure with Arbitrary Rectangular Holes

The Continuous Scanning Laser Doppler Vibrometry (CSLDV) developed on the base of the galvanometer scanner system has made it possible to quickly obtain the full-field vibration responses within an intact structure without any hole. In order to make CSLDV also suitable for testing structures with arbitrary rectangular holes, a novel method of area-partitioned continuous scanning test and mode reconstruction is further proposed in this paper. In the first step, the surface of a flat plate structure with arbitrary rectangular holes is divided into several rectangular sub regions by an appropriate area-partitioned method, and the operational deflection shape(ODS) of each sub region is obtained by a continuous scanning at constant speed. Then, considering that the ODSs of all sub regions are spliced and the relative phases among them are different, the operational deflection shape of the whole structure is reconstructed based on linear function and other process. Finally, a plate structure with two arbitrary rectangular holes was taken as an example for modal test using the proposed method. At the same time, a validated experiment was performed using the SLDV measurement. The results show the mode shapes derived from the extended CSLDV are in agreement with those from SLDV and the Modal Assurance Criterion (MAC) between the two are all greater than 0.95. This method has great potential in practical engineering applications with the advantages of high efficiency and the full-field measurement.

Extension of continuous scanning laser Doppler vibrometry measurement for complex structures with curved surfaces

The continuous Scanning Laser Doppler Vibrometry (SLDV) developed on the base of the galvanometer scanner system has made it possible to quickly obtain the full field vibration responses within a rectangular area of the structure. In this paper, an arbitrary continuous scanning path generating method for Continuous Scanning Laser Doppler Vibometry (CSLDV) is further put forward in order to allow the CSLDV suitable for testing structures featured by complex shapes not just for regular areas. In the first step, the relationship between position of laser spot and the driving voltages of galvanometer scanner system has been described by a mathematical modeling. Then, a novel arbitrary scanning path generating strategy based on CSLDV is presented by deforming a normalization rectangular scanning path to an arbitrary continuous scanning path. The mapping relation between the normalization rectangular scanning path and arbitrary continuous scanning path is established using the reference points. In the second step, a compressor blade with curved surface was taken as an example for modal test using the proposed method. At the same time, a validated experiment was performed in SLDV. The results show the mode shapes derived from the extended CSLDV are in agreement with those from SLDV and the Modal Assurance Criterion (MAC) between the two are all greater than 0.96. They also demonstrate the feasibility and effectiveness of the proposed method for CSLDV test and show strong potential on further practical engineering applications.

Mode matching of Continuous Scanning Laser Doppler Vibration data in the frequency domain

Applications as structural diagnostics, condition monitoring and fatigue testing are requiring the development of vibration tests characterized by reduced testing time, fine spatial resolution and high Signal to Noise Ratio (SNR). In this context, Continuous Scanning Laser Doppler Vibrometry (CSLDV) can have a great impact as a substitute of classic Discrete Scanning Laser Doppler Vibrometry (SLDV). In fact, CSLDV makes it possible to measure the target structural vibration much faster and with finer spatial resolution than SLDV, as well keeping an acceptable level of SNR. CSLDV joins together the spatial and time information, because the vibration datum obtained from the laser, which continuously scans (over time and space) the structure under test, is modulated by the Operational Deflection Shape (ODS) excited during the experiment. This results in a spectrum characterized by sideband patterns uniquely associated to the ODS excited. However, the current drawback in fully exploiting CSLDV in everyday testing is related to the necessity of being managed by an expert operator who knows how to extract meaningful information from data measured. This paper proposes a procedure which aims to automatize the information extraction process from CSLDV signals, in order to ease the utilization of CSLDV in vibration laboratories. The idea starts from a simple observation: if the mode shapes of the structure under test are known a priori, e.g. from a numerical model, an analytical formulation or previous measurements, as is the case for fatigue tests, it is possible to settle a procedure that searches for similarities between those known mode shapes (the candidate mode shapes) and ODSs that actually modulate the signal measured. This procedure can therefore be considered a pattern matching technique that is able to identify the resonance frequency related to each ODS and the mode shapes that better match with the ODSs excited. A detailed description of the algorithm is given in this paper. Moreover, the procedure is analyzed in order to discuss its sensitivity to noise, overlapping of resonance frequencies (close modes situation) and ODS complexity. The application of the approach to experimental data is also discussed.

Delamination detection by Multi-Level Wavelet Processing of Continuous Scanning Laser Doppler Vibrometry data

A novel non-destructive testing procedure for delamination detection based on the exploitation of the simultaneous time and spatial sampling provided by Continuous Scanning Laser Doppler Vibrometry (CSLDV) and the feature extraction capability of Multi-Level wavelet-based processing is presented in this paper. The processing procedure consists in a multi-step approach. Once the optimal mother-wavelet is selected as the one maximizing the Energy to Shannon Entropy Ratio criterion among the mother-wavelet space, a pruning operation aiming at identifying the best combination of nodes inside the full-binary tree given by Wavelet Packet Decomposition (WPD) is performed. The pruning algorithm exploits, in double step way, a measure of the randomness of the point pattern distribution on the damage map space with an analysis of the energy concentration of the wavelet coefficients on those nodes provided by the first pruning operation. A combination of the point pattern distributions provided by each node of the ensemble node set from the pruning algorithm allows for setting a Damage Reliability Index associated to the final damage map. The effectiveness of the whole approach is proven on both simulated and real test cases. A sensitivity analysis related to the influence of noise on the CSLDV signal provided to the algorithm is also discussed, showing that the processing developed is robust enough to measurement noise. The method is promising: damages are well identified on different materials and for different damage-structure varieties.

Exploiting Continuous Scanning Laser Doppler Vibrometry in timing belt dynamic characterisation

Dynamic behaviour of timing belts has always interested the engineering community over the years. Nowadays, there are several numerical methods to predict the dynamics of these systems. However, the tuning of such models by experimental approaches still represents an issue: an accurate characterisation does require a measurement in operating conditions since the belt mounting condition might severely affect its dynamic behaviour. Moreover, since the belt is constantly moving during running conditions, non-contact measurement methods are needed. Laser Doppler Vibrometry (LDV) and imaging techniques do represent valid candidates for this purpose.This paper aims at describing the use of Continuous Scanning LDV (CSLDV) as a tool for the dynamic characterisation of timing belts in IC (Internal Combustion) engines (cylinder head). The high-spatial resolution data that can be collected in short testing time makes CSLDV highly suitable for such application. The measurement on a moving surface, however, represents a challenge for CSLDV. The paper discusses how the belt in-plane speed influences CSLDV signal and how an order-based multi-harmonic excitation might affect the recovery of Operational Deflection Shapes in a CSLDV test. A comparison with a standard Discrete Scanning LDV measurement is also given in order to show that a CSLDV test, if well designed, can indeed provide the same amount of information in a drastically reduced amount of time.

Re-sampling of continuous scanning LDV data for ODS extraction

The paper presents a method for processing data obtained by Continuous Scanning Laser Doppler Vibrometry (CSLDV) that makes it possible to measure vibrations combining spatial and temporal information. Since the laser continuously scans the structure under test over time and space, it measures a vibration signal modulated by the Operational Deflection Shape (ODS) of the structure. CSLDV time history can then be re–sampled at an arbitrary spatial resolution in order to recover the vibration data at several positions on the trajectory swept by the laser. The main drawback of such method is that the actual sampling rate of the re–sampled vibration signals is dictated by frequencies of the mirrors, which determine the laser scanning speed. The paper presents a method for the automatic recognition of ”aliased” natural frequencies and recovery of the actual un–aliased natural frequencies. Once those frequencies are determined, the corresponding ODSs are recovered.

Wavelet Processing of Continuous Scanning Laser Doppler Vibrometry data in Non-Destructive Testing

The present paper proposes a novel non-destructive testing procedure based on the exploitation of the simultaneous time and spatial sampling provided by Continuous Scanning Laser Doppler Vibrometry (CSLDV) and the feature extraction capabilities of wavelet-based processing. Two criteria for selecting in an objective way the mother-wavelet to be used in the decomposition procedure, the Relative Wavelet Energy and Energy to Shannon Entropy Ratio, are compared in terms of capability of best locating the damage. The paper demonstrates the applicability of the procedure for the identification of superficial and in-depth defects in simulated and real test cases when an area scan is performed over the test sample. The method shows promising results, since defects are identified in different severity conditions.

Multi-frequency, 3D ODS measurement by continuous scan laser Doppler vibrometry

Continuous scan laser Doppler vibrometry (CSLDV) is a technique which has been described and explored in the literature for over two decades, but remains niche compared to SLDV inspection by a series of discrete-point measurements. This is in part because of the unavoidable phenomenon of laser speckle, which deteriorates signal quality when velocity data is captured from a moving spot measurement. Further, applicability of CSLDV has typically been limited to line scans and rectangular areas by the application of sine, step, or ramp functions to the scanning mirrors which control the location of the measurement laser spot. In this paper it is shown that arbitrary functions to scan any area can easily be derived from a basic calibration routine, equivalent to the calibration performed in conventional discrete-point laser vibrometry. This is extended by performing the same scan path upon a test surface from three independent locations of the laser head, and decomposing the three sets of one-dimensional deflection shapes into a single set of three-dimensional deflection shapes. The test was performed with multi-sine excitation, yielding 34 operating deflection shapes from each scan.

Exploiting continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation methods for noise source identification

This paper proposes the use of continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation techniques that aim at characterizing the structure-borne contributions of the noise emission of a mechanical system. The time domain correlation technique presented in this paper is based on the use of FIR (finite impulse response) filters obtained from the vibro-acoustic transfer matrix when vibration data are collected by laser Doppler vibrometry (LDV) exploited in continuous scan mode (CSLDV). The advantages, especially in terms of source decorrelation capabilities, related to the use of CSLDV for such purpose, with respect to standard discrete scan (SLDV), are discussed throughout the paper. To validate this approach, vibro-acoustic measurements were performed on a planetary gear motor for home appliances. The analysis of results is also supported by a simulation.

Laser Doppler Vibrometry for Structural Dynamic Characterization of Rotating Machinery

Laser Doppler Vibrometry (LDV) is a well established technique able to accurately measure vibration velocity of any kind of structure in remote, i.e. non-intrusive way, this allowing to overcome the problem of mass loading, typical of contact sensors as accelerometers and strain-gauges, which has strong influence in case of lightweight structures. Moreover, the possibility of driving automatically the laser beam, by means of moving mirrors controlled with galvanometer servo-actuators, permits to perform scanning measurements at different locations with high spatial resolution and reduced testing time and easily measure the operational deflection shapes (ODS) of the scanned surface. The exploitation of the moving mirrors has allowed to drive the laser beam in a continuous way making it to scan continuously over the structure surface and cover it completely. This way of operation, named Continuous Scanning LDV, permits to perform full-field measurements, the LDV output carrying simultaneously the time-and spatial-dependent information related to the structural vibration. A complementary strategy making use of the LDV coupled with moving mirrors is the so called Tracking LDV, where the laser beam is driven to follow a moving object whose trajectory must be known a priori or measured during operation (e.g. via an encoder in the case of rotating structures). In this paper some applications of the Tracking Laser Doppler Vibrometry (TLDV) and Continuous Scanning Laser Doppler Vibrometry (CSLDV) will be described they concerning, specifically modal and vibrational analysis of rotating structures.

Six degrees of freedom measurement with continuous scanning laser doppler vibrometer

The dynamic response of a structure can consist of up to six degrees of freedom (DOF). Not all of them may be present at all times in a structure, but for a detailed finite element model validation it is important to provide as much information as possible. Current contact transducers can pick up a combination of some of the mentioned DOFs, but are not able to provide all six DOFs from a single transducer. Several contact-less transducers, such as a Scanning Laser Doppler Vibrometer (SLDV), can be combined to a single measurement system that allows the determination of up to three translational degrees of freedom at a point, but they are still not providing rotational information as the laser beam is fixed to a point. Continuously Scanning Laser Doppler Vibrometer (CSLDV) methods can partly overcome this issue by travelling in small circles around the measurement location and thus also sensing the rotation around the three axes at a point. An introduction of CSLDV methods for measuring DOFs at a point is given, focussing on a method for capturing up to 5 DOFs using a lens. However, the lens can make the measurement of the in-plane rotation very difficult and impractical. This paper presents a new type of scanning device which allows measuring all six degrees of freedom at a measurement location with a single setup and a single point LDV, by combining variable small circle scans. Experimental validation of the scanning head is presented and the measurement of the last DOF, the in-plane rotation, is demonstrated.

Surface wave measurements using a single continuously scanning laser Doppler vibrometer: Application to elastography

A continuous scanning laser Doppler vibrometry (CSLDV) obtained sweeping a single laser beam along a periodic scan pattern allows measuring surface vibrations at many points simultaneously by demultiplexing the CSLDV signal. This known method fundamentally differs from conventional scanning laser vibrometry techniques in which the laser beam is kept at a fixed point during each measurement and then moved to a new position prior to the next measurement. This article demonstrates the use of a CSLDV for measuring in a non-contact fashion the velocity of low-frequency surface waves (f < 100 Hz) propagating over soft materials, namely here gel surfaces-mimicking human body soft tissues-and skeletal muscles, to develop an affordable and noninvasive elastography modality. The CSLDV vibration measurements obtained with a single laser beam, linearly scanned over the test surface at 200 Hz over lengths up to 6 cm, were validated using an array of three fixed laser Doppler vibrometers distributed along the same scan line. Furthermore, this CSLDV setup was used to measure the increase in surface wave velocity over the biceps brachii muscle which was directly correlated to the actual stiffening of the biceps occurring while a subject was performing voluntary contractions at an increasing level.

Broadband measurement of translational and angular vibrations using a single continuously scanning laser Doppler vibrometer

A continuous scanning laser Doppler velocimetry (CSLDV) technique is used to measure the low frequency broadband vibrations associated with human skeletal muscle vibrations (typically f < 100 Hz) by continuously varying the orientation of laser beam over distances that are short compared to the characteristic wavelengths of the vibrations. The high frequency scan (compared to the vibration frequency) enables the detection of broadband translational and angular velocities at a single point using amplitude demodulation of the CSDLV signal. For instance, linear scans allow measurement of the normal surface velocity and one component of angular velocity vector, while circular scans allow measurement of an additional angular velocity component. This CSLDV technique is first validated here using gel samples mimicking soft tissues and then applied to measure multiple degrees of freedom (DOF) of a subject's hand exhibiting fatigue-induced tremor. Hence this CSLDV technique potentially provides a means for measuring multiple DOF of small human body parts (e.g., fingers, tendons, small muscles) for various applications (e.g., haptic technology, remote surgery) when the use of skin-mounted sensors (e.g. accelerometers) can be problematic due to mass-loading artifacts or tethering issues.

Performance analysis of continuous tracking laser Doppler vibrometry applied to rotating structures in coast-down

In this paper a performance analysis of the so-called tracking continuous scanning laser Doppler vibrometry (TCSLDV) exploited in coast-down has been performed. This non-contact measurement system is able to scan continuously over a rotating surface during coast-down and to determine vibration operational deflection shapes (ODSs) and natural frequencies in short time, i.e. the temporal extent of the coast-down. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the whole rotor surface synchronously with its rotation, so that the LDV output is modulated by the structure's ODSs. This technique has a full-field nature that enables it to measure simultaneously the time and spatial dependence of the vibration in a unique measurement. However, the TCSLDV presents some criticalities in practical applications, especially when applied to rotary transient and fast processes. In fact, if the vibration is transient and decays very fast, then the laser beam could not have had the time to scan the complete structure surface and the modulation of the ODS could be partial. An analytical model reproducing a representative experiment has been developed in order to evaluate the sensitivity of results to testing conditions. The laser beam trajectory in both the fixed and rotating reference systems has been synthesized showing its dependence on experimental parameters as the rotation speed variation during coast-down. It has been demonstrated the decrease in speed induces the deformation of the laser trajectory influencing the LDV output time history, spectrum and consequently the recovered ODS.

Investigation of continuous scanning laser Doppler vibrometry for non-contact measurement of linear and angular surface deflections.

Laser Doppler vibrometry (LDV) is a non-contact technique for sensing surface vibrations. Traditionally LDV uses one or several fixed beams to measure vibrational velocity of specific points and orientations. Measurement of angular velocity requires at least two beams. Instead, we develop a technique, continuous scanning laser Doppler vibrometry (CSLDV), using a single laser beam continuously sweeping the area of interest using a scanning mirror. Linear scans allow the measurement of normal and angular velocities while circular scans allow the measurement of normal and two angular velocities. We validated the CSLDV technique for measuring low frequency (less than 50 Hz) broadband vibrations of gel samples which mimics natural vibration of human body. Such system could potentially be used to monitor multiple DOF of the skin surface for haptic or remote control applications. There is a disadvantage of using CSLDV which is speckle noise that is generated when coherent light source is reflected back from an optically rough surface. We will discuss the effects of scan lengths, scanning frequency, target to sensor distance, and the excitation amplitude on the performance of CSLDV.