Laser Vibrometry Measurements Research Articles

PCB-based miniature vibro-tactile display for the visually impaired

An estimated 2.2 billion people worldwide are visually impaired (WHO, 2019), yet current Braille readers are limited to one or two rows of text and cannot display images. Thus, a low-cost, electronically refreshable tactile display for accessing graphical and textual information is needed. We are developing such a display using a printed circuit board (PCB) as the substrate and bottom electrode array, and a metalized Kapton film as the vibrating membrane. Punched foam tape is used as a spacer to create a gap between the board and film so the pixels can be actuated electrostatically. The current prototype has 6 pixels at 3.5 mm spacing, Vibration amplitudes of 8 microns peak-to-peak were achieved in a prototype using a drive voltage of 600V at 200 Hz. This is sensible to the human touch. Further optimization requires matching the mechanical impedance of the finger, and maximizing vibration amplitude in the preferred sensible frequency range between 50 and 300 Hz. Finite element analysis (FEA), laser vibrometry, and nano-newton force-displacement measurements have been used to characterize the system. Future versions will increase amplitude, reduce drive voltages, and increase display resolution to larger arrays of more closely spaced pixels through further miniaturization.

Experimental setup for laser vibrometry measurements of the vibrating horn in Ultrasonic Metal Welding

Ultrasonic Metal Welding is a type of welding used in the production of electric car batteries. Currently, it suffers from quality fluctuations, and there is a need for online quality control techniques. One technique being investigated involves the measurement of the vibrations of the welding horn using laser vibrometry. However, this comes with two obstacles. First, measurements close to the welding site, which contain the most information, are difficult to do. This is due to the interference of particles ejected during welding, which interfere with the laser beam. Second, it is possible that the use of reflective tape for vibrometry changes the vibrations measured, due to heating of the horn during welding. This work investigates these problems and presents some solutions. The effects of the reflective tape on the measurement is investigated, by measuring the vibrations of the horn during welding and free run, with and without reflective tape, and with and without prior heating of the tape. Furthermore, the interference of ejected particles is minimized using a special experimental setup, and measurements are done without any reflecting tape.

Improved thermoacoustic sound projectors by vibration mode modification

The problem of coupled panel-cavity vibrations is addressed by studying the resonance modes in high power, low frequency, encapsulated thermoacoustic sound projectors using scanning laser vibrometry and sound pressure measurements. We fabricated and analyzed the performance of large area thermoacoustic sound projectors based on freestanding carbon nanotube sheets, encapsulated between a heat sink and various rigid vibrating plates. For thin, simply-supported vibrating plates we observed a strong deviation of resonance frequency and sound intensity from theoretical prediction for in vacuo plates. The domination of symmetric, odd modes for thin plates, with substantially improved sound radiation, is attributed to the coupled panel-cavity modes produced by harmonically varying pressure within the cavity. The observed modified mode shapes result from a minimum energy principle, wherein the potential energy stored in the entrained gas is minimized by reducing the overall change of the cavity gas volume. The effect of plate and cavity thickness on thermoacoustic projector performance in air is described for various devices. The optimized ultralight, low volume sound projectors can generate a remarkably high sound pressure level in air of over 120 dB re 20 μPa @ 1 m in a frequency range of 500 - 3000 Hz.

Superposition model of mode shapes composed of travelling torsional guided waves excited by multiple circular transducer arrays in pipes

In pipe inspection using ultrasonic guided wave technique, the current commercial transmitters are designed for the unidirectional guided wave excitation using multiple circular piezoelectric transducers arrays in the axial direction. However, the source with many individual transducer elements in arrays has difficulty in achieving an axisymmetric loading perfectly for defect detection. Therefore, a quasi-axisymmetric wave is formed due to many undesired wave modes are launched instead of a pure axisymmetric wave at a given excitation frequency. In this paper, a realistic superposition model of axial multiple transducer arrays is proposed. The model has many potential applications; one example is investigating the source influence on the generated quasi-axisymmetric wave effect. The analytical model is employed to achieve the predictions for investigating a transmitter’s influence for the unidirectional enhancement of torsional T(0,1) guided wave mode excitation in a pipe inspection system composing of three piezoelectric transducer ring arrays. The excitation function with variable power levels among transducers in arrays is also introduced. The predictive results using the analytical model for the distribution of circumferential displacement amplitudes over time are verified using the finite element method and a CLV-3D laser vibrometry measurement on a 219.1-mm-outer-diameter steel pipe without defect. A comparison between calculated and test results has been analysed quantitatively. The respective results are in good agreement. Thus, predictions for the superposed wavefield can be used to analyse the realistic characterisation of the excitation function in axial multiple transducer arrays. Additionally, a sensitivity analysis for part-circumferential crack detection using the quasi-axisymmetric torsional modes generated is also evaluated using finite element modelling.

Remote vibrometry recognition of nonlinear eigen-states for object coverage of randomly large size

For objects of “large” vibration size such as waves on the sea surface, the choice of measurement method can create different understandings of system behavior. In one case, laser vibrometry measurements of a vibrating bar in a controlled laboratory setting, variation in probe spot size can omit or uncover crucial structural vibration mode coupling data. In another case, a finite element simulation of laser vibrometry measures a nonlinearly clattering armor plate system of a ground vehicle. The simulation shows that sensing the system dynamics simultaneously over the entire structure reveals more vibration data than point measurements using a small diameter laser beam spot, regardless of the variation of footprint (coverage) boundaries. Furthermore, a simulation method described herein allows calculation of transition probabilities between modes (change-of-state). Wideband results of both cases demonstrate the 1/f trend explained within – that the energy of discrete structural vibration modes tends to decrease with increasing mode number (and frequency), and why. These results quantify the use of less expensive non-imaging classification systems for vehicle identification using the remote sensing of surface vibrations while mitigating spectral response distortion due to coverage variation on the order of the structural wavelength (spectral reduction or elimination).

Incremental scattering of the A0 Lamb wave mode from a notch emanating from a through-hole

Lamb wave scattering from a crack originating at a through-hole is of practical importance because of the abundance of fastener holes used in engineering structures. Notches are often used to simulate cracks so that Lamb wave methods can be more conveniently investigated in the laboratory. A linear, three-dimensional finite element model is employed in this paper to study incremental scattering of the fundamental anti-symmetric (A0) Lamb wave mode from notches emanating from through-holes. The term “incremental scattering” refers to the change in scattering caused by introduction of the notch and is motivated by structural health monitoring for which transducers are fixed and signal changes are interpreted to detect damage. Far-field angular scattering patterns are generated for multiple incident angles and frequencies, and such patterns are experimentally validated at one frequency by laser vibrometry measurements. Comparisons are made between a vertical notch alone (no hole) and notches located above and below the through-hole. Additionally, holes of different sizes are considered to investigate the effect of hole diameter on incremental scattering patterns. Results show that the presence, location and size of the through-hole affect both the shape and strength of notch incremental scattering patterns.

Nonlinear elastic imaging of barely visible impact damage in composite structures using a constructive nonlinear array sweep technique

Linear and nonlinear ultrasound imaging methods highlight different damage features: the linear method detects large stiffness changes, while the nonlinear technique identifies small impedance mismatches, such as microcracks or closed delaminations. Typically, nonlinear ultrasound techniques detect damage/defects in materials by measuring higher order harmonics. These harmonics can be difficult to measure due to low magnitude and signal to noise ratios (SNR): hence large excitation amplitudes are needed, which can further complicate the reliability of these methods as equipment nonlinearities can be generated. To overcome these issues, exciting at specific frequencies, known as local defect resonances (LDR), produce a much larger displacements at the damaged regions. However, estimation of LDR is time-consuming, complex and not an easily automated process.A coupled baseline-free linear and nonlinear ultrasonic imaging approach is proposed, using a Constructive Nonlinear Array Sweep excitation and an image subtraction method for identifying damage in layered materials. The signal sweep method uses a narrow band frequency excitation to increase the probability of detection of a LDR frequency. The novel imaging approach was employed using laser vibrometry measurements in various complex composite structures to assess barely visible impact damage, critical for the aircraft industry. The results showed better estimation of impact damage when compared to classical linear or nonlinear ultrasonic methods leading to improved reliability of aircraft inspections.

Minimally invasive laser vibrometry (MIVIB) with a floating mass transducer – A new method for objective evaluation of the middle ear demonstrated on stapes fixation

Ossicular fixation through otosclerosis, chronic otitis media and other pathologies, especially tympanosclerosis, are treated by surgery if hearing aids fail as an alternative. However, the best hearing outcome is often based on knowledge of the degree and location of the fixation. Objective methods to quantify the degree and position of the fixation are largely lacking. Laser vibrometry is a known method to detect ossicular fixation but clinical applicability remains limited. A new method, minimally invasive laser vibrometry (MIVIB), is presented to quantify ossicle mobility using laser vibrometry measurement through the ear canal after elevating the tympanic membrane, thus making the method feasible in minimally invasive explorative surgery. A floating mass transducer provides a clinically relevant transducer to drive ossicular vibration. This device was attached to the manubrium and drove vibrations at the same angle as the longitudinal axis of the stapes and was therefore used to assess ossicular chain mobility in a fresh-frozen temporal bone model with and without stapes fixation. The ratio between the umbo and incus long process was shown to be useful in assessing stapes fixation. The incus-to-umbo velocity ratio decreased by 15 dB when comparing the unfixated situation to stapes fixation up to 2.5 kHz. Such quantification of ossicular fixation using the incus-to-umbo velocity ratio would allow quick and objective analysis of ossicular chain fixations which will assist the surgeon in surgical planning and optimize hearing outcomes.

A model‐based method for damage detection with guided waves

Summary Defect detection techniques, which utilise guided waves, have received significant attention over the past twenty years. Many of these techniques implement the baseline signal subtraction approach for damage diagnosis. In this approach, the baseline signal previously recorded for a defect-free structure is compared with/or subtracted from the actual signal recorded during routine inspections. A significant deviation between these two signals (or residual signal/time-trace) can be treated as an indication of the presence of critical damage. However, the accuracy of this common approach can be compromised by various uncontrolled factors, which include ambient temperature variations, unavoidable inconsistencies in the PZT installation procedure and degradation of mechanical properties with time. This paper presents a new method for reconstruction of the baseline signal, which can compensate for the above influences and improve the accurateness of damage diagnosis. The method utilises 3D laser vibrometry measurements in conjunction with high-fidelity FE simulations. This paper also describes an application of this method to the reconstruction of the baseline signal and detection of damage in beam and plate structures. Copyright © 2016 John Wiley & Sons, Ltd.

Estimation of the in-plane vibrations of a rotating spindle, using out-of-plane laser vibrometry measurements

A method for estimating the in-plane vibrations of a rotating spindle using out-of-plane laser vibrometry measurements is described. This method enables the possibility to obtain the two orthogonal radial vibration components of a rotating spindle. The method uses the fact that the laser vibrometer signal is a total surface velocity of the measurement point in the laser direction.Measurements are conducted on a rotating milling machine spindle. The spindle is excited in a controlled manner by an active magnetic bearing and the response is measured by laser vibrometer in one of the two orthogonal directions and inductive displacement sensors in two orthogonal directions simultaneously. The work shows how the laser vibrometry crosstalk can be used for resolving the in-plane vibration component, that is the vibrations in the laser vibrometer cross direction. The result is compared to independent measurement signals from the displacement sensors.The measurement method can be used for vibration measurements on rotating parts, for example, where there is lack of space for orthogonal measurements.

Hybrid active focusing with adaptive dispersion for higher defect sensitivity in guided wave inspection of cylindrical structures

Ultrasonic guided wave inspection is widely used for scanning prismatic structures such as pipes for metal loss. Recent research has investigated focusing the sound energy into predetermined regions of a pipe in order to enhance the defect sensitivity. This paper presents an active focusing technique which is based on a combination of numerical simulation and time reversal concept. The proposed technique is empirically validated using a 3D laser vibrometry measurement of the focal spot. The defect sensitivity of the proposed technique is compared with conventional active focusing, time reversal focusing and synthetic focusing through an empirically validated finite element parametric study. Based on the results, the proposed technique achieves approximately 10 dB improvement of signal-to-coherent-noise ratio compared to the conventional active focusing and time reversal focusing. It is also demonstrated that the proposed technique to have an amplitude gain of around 5 dB over synthetic focusing for defects <0.5λs. The proposed technique is shown to have the potential to improve the reliably detectable flaw size in guided wave inspection from 9% to less than 1% cross-sectional area loss.

Noise and Vibration Measurements for the Assessment of Damping Efficiency in Power Transformers

Power transformers emit a very characteristic harmonic sound during operation. The sound spectrum consists of the harmonics of double frequency of the power grid with the fundamental of 100Hz or 120Hz in different countries around the world. This fundamental frequency dominates in the sound spectrum during the load condition while the higher harmonics are representative for the no-load operating state. Regardless of loading conditions, all the harmonics build rather unpleasant noise, which is difficult to suppress. Moreover, this noise is sometimes difficult to measure and interpret, due to the harmonic nature and a content of harmonics of relatively low frequencies.This paper presents the exemplary study on the assessment of noise damping efficiency, performed with the use of laser vibrometry and sound intensity measurements, which can provide the most complete information, when it comes to the identification of the sources of noise as well as to the proper evaluation of damping solutions.

A methodology for estimating guided wave scattering patterns from sparse transducer array measurements.

Ultrasonic guided waves are one of the primary methods being investigated for structural health monitoring of plate-like components. A common practice is to collect measurements from a sparse transducer array using the pitch-catch method, which enables interrogation of defects from multiple directions. Thus, knowledge of how guided waves scatter from defects is very useful for detection, localization, and characterization of damage. One way to describe scattering patterns is with a matrix indexed by incident angle and scattered angle, and sparse array measurements essentially sample this matrix. A methodology is proposed in this paper to estimate the complete scattering matrix from these limited array measurements. First, recorded array signals are compensated for geometric spreading loss, wave packet spreading loss, and transducer differences. Initial scattering values are then extracted from the scattered wave packets after baseline subtraction and are augmented using transducer reciprocity and any a priori knowledge of defect geometric symmetry. Finally, radial basis function interpolation is performed on these values to obtain the complete scattering matrix. Scattering matrices are generated from experimental data by cutting notches of different lengths originating from a through-hole in an aluminum plate specimen that is instrumented with a sparse transducer array. The methodology is validated by laser vibrometry measurements performed on a nominally identical specimen for one notch length.

Pilot scale ultrasound enabled plate heat exchanger – Its design and potential to prevent biofouling

A pilot scale 100–150L/h plate heat exchanger (PHE) was retrofitted with ultrasound transducers without changing the main PHE design. The ultrasound enabled PHE (UPHE) and parallel split stream reference PHE (RPHE) were used to investigate the potential of ultrasound to prevent biofouling during processing of cheese milk inoculated with 103CFU/mL Streptococcus thermophilus (TRS). Scanning laser vibrometry measurements showed that, for a 15-plate package, ultrasound conditions of 20kHz and 70W produced vibration magnitudes in the range of 500–1200nm, corresponding to the size of the TRS. In one experiment, application of these ultrasound conditions was found to delay the onset of the increase in product outlet TRS concentration by 3h compared to the RPHE. Application of conditions where only micro-streaming occurred showed no effect on biofouling. The experimental result is a first indication that surface vibrations of the same order magnitude as the adhering microorganism may be a mechanism by which ultrasound can prevent biofouling in plate heat exchangers.

Equivalent circuit modeling and vibrometry measurements of the Nigerian-origin Udu Utar drum

The Udu drum, sometimes called the water pot drum, is a traditional Nigerian instrument. Musicians who play the Udu exploit its aerophone and idiophone resonances. This paper will discuss an electrical equivalent circuit model for the Udu Utar, a modern innovation of the traditional Udu, to predict the low frequency aerophone resonances and will also present scanning laser vibrometer measurements to determine the mode shapes of the dominant idiophone resonances. These analyses not only provide an understanding of the unique sound of the Udu instrument but may also be used by instrument designers to create instruments with resonance frequencies at traditional musical intervals for the various tones produced and to create musical harmonic ratios. The information, specifically the laser vibrometry measurements, may also be useful to musicians in knowing the best places to strike the Udu to excite musical tones.

Generated Vibration Modes in Ultrasonic Scaler Transducer Components

Magnetostrictive materials have been utilized for the production of resonant ultrasonic-frequency vibrations for a number of years. This effect has been utilised in laminated nickel-based ultrasonic dental scaler cleaning systems. Laser vibrometry measurements were undertaken that showed that a typical dental ultrasonic scaler resonated at 29 kHz due to dynamic magnetostriction of the insert's laminated nickel 'stack.' This, in turn, excited insert probe vibrations that are used to remove dental calculus. 3D Femlab™ mechanical finite element models were created to explore the flexural vibration modes associated with the whole insert at the vibration frequency. These models indicated that it was possible to reproduce similar probe vibration mode-shapes to those seen via scanning laser vibrometry measurements. Also, the models indicate that vibrations, useful for dental scaling, can only be achieved if the coupled flexural and longitudinal resonant modes are stimulated at the same drive frequency.

Specialization for underwater hearing by the tympanic middle ear of the turtle, Trachemys scripta elegans.

Turtles, like other amphibious animals, face a trade-off between terrestrial and aquatic hearing. We used laser vibrometry and auditory brainstem responses to measure their sensitivity to vibration stimuli and to airborne versus underwater sound. Turtles are most sensitive to sound underwater, and their sensitivity depends on the large middle ear, which has a compliant tympanic disc attached to the columella. Behind the disc, the middle ear is a large air-filled cavity with a volume of approximately 0.5 ml and a resonance frequency of approximately 500 Hz underwater. Laser vibrometry measurements underwater showed peak vibrations at 500-600 Hz with a maximum of 300 µm s(-1) Pa(-1), approximately 100 times more than the surrounding water. In air, the auditory brainstem response audiogram showed a best sensitivity to sound of 300-500 Hz. Audiograms before and after removing the skin covering reveal that the cartilaginous tympanic disc shows unchanged sensitivity, indicating that the tympanic disc, and not the overlying skin, is the key sound receiver. If air and water thresholds are compared in terms of sound intensity, thresholds in water are approximately 20-30 dB lower than in air. Therefore, this tympanic ear is specialized for underwater hearing, most probably because sound-induced pulsations of the air in the middle ear cavity drive the tympanic disc.

Laser vibrometry measurements of acoustic coupling across alligator eardrums

Laser vibrometry measurements of acoustic coupling across alligator eardrums

Air-coupled linear and sparse cMUT array manufactured using MUMPs process

An assessment of the standard fabrication Micro-Electro-Mechanical Systems (MEMS) process Multi-User MEMS Processes (MUMPs) for complex air-coupled capacitive Micromachined Ultrasonic Transducer array aperture manufacture is reported. A 1-D linear array and a 2-D sparse symmetric binned-array have been designed and manufactured, and then characterised experimentally using electrical impedance measurements, laser vibrometry and air-coupled field measurement; the experimental data are supported by simulated data using Finite Element technique and field simulation based on Huygens’ principle. A methodology for the manufacture of the array structures using the MUMPs process is described. Electrical characterisation shows the devices operation at 770 kHz and the existence of large parasitic capacitances and electrical losses. Mechanical crosstalk of array substrate has been measured at −40 dB using laser vibrometry. Moreover, the laser vibrometry measurement and the field characteristics of one element reveal that each element operates as a piston radiator.

Comparison of modal analysis results of laser vibrometry and nearfield acoustical holography measurements of an aluminum plate.

Two methods for non-contact vibration measurements are compared using modal analysis. Laser vibrometry and nearfield acoustical holography are used to measure mode shapes of an aluminum plate excited by a mechanical shaker with random noise. Laser vibrometry provides a direct measurement of the panel surface velocity, while the nearfield acoustical holography measurements are used to calculate the surface velocity indirectly. Both data sets are compared in frequency, damping, and mode shapes. The results are also compared to finite element and theoretical analyzes. Results are presented from all four approaches.