Piezoelectric Sensor Array Research Articles

GW28-e0890 Validation of a piezoelectric sensor array based device for measurement of carotid-femoral pulse wave velocity: the Philips Prototype

GW28-e0890 Validation of a piezoelectric sensor array based device for measurement of carotid-femoral pulse wave velocity: the Philips Prototype

Acoustic Emission Signatures of Fatigue Damage in Idealized Bevel Gear Spline for Localized Sensing

In many rotating machinery applications, such as helicopters, the splines of an externally-splined steel shaft that emerges from the gearbox engage with the reverse geometry of an internally splined driven shaft for the delivery of power. The splined section of the shaft is a critical and non-redundant element which is prone to cracking due to complex loading conditions. Thus, early detection of flaws is required to prevent catastrophic failures. The acoustic emission (AE) method is a direct way of detecting such active flaws, but its application to detect flaws in a splined shaft in a gearbox is difficult due to the interference of background noise and uncertainty about the effects of the wave propagation path on the received AE signature. Here, to model how AE may detect fault propagation in a hollow cylindrical splined shaft, the splined section is essentially unrolled into a metal plate of the same thickness as the cylinder wall. Spline ridges are cut into this plate, a through-notch is cut perpendicular to the spline to model fatigue crack initiation, and tensile cyclic loading is applied parallel to the spline to propagate the crack. In this paper, the new piezoelectric sensor array is introduced with the purpose of placing them within the gearbox to minimize the wave propagation path. The fatigue crack growth of a notched and flattened gearbox spline component is monitored using a new piezoelectric sensor array and conventional sensors in a laboratory environment with the purpose of developing source models and testing the new sensor performance. The AE data is continuously collected together with strain gauges strategically positioned on the structure. A significant amount of continuous emission due to the plastic deformation accompanied with the crack growth is observed. The frequency spectra of continuous emissions and burst emissions are compared to understand the differences of plastic deformation and sudden crack jump. The correlation of the cumulative AE events at the notch tip and the strain data is used to predict crack growth. The performance of the new sensor array is compared with the conventional AE sensors in terms of signal to noise ratio and the ability to detect fatigue cracking.

On-Line Multi-Damage Scanning Spatial-Wavenumber Filter Based Imaging Method for Aircraft Composite Structure.

Structural health monitoring (SHM) of aircraft composite structure is helpful to increase reliability and reduce maintenance costs. Due to the great effectiveness in distinguishing particular guided wave modes and identifying the propagation direction, the spatial-wavenumber filter technique has emerged as an interesting SHM topic. In this paper, a new scanning spatial-wavenumber filter (SSWF) based imaging method for multiple damages is proposed to conduct on-line monitoring of aircraft composite structures. Firstly, an on-line multi-damage SSWF is established, including the fundamental principle of SSWF for multiple damages based on a linear piezoelectric (PZT) sensor array, and a corresponding wavenumber-time imaging mechanism by using the multi-damage scattering signal. Secondly, through combining the on-line multi-damage SSWF and a PZT 2D cross-shaped array, an image-mapping method is proposed to conduct wavenumber synthesis and convert the two wavenumber-time images obtained by the PZT 2D cross-shaped array to an angle-distance image, from which the multiple damages can be directly recognized and located. In the experimental validation, both simulated multi-damage and real multi-damage introduced by repeated impacts are performed on a composite plate structure. The maximum localization error is less than 2 cm, which shows good performance of the multi-damage imaging method. Compared with the existing spatial-wavenumber filter based damage evaluation methods, the proposed method requires no more than the multi-damage scattering signal and can be performed without depending on any wavenumber modeling or measuring. Besides, this method locates multiple damages by imaging instead of the geometric method, which helps to improve the signal-to-noise ratio. Thus, it can be easily applied to on-line multi-damage monitoring of aircraft composite structures.

A continuous gas leakage localization method based on an improved beamforming algorithm

A continuous gas leakage localization method based on an improved beamforming algorithm is proposed and experimentally demonstrated. The ultrasonic signals excited by the leakage are collected by the contact-coupled piezoelectric ultrasonic sensor array, afterward, the time-space matrix of the leakage signal is calculated, and through analyzing the matrix, the position of the leakage hole can be obtained. The measured phase velocity is taken into account in the method to compensate the effect of frequency dispersion. Localization experiments were carried out on a flat plate and a stiffened plate, demonstrating that the proposed method can successfully locate the continuous gas leakage in both the near and far field cases. In addition, the experiment results indicate that frequency dispersion compensation can reduce the localization error efficiently. The method is demonstrated to achieve the mean errors of 3.74mm and 8.59mm, for an area of 1m×1m on the flat and stiffened plates, respectively.

極薄圧電ひずみセンサを用いたフレキシブルセンサアレイシートの開発

In this paper, we present a novel flexible sensor array manufacturing process that involves transfer printing methods using a chip mounter with a vacuum collet. We successfully transfer-printed continuously very fragile microelectromechanical systems (MEMS)-based 5-mm-long, 1-mm-wide, 5-µm-thick high-aspect-ratio ultra-thin PZT(1.9µm)/Si(3µm) strain sensors onto a polyimide based flexible printed-circuit (FPC) substrate with etched Cu wiring. Then, we connected the sensors to the Cu wiring by printing insulating and conductive pastes using a screen printer. The output voltage based on the deformation behavior of the test plate was generated from the flexible piezoelectric strain sensor array attached to the plate. Therefore, the developed piezoelectric sensor array is capable of easily performing the distribution measurement of the strain leading to damage such as cracks.

Wireless gas sensing based on a passive piezoelectric resonant sensor array through near-field induction

We developed a wireless and passive piezoelectric resonant sensor for contimuous volatile organic compound detection. An equivalent circuit is proposed to model the sensing system, and Lamb wave resonators are adopted to demonstrate the wireless interrogation achieved by near-field inductive coupling. The wireless sensing system is employed to monitor the ethanol vapor concentration, and the sensitivity of the wireless sensor barely degrades compared to that of the wired one. Further, we simultaneously and wirelessly tracked several resonance frequencies of a monolithic sensor array, which demonstrates its potential for high-throughput and real-time point-of-care test.

Damage detection based on sparse virtual element boundary measurement using metal-core piezoelectric fiber

Pseudo-excitation approach is a recently developed vibration-based damage detection method, exhibiting some appealing features for structural health monitoring applications. However, two main bottlenecking problems, that is, dense measurement points and venerable noise immunity, hamper its use in practical applications. This article tackles these problems by proposing a novel method based on sparse virtual element boundary measurement using metal-core piezoelectric fiber sensors. Different from the local “point-by-point” interrogation modality used in the original pseudo-excitation approach, the proposed method divides the entire structure into several virtual elements to construct a damage location index, describing the damage-induced dynamic perturbation in the corresponding virtual element. To avoid the high-order derivative calculation, which is mainly responsible to the low noise robustness of the original pseudo-excitation approach, metal-core piezoelectric fiber sensors are used to directly measure the surface strains, but only at the virtual element boundaries, leading to a significantly reduced number of measurement points. Experiment is designed and carried out using a cantilever beam, in which a 10-metal-core piezoelectric fiber sensor array is embedded in the structure. Along with the sparse laser Doppler vibrometer measurement, a normalized damage location index is constructed. Results demonstrate that the proposed method not only enhances the noise robustness but also allows a significant reduction in the number of measurement points.

Ultra-Thin Piezoelectric Strain Sensor Array Integrated on a Flexible Printed Circuit Involving Transfer Printing Methods

In this paper, we present a novel sensor array manufacturing process that involves transfer printing methods using a chip mounter with a vacuum collet. Using these methods, one can mount not only ultra-thin microsensors but also microcontroller and amplifier chips required for sensor device fabrication. We successfully transfer-printed a very fragile microelectromechanical systems-based 5-mm-long, 1-mm-wide, 5- $\mu \text{m}$ -thick high-aspect-ratio ultra-thin Pb(Zr, Ti)O3 (PZT) (1.9 $\mu \text{m}$ )/Si (3 $\mu \text{m}$ ) strain sensor onto a flexible printed-circuit (FPC) substrate with etched Cu wiring. Then, we connected the sensor to the wiring by printing a conductive paste using a screen printer. Large ferroelectric polarization–voltage hysteresis curves were obtained even after the transfer printing process. Since an output voltage corresponding to the magnitude of the strain from the developed sensor was generated, it was confirmed that ultra-thin sensors could be integrated to the FPC substrate by these transfer and screen printing techniques without damage. The PZT thin films have shown 0.18 mV/ $\mu \varepsilon $ of dynamic strain sensitivity according to the resonant vibration of the stainless plate.

An Intelligent Street Light System based on Piezoelectric Sensor Networks

Objectives: The project aims at saving energy by detecting the vehicle movement on highways and switching on the block of street light ahead of it and simultaneously switching off the trailing lights. Methods: This project requires a sensor to detect the day and night and an array of piezoelectric sensors to detect the vehicle movements and accordingly switching on the lights ahead of it. So when there are no vehicles on the highway, all the lights remains off. Sensors used on either side of the road-senses vehicle movement and sends logic commands to microcontroller to change the intensity of the lamps accordingly. Findings: In current years, the street light control is automated by changing the resistance using the light-sensitive device. The reliability of this method is less. Though it reduces the man power, it does not conserve energy spent during night time. It cannot meet the needs of the growing street lamp information and intelligent management. The aim of smart street lights is to save energy, and to save cost. In our project we make use of the piezoelectric sensors, which is cost effective. ZigBee technology is used to communicate between the lights and internet of things to calculate parameters as well as store the information about the vehicles. Statistical analysis is also done on the data obtained from the sensors. Experiments were conducted on the road and with different types of vehicles. Sufficient data were obtained and were consolidated to come to the conclusions. All these data were processed using MATLAB Software to obtain the graphical results. The shape of the graph clearly gives us some strong conclusions. Improvements: The entire setup can be connected to the IOT (Internet of Things) for processes like data extraction, warning vehicles and is particularly useful for places like a 4-way junctions etc.

Structural Health Monitoring Using Lamb Wave Reflections and Total Focusing Method for Image Reconstruction

This investigation aimed to adapt the total focusing method (TFM) algorithm (originated from the synthetic aperture focusing technique in digital signal processing) to accommodate a circular array of piezoelectric sensors (PZT) and characterise defects using guided wave signals for the development of a structural health monitoring system. This research presents the initial results of a broader study focusing on the development of a structural health monitoring (SHM) guided wave system for advance carbon fibre reinforced plastic (CFRP) composite materials. The current material investigated was an isotropic (aluminium) square plate with 16 transducers operating successively as emitter or sensor in pitch and catch configuration enabling the collection of 240 signals per assessment. The Lamb wave signals collected were tuned on the symmetric fundamental mode with a wavelength of 17 mm, by setting the excitation frequency to 300 kHz. The initial condition for the imaging system, such as wave speed and transducer position, were determined with post processing of the baseline signals through a method involving the identification of the waves reflected from the free edge of the plate. The imaging algorithm was adapted to accommodate multiple transmitting transducers in random positions. A circular defect of 10 mm in diameter was drilled in the plate, which is similar to the delamination size introduced by a low velocity impact event in a composite plate. Images were obtained by applying the TFM to the baseline signals, Test 1 data (corresponding to the signals obtained after introduction of the defect) and to the data derived from the subtraction of the baseline to the Test 1 signals. The result shows that despite the damage diameter being 40 % smaller than the wavelength, the image (of the subtracted baseline data) demonstrated that the system can locate where the waves were reflected from the defect boundary. In other words, the contour of the damaged area was highlighted enabling its size and position to be determined.

Development of single channeled serial-connected piezoelectric sensor array and damage visualization based on multi-source wave propagation imaging

Piezoelectric sensors have been widely used for structural health monitoring. However, a single piezoelectric sensor has limited coverage, and multi-channel piezoelectric sensors require massive cabling and channeling devices. This article presents a serial-connected piezoelectric sensor array equipped with conductive fabric tape for damage diagnosis on a wide area, structure using an ultrasonic propagation imaging system. The sensor array was developed as a single channel sensing technique with multiple sensors. Using conductive fabric tape for installation provided convenient and compact installation, light weight, and comparatively low attenuation, compared with the lead wire for the easy smartilization of structures. An aluminum plate with various artificial damages was tested with the proposed sensor array combined with the ultrasonic propagation imaging system. The ultrasonic propagation imaging system stimulated the sensitive areas of the serially connected piezoelectrics and generated multi-wave sources covering a wide area. The multi-source wave propagation video visualized the damages in the form of anomalous waves. Consequently, multi-cabling and multi-channeling are not required because multiple piezoelectrics can be deployed by serial connections using conductive fabric tapes because of the multi-source wavefield generating capability of the ultrasonic propagation imaging system. Therefore, light-weight, compact, and easy installation of the sensor network and inspection for full-scale structures become possible.

Structural impact detection with vibro-haptic interfaces

This paper presents a new sensing paradigm for structural impact detection using vibro-haptic interfaces. The goal of this study is to allow humans to ‘feel’ structural responses (impact, shape changes, and damage) and eventually determine health conditions of a structure. The target applications for this study are aerospace structures, in particular, airplane wings. Both hardware and software components are developed to realize the vibro-haptic-based impact detection system. First, L-shape piezoelectric sensor arrays are deployed to measure the acoustic emission data generated by impacts on a wing. Unique haptic signals are then generated by processing the measured acoustic emission data. These haptic signals are wirelessly transmitted to human arms, and with vibro-haptic interface, human pilots could identify impact location, intensity and possibility of subsequent damage initiation. With the haptic interface, the experimental results demonstrate that human could correctly identify such events, while reducing false indications on structural conditions by capitalizing on human’s classification capability. Several important aspects of this study, including development of haptic interfaces, design of optimal human training strategies, and extension of the haptic capability into structural impact detection are summarized in this paper.

Flexible Piezoelectric Tactile Sensor Array for Dynamic Three-Axis Force Measurement.

A new flexible piezoelectric tactile sensor array based on polyvinylidene fluoride (PVDF) film is proposed for measuring three-axis dynamic contact force distribution. The array consists of six tactile units arranged as a 3 × 2 matrix with spacing 8 mm between neighbor units. In each unit, a PVDF film is sandwiched between four square-shaped upper electrodes and one square-shaped lower electrode, forming four piezoelectric capacitors. A truncated pyramid bump is located above the four piezoelectric capacitors to improve force transmission. A three-axis contact force transmitted from the top of the bump will lead to the four piezoelectric capacitors underneath undergoing different charge changes, from which the normal and shear components of the force can be calculated. A series of dynamic tests have been carried out by exerting sinusoidal forces with amplitudes ranging from 0 to 0.5 N in the x-axis, 0 to 0.5 N in the y-axis, and 0 to 1.5 N in the z-axis, separately. The tactile units show good sensitivities with 14.93, 14.92, and 6.62 pC/N in the x-, y-, and z-axes, respectively. They can work with good linearity, relatively low coupling effect, high repeatability, and acceptable frequency response in the range of 5–400 Hz to both normal and shear load. In addition, dynamic three-axis force measurement has been conducted for all of the tactile units. The average errors between the applied and calculated forces are 10.68% ± 6.84%. Furthermore, the sensor array can be easily integrated onto a curved surface, such as robotic and prosthetic hands, due to its excellent flexibility.

Rapid Multi-Damage Identification for Health Monitoring of Laminated Composites Using Piezoelectric Wafer Sensor Arrays.

Through the use of the wave reflection from any damage in a structure, a Hilbert spectral analysis-based rapid multi-damage identification (HSA-RMDI) technique with piezoelectric wafer sensor arrays (PWSA) is developed to monitor and identify the presence, location and severity of damage in carbon fiber composite structures. The capability of the rapid multi-damage identification technique to extract and estimate hidden significant information from the collected data and to provide a high-resolution energy-time spectrum can be employed to successfully interpret the Lamb waves interactions with single/multiple damage. Nevertheless, to accomplish the precise positioning and effective quantification of multiple damage in a composite structure, two functional metrics from the RMDI technique are proposed and used in damage identification, which are the energy density metric and the energy time-phase shift metric. In the designed damage experimental tests, invisible damage to the naked eyes, especially delaminations, were detected in the leftward propagating waves as well as in the selected sensor responses, where the time-phase shift spectra could locate the multiple damage whereas the energy density spectra were used to quantify the multiple damage. The increasing damage was shown to follow a linear trend calculated by the RMDI technique. All damage cases considered showed completely the developed RMDI technique potential as an effective online damage inspection and assessment tool.

A health management technology for multisite cracks in an in-service aircraft fuselage based on multi-time-frame laser ultrasonic energy mapping and serially connected PZTs

The problems of multisite fatigue crack growth and multiple interacting cracks in aircraft fuselage structures may incur catastrophic failure. This article proposes a structural health management (SHM) strategy for multisite cracks in the skin and the vicinity of fastener holes in lap joints and demonstrates that the strategy is effective in an in-service metallic aircraft fuselage. To this end, the present article uses a conductive fabric tape-equipped serially connected piezoelectric sensor array combined with an ultrasonic propagation imaging (UPI) system. The serially connected piezoelectric sensor array is defined as a single sensor that has multiple sensor nodes with one electrical output. It was connected with conductive fabric tape instead of lead wire due to its comparatively low attenuation, light weight, and compact and easy installation on the internal structures of the complex fuselage. This technology does not require multi-cabling and multi-channeling because of its ultrasonic propagation imaging capability from multiple sources of the UPI system. The fuselage side skin of a Cessna-150 with various crack locations and orientations was tested using the proposed SHM technology. The ultrasonic wave propagation imaging video showed multisource wave propagation, and all of the crack-induced scattering wavefields appeared at different times because of the large inspection area. Therefore, we have developed a multi-time-frame ultrasonic energy mapping algorithm, which was processed in the wavenumber domain to visualize the cracks. Consequently, the lightweight, easy, and compact installation of the serially connected PZTs and their combination with the UPI system were shown, demonstrating that the SHM strategy for multisite cracks in an in-service aircraft fuselage is effective.

Selection of a piezoelectric sensor array for detecting volatile organic substances in water

Sensitive coatings for piezoelectric sensors used to assess the presence of anthropogenic volatile organic substances in the equilibrium gas phase over natural water have been selected using an array of measuring elements, and the data have been processed using principal component analysis. Groups of piezoelectric sensors with similar characteristics for substance identification have been determined based on the correlation between piezoelectric sensor responses when vapors of organic substances were detected. The stepby-step optimization of the piezoelectric sensor array has been carried out in order to identify the greatest number of organic compounds (vapors) in the sample. Volatile organic compounds can be identified in their aqueous solutions and natural water using the optimized piezoelectric sensor array.

A Location Method Using Sensor Arrays for Continuous Gas Leakage in Integrally Stiffened Plates Based on the Acoustic Characteristics of the Stiffener.

This paper proposes a continuous leakage location method based on the ultrasonic array sensor, which is specific to continuous gas leakage in a pressure container with an integral stiffener. This method collects the ultrasonic signals generated from the leakage hole through the piezoelectric ultrasonic sensor array, and analyzes the space-time correlation of every collected signal in the array. Meanwhile, it combines with the method of frequency compensation and superposition in time domain (SITD), based on the acoustic characteristics of the stiffener, to obtain a high-accuracy location result on the stiffener wall. According to the experimental results, the method successfully solves the orientation problem concerning continuous ultrasonic signals generated from leakage sources, and acquires high accuracy location information on the leakage source using a combination of multiple sets of orienting results. The mean value of location absolute error is 13.51 mm on the one-square-meter plate with an integral stiffener (4 mm width; 20 mm height; 197 mm spacing), and the maximum location absolute error is generally within a ±25 mm interval.

Impact imaging of aircraft composite structure based on a model-independent spatial-wavenumber filter

The spatial-wavenumber filtering technique is an effective approach to distinguish the propagating direction and wave mode of Lamb wave in spatial-wavenumber domain. Therefore, it has been gradually studied for damage evaluation in recent years. But for on-line impact monitoring in practical application, the main problem is how to realize the spatial-wavenumber filtering of impact signal when the wavenumber of high spatial resolution cannot be measured or the accurate wavenumber curve cannot be modeled. In this paper, a new model-independent spatial-wavenumber filter based impact imaging method is proposed. In this method, a 2D cross-shaped array constructed by two linear piezoelectric (PZT) sensor arrays is used to acquire impact signal on-line. The continuous complex Shannon wavelet transform is adopted to extract the frequency narrowband signals from the frequency wideband impact response signals of the PZT sensors. A model-independent spatial-wavenumber filter is designed based on the spatial-wavenumber filtering technique. Based on the designed filter, a wavenumber searching and best match mechanism is proposed to implement the spatial-wavenumber filtering of the frequency narrowband signals without modeling, which can be used to obtain a wavenumber-time image of the impact relative to a linear PZT sensor array. By using the two wavenumber-time images of the 2D cross-shaped array, the impact direction can be estimated without blind angle. The impact distance relative to the 2D cross-shaped array can be calculated by using the difference of time-of-flight between the frequency narrowband signals of two different central frequencies and the corresponding group velocities. The validations performed on a carbon fiber composite laminate plate and an aircraft composite oil tank show a good impact localization accuracy of the model-independent spatial-wavenumber filter based impact imaging method.

Design and Experimentation of Piezoelectric Crystal Sensor Array for Grain Cleaning Loss

In order to improve the accuracy and reliability for detection of grains cleaning loss, a piezoelectric crystal sensor array was proposed in full width of distribution to realize the online multipoints detection. The dynamic model for grain collision sensor array was evaluated by the method of experimental model analysis. The distribution of low-level vibration and deformation was tested for the sensitive elements of detection array using dynamic signal tester. The location and quantity of the piezoelectric crystal units were determined. The simulation was done based on the software ANSYS for the impacts of grains on the sensitive elements. A quantitative analysis was developed to correct the positions of the piezoelectric crystal units and the construction was adjusted according to the deformation contours. The exact positions were quantitatively analyzed and corrected based on the numerical results for the deformation placement, which was used for optimizing the array construction. Then, the grain impact experiment was established on the test bench. The results showed that the performance was stable for the piezoelectric sensor array, and output signal amplitude was about 2.5 V, with uniform sensitivity in the whole range. The present array sensor helps to provide a technical foundation for detection of grain cleaning loss.

Damage localization using warped frequency transform in active structural health monitoring

A spatially distributed array of piezoelectric sensors is widely used in structural health monitoring (SHM) for active ultrasonic methods. Owing tomanyadvantages of delay-and-sum algorithm such asintuition, lowcomputation cost and accurate imaging result, it is very suitable for damage localization. However, due to the dispersion effect of Lamb waves, the wave packet becomes wide and distorted as it propagates in the structure which will have a negative effect in the spatial resolution of the damage imaging. This paper presents an active imaging method using warped frequency transform (WFT) to improve the spatial resolution of the imaging result. In WFT, the warped frequency range is limited in narrowband and the central frequency is fixed to keep the wave packet undistorted. Then, the dispersion effect can be compensated and the velocity of the wave packet is converted into phase velocity which is much lower than group velocity. Due to the compensated wave packet and low wave velocity, the spatial resolution of the delay-and-sum algorithm is improved. Both single damage and dual adjacent damages are detected in the experiments. The comparisons between delay-and-sum algorithm with and without WFT are made to demonstrate the proposed method.