Polyvinylidene Fluoride Film Sensors Research Articles

Long Short-Term Memory Networks for Unconstrained Sleep Stage Classification Using Polyvinylidene Fluoride Film Sensor.

Sleep stage scoring is the first step towards quantitative analysis of sleep using polysomnography (PSG) recordings. However, although PSG is a gold standard method for assessing sleep, it is obtrusive and difficult to apply for long-term sleep monitoring. Further, because human experts manually classify sleep stages, it is time-consuming and exhibits inter-rater variability. Therefore, this article proposes a long short-term memory (LSTM) model for automatic sleep stage scoring using a polyvinylidene fluoride (PVDF) film sensor that can provide unconstrained long-term physiological monitoring. Signals were recorded using a PVDF sensor during PSG. From 60 recordings, 30 were used for training, 10 for validation, and 20 for testing. Sixteen parameters, including movement, respiration-related, and heart rate variability, were extracted from the recorded signals and then normalized. From the selected LSTM architecture, four sleep stage classification performances were evaluated for a test dataset and the results were compared with those of conventional machine learning methods. According to epoch-by-epoch (30 s) analysis, the classification performance for the four sleep stages had an average accuracy of 73.9% and a Cohen's kappa coefficient of 0.55. When compared with other machine learning methods, the proposed method achieved the highest classification performance. The use of LSTM networks with the PVDF film sensor has potential for facilitating automatic sleep scoring, and it can be applied for long-term sleep monitoring at home.

Transient behavior of a cantilever plate subjected to impact loading: Theoretical analysis and experimental measurement

This study employed theoretical analysis and experimental measurements in an exploration of the transient behavior of a cantilever plate subjected to a dynamic impact force. The solution of transient displacement is established as a product of the time function and mode shapes for the cantilever plate. The superposition method was used to obtain the mode shapes and resonant frequencies for free vibrations, and the orthogonality of the mode shapes was used to determine the time function. We demonstrate that the results of transient behaviors including transient displacement, strain, velocity and acceleration of the cantilever plate can be explicitly presented in closed forms. In the experiment, the dynamic external force was produced by the impact of a steel ball on the cantilever plate, and a pair of Polyvinylidene fluoride (PVDF) sensors are used to measure the force history of the impact loading. Fotonic sensor (FS) was also used to obtain the time signals of transient displacement, and PVDF film sensors were used to determine transient strain in the cantilever plate.Our results obtained in the theoretical analysis are highly consistent with experimental measurement and FEM simulations in the time domain. This is a clear demonstration of the effectiveness of pairing theoretical analysis with impact loading histories measured using PVDF sensors in the representation of transient wave propagation behavior in a cantilever plate. Based on the proposed theoretical solution, we established the relationship between transient behaviors and vibrational characteristics that allow us to get more information of the physical phenomenon of the transient response. In transient signal, the contribution of each mode is related to the position of impact point and observation point. The position of exciting force and observation near the nodal lines of particular mode shapes can reduce the contribution of those modes in the transient waveform. We also investigated the influence on the transient response of a cantilever plate subjected to various impact loading duration, and indicate that the amplitude of transient displacement after the release of external force is depended on the impact force duration. Setting the force released at the time of period for specific mode, which provide the maximum contribution, can suppress the amplitude of oscillation in subsequent transient displacement.

Experimental investigation on characteristics of stress wave propagation and plasma discharge induced by high-velocity impact solar array

To investigate the relationship between stress wave propagation and the plasma discharge generated by a projectile high-velocity impact on solar array with 2A12 aluminum substrate structures at the incidence angle of 60°. Three sets of experiments about stress wave propagating and plasma discharge in composite structure of solar array have been performed by taking advantage of PVDF (polyvinylidene fluoride) film sensor, plasma characteristic parameter diagnostic system, discharge measurement system and two-stage light gas gun loading system established by ourselves. Experimental results showed that the duration of stress wave was about 30±5μs, and maximum stresses are about 1400±200MPa in experiments. Furthermore, the discharge phenomenon did happen and the duration of discharge current was also different in solar array at the different impact velocities. The basic consistency between stress wave propagation and discharge duration is also confirmed by experiments.

Prevalence of sleep-disordered breathing among women working in the aged care services in Japan.

The aim of this study was to examine the prevalence of sleep-disordered breathing (SDB) in women working in the field of aged care in Japan. A cross-sectional study was conducted for female employees aged 18-60years in aged care facilities in Japan. The analyzed set consisted of 712 participants with complete data. SDB was determined by respiratory disturbance index (RDI), measured using an ambulatory airflow monitor with a polyvinylidene fluoride film sensor to detect nasal and oral airflow overnight at home. Based on the findings of previous studies, RDI 10 was considered equivalent to apnea-hypopnea index (AHI) 15. The prevalence of moderate-to-severe SDB (RDI ≥ 10) was 22.8%. The mean age was 38.1years, and mean sleeping time of participants was 6.1h. The median body mass index (BMI) was 22.0. Women with moderate-to-severe SDB (RDI ≥ 10) had a higher age, neck circumference, neck-height ratio, BMI, systolic blood pressure, and increased prevalence of hypertension, short sleepers (amount of sleep on the SDB testing day < 6h), and habitual snorers, compared to women with no or mild SDB (RDI < 10). Our study found that women working in the aged care services in Japan were heavily burdened by SDB even though they were relatively young and slim. To prevent CVD from developing in the future, programs through which workplaces can help their employees improve their lifestyle, and early diagnosis and treatment of SDB are highly recommended.

Fabrication of PVDF film sensors for fatigue damage monitoring of single-lap adhesive joints

Polyvinylidene fluoride (PVDF) has been widely used in piezoelectric applications because of its outstanding properties such as low permittivity, high thermal stability, high chemical resistance, and flexibility. In this study, PVDF film sensors were prepared for fatigue damage monitoring of single-lap adhesive joints. PVDF films were fabricated using the solvent casting method, and post treatment, including mechanical stretching and heat setting, was conducted to increase the β-phase crystallinity of the films. Subsequently, the PVDF films were polarized to align the molecular dipoles. The crystal structures of the PVDF films were investigated using X-ray diffraction (XRD) and the thermal behaviors were analyzed using differential scanning calorimetry (DSC). The piezoelectric constants of the prepared PVDF films were measured using a d33 meter. The sensing performance of the films for fatigue damage monitoring of single-lap adhesive joints was assessed by fatigue tests under various stress levels. In addition, a fatigue failure manual based on the PVDF sensor signal, which can predict the failure of the adhesive joints, was constructed.

Investigation of Transient Behavior of a Cantilever Plate Based on Elastic Impact Loading History Detection Using Polyvinylidene Fluoride Film Sensors

In this paper, we investigate elastic impact loading history detection based on polyvinylidene fluoride (PVDF) film sensors and demonstrate its applications on simulating flexible structural responses or investigating dynamic measurement performances of sensors. A pair of PVDF film sensors, bonded collocatedly on two opposite surfaces of a cantilever plate, is used to extract steel ball-generated elastic impact loadings. Impact detecting performance is first demonstrated by comparing the transient displacement responses of the cantilever plate with finite-element method (FEM) prediction based on the obtained impact loading history. Transient strain responses of the plate, also detected by the PVDF film sensors, are then compared with the FEM computation to discuss the cross-sensitivity effect (i.e., influence of the strain in orthogonal direction) of the PVDF film sensors. In addition to the comparison of the time-domain measurement results, sensing results in frequency domain are discussed with the nodal lines in the plate mode shapes. The experimental results in both time and frequency domain show that the commercial PVDF film sensors have the capability for obtaining elastic impact loading history on flexible structures that can be employed for further dynamic structural or dynamic sensing performance investigation.

PVDF 필름 기반 센서를 이용한 무구속적인 누운 자세 추정

Abstract: As body postures on bed affects various sleep related diseases, it is considered as important informationwhen monitoring sleeping in daily life. Though there have already been a few approaches to monitor body postureson bed conventionally, the development for simple and unconstrained methods is still needed to realize the long-termdaily monitoring. Focusing on the fact that ballistocardiogram changes depending on the body postures on bed, wedeveloped a novel method to estimate body posturesusing extremely simple, film-type ballistocardiogram sensorwhich is based on polyvinylidene fluoride(PVDF) film. With 10 subjects, we performed two experiments. One wasfor an estimation test to show that body postures on bed can be estimated by ballistocardiogram, and the other wasfor a reproducibility test to present the feasibility of ballistocardiogram based body postures monitoring. To estimatebody postures on bed, we made an individual template set of body postures by designating one ballistocardiogram(BCG) sample as a template in each postures. Then, we calculated Pearson’s correlation coefficients between a sam-ple and each templates and estimated the body posture of the sample by choosing a posture which corresponds tothe most significant correlation coefficients. As a result, we estimated body postures on bed with 99.2% accuracyin average and found that the estimation using ballistocardiogram is reproducible. Key words: Ballistocardiogram, sleeping position, PVDF film sensor

Response of impedance measured by polyvinylidene fluoride film sensors to damage propagation for wind turbine blade

To establish an effective operation and maintenance system for the wind turbine system, techniques for early detecting the damage, which may occur during its operation, may be required. In this study, responses of impedance, which can be a technique to early detect the damage, to the damage extension in the wind turbine blade were investigated through the full-scale structural test of the 100 kW wind blade. The impedance was measured with polyvinylidene fluoride film sensors with dimension of 25 × 20 mm2 patched on the blade skin. The damages were initiated along the trailing and leading edges and extended with increasing static load level. Impedance signatures varied with increasing the damage size were analyzed in the frequency ranging from 1 to 300 MHz. It was found that the patterns of the impedance signatures are remarkably dependent upon the static load level and sensor location. The change in the signatures was quantitatively quantified by the damage metric. The scalar metric values were closely correlated to the damage extension size and the sensor locations to the extended damage.

Survivability of integrated PVDF film sensors to accelerated ageing conditions in aeronautical/aerospace structures

This work validates the use of integrated polyvinylidene fluoride (PVDF) film sensors for dynamic testing, even after being subjected to UV-thermo-hygro-mechanical accelerated ageing conditions. The verification of PVDF sensors’ survivability in these environmental conditions, typically confronted by civil and military aircraft, is the main concern of the study. The evaluation of survivability is made by a comparison of dynamic testing results provided by the PVDF patch sensors subjected to an accelerated ageing protocol, and those provided by neutral non-aged sensors (accelerometers). The available measurements are the time-domain response signals issued from a modal analysis procedure, and the corresponding frequency response functions (FRF). These are in turn used to identify the constitutive properties of the samples by extraction of the modal parameters, in particular the natural frequencies. The composite specimens in this study undergo different accelerated ageing processes. After several weeks of experimentation, the samples exhibit a loss of stiffness, represented by a decrease in the elastic moduli down to 10%. Despite the ageing, the integrated PVDF sensors, subjected to the same ageing conditions, are still capable of providing reliable data to carry out a close followup of these changes. This survivability is a determinant asset in order to use integrated PVDF sensors to perform structural health monitoring (SHM) in the future of full-scale composite aeronautical structures.

Application of a Fiber Bragg Grating-Based Sensing System on Investigating Dynamic Behaviors of a Cantilever Beam Under Impact or Moving Mass Loadings

By setting up a fiber Bragg grating (FBG) displacement and strain sensing system, this paper experimentally investigates dynamic behaviors of a cantilever beam under impact or moving mass loadings. Dynamic sensing ability of the FBG displacement sensor is demonstrated by simultaneously measuring with a noncontact Fotonic sensor. Responses obtained from the FBG strain sensor are compared with those obtained from polyvinylidene fluoride film sensors and a strain gauge. First, an impact loading is generated by an impact hammer, and transient out-of-plane displacement responses obtained from the proposed FBG displacement sensor are compared with finite element simulations. Then, vibrations of the cantilever beam subjected to impact loadings by a steel ball are investigated. Finally, with a high-speed camera to detect the moving speed, experimental study of the cantilever beam under a moving mass is performed. By using the FBG displacement and strain sensing system, three kinds of modes of the cantilever beam (i.e., bending modes, lateral bending modes, and torsional modes) are detected under impact or moving mass loadings. With the high sensitivity of the proposed FBG sensing system, relationships between the transient responses induced by impact or moving mass loading and the vibration modes are discussed.

Experimental Investigation of the Cross-Sensitivity and Size Effects of Polyvinylidene Fluoride Film Sensors on Modal Testing

Due to advantages such as light weight, flexibility, and low cost, polyvinylidene fluoride (PVDF) films have been widely used in engineering applications as sensors for detecting strain, pressure, or micro-force. However, it is known that PVDF strain sensors have strain cross-sensitivity in mutually orthogonal directions. Furthermore, the size of the PVDF film sensor would also affect the dynamic strain sensing performance. In this paper, to investigate the cross-sensitivity and size effects experimentally, we employ PVDF film sensors to perform dynamic measurements on a cantilever beam. Since the vibrations of the cantilever beam are excited by impacts of a steel ball, the induced highly repeatable transient responses contain a wide range of resonant frequencies and thus can be used to investigate both the size and cross-sensitivity effects of the PVDF film sensors in a dynamic sensing environment. Based on the experimental results of the identified resonant frequencies compared with results obtained from a strain gauge, finite element calculations, and theoretical predictions, suggestions for the use of the PVDF strain sensor in modal testing are given in this paper.

Finger-Mounted Tactile Sensor for Evaluating Surfaces

This paper presents a finger-mounted tactile sensor for extracting information on fine surface properties of objects such as textile fabrics. The prototype sensor has a thin structure composed of a sheet of PVDF (polyvinylidene fluoride) film sensor and some metal parts for converting compressive forces into area expansion of the PVDF film. By using a signal processing program based on the FFT (fast Fourier transform), voltage signal sequences from nine different fabrics were distinguished, even in the presence of variations in the pressing force and the speed of rubbing motion induced by the fluctuations in the user’s hand motion. In addition, the signal sequences from abraded fabrics were sorted by their levels of abrasion by extracting a signal component correlated with the abrasion level.11. This paper is the full translation from the transactions of JSME, Series C, Vol.77, No.784, 2011.

Polyvinylidene fluoride film sensors in collocated feedback structural control: application for suppressing impact-induced disturbances

Polyvinylidene fluoride (PVDF) films are light, flexible, and have high piezoelectricity. Because of these advantages, they have been widely used as sensors in applications such as underwater investigation, nondestructive damage detection, robotics, and active vibration suppression. PVDF sensors are especially preferred over conventional strain gauges in active vibration control because the PVDF sensors are easy to cut into different sizes or shapes as piezoelectric actuators and they can then be placed as collocated pairs. In this work, to focus on demonstrating the dynamic sensing performance of the PVDF film sensor, we revisit the active vibration control problem of a cantilever beam using a collocated lead zirconate titanate (PZT) actuator/PVDF film sensor pair. Before applying active vibration control, the measurement characteristics of the PVDF film sensor are studied by simultaneous comparison with a strain gauge. The loading effect of the piezoelectric actuator on the cantilever beam is also investigated in this paper. Finally, four simple, robust active vibration controllers are employed with the collocated PZT/PVDF pair to suppress vibration of the cantilever beam subjected to impact loadings. The four controllers are the velocity feedback controller, the integral resonant controller (IRC), the resonant controller, and the positive position feedback (PPF) controller. Suppression of impact disturbances is especially suitable for the purpose of demonstrating the dynamic sensing performance of the PVDF sensor. The experimental results also provide suggestions for choosing between the previously mentioned controllers, which have been proven to be effective in suppressing impact-induced vibrations.

Finger-Mounted Tactile Sensor for Evaluating Surfaces

This paper presents a finger-mounted tactile sensor for extracting information on fine surface properties of objects such as textile fabrics. The prototype sensor has a thin structure composed of a sheet of PVDF (Polyvinylidene Fluoride) film sensor and some metal parts for converting compressive forces into area expansion of the PVDF film. By using a signal processing program based on FFT (fast Fourier transform), voltage signal sequences from nine different fabrics were distinguished, in spite of the variation in the pressing force and the speed of rubbing motion induced by the fluctuation of the user's hand motion. In addition, the signal sequences from abraded fabrics were sorted in the order of the level of abrasion, by extracting a signal component correlated with the abrasion level.

Development of QRS detection algorithm designed for wearable cardiorespiratory system

An in-home sleep monitoring system was developed previously in our laboratory for monitoring electrocardiography (ECG) and respiratory signals. However, the ECG signal acquired with this system is prone to high-grade noise caused by motion artifact. Since the detection of the QRS complexes with high accuracy is very important in a computer-based analysis of the ECG, a high accuracy QRS detection algorithm is developed and based on the combination of heart rate indicators and morphological ECG features. The proposed algorithm is tested both on 16 h data acquired using the two sensors of our cardiorespiratory belt system, i.e., the polyvinylidene fluoride (PVDF) film and the conductive fabric sheets, and on all 48 records of the MIT/BIH Arrhythmia Database. Satisfying results are obtained for both databases, the sensitivity S e and positive predictivity P + were calculated for each case and results show S e = [96.98%, 93.76%] and P + = [97.81%, 99.48%] for conductive fabric and PVDF film sensors, respectively, and S e = 99.77% and P + = 99.64% in the case of the MIT/BIH Arrhythmia Database. Further, heart rate variability (HRV) measures were calculated using our system and a commercial system. A comparison between systems’ results is done to show the usefulness of our developed algorithm used with our cardiorespiratory belt sensor.

Heartbeat Monitoring Technique Based on Corona-Poled PVDF Film Sensor for Smart Apparel Application

Flexible piezoelectric polymer materials for smart apparel and wearable computer applications are of great interest. Among known ferroelectric and piezoelectric polymers, polyvinylidene fluoride (PVDF) exhibit β-phase under poling and is known to give highest piezo-, pyro-, and ferroelectric properties. Previous reports suggests that, during corona poling of the PVDF film, a high surface electric potential is generated resulting in a high internal electric field within the polymer film causing the polarization of the dipoles along the direction of the applied electric field. The resultant phase change from α- to β-phase and the dipole switching generates displacement of charges or piezoelectricity. And also mechanical variation would change dipole density of PVDF film. In this report, we measured human heartbeat signal from an DAQ interfaced with a custommade voltage-amplifier with specific frequency filtering function using the corona-poled PVDF film of various sizes and thickness as a piezoelectric sensor and analyzed it. We employed elastic textile band to sensor system for comfortable fit on wrist or ankle. And then, we found the feasibility of applying flexible PVDF film sensor to smart apparel application which can sense heartbeat rate, blood pressure, respiration rate, accidental external impact on human body, etc.

Damage Mechanisms in Loaded Aramid Composites by Means of Embedded PVDF Acoustic Emission Sensors

Cost-effective and reliable damage detection is critical for the utilization of composite materials due to the relatively localised nature of damage formation and the resultant reduction in structural integrity. Of the methods available, Acoustic Emission (AE) is considered as one potential technology for on-line and in situ monitoring of structural degradation of composite materials. Purpose of this work was to study the interaction between embedded PVDF (polyvinylidene fluoride) transducers and composite samples as well as detect and characterize the failure mechanisms in aramid/epoxy flexural test specimens using acoustic emission data obtained by embedded PVDF film sensors. Furthermore, it has been realized a comparison with surface mounted PVDF data. Results of our previous works (Caneva et al., 2005) dealing with monitoring tensile and flexural behaviour of glass/epoxy composites enabled to extend this methodology to aramid/epoxy composites. The use of Acoustic Emission and Scanning Electron Microscopy (SEM) observations enabled to identify and understand the failure mechanisms of the composites tested. Furthermore, satisfactory results of this work highlighted that the application of PVDF shows promise as a suitable acoustic emission transducer for fibrous composite materials.

Parametric Analysis of Acoustic Emission Signals for Evaluating Damage in Composites Using a PVDF Film Sensor

With the increased utilization of advanced composites in strategic industries, the concept of Structural Health Monitoring (SHM) with its inherent advantages is gaining ground over the conventional methods of NDE and NDI. The most attractive feature of this concept is on-line evaluation using embedded sensors. Consequently, development of methodologies with identification of appropriate sensors such as PVDF films becomes the key for exploiting the new concept. And, of the methods used for on-line evaluation acoustic emission has been most effective. Thus, Acoustic Emission (AE) generated during static tensile loading of glass fiber reinforced plastic composites was monitored using a Polyvinylidene fluoride (PVDF) film sensor. The frequency response of the film sensor was obtained with pencil lead breakage tests to choose the appropriate band of operation. The specimen considered for the experiments were chosen to characterize the differences in the operation of the failure mechanisms through AE parametric analysis. The results of the investigations can be characterized using AE parameter indicating that a PVDF film sensor was effective as an AE sensor used in structural health monitoring on-line.

A PVDF piezopolymer sensor for unconstrained cardiorespiratory monitoring during sleep

This paper presents an unconstrained sensing technique for the detection of the respiration and heartbeats during sleep using a piezopolymer PVDF film sensor with the aim that the sensor can be used on the ordinary bed together with the condition that the use of the sensor does not interfere the daily sleep of the patient under detection. A polyvinylidene fluoride (PVDF) film is used as the sensory material in the sensor system. The film is placed under the sheet at the location of the thorax to pick up the fluctuation of the pressure on the bed caused by the respiratory movement and heartbeats. Fundamental principle and the functional verification of the sensor system are described in details in the paper. It is shown that the respiration and heartbeats can simultaneously be detected by the sensor with the use of the wavelet decomposition analysis.

Wireless Monitoring of Cable Tension of Cable-Stayed Bridges Using PVDF Piezoelectric Films

For cable-stayed bridges, cable tension and its variation are two of the most important factors for judging whether the bridges are functional or not. Therefore, the in-situ monitoring of the stayed cables’ tension of the cable-stayed bridge is very important and necessary, from which we can determine the health of the bridge. However, how to acquire and retrieve the signal is the key problem to determine whether the method is suitable for in-situ monitoring of the cable tension of cable-stayed bridges. In this paper, polyvinylidene fluoride (PVDF) piezoelectric films are used as the sensors to acquire the dynamic responses of the stayed cables, from which the tension of the stayed cables can be determined. Signal retrieval from the PVDF film sensors via wireless transmission technology is developed, which could be used for the wireless in-situ monitoring of the cable tension. The principles, feasibility, and configuration of the wireless system for monitoring the stayed cable tension of cable-stayed bridges are investigated. Also the validating experiment is performed on a small-scale stayed cable model in the laboratory. Results show that the proposed wireless monitoring system is feasible for the in-situ monitoring of the stayed cable tension. The method and transmission system can be applied to wireless health monitoring of civil infrastructures with various sensors.