High-resolution Displacement Sensor Research Articles

Experimental Study on Monitoring Damage Progression of Basalt-FRP Reinforced Concrete Slabs Using Acoustic Emission and Machine Learning.

Basalt fiber-reinforced polymer (BFRP) reinforced concrete is a new alternative to conventional steel-reinforced concrete due to its high tensile strength and corrosion resistance characteristics. However, as BFRP is a brittle material, unexpected failure of concrete structures reinforced with BFRP may occur. In this study, the damage initiation and progression of BFRP-reinforced concrete slabs were monitored using the acoustic emission (AE) method as a structural health monitoring (SHM) solution. Two simply supported slabs were instrumented with an array of AE sensors in addition to a high-resolution camera, strain, and displacement sensors and then loaded until failure. The dominant damage mechanism was concrete cracking due to the over-reinforced design and adequate BFRP bar-concrete bonding. The AE method was evaluated in terms of identifying the damage initiation, progression from tensile to shear cracks, and the evolution of crack width. Unsupervised machine learning was applied to the AE data obtained from the first slab testing to develop the clusters of the damage mechanisms. The cluster results were validated using the k-means supervised learning model applied to the data obtained from the second slab. The accuracy of the K-NN model trained on the first slab was 99.2% in predicting three clusters (tensile crack, shear crack, and noise). Due to the limitation of a single indicator to characterize complex damage properties, a Statistical SHapley Additive exPlanation (SHAP) analysis was conducted to quantify the contribution of each AE feature to crack width. Based on the SHAP analysis, the AE duration had the highest correlation with the crack width. The cumulative duration of the AE sensor near the crack had close to 100% accuracy to track the crack width. It was concluded that the AE sensors positioned at the mid-span of slabs can be used as an effective SHM solution to monitor the initiation of tensile cracks, sudden changes in structural response due to major damage, damage evolution from tensile to shear cracks, and the progression of crack width.

Theoretical and Experimental Investigation of Intracavity Displacement-Sensor Based on All-Single-Mode Fiber

An all-single-mode fiber intracavity displacement sensor based on U-shaped single-mode fiber interferometer (U-SMFI) is demonstrated theoretically and experimentally. The theoretical model for a cavity-loss-modulated Er-doped fiber ring laser is proposed to analyze the linewidth-narrowing and signal-to-noise ratio (SNR) enhancement mechanism. The evolutionary process of the spectra is calculated through solving the rate equations to investigate the role that the intracavity spectroscopy plays in narrowing the linewidth and reducing the bandwidth-broadening effect theoretically for the first time. Experimental setup for intracavity displacement sensing via a U-SMFI is established to verify the above principle. According to the experimental results, the linewidth and SNR characteristics have been improved a lot compared with the passive U-SMFI. Moreover, it is indicated that the sensitivity of the intracavity displacement-sensor can be optimized through adjusting the bending radius of the U-SMFI. The mode-hopping phenomenon is investigated and eliminated to make the wavelength stability of the output laser comparable to the highest resolution of the optical spectrum analyzer. When the displacement applied to the U-SMFI with a radius of 5.95 mm is changed from 0 to 200 μm, the sensitivity of the proposed intracavity-sensor is 52.92 pm/μm, which can be enhanced through a further optimization to the radius of the U-SMFI. Our proposed sensor design is expected to emerge as a new and promising alternative approach to achieve high-resolution displacement sensor.

A simple and high-resolution POF displacement sensor based on face-coupling method

A Various fiber-optics displacement sensor methods are presented yearly, and a high-resolution displacement sensor with cost-effective and straightforward operation is an essential demand. Here, we present a novel sensor structure for displacement measurement. The design is based on a hole in the one fiber's core, whereas a second fiber is utilized to move inside the hole made on the first fiber. The optical power coupling varies with the distance between the first and second fiber. The drilling length is investigated, and the four sensors are fabricated with different displacement ranges. The results show drilling length influences sensor's range, and the sensor has a nanometer resolution of about 8.647 nm with a sensitivity of 11.56 nW/µm in between 0 µm and 2500 µm. Moreover, a maximum range of 4 mm is achieved. The designed structure could be used not only for displacement, but also for vibration, angle, and pressure sensor.

Single-detecting-path high-resolution displacement sensor based onself-interference effect of a single submicrometer grating.

On the basis of the self-interference effect between ±1 st-order diffraction beams from a single optical submicrometer grating, we demonstrate a single-detecting-path optical displacement sensor with high resolution. Using a quadrant optoelectronic detector, a single-detecting-path system without any wave plates is realized experimentally. Combined with an interpolation circuit, we demonstrate the system for displacement measurement within a range of 200 µm. The results indicate a detecting sensitivity of 905.4°/µm and an accuracy of ±1.9µm. It is worth mentioning that, considering a maximum subdividing factor of 9674 used in experiment, the resolution goes down to 41.1 pm in principle. We demonstrate a compact optical sensor with high resolution, which is promising in developing miniaturized displacement systems.

Simulation and Optimization of SNAP-Taper Coupling System in Displacement Sensing.

Sensing applications based on whispering gallery mode (WGM) microcavities have attracted extensive attention recently, especially in displacement sensing applications. However, the traditional displacement sensing scheme based on shift in a single resonance wavelength, has a lot of drawbacks. Herein, a novel displacement sensing scheme based on the surface nanoscale axial photonics (SNAP) is proposed to achieve a wide range and high-resolution displacement sensor through analyzing the transmittance of multiple axial modes. By analyzing the surface plot of the resonance spectrum with different coupling positions, the ideal coupling parameters and ERV for displacement sensing are obtained. In the following, displacement sensing with high sensitivity and a wide range is theoretically realized through adjusting the sensitivity threshold and the number of modes. Finally, we present our views on the current challenges and the future development of the displacement sensing based on an SNAP resonator. We believe that a comprehensive understanding on this sensing scheme would significantly contribute to the advancement of the SNAP resonator for a broad range of applications.

Experimental and numerical analysis on the equilibrium shape of sessile droplets on elastic thin membranes

The equilibrium shape of sessile droplets on elastic thin membranes is determined by the balance among three types of surface tensions at the interface, namely vapor-liquid, liquid-solid, and solid-vapor. We studied the cross-sectional contour of the membrane equilibrium state and sessile drops by focusing on the global characteristics of the deformation, both numerically and experimentally. To determine the initial tension, which is required for numerical analysis, we carried out a modal test. A laser Doppler vibrometer (LDV) was used to measure the velocity of the membrane vibration on a table isolated from external forces. With the measured the first and second modal frequencies, the initial tension of the membrane was derived using a circular membrane vibration equation by solving a first kind of Bessel function. Furthermore, a high-resolution reflective displacement sensor and micro linear actuator with micrometer accuracy were used to measure the cross-section deformation of the membrane on the sessile drop, and the droplet contact radius in the experimental environment. The membrane sample was assembled on a thin, light aluminum ring with a resonance frequency of about 700 Hz, and three different diameters were employed. Using this technique, we investigated the trend in membrane deformation due to the increase in the weight of the water droplets and the surface tension, which is an intrinsic property of the membrane. In addition, we found that the experimental results related to the contact radius and contour of a water drop agreed with the simulation data well.

High-Resolution Displacement Sensor Based on a Chirped Fabry–Pérot Interferometer Inscribed on a Tapered Microfiber

In this study, a high-resolution optical fiber Fabry–Pérot (FP) interferometer displacement sensor with chirped spectral characteristics based on a tapered microfiber is theoretically discussed and experimentally implemented. Instead of inscribing two fiber Bragg gratings (FBGs) on the symmetric position of the microfiber, we continuously inscribed one long FBG along the microfiber region to reduce the cavity length. The bandwidth of the interferometer is over 35 nm, and its displacement sensitivity is as high as 36.5 nm/mm at the tension state.

Application of double metal/dielectric gratings in optical displacement detection.

A high-resolution displacement detection based on in-plane and out-of-plane moving gratings is discussed in this paper. To improve the detective resolution and stability, a model of a double metal/dielectric optical grating is established to investigate the tolerance of grating parameters. The zero-order optical diffraction efficiency of the grating is calculated by rigorous coupled-wavelength analysis, which depends on the grating parameters. The dependency relation curves are a cosine and pulse curve, respectively, for in-plane movement with the highest slope superior to 0.29%/nm and the highest resolution can reach to 0.01nm, and for out-of-plane movement with the highest slope superior to 1.63%/nm and the optimum resolution can reach to 0.002nm. These results indicate that the in-plane effect is feasible for a long-range and high-resolution displacement detection, and the out-of-plane effect is suitable for small-range and high-resolution displacement detection. Both kinds of effects provide a designed solution for the development of a high-resolution displacement sensor.

Integration of High-Resolution Laser Displacement Sensors and 3D Printing for Structural Health Monitoring.

This paper presents a novel experimental design for complex structural health monitoring (SHM) studies achieved by integrating 3D printing technologies, high-resolution laser displacement sensors, and multiscale entropy SHM theory. A seven-story structure with a variety of composite bracing systems was constructed using a dual-material 3D printer. A wireless Bluetooth vibration speaker was used to excite the ground floor of the structure, and high-resolution laser displacement sensors (1-μm resolution) were used to monitor the displacement history on different floors. Our results showed that the multiscale entropy SHM method could detect damage on the 3D-printed structures. The results of this study demonstrate that integrating 3D printing technologies and high-resolution laser displacement sensors enables the design of cheap, fast processing, complex, small-scale civil structures for future SHM studies. The novel experimental design proposed in this study provides a suitable platform for investigating the validity and sensitivity of SHM in different composite structures and damage conditions for real life applications in the future.

Atomic force microscopy combined with optical tweezers (AFM/OT)

The role of mechanical properties is essential to understand molecular, biological materials, and nanostructures dynamics and interaction processes. Atomic force microscopy (AFM) is the most commonly used method of direct force evaluation, but due to its technical limitations this single probe technique is unable to detect forces with femtonewton resolution. In this paper we present the development of a combined atomic force microscopy and optical tweezers (AFM/OT) instrument. The focused laser beam, on which optical tweezers are based, provides us with the ability to manipulate small dielectric objects and to use it as a high spatial and temporal resolution displacement and force sensor in the same AFM scanning zone. We demonstrate the possibility to develop a combined instrument with high potential in nanomechanics, molecules manipulation and biological studies. AFM/OT equipment is described and characterized by studying the ability to trap dielectric objects and quantifying the detectable and applicable forces. Finally, optical tweezers calibration methods and instrument applications are given.

Micrograting Displacement Sensor with Integrated Electrostatic Actuation

A high-resolution micro-grating displacement sensor with diffraction-based and integrated electrostatic actuation is proposed and experimentally demonstrated. The Al reflecting membrane is fabricated at the bottom of a silicon moving part and the Au micro-gratings are patterned on a transparent substrate. This structure forms a phase sensitive diffraction grating, providing the displacement sensitivity of the micro-grating interferometer. It shows sensitivity adjustment and self-calibration capabilities with electrostatic actuation. Additional system components include a coherent light source, photodiodes, and required electronics. Experimental results show that the displacement sensor has a sensitivity of about 1.8 mV/nm and a resolution of less than 1 nm in the linear region. This displacement sensor is very promising in the fields requiring high sensitivity, broad dynamic range, and immunity to electromagnetic interference.

An Advanced Multiconfiguration Stator Well Cooling Test Facility

Optimization of cooling systems within gas turbine engines is of great interest to engine manufacturers seeking gains in performance, efficiency, and component life. The effectiveness of coolant delivery is governed by complex flows within the stator wells and the interaction of main annulus and cooling air in the vicinity of the rim seals. This paper reports on the development of a test facility which allows the interaction of cooling air and main gas paths to be measured at conditions representative of those found in modern gas turbine engines. The test facility features a two stage turbine with an overall pressure ratio of approximately 2.6:1. Hot air is supplied to the main annulus using a Rolls-Royce PLC Dart compressor driven by an aero-derivative engine plant. Cooling air can be delivered to the stator wells at multiple locations and at a range of flow rates which cover bulk ingestion through to bulk egress. The facility has been designed with adaptable geometry to enable rapid changes of cooling air path configuration. The coolant delivery system allows swift and accurate changes to the flow settings such that thermal transients may be performed. Particular attention has been focused on obtaining high accuracy data, using a radio telemetry system, as well as thorough through-calibration practices. Temperature measurements can now be made on both rotating and stationary disks with a long term uncertainty in the region of 0.3 K. A gas concentration measurement system has also been developed to obtain direct measurement of re-ingestion and rim seal exchange flows. High resolution displacement sensors have been installed in order to measure hot running geometry. This paper documents the commissioning of a test facility which is unique in terms of rapid configuration changes, nondimensional engine matching, and the instrumentation density and resolution. Example data for each of the measurement systems are presented. This includes the effect of coolant flow rate on the metal temperatures within the upstream cavity of the turbine stator well, the axial displacement of the rotor assembly during a commissioning test, and the effect of coolant flow rate on mixing in the downstream cavity of the stator well.

A high-resolution displacement sensor based on a grating interferometer with the phase modulation technique

A high-resolution displacement sensor based on a grating interferometer with the phase modulation technique is demonstrated in this paper. The grating interferometer is formed with a grating and a mirror, which is fixed on a piezoelectric translator to generate phase modulation. The detected signal is intensity-modulated and processed with a lock-in amplifier for demodulation. Experimental results show that this displacement sensor has a displacement sensitivity of 19.8 mV nm−1 and a high-resolution displacement detection of 0.05 nm in the linear region.

Development of An Automatic Approaching System for Electrochemical Nanofabrication Using Visual and Force-Displacement Sensing

In this paper, a fast automatic precision approaching system is developed for electrochemical nanofabrication using visual and force-displacement sensing. Before the substrate is fabricated, the template should approach the substrate accurately to establish the initial gap between the template and substrate. During the approaching process, the template is first quickly moved towards the substrate by the stepping motor until a specified gap is detected by the visual feedback. Then, the successive approach using the switch of macro-micro motion with a force-displacement sensing module is triggered to make the template contact with the substrate to nanometre accuracy. The contact force is measured by the force-displacement sensing module which employs the high-resolution capacitive displacement sensor and flexure compliant mechanism. The high sensitivity of this capacitive displacement sensor ensures high accuracy of the template-substrate contact. The experimental results show that the template can reach the substrate accurately and smoothly, which verifies the effectiveness of the proposed approaching system with the visual and the force-displacement sensing modules.

Intense fracturing and fracture sealing induced by mineral growth in porous rocks

Mineral precipitation in the pores of a rock may exert a force, which is called crystallization pressure. This process has been studied experimentally and results bring a new look on the way fractures may develop and seal in natural systems. Cylindrical core samples of porous limestone and sandstone were left for several weeks in contact with an aqueous solution saturated with sodium chloride, at 30 or 45 °C, under axial normal stress in the range 0.02–0.26 MPa. The fluid was allowed to rise in the core samples by capillary forces, up to a controlled height where evaporation and precipitation occurred. The uniaxial deformation of the samples was measured using high-resolution displacement sensors. The samples were characterized using computed X-ray tomography, allowing therefore imaging in 3D the intensity and localization of the damage. Two kinds of damage could be observed. Firstly, small rock fragments were peeled from the sample surface. Secondly, and more interestingly, fracture networks developed, by nucleation of microcracks at the interface where evaporation occurred, and propagation to the free surface. Two families of fractures could be identified. A first set of sealed fracture parallel to the evaporation front is directly induced by crystallization pressure. A second fracture network, perpendicular to the evaporation front, accommodates the first set of fractures. An analytical model where fluid flow is coupled to evaporation, vapour transport, and localization of mineral precipitation explains the shape of this fracture network.

Laser-detected lateral muscle displacement is correlated with force fluctuations during voluntary contractions in humans

Fluctuations in muscle force during steady voluntary contractions result from the summation of twitch forces produced by asynchronous activation of multiple motor units. We hypothesized that oscillatory lateral muscle displacement, measured with a non-contact high-resolution laser displacement sensor, is correlated with force fluctuations during steady, voluntary contractions with a human muscle. Eight healthy young adults (20–33 yrs) performed steady isometric contractions with the first dorsal interosseus muscle. Contraction intensity ranged from 2.5% to 60% of the maximal voluntary contraction force. Oscillatory lateral displacement of the muscle surface was measured with a high-resolution laser displacement sensor (0.5 μm resolution), concurrently with abduction force of the index finger. In the time-domain analysis, there was a significant positive peak in the cross-correlation function between lateral muscle displacement and force fluctuations. In addition, the amplitude increased linearly with contraction intensity in both signals. In the frequency-domain analysis, frequency content was similar in both signals, and there was significant coherence between signals for the major frequency range of the signals (<5 Hz). In conclusion, laser-detected lateral displacement of a hand muscle is correlated with force fluctuations across a wide range of contraction intensity during steady voluntary contractions in humans.

Frequency features of mechanomyographic signals of human soleus muscle during quiet standing

The purpose of the present study was to determine whether the frequency features of the MMG signals during quiet standing reflect body sway as well as recurring muscle activity. Twenty healthy men maintained quiet standing in a barefoot position with their eyes open or closed. During quiet standing, MMG detected using uniaxial piezoresistive accelerometer and surface electromyogram (EMG) signals were recorded from the soleus (SOL) muscle, and the center of mass (CoM) displacement (CoMdis) in the anteroposterior direction was measured by a high-resolution laser displacement sensor. In addition, CoMdis was time-differentiated to yield CoM velocity (CoMvel). Cross-spectral analysis revealed that significant coherency spectra from MMG to CoMdis and from MMG to rectified EMG of SOL were observed below 2 Hz and 8–12 Hz frequency band, respectively. Furthermore, we revealed that the trajectories of MMG and the calculated dMMG/d t were significantly correlated to CoMdis and CoMvel, respectively. These results suggest that kinematic and physiological parameters of postural control during quiet standing can be quantified by frequency features of the MMG.

Feedback control of tri-layer polymer actuators to improve their positioning ability and speed of response

Improving positioning ability of electroactive polymer actuators typified by polypyrrole (PPy) through feedback control of their position has been considered. The actuators operate in air, as opposed to their predecessors. One of the problems associated with the actuators is the forward relaxation/creep or drift, which occurs after the full reduction and oxidation processes are completed. A high-resolution laser displacement sensor was used to detect the position of the cantilever-type polymer actuators and the classical, yet effective, Proportional, Integral and Derivative (PID) control method has been employed to generate the appropriate control input in order to make sure that no drift occurs in the user-specified position of the actuators. A set of experimental results with and without feedbacking the position data are presented to demonstrate the efficacy of the control method for improving the positioning ability and the speed of response. The rise time is reduced by more than 500 times and the position tracking error is less than 10% for a time-varying user-specified position command.

Small Lateral Displacement of the Muscle Represents Low-Frequency Force Fluctuations During Voluntary Contractions in Humans

Small lateral vibration of the muscle surface, termed as muscle sound or mechanomyogram, has been recorded with accelerometers or displacement sensors. The amplitude of this vibration depends on the development of fusion of motor units during electrically stimulated contractions in humans (Yoshitake et al. J. Appl. Physiol. 93:1744–1752,2002). PURPOSE: To examine if there is a temporal correlation between force fluctuations and small lateral displacement of the muscle surface during voluntary contractions of a muscle in humans. METHODS: Seven healthy young adults (20–33 yrs) performed steady isometric contractions with the first dorsal interosseus muscle. Target force ranged from 2.5 to 80% of the maximal voluntary contraction force. Small lateral displacement of the muscle surface was measured with a high-resolution laser displacement sensor (LK-G155, Keyence, Japan; 0.4 urn resolution). Abduction force of the index finger and small lateral displacement of the muscle surface were recorded simultaneously. The standard deviations (SD) of force and muscle displacement were calculated after the linear trend was removed. To examine the extent of temporal correlation between force and muscle displacement, cross-correlation function (CCF) between these signals was calculated. RESULTS: The major frequency range in signals was <5 Hz for both force and muscle displacement across target forces. The SD of force increased with target force from 47.0 ± 5.7mN to 606.6 ± 77.1 mN. Similarly, the SD of muscle displacement increased with target force from 21.3 ± 2.1 um to 57.7 ± 9.3 urn. A significant linear correlation was found between the SD of force and the SD of muscle displacement (r = 0.60, P <0.0001). There was a significant temporal correlation between the force and muscle displacement as shown by a positive peak in the CCF at all target forces (P <0.01). The peak value of the CCF and its time lag ranged from 0.49 to 0.59 and from −0.60 to −0.34 ms, respectively. These values were not influenced by target force (P > 0.05). CONCLUSION: Temporal characteristics of low-frequency force fluctuations in a muscle were represented by small lateral displacement of the muscle surface across a wide range of contraction intensity.

Simple force balance accelerometer/seismometer based on a tuning fork displacement sensor

Seismometers and microelectromechanical system accelerometers use the force-balance principle to obtain measurements. In these instruments the displacement of a mass object by an unknown force is sensed using a very high-resolution displacement sensor. The position of the object is then stabilized by applying an equal and opposite force to it. The magnitude of the stabilizing force is easily measured, and is assumed to be equivalent to the unknown force. These systems are critically dependent on the displacement sensor. In this article we use a resonant quartz tuning fork as the sensor. The tuning fork is operated so that its oscillation is lightly damped by the proximity of the movable mass object. Changes in the position of the mass object cause changes in the phase of the fork’s resonance; this is used as the feedback variable in controlling the mass position. We have developed an acceleration sensor using this principle. The mass object is a piezoelectric bimorph diaphragm which is anchored around its perimeter, allowing direct electronic control of the displacement of its center. The tuning fork is brought very close to the diaphragm center, and is connected into a self-oscillating feedback circuit which has phase and amplitude as outputs. The diaphragm position is adjusted by a feedback loop, using phase as the feedback variable, to keep it in a constant position with respect to the tuning fork. The measured noise for this sensor is approximately 10.0 mg in a bandwidth of 100 Hz, which is substantially better than commercial systems of equivalent cost and size.