2D Sensor Array Research Articles

High-conductivity and long-term stability strain sensor based on silk fibroin and polyvinyl alcohol hydrogels

Conductive hydrogels as promising candidates of wearable electronic have attracted considerable interest in health monitoring, multifunctional electronic skins, and human-machine interfaces. However, to simultaneously achieve excellent electrical properties, long-term stability, and water vapor permeability of conductive hydrogels remains a challenge. Herein, a highly conductive hydrogel (MPRS) with high breathability (5.90 mg∙cm−2∙h−1), excellent electrical conductivity (6.67 S/m), high elongation (315%), and remarkable water retention properties was prepared adopting cold freeze-thawing method. MXene, poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS), and reduced graphene oxide were used as conductive materials, while silk fibroin (SF) and polyvinyl alcohol (PVA) were used as the gel matrix. Lithium chloride solution was used as a solvent during the preparation process. The strain sensor based on this hydrogel demonstrates rapid response (0.33 s) and recovery ability (0.31 s) to strain stimuli. Upon 250 cycles of stretching at 35% strain, the output signal remains stable and highly repeatable, ensuring accurate detection of human joint movements and facial expressions. The fabrication of flexible electrodes using the hydrogel enabled real-time monitoring of human electromyography and electrocardiogram signals. The applications in a 2D sensor array and human-machine interaction interfaces also proved the excellent performance of this conductive hydrogels in wearable sensor.

Direction-of-Arrival Estimation With Planar Luneburg Lens and Waveguide Metasurface Absorber

Direction of arrival (DOA) estimation using array antennas is known to have high resolution. However, these algorithms require high computational resources. This paper proposes a unique DOA estimation system without a sophisticated algorithm, using a planar Luneburg lens and a waveguide metasurface absorber. The planar Luneburg lens consists of a convex lens and a waveguide that separates incoming electromagnetic waves from different directions and focuses them at different points on the sides of the convex lens. The metasurface absorber consists of a mushroom-shaped structure that, when placed at the focal point of the lens, behaves like a compact 2D sensor array. Information regarding the intensity and angle of the incident electromagnetic wave can be obtained from the power and position of the sensor element. The resolution of the system is 2.2–2.4 degrees (depending on the sensor position), and the maximum estimation error is within 2.4 degrees in the 5.65 GHz frequency band.

High-throughput imaging surface plasmon resonance biosensing based on ultrafast two-point spectral-dip tracking scheme

Wavelength interrogation surface plasmon resonance imaging (λSPRi) has potential in detecting 2-dimensional (2D) sensor array sites, but the resonance wavelength imaging rate limits the application of detecting biomolecular binding process in real time. In this paper, we have successfully demonstrated an ultrafast λSPRi biosensor system. The key feature is a two-point tracking algorithm that drives the liquid crystal tunable filter (LCTF) to achieve fast-tracking of the resonance wavelength movement caused by the binding of target molecules with the probe molecules on the sensing surface. The resonance wavelength measurement time is within 0.25s. To date, this is the fastest speed ever reported in λSPRi. Experiment results show that the sensitivity and dynamic are 2.4 × 10-6 RIU and 4.6 × 10-2 RIU, respectively. In addition, we have also demonstrated that the system has the capability of performing fast high-throughput detection of biomolecular interactions, which confirms that this fast real-time detecting approach is most suitable for high-throughput and label-free biosensing applications.

Biomimetic epidermal sensors assembled from polydopamine-modified reduced graphene oxide/polyvinyl alcohol hydrogels for the real-time monitoring of human motions.

Conductive hydrogel-based epidermal strain sensors can generate repeatable electrical changes upon mechanical deformations for indication of the skin's physiological condition. However, this remains challenging for many conductive hydrogel sensors due to biomechanical mismatch with skin tissues and an unstable resistance variation response, resulting in non-conformable deformations with the epidermis and dermis, and consequently generating inaccurate monitoring of human movements. Herein, a conductive hydrogel that highly matches the skin is fabricated from dynamically hydrogen-bonded nanocrystallites of polydopamine-modified reduced graphene oxide (PDA-rGO) nanosheets composited with polyvinyl alcohol, namely the PDA-rGO/PVA hydrogel. PDA-rGO provides a large number of dynamic hydrogen-bonding interactions in the hydrogel, resulting in a skin-matching modulus (78 kPa) and stretchability. Moreover, the resultant hydrogel possesses excellent cytocompatibility and conductivity (0.87 S m-1), high sensitivity (gauge factor of compression: 20) at low strain and outstanding linearity at high strain as well as a stable resistance variation response. These desirable properties enable the application of the PDA-rGO/PVA hydrogel as a skin-friendly wearable sensor for real-time and accurate detection of both large-scale joint movements and tiny physiological signals, including the bending and relaxing of fingers, the wrist, elbow and knee joints, and wrist pulse and swallowing. Moreover, this hydrogel is integrated into a 2D sensor array that monitors strains or pressures in two dimensions, which is promising for electronic skin, biosensors, human-machine interfaces, and wearable electronic devices.

Joint 3D localization and classification of space debris using a multispectral rotating point spread function.

We consider the problem of joint three-dimensional (3D) localization and material classification of unresolved space debris using a multispectral rotating point spread function (RPSF). The use of RPSF allows one to estimate the 3D locations of point sources from their rotated images acquired by a single 2D sensor array, since the amount of rotation of each source image about its x, y location depends on its axial distance z. Using multispectral images, with one RPSF per spectral band, we are able not only to localize the 3D positions of the space debris but also classify their material composition. We propose a three-stage method for achieving joint localization and classification. In stage 1, we adopt an optimization scheme for localization in which the spectral signature of each material is assumed to be uniform, which significantly improves efficiency and yields better localization results than possible with a single spectral band. In stage 2, we estimate the spectral signature and refine the localization result via an alternating approach. We process classification in the final stage. Both Poisson noise and Gaussian noise models are considered, and the implementation of each is discussed. Numerical tests using multispectral data from NASA show the efficiency of our three-stage approach and illustrate the improvement of point source localization and spectral classification from using multiple bands over a single band.

A Speckle-Free Angular Interrogation SPR Imaging Sensor Based on Galvanometer Scan and Laser Excitation

A speckle-free fast angular interrogation surface plasmon resonance imaging (SPRi) sensor based on a diode-pumped all-solid-state laser and galvanometer is reported in this work. A bidirectional scan using a galvanometer realizes the fast scanning of the incidence angle. The experimental results showed that the time needed for completing an SPR dip measurement was decreased to 0.5 s. And through cascading an immovable diffuser and two diffusers rotating in opposite directions, laser speckle was eliminated. The dynamic detection range and the sensitivity reached 4.6 × 10−2 and 1.52 × 10−6 refractive index unit (RIU), respectively, in a 2D array sensor when the angle scanning range was set as 7.5°. More importantly, the results demonstrated that the angular interrogation SPR imaging sensor scheme had the capability to perform fast and high-throughput detection of biomolecular interactions at 2D sensor arrays.

Three-dimensional evolution of damage in sandstone Brazilian discs by the concurrent use of active and passive ultrasonic techniques

Active and passive ultrasonic techniques were used to study three-dimensional evolution of damage in sandstone discs under Brazilian test. A standard disc, 50 mm in diameter and 25 mm in thickness, was instrumented with a 3D sensor array containing 12 Nano30 sensors and was diametrically compressed under line loads quasi-statically. P-wave velocities along 64 ray paths within the disc sample were measured before and during the experiment, which were found to be strongly anisotropic and were then used to construct a time-dependent transversely isotropic velocity model. Twelve channels of full waveform data were recorded continuously by acoustic emission (AE) monitoring until final failure, from which discrete AE events were triggered. Based on the constructed velocity model, a total of 1775 AE events were located successfully by the collapsing grid search algorithm. Two different classification methods were applied to the decomposed moment tensors, revealing a mixture of tensile and shear microcracks clustered in the crack initiation point. Orientations of P and T axes were also found to be a not perfect but passable indicator of stress state of the disc sample. Furthermore, a similar Brazilian test was performed with a 2D sensor array for comparison. Array analysis indicated that the 3D sensor array generally performs better in the three-dimensional space due to its full-size spatial coverage, while the 2D sensor array has higher location accuracy on the plane where sensors are. Both cases were found to have similar crack initiation positions, about 10 mm away from the disc centre, suggesting that the obtained tensile strength may underestimate the true value.

NIR hyperspectral compressive imager based on a modified Fabry–Perot resonator

The acquisition of hyperspectral (HS) image datacubes with available 2D sensor arrays involves a time consuming scanning process. In the last decade, several compressive sensing (CS) techniques were proposed to reduce the HS acquisition time. In this paper, we present a method for near-infrared (NIR) HS imaging which relies on our rapid CS resonator spectroscopy technique. Within the framework of CS, and by using a modified Fabry–Perot resonator, a sequence of spectrally modulated images is used to recover NIR HS datacubes. Owing to the innovative CS design, we demonstrate the ability to reconstruct NIR HS images with hundreds of spectral bands from an order of magnitude fewer measurements, i.e. with a compression ratio of about 10:1. This high compression ratio, together with the high optical throughput of the system, facilitates fast acquisition of large HS datacubes.

Wavelength-Scanning SPR Imaging Sensors Based on an Acousto-Optic Tunable Filter and a White Light Laser.

A fast surface plasmon resonance (SPR) imaging biosensor system based on wavelength interrogation using an acousto-optic tunable filter (AOTF) and a white light laser is presented. The system combines the merits of a wide-dynamic detection range and high sensitivity offered by the spectral approach with multiplexed high-throughput data collection and a two-dimensional (2D) biosensor array. The key feature is the use of AOTF to realize wavelength scan from a white laser source and thus to achieve fast tracking of the SPR dip movement caused by target molecules binding to the sensor surface. Experimental results show that the system is capable of completing a SPR dip measurement within 0.35 s. To the best of our knowledge, this is the fastest time ever reported in the literature for imaging spectral interrogation. Based on a spectral window with a width of approximately 100 nm, a dynamic detection range and resolution of 4.63 × 10−2 refractive index unit (RIU) and 1.27 × 10−6 RIU achieved in a 2D-array sensor is reported here. The spectral SPR imaging sensor scheme has the capability of performing fast high-throughput detection of biomolecular interactions from 2D sensor arrays. The design has no mechanical moving parts, thus making the scheme completely solid-state.

Subpixel characterization of a PIV-CCD camera using a laser spot

We present a simple method for charge-coupled device (CCD; or CMOS) sensor characterization by using a subpixel laser spot. This method is used to measure the variations in sensitivity of the 2D sensor array systems equipped with a microlens array. The experimental results show that there is variation in the sensitivity for each position on the CCD of the camera, and the pixel optical center error with respect to the geometrical center is in the range of one-tenth that of a pixel. The disparity observed is attributed to the coherence of the laser light used that generates interference at the scale of the pixel. This may have significant consequences for coherent light imaging using CCD (or CMOS) such as particle image velocimetry.

2D hall sensor array for measuring the position of a magnet matrix

Hall sensor array is used to measure the position of a magnet array and needs compensation of errors caused by manufacturing tolerances as well as harmonic components of the magnetic flux of the magnet array. This paper presents a 2D hall sensor array to measure the position of a magnet matrix by filtering harmonic components. First, 1D sensor array to filter out arbitrary number of harmonic signals is proposed and extended to 2D sensor array for the position measurement of a magnet matrix. A 2D hall sensor array for filtering both fundamental and 2nd harmonic components of the magnetic flux and its conditioning circuit board are built to measure the position of the magnet matrix for a planar motor. Finally, performances of filtering harmonics and measuring position of the magnet matrix are experimentally verified with a XY linear motion stage. The proposed 2D hall sensor array is compatible to a sine encoder and is directly applicable to commercial motion controllers.

Static Hyperspectral Fluorescence Imaging of Viscous Materials Based on a Linear Variable Filter Spectrometer

This paper presents a low-cost hyperspectral measurement setup in a new application based on fluorescence detection in the visible (Vis) wavelength range. The aim of the setup is to take hyperspectral fluorescence images of viscous materials. Based on these images, fluorescent and non-fluorescent impurities in the viscous materials can be detected. For the illumination of the measurement object, a narrow-band high-power light-emitting diode (LED) with a center wavelength of 370 nm was used. The low-cost acquisition unit for the imaging consists of a linear variable filter (LVF) and a complementary metal oxide semiconductor (CMOS) 2D sensor array. The translucent wavelength range of the LVF is from 400 nm to 700 nm. For the confirmation of the concept, static measurements of fluorescent viscous materials with a non-fluorescent impurity have been performed and analyzed. With the presented setup, measurement surfaces in the micrometer range can be provided. The measureable minimum particle size of the impurities is in the nanometer range. The recording rate for the measurements depends on the exposure time of the used CMOS 2D sensor array and has been found to be in the microsecond range.

LOW-COST DEVICE FOR C-SCAN PHOTOACOUSTIC IMAGING

The prostate gland or other region of interest is stimulated with laser light, resulting in ultrasound waves (photoacoustic effect) which are focused by an acoustic lens and captured by a specific 1- or 2D sensor array and subsequently displayed as a C-scan on a computer screen. The amplitude of the ultrasound waves generated by laser stimulation is proportional to the optical absorption of the tissue element at that spatial location. Variability in tissue absorption results in C-scan image contrast. The photoacoustic imaging is combined with an ultrasound C-scan image produced with a plane ultrasound wave applied to the region of interest.

Space Charge Measurement System Using Sensor Array and Semiconductor Analog Switch

Real time space charge measurement systems are also required to observe the localized space charge distribution in insulating materials. Since the space charge behavior is generally transient, a real time measurement system will be also required in the two or three dimensional (2D or 3D) space charge measurement systems. A 2D sensor array PEA system has been developed. However, the number of sensors is limited by A/D converter channel, because the cost of A/D converter is expensive. So, an improved 2D space charge measurement system in lateral and thickness direction for transient space charge has been developed by using the sensor array and semiconductor analog switch unit in the PEA method.

Exploiting sparsity in time-of-flight range acquisition using a single time-resolved sensor

Range acquisition systems such as light detection and ranging (LIDAR) and time-of-flight (TOF) cameras operate by measuring the time difference of arrival between a transmitted pulse and the scene reflection. We introduce the design of a range acquisition system for acquiring depth maps of piecewise-planar scenes with high spatial resolution using a single, omnidirectional, time-resolved photodetector and no scanning components. In our experiment, we reconstructed 64 × 64-pixel depth maps of scenes comprising two to four planar shapes using only 205 spatially-patterned, femtosecond illuminations of the scene. The reconstruction uses parametric signal modeling to recover a set of depths present in the scene. Then, a convex optimization that exploits sparsity of the Laplacian of the depth map of a typical scene determines correspondences between spatial positions and depths. In contrast with 2D laser scanning used in LIDAR systems and low-resolution 2D sensor arrays used in TOF cameras, our experiment demonstrates that it is possible to build a non-scanning range acquisition system with high spatial resolution using only a standard, low-cost photodetector and a spatial light modulator.

Exploiting AML algorithm for multiple acoustic source 2D and 3D DOA estimations

The approximate maximum likelihood (AML) algorithm shows promises for joint estimations of acoustic source spectrum and direction-of-arrival (DOA). For the multisource case, the AML algorithm remains feasible as one considers an alternating projection procedure based on sequential iterative search on single source parameters. In order to perform multisource beamforming operations, earlier, we used a two-dimensional (2D) sensor array with 2D AML to obtain the DOA estimations for sources in the far field of the array. Now, the 3D AML algorithm enables a 3D sensor array to collect acoustic data for estimation of DOA represented in azimuth and elevation angles. In this paper, we consider two acoustic sources being simultaneously active and derive Cramer-Rao bound (CRB) on DOA estimation given by a 3D array. Also, we compare the two-source DOA estimation results of 2D AML and 3D AML, respectively. To construct reasonable constraints on sensor topology, we adopt the concept of isotropic array and decouple the interplay between azimuth and elevation angle variations. The 3D AML algorithm exhibits superior spatial separation over its 2D counterpart for multiple sources located at different azimuth and elevation angles. Although the 3D AML algorithm requires more demanding computational complexity, we have developed a number of efficiency enhancements to reduce the loading for the grid search.

Geometrical camera calibration with diffractive optical elements

Traditional methods for geometrical camera calibration are based on calibration grids or single pixel illumination by collimated light. A new method for geometrical sensor calibration by means of diffractive optical elements (DOE) in connection with a laser beam equipment is presented. This method can be especially used for 2D-sensor array systems but in principle also for line scanners.