Bending Sensor Research Articles

Numerical framework for anisotropic flexible piezoelectrics with large deformation

Flexible piezoelectric materials with high stretchability have been realized in industrialization production. Among them, piezoelectric fiber composites made of elastomers and piezoelectric polymers have gained widespread attentions in smart applications. In general, piezoelectric fiber composites can be considered as anisotropic materials. However, most existing piezoelectric constitutive models, especially those integrated into commercial software, only account for small deformation conditions, which are not suitable for anisotropic flexible piezoelectrics. In the present work, a novel model for anisotropic flexible piezoelectric material is established based on the anisotropic Neo-Hookean material model and electromechanical theory. The finite element framework is implemented in the commercial finite element software ABAQUS via a user element subroutine, with a monolithic algorithm procedure employed to solve the nonlinear governing equations. To validate the accuracy of finite element implementation, analytical solutions for equibiaxial stretching of transversely isotropic piezoelectric have been compared with simulated results. Furthermore, the influences of anisotropy level on electromechanical response are also discussed. Subsequently, numerical simulations of bending actuators and sensors further demonstrate the robustness of the developed elements. The obtained results show that the anisotropy direction and level have both favorable and unfavorable influences on structure performances, which could provide a reference for the design of anisotropic flexible piezoelectric structures.

Directional Bending Sensor and Multiparameter Sensor Based on D-Core Multimode Fiber

For the sensor with interference between fiber core mode and cladding mode, the directional bending sensor can be realized by processing asymmetric structure on the cladding or using eccentric fiber, but the temperature and strain crosstalk of the sensor with interference between fiber core mode and cladding mode is large. In this article, a kind of fiber intermode interference sensor with asymmetric structure is proposed, which can realize vector bending sensing and reduce temperature and strain crosstalk. The designed D-core multimode fiber has cladding and air on both sides of the fiber core. When 0° and 180° bending are applied to it, the interference spectrum moves in different directions, thus realizing the recognition of bending direction. For curvature detection, in the curvature range of 0–0.62 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , the sensitivity of 0° bending is 11.00 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , and that of 180° bending is −11.40 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> . For liquid level detection, the sensing sensitivity is 234 pm/mm with the range of 0–4 cm, and the linearity is 0.993. In addition, the temperature sensitivity is as low as 10 pm/°C, and the strain sensitivity is as low as −1.28 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> . The proposed sensor realized the directional bending sensing and liquid level sensing of intermodal interference. Its low sensitivity to temperature and strain can be used to avoid the cross-sensitivity problems. In the field of water flow detection, the simultaneous detection of water level and water flow direction can be realized by the proposed sensor.

Personalized and Safe Soft Glove for Rehabilitation Training

Traditional hand rehabilitation devices present a challenge in providing personalized training that can lead to finger movements exceeding the safe range, resulting in secondary injuries. To address this issue, we introduce a soft rehabilitation training glove with the function of safety and personalization, which can allow patients to select training modes based on rehabilitation and provide real-time monitoring, as well as feedback on finger movement data. The inner glove is equipped with bending sensors to access the maximum/minimum angle of finger movement and to provide data for the safety of rehabilitation training. The outer glove contains flexible drivers, which can drive fingers for different modes of rehabilitation training. As a result, the rehabilitation glove can drive five fingers to achieve maximum extension/flexion angles of 15.65°/85.97°, 15.34°/89.53°, 16.78°/94.27°, 15.59°/88.82°, and 16.73°/88.65°, from thumb to little finger, respectively, and the rehabilitation training frequency can reach six times per minute. The safety evaluation result indicated an error within ±6.5° of the target-motion threshold. The reliability assessment yielded a high-intra-class correlation coefficient value (0.7763–0.9996). Hence, the rehabilitation glove can achieve targeted improvement in hand function while ensuring safety.

Printed Directional Bending Sensor with High Sensitivity and Low Hysteresis for Human Motion Detection and Soft Robotic Perception.

We present a high-performance flexible bending strain sensor for directional motion detection of human hands and soft robotic grippers. The sensor was fabricated using a printable porous conductive composite composed of polydimethylsiloxane (PDMS) and carbon black (CB). The utilization of a deep eutectic solvent (DES) in the ink formulation induced a phase segregation between the CB and PDMS and led to a porous structure inside the printed films after being vapored. This simple and spontaneously formed conductive architecture provided superior directional bend-sensing characteristics compared to conventional random composites. The resulting flexible bending sensors displayed high bidirectional sensitivity (gauge factor of 45.6 under compressive bending and 35.2 under tensile bending), negligible hysteresis, good linearity (>0.99), and excellent bending durability (over 10,000 cycles). The multifunctional applications of these sensors, including human motion detection, object-shape monitoring, and robotic perceptions, are demonstrated as a proof-of-concept.

Femtosecond Laser Line-by-Line Inscribed Seven Core Fiber Cascaded Fabry–Perot Cavity and Its Vectorial Bending Sensing Application

Multi-core fibers have been widely used for vector-bending sensing due to their off-axis distributed cores. In contrast to vector-bending sensors based on Bragg gratings, fiber Fabry–Perot (F–P) interferometers are more advantageous due to their ease of fabrication and potential for introducing the Vernier effect to further improve sensitivity. We propose and experimentally demonstrate a cascaded Fabry–Perot (F–P) cavity vector bending sensor. From the experimental results, the sensor has a strong bending dependence with a maximum sensitivity of 123.12 pm/m−1, and the curvature magnitude and direction can be reconstructed from the tilted wavelength shift of the asymmetric fiber-core F–P cavities.

Theoretical and experimental study on FBG bending sensor for quantitative monitoring of bolt looseness

A quantitative monitoring method for bolt looseness using an elastic substrate fiber Bragg grating (FBG) bending sensor is presented in this paper, which is achieved by measuring the wavelength shift of the FBG sensor response to the bending curvature change induced by bolt loose angle variation. The theoretical sensing principle is established based on the deformation model of an inextensible elastic beam under prescribed end rotations. The influence parameters on the FBG bending sensor are investigated numerically under the bolt loose angle variation from 0° to 15°, and the position on the sensor with maximum curvature difference varying linearly to bolt loose angle variation is obtained and selected to arrange the FBG. Based on the theoretical and numerical analysis, a general solution of the wavelength shifts of the FBG bending sensor versus bolt loose angle under an optimal initial installation condition (i.e., d/l=0.9, and s/l = 0.85) is established for quantitative monitoring bolt looseness. The feasibility and performance of the proposed method are experimentally verified and the maximum error between numerical and experimental results is less than 6.19%. The results indicate that the proposed method can quantitatively monitor bolt loose angle with high resolution (about 0.005°), also prove that the theoretical derivation in this paper and the proposed design criteria can theoretically guide the monitoring of bolt looseness using FBG bending sensor.

Self-Cleaning Bending Sensors Based on Semitransparent ZnO Nanostructured Films.

The design of multifunctional nanostructured materials is the key to the development of smart wearable devices. For instance, nanostructures endowed with both piezoelectric and photocatalytic activities could well be the workhorse for solar-light-driven self-cleaning wearable sensors. In this work, a simple strategy for the assembly of a flexible, semitransparent piezophotocatalytic system is demonstrated by leveraging rational wet chemistry synthesis of ZnO-based nanosheets/nanoflowers (NSs/NFs) under basic pH conditions onto flexible ITO/PET supports. A KMnO4 pretreatment before the ZnO synthesis (seeded ZnO) allows for the control of the density, size, and orientation of the NSs/NFs systems compared to the systems produced in the absence of seeding (seedless ZnO). The electrical response of the sensors is extracted at a 1 V bias as a function of bending in the interval between 0 and 90°, being the responsivity toward bending significantly enhanced by the KMnO4 treatment effect. The photocatalytic activity of the sensors is analyzed in aqueous solution (methylene blue, 25 μM) by a solar simulator, resulting in similar values between seedless and seeded ZnO. Upon bending the sensor, the photocatalytic activity of seedless ZnO is almost unaffected, whereas that of seeded ZnO is improved by about 25%. The sensor's reusability and repeatability are tested in up to three different cycles. These results open up the way toward the seamless integration of bending sensitivity and photocatalysis into a single device.

Wearable bending wireless sensing with autonomous wake-up by piezoelectric and triboelectric hybrid nanogenerator

The Internet of Things calls for the demand for wearable self-powered sensors for human motion monitoring and interaction. However, the redundant data and large amount of power consumption limit the application in sustainable wireless sensing. In this study, a flexible hybrid nanogenerator with bridge structure design for self-powered wearable sensor is proposed. When harvesting energy from the finger bent motion, it combines contact separation mode for triboelectric nanogenerator (TENG) with bending d31 mode for piezoelectric generator (PEG). To highlight the significance, we propose an autonomous wake-up wireless sensing method by hybrid generator in wearable bending. Different from recording and transmitting data all the time, the TENG signal as a trigger to record the PEG voltage amplitude as angle sensing data for wireless transmitting. Concise and accurate sensing data with autonomous wake-up is expected to reduce the computational burden and power consumption for wireless sensing and provide more possibilities for wearable wireless monitoring and human-computer interaction. The wireless manipulator interaction experiment based on this hybrid nanogenerator as wearable bending sensor verifies the feasibility of this scheme, which has a great application prospect in wearable self-powered sensors field such as virtual reality and robot control.

Multifunctional Ultraviolet Laser Induced Graphene for Flexible Artificial Sensory Neuron

AbstractNeuromorphic perceptual system (NPS) is inspired by the interaction between organisms and environment, which has boosted many emerging fields like human–machine interfaces and bionic robots. Laser‐induced graphene (LIG) can be a fast, low cost, and accurate patterning technology toward a flexible and biological plausible NPS. While, to unleash the full potential of LIG, thorny issues like low endurance and limited stretchability should be addressed. Herein, high‐performance ultraviolet LIG (UV‐LIG) based sensors and electrodes are introduced for building flexible artificial sensory neuron (ASN). The UV‐LIG can be fabricated with a fine linewidth of ≈75 µm and a high conductivity of 3900 ± 150 S cm−1, which facilitates the demonstration of LIG toolbox, containing bending sensors, flexible heaters, etc. The transferred LIG electrodes retain good conducting property (only ≈12.7% increment in resistance) and enable large stretchability (up to 150% strain), outperforming most of the LIG‐based strain sensors. As a proof‐of‐concept, an artificial sensory neuron that is able to mimic the strain perception of somatosensory system is realized based on the integration of LIG‐based functional components and an InGaZnO (Indium‐Gallium‐Zinc Oxide, IGZO)‐based synaptic transistor. This work can provide an efficient patterning methodology as well as essential components for neuromorphic perceptual systems.

3-D Parallel Fiber Bragg Gratings Bending Sensor Based on Single-Channel Measurement

A bending sensor is proposed to demodulate the multichannel bending signals of multicore fiber (MCF) to single-channel signals by using the phase mask method combined with the thermal diffusion technique (TDT). A four-core fiber (FCF) bending sensor was fabricated. Each fiber core of the FCF had different resonant wavelengths through the method of core-by-core inscribed fiber Bragg gratings (FBGs) at the same axial position. The FCF was coupled with a single-mode fiber (SMF) by the TDT method. Finally, the single-channel measurement of the 3-D parallel FCF FBGs signals was realized. The FBGs on four cores of FCF were equivalent to the FBGs on two orthogonal cores by a different operation. The experimental results showed that the maximum bending sensitivities of the equivalent FBGs were 187.2 and −192.6 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , respectively. This sensor realized single-channel demodulation of MCF signals with the elimination of the crosstalk caused by temperature and strain. The proposed sensor fabrication technology with high integration, high sensitivity, and reduced crosstalk, is expected to be applied in the field of 3-D shape measurement or wearable artificial electronics.

In-Fiber Thermally Diffused Coupler and Fiber Bragg Grating Inscribed in Twin-Core Fiber for Sensitivity-Enhanced Vector Bending Sensing

A vector bending fiber sensor based on core-by-core inscribed fiber Bragg gratings in a twin-core fiber has been proposed and experimentally demonstrated. An in-fiber integrated vector bending sensor is realized by using the thermal diffusion technique to fabricate the coupler. The characteristics of the coupler fabricated by thermal diffusion are simulated and experimented. By inscribing fiber Bragg gratings with different reflection wavelengths in the two cores of a symmetrical twin-core fiber, the curvature sensitivity can be enhanced by tracking the wavelength difference between the fiber Bragg gratings of the two cores. The measured bending sensitivity of the fiber Bragg grating ranges from −161.6 pm/m−1 to +165.5 pm/m−1. The differential sensitivity of the two cores is twice that of a conventional single grating, and the temperature-induced crosstalk is also reduced. The bending sensor proposed in this paper has the advantages of high integration, enhancing the sensitivity and two-dimensional orientation recognizability, and reducing temperature crosstalk, which can be a promising candidate for structural health monitoring or wearable artificial electronics applications.

Ionic liquid crystal elastomers for actuators, sensors, and organic transistors

ABSTRACTIn this paper, we discuss the preparation and several electrical properties of two ionic Liquid Crystal Elastomers. We show that iLCEs can be used as electric actuators, bending sensors and as solid electrolytes of organic electrochemical transistors. We summarise the results using one type of LCE and two types of ionic liquids. It is found that the ionic liquid that phase separates from the reactive mesogenic monomers and forms micron size ionic channels performs better than the less phase separating ionic liquid. The switching time of the order of a second was found to be possible using the ionic liquid with smaller ions and larger channels. These results provide many exciting opportunities in various areas and impact future applications, although they are far from optimised. Fine tuning of the high-performance iLCEs require an intricate compatibility between the LCE and the electrodes and appears to be laborious. On the other hand, the first promising results justify the efforts to study more type of liquid crystal elastomers, ionic liquids and compliant electrodes.

Evaluation of polymer-based eccentric FBG bending sensor for humidity, strain, temperature and torsion

We analyse and evaluate the sensitivity and robustness of an eccentric fiber Bragg grating sensor micro-structured into a polymer optical fiber under different relative humidity, temperature, strain and torsion conditions. Relative humidity and temperature conditions are established with a climate test chamber and prepared salt solutions. Though made of polymer, the cross-sensitivity of the sensor to relative humidity is low, enabling usage in rapidly changing environment applications, e.g., for movement detecting gloves or in vehicles, but also in high moisture situations. We find a linear dependence on temperature, so that either bending or temperature can be measured separately from each other. After rotation of one end, the sensor measures torsion by observing the light intensity and the change of the full width at half maximum. Strain measurement shows multiple elastic strain regions before final plastic deformations occur. In the next step, the flexible sensor system will be implemented in a sensor glove to monitor finger movement and detect different hand gestures.

Electrospun Core–Sheath PVDF Piezoelectric Fiber for Sensing Application

In recent years, piezoelectric polymer sensors are used in energy harvesting and self-powered sensing due to their flexibility, low density, and high piezoelectric constant, and their performances may be improved through a careful architectural design. Herein, we reported a facile strategy for fabricating core-sheath piezoelectric fiber (C-PEF) by directly electrospinning poly(vinylidene fluoride) (PVDF) onto the stainless steel wires. Such C-PEF can well respond to bending deformation with different degrees, and therefore it can be assembled into a piezoelectric bending sensor for airflow speed sensing. Furthermore, spring-like structured C-PEF (S-C-PEF) can serve as a piezoelectric spring sensor and in a sophisticated manner monitor human sleep behavior. This work paves a way for developing multigeometric piezoelectric sensors though a low-cost, facile, and reliable method, showing potential applications including bending sensing and health monitoring.

High-sensitivity vector bend sensor based on a fiber directional coupler inscribed by a femtosecond laser.

In this Letter, we demonstrate a high-sensitivity vector bend sensor based on a fiber directional coupler. The fiber directional coupler is composed of two parallel waveguides inscribed within a no-core fiber (NCF) by a femtosecond laser. Since the two written waveguides have closely matched refractive indices and geometries, the transmission spectrum of the fiber directional coupler possesses periodic resonant dips. Such a fiber directional coupler exhibits a good bending-dependent spectral shift response due to its asymmetric structure. Experimental results show that bending sensitivities of -97.11 nm/m-1 and 58.22 nm/m-1 are achieved for the 0° and 180° orientations in the curvature range of 0-0.62 m-1, respectively. In addition, the proposed fiber directional coupler is shown to be insensitive to external humidity changes, thus improving its suitability in high-accuracy bending measurements.

Hydrometer Design Based on Thin-Film Resistive Sensor for Water Measurement in Residential Buildings

Because of economic, population, and consumption patterns changes, the use of freshwater has increased significantly in the last 100 years. Notably, measurement is essential to encourage water conservation. Thus, the present study aims to evaluate the applicability of a thin-film resistive sensor (bend sensor) with different coatings for implementation in individualized water measurement systems. The motivation of this work is to propose a volumetric meter using flow control valves that ordinarily are already present in a building’s hydraulic installations. Methodologically, the following are presented: the system developed for the electromechanical and thermal characterization of the sensor, the sensor computational simulation performed using Ansys® software, and for the electronic circuit designed in LTSpice® software, the artificial neural network used to estimate the flow and the volume estimates from the trapezoidal pulses. The results obtained allowed us to assess that, taking into account the type of coating, the sensor coated with polyester has better behavior for the proposed hydrometer. In addition, this evaluation allowed us to conclude that the bend sensor demonstrated its feasibility to be used as a transducer of this novel type of volumetric meter and can be easily inserted inside a hydraulic component, such as a flow control valve, for example.

Multifunctional Electronic Textiles by Direct 3D Printing of Stretchable Conductive Fibers

AbstractThe integration of functional fibers into wearable devices by traditional methods is commonly completed in weaving. A new post‐weaving method of integrating fiber devices into textiles is needed to address the challenge of incorporating functional fiber into ready‐made garments without tearing down the clothing and re‐weaving. A 3D printing method to simultaneously fabricate and integrate highly stretchable conductive fiber into ready‐made garments with designed patterns is presented. The fabricated sheath–core fiber consists of a styrene–ethylene–butylene–styrene (SEBS) shell and a Ga–In–Sn alloy liquid metal core. The SEBS shell guarantees the high stretchability (up to 600%) and flexibility, while the liquid metal core offers a high conductivity maintained at large deformation. It is shown that sophisticated patterns, which have millimeter‐level‐resolution that are difficult to be integrated into textiles by weaving, and even more laborious to be incorporated into ready‐made garments, can now be easily modified and implemented into both textiles and ready‐made garments by a time‐saving and low‐cost 3D printing method. Utilizing the electrical characteristics of the fiber in pre‐designed patterns, on‐clothing soft electronics can be printed directly. A printed on‐clothing strain sensor, bending sensor, wireless charging coil, and a touch‐sensing network are demonstrated to show the potential applications in wearable electronics.

Two-dimensional displacement (bending) sensor based on cascaded Fabry-Perot interferometers fabricated in a seven-core fiber.

We demonstrated a two-dimensional vector displacement (bending) sensor with high angular resolution based on Vernier effect generated by two cascaded Fabry-Perot interferometers (FPI) in a seven-core fiber (SCF). To form the FPI, plane-shaped refractive index modulations are fabricated as the reflection mirrors in the SCF using slit-beam shaping and femtosecond laser direct writing. Three pairs of cascaded FPIs are fabricated in the center core and the two non-diagonal edge cores of the SCF and applied to the vector displacement measurement. The proposed sensor exhibits high displacement sensitivity with significant direction dependence. The magnitude and direction of the fiber displacement can be obtained via monitoring the wavelength shifts. Moreover, the source fluctuations and the temperature cross-sensitivity can be referenced out by monitoring the bending-insensitive FPI of the center core.

Spatially-resolved bending recognition based on a learning-empowered fiber specklegram sensor.

Fiber specklegram sensors do not rely on complex fabrication processes and expensive sensor interrogation schemes and provide an alternative to routinely used fiber sensing technologies. Most of the reported specklegram demodulation schemes focus on correlation calculation based on statistical properties or classification according to features, resulting in limited measurement range and resolution. In this work, we propose and demonstrate a learning-empowered spatially resolved method for fiber specklegram bending sensors. This method can learn the evolution process of speckle patterns through a hybrid framework constructed by a data dimension reduction algorithm and regression neural network, which can simultaneously identify the curvature and perturbed position according to the specklegram, even for the unlearned curvature configuration. Rigorous experiments are performed to verify the feasibility and robustness of the proposed scheme, and the results show that the prediction accuracy for the perturbed position is 100%, and the average prediction errors for the curvature of the learned and unlearned configurations are 7.79 × 10-4 m-1 and 7.02 × 10-2 m-1, respectively. The proposed method promotes the application of fiber specklegram sensors in the practical scene and provides insights for the interrogation of sensing signals by deep learning.

Design of a SMA-based soft composite structure for wearable rehabilitation gloves.

The combination of smart soft composite structure based shape memory alloy (SMA) and exoskeleton technology has the advantages of light weight, energy saving, and great human-exoskeleton interaction. However, there are no relevant studies on the application of SMA-based soft composite structure (SSCS) in hand exoskeletons. The main difficulty is that directional mechanical properties of SSCS need to comply with fingers movement, and SSCS can deliver enough output torque and displacement to the relevant joints. This paper aims to study the application of SSCS for wearable rehabilitation gloves and explore its bionic driving mechanism. This paper proposes a soft wearable glove (Glove-SSCS) for hand rehabilitation actuated by the SSCS, based on finger force analysis under different drive modes. The Glove-SSCS can support five-finger flexion and extension, weighs only 120 g, and adopts modular design. Each drive module adopts a soft composite structure. And the structure integrates actuation, sensing and execution, including an active layer (SMA spring), a passive layer (manganese steel sheet), a sensing layer (bending sensor) and connection layers. To obtain a high-performance SMA actuators, the performance of SMA materials was tested in terms of temperature and voltage, temperature at the shortest length, pre-tensile length and load. And the human-exoskeleton coupling model of Glove-SSCS is established and analyzed from force and motion. The results show that the Glove-SSCS can realize bidirectional movements of fingers flexion and extension, with ranges of motion are 90-110° and 30-40°, and their cycles are 13-19 s and 11-13 s. During the use of Glove-SSCS, the temperature of gloves is from 25 to 67°C, and the surface temperature of hands is from 32 to 36°C. The temperature of Glove-SSCS can be kept at the lowest temperature of SMA operation without much impact on the human body.