Compliant Sensor Research Articles

Freezing from the heat: Building overcooling in Qatar

Qatar has the unfortunate reputation of the country with the highest per-capita carbon emissions in the world, 15% of which is due to space cooling demand. We investigate the extent to which this demand is driven by ‘overcooling’ in non-domestic buildings, that is low indoor set-points resulting in wasted energy and cold thermal discomfort. Using a recently developed overcooling metric, ISO 7730 compliant sensors and occupant survey data comprising 2,472 responses from eight morphologically diverse office buildings, we find that 32% of occupants can be classed as uncomfortably cold. Our analysis implicates the application of the ‘international’ ASHRAE 55 thermal comfort standard in the observed overcooling. Using computer models of the studied buildings, we find that this overcooling is responsible for 27% of their cooling energy demand, translating to 4% of national cooling energy demand and 2% of carbon for all non-domestic buildings. Thus, a simple upward adjustment of set-point temperatures by ∼ 2°C in non-domestic buildings would greatly improve comfort and reduce energy consumption and carbon emissions without changing the building or its systems. This suggests an urgent need for a new localised thermal comfort standard, with wider regional applicability due to similar culture and climates. Practical application Eliminating building overcooling yields in a notable reduction of 8.5% of cooling energy demand for every 1°C increase in the indoor setpoint temperature. This reduction results in an average of 32.3 kW·h/m2/yr in a typical office building in Qatar. A comfort temperature of 24.9°C is suggested which is on average of 1.5°C warmer than the currently applied indoor temperature setpoint, resulting in substantial cooling reductions and increased thermal comfort. In practice, raising the indoor setpoint temperature by 2°C could prevent building overcooling, improve indoor thermal comfort and reduce cooling energy demand across in warm climates.

Anisotropy in magnetic materials for sensors and actuators in soft robotic systems.

The field of soft intelligent robots has rapidly developed, revealing extensive potential of these robots for real-world applications. By mimicking the dexterities of organisms, robots can handle delicate objects, access remote areas, and provide valuable feedback on their interactions with different environments. For autonomous manipulation of soft robots, which exhibit nonlinear behaviors and infinite degrees of freedom in transformation, innovative control systems integrating flexible and highly compliant sensors should be developed. Accordingly, sensor-actuator feedback systems are a key strategy for precisely controlling robotic motions. The introduction of material magnetism into soft robotics offers significant advantages in the remote manipulation of robotic operations, including touch or touchless detection of dynamically changing shapes and positions resulting from the actuations of robots. Notably, the anisotropies in the magnetic nanomaterials facilitate the perception and response with highly selective, directional, and efficient ways used for both sensors and actuators. Accordingly, this review provides a comprehensive understanding of the origins of magnetic anisotropy from both intrinsic and extrinsic factors and summarizes diverse magnetic materials with enhanced anisotropy. Recent developments in the design of flexible sensors and soft actuators based on the principle of magnetic anisotropy are outlined, specifically focusing on their applicabilities in soft robotic systems. Finally, this review addresses current challenges in the integration of sensors and actuators into soft robots and offers promising solutions that will enable the advancement of intelligent soft robots capable of efficiently executing complex tasks relevant to our daily lives.

Piezoresistive Performance of a Compliant Scalable Sensor Made of Low‐Cost Exfoliated Graphite Polymer Composites

Compliant sensors have drawn considerable interests in human–robot interactions. However, it is still challenging to obtain reliable and reproducible performance at reduced costs, especially for scaled‐up sensing applications. This work investigates the piezoresistive performance of a low‐cost, scalable sensor, which is made by spray coating exfoliated graphite natural rubber latex composites (EG/latex) over an elastomeric substrate. The EG/latex sensor provides a uniform distribution in the sheet resistance (variation below 4%) over a large 10 cm × 10 cm area. The stabilized piezoresistive response under cyclic tests is found highly related to both the filler concentration and the strain region. The 8 wt% and the 10 wt% sensors show nonlinear piezoresistive response, while the 25 wt% sensor exhibits the best linearity with a high gauge factor (7.6) and the lowest hysteresis (0.043) under a maximum strain of 60%. The distinct piezoresistive behavior is found attributed to the different levels of microbreakage under the applied strain. Scalable sensing performance is demonstrated on a hand‐size glove spray‐coated with 25 wt% EG/latex. Effective localization of contacts over the continuous sensing glove is achieved via the technique of electrical impedance tomography, validating the potential of the cost‐competitive EG/latex sensor for scalable sensing applications.

Properties and special phenomena of strain sensors made of carbon particle-filled elastomers

Abstract Elastomers with a percolative network of carbon particles are a frequently studied class of materials for applications requiring high elongation and compliant sensors. For novel applications such as soft robots or smart textiles, these have some advantages over traditional strain gauges. However, their functionality is not fully understood. In this work, such materials are investigated as strain sensors in terms of their dynamic behavior, and their current limitations are demonstrated. It becomes clear that such sensors exhibit a non-monotonic behavior under dynamic loads that differs significantly from that observed in quasi-static tests. Two strategies for improving sensor characteristics are derived, modeled, and experimentally tested using the results and an electro-mechanical network model. First, a melt-spinning process that orients the carbon nanotube particles in the process direction creates different degrees of anisotropy. Second, to generate a local negative transverse contraction, an additional auxetic support structure is used. While the resulting anisotropy is insufficient to improve sensor properties, the auxetic structure can significantly improve strain sensitivity.

Finite Element Analysis of a High Compliant Balloon with Strain Sensing Segments for the Identification of Biomechanical Properties within Tubular Vessels

Conventional methods like Optical Coherence Tomography, Angiography, Fractional Flow Reduction, and Intravascular Ultrasound are used to identify stenosis within arterial vessel segments. These modalities are in fact not ideal to identify the diverse mechanical characteristics of the local tissue. To address the shortcomings, a strain sensing high compliant balloon system for tactile assessment of the vessel lumen is currently investigated in our groups. In this study, four exemplary balloon tissue interactions based on its corresponding 2D traverse cross section are simulated within a Finite Element Analysis (FEA). The simulations were analyzed in respect to the evolving strain states in 5 neighboring sensor elements along the balloon surface.The conducted discussion evaluates the relevance of the obtained interdependencies in respect to an interpretation of the sensor segments, aiming at determining biomechanical properties of the surrounding vessel segment. Thereby, comparisons among the sensor segments and with respect to inflation sequences in the other vessel segments are discussed. Finally, relevant dependencies and design guidelines for strain sensing high compliant balloons are presented, paving the way for future refined FEA studies which ultimately aims for an intraoperative parameter identification with such high compliant sensor balloons.

Novel Calibration Methodologies for Compliant, Multiaxis, Fiber-Optic-Based Force/Torque Sensors

In this article, we present a number of novel calibration methodologies for compliant, multiaxis, fiber-optic-based force/torque sensors and evaluate the performance of these methods in real-time experiments with our custom-designed sensors. These methods address the challenges arising from the complex dynamic behavior of a compliant sensor and the nonlinearities in the force–deflection relationship. This article also investigates compliant sensor performance against its response characteristics, such as the impact of compliance on the sensor’s bandwidth using dynamic modeling and identification process. A powerful hysteresis compensation solution using a dynamic estimation model is also presented. Furthermore, we propose and apply a new calibration strategy building on the combination of a linear dynamic model combined with a static nonlinear model. This includes a state space (SS) model with Gaussian Process Regression (GPR) model named SSGPR. The results achieved from the proposed calibration method have revealed an improvement from an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> -squared value of 93.86%–100% when compared to data obtained using a linear dynamic model.

Magneto-Active Elastomer Filter for Tactile Sensing Augmentation Through Online Adaptive Stiffening

The mechanical properties of a sensor strongly affect its tactile sensing capabilities. The role of morphology and stiffness on the quality of the tactile data has already been the subject of several studies, which focus mainly on static sensor designs and design methodologies. However, static designs always come with trade-offs: considering stiffness, soft compliant sensors ensure a better contact, but at the price of mechanically filtering and altering the detected signal. Conversely, online adaptable filters can tune their characteristics, becoming softer or stiffer when needed. We propose a magneto-active elastomer filter which, when placed on top of the tactile unit, allows the sensor to change its stiffness on demand. We showcase the advantages provided by online stiffening adaptation in terms of information gained and data structure. Moreover, we illustrate how adaptive stiffening influences classification, using 9 standard machine learning algorithms, and how adaptive stiffening can increase the classification accuracy up to 34% with respect to static stiffness control.

Vulnerable Road User Protection from Heavy Goods Vehicles Using Direct and Indirect Vision Aids

In Europe, heavy goods vehicles (HGVs) are disproportionately involved in serious and fatal collisions with vulnerable road users (VRUs). An interrogation of 2019 national crash data for Great Britain (Stats19) suggested that detection of cyclists and pedestrians in the nearside and front blind spots of HGVs is still a significant problem during forward or left-turn manoeuvres of the HGV. To improve detection, Transport for London introduced Direct Vision and Safe System Standards in 2021 for HGVs entering the Greater London area. This research assessed the efficacy of one of the Safe System requirements—the fitment of sensors to detect vulnerable road users on the nearside of the vehicle. A physical testing procedure was developed to determine the performance of a sensor system meeting the Transport for London Safe System requirements. Overall, the Safe System compliant sensor system missed 52% of expected detection nodes on the nearside of the vehicle. A total of 56% of the “stop vehicle” nodes, 45% of the “slow down” and 48% of the “proceed with caution” nodes were not recognised. The most forward sensor did not fully cover the front-left corner blind spot, missing 70% of the desired detection nodes. Nearside sensor systems fitted to Safe System requirements may cover a reasonable area but could still leave many undetected zones to the left and front of the vehicle. Standardising sensor range and location could help to eliminate sensor blind spots. Mandating additional front sensors would help cover the blind spot at the front-left corner of the HGV.

Soft Elastomeric Capacitor for Strain and Stress Monitoring on Sutured Skin Tissues.

Sutures are ubiquitous medical devices for wound closures in human and veterinary medicine, and suture techniques are frequently evaluated by comparing tensile strengths in ex vivo studies. Direct and nondestructive measurement of tensile force present in sutured biological skin tissue is a key challenge in biomechanical fields because of the unique and complex properties of each sutured skin specimen and the lack of compliant sensors capable of monitoring large levels of strain. The authors have recently proposed a soft elastomeric capacitor (SEC) sensor that consists of a highly compliant and scalable strain gauge capable of transducing geometric variations into a measurable change in capacitance. In this study, corrugated SECs are used to experimentally characterize the inherent biomechanical properties of canine skin specimens. In particular, an SEC corrugated with a re-entrant hexagonal honeycomb pattern is studied to monitor strain and stresses for three specific suture patterns: simple interrupted, cruciate, and intradermal patterns. Stress is estimated using constitutive models based on the Fractional Zener and the Kelvin-Voigt models, parametrized using a particle swarm algorithm from experimental data and results from a validated finite element model. Results are benchmarked against findings from the literature and show that SECs are valuable for clinical evaluation of tensile force in biological skins. It was found that both the ranking of suture pattern performance and the sutured skin's Young's modulus using the proposed approach agreed with data reported in the literature and that the estimated stress at the suture level closely matched that of an approximate finite element model.

Bringing flexibility to giant magnetoresistive sensors directly grown onto commercial polymeric foils

The emergence of augmented reality, robotics and point-of-care biosensors has pushed forward the frontiers of compliant sensors with mechanical resilience and capability of being arbitrarily shaped upon demand. Here, we report exchange-biased spin valve structures directly fabricated on 25 μm thick commercial polymeric substrates. Linear electrical response with low coercivity was demonstrated for sensors grown on polymers. Despite the higher roughness of polymers ≃0.75 nm, a Néel coupling field comparable to that of samples grown on conventional SiO2 substrates was shown. Nevertheless, significant changes in the linear range of polymeric samples were observed, together with changes in the shift of the transfer curve. The measurements also indicate deviations from fully orthogonal magnetization orientation achieved in patterned linear sensors. Such results were ascribed to the presence of a non-negligible magnetostrictive component, most likely due to residual mechanical stress in the sensor's free- and pinned-layers. To support the study, a macrospin model was developed, considering the magnetoelastic anisotropy, to address in particular the impact of mechanical stress on final sensor output.

F-TOUCH Sensor: Concurrent Geometry Perception and Multi-Axis Force Measurement

Force and tactile sensing has experienced a surge of interest over recent decades. It conveys a range of information through physical interaction, including force, pressure, texture and vibration. Even so, existing tactile sensors have difficulty in precisely acquiring force signals to capture magnitude and direction. This is because vision-based tactile sensors, such as the popular GelSight sensor, estimate force and torque from discrete markers embedded in a compliant sensor interface and detected via image processing, resulting in force errors that are sizeable. This paper presents a novel design for a force/tactile sensor, namely F-TOUCH (Force and Tactile Optically Unified Coherent Haptics), that signifies an appreciable advance on current state-of-the-art tactile sensors. In addition to the acquisition of geometric features via the (silicone) elastomeric component at a high spatial resolution, our sensor incorporates a deformable spring-mechanism structure, allowing us to measure translational and rotational force and torque along six axes with high accuracy. The proposed sensor contains three principal components: a coated elastomer layer acting as the compliant sensing medium, spring mechanisms acting as deformable structural elements, and a camera for image capture. The camera records the deformation of the structural elements as well as the distortion of the compliant sensing medium, concurrently acquiring force and tactile information. The sensor is calibrated with the aid of a commercial ATI Industrial Automation <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> 6-axis force/torque sensor. An experimental study reveals that the F-TOUCH sensor outperforms GelSight as a force sensor.

Surface Textures for Stretchable Capacitive Strain Sensors

Advances in smart materials and electronics have enabled compliant sensing systems mimicking nature. In structural health monitoring (SHM) applications, the technology still lacks accuracy in tracking measurements over large geometries. Herein we report a facile fabrication for a compliant sensor based on an elastomeric capacitive sensor technology, augmented with a scalable surface texture inspired by various grid geometries. Texture designs are selected to direct, and therefore improve, the strain sensing capabilities and thus the sensor’s gauge factor. We showcase selected designs based on a detailed engineering analysis. The selected surface patterns are investigated under increasing strain targets and strain frequency sweeps. Results confirm that a stretchable thermoplastic composite sensor with a textured pattern embossed into the hyper-elastic matrix yields approximately 30% increase in the gauge factor and 35% in signal accuracy, great linearity up to 30% strain, and overall signal stability to empower SHM applications.

Numerical Investigation of Auxetic Textured Soft Strain Gauge for Monitoring Animal Skin.

Recent advances in hyperelastic materials and self-sensing sensor designs have enabled the creation of dense compliant sensor networks for the cost-effective monitoring of structures. The authors have proposed a sensing skin based on soft polymer composites by developing soft elastomeric capacitor (SEC) technology that transduces geometric variations into a measurable change in capacitance. A limitation of the technology is in its low gauge factor and lack of sensing directionality. In this paper, we propose a corrugated SEC through surface texture, which provides improvements in its performance by significantly decreasing its transverse Poisson’s ratio, and thus improving its sensing directionality and gauge factor. We investigate patterns inspired by auxetic structures for enhanced unidirectional strain monitoring. Numerical models are constructed and validated to evaluate the performance of textured SECs, and to study their performance at monitoring strain on animal skin. Results show that the auxetic patterns can yield a significant increase in the overall gauge factor and decrease the stress experienced by the animal skin, with the re-entrant hexagonal honeycomb pattern outperforming all of the other patterns.

Sustainable manufacturing of sensors onto soft systems using self-coagulating conductive Pickering emulsions.

Compliant sensors based on composite materials are necessary components for geometrically complex systems such as wearable devices or soft robots. Composite materials consisting of polymer matrices and conductive fillers have facilitated the manufacture of compliant sensors due to their potential to be scaled in printing processes. Printing composite materials generally entails the use of solvents, such as toluene or cyclohexane, to dissolve the polymer resin and thin down the material to a printable viscosity. However, such solvents cause swelling and decomposition of most polymer substrates, limiting the utility of the composite materials. Moreover, many such conventional solvents are toxic or otherwise present health hazards. Here, sustainable manufacturing of sensors is reported, which uses an ethanol-based Pickering emulsion that spontaneously coagulates and forms a conductive composite. The Pickering emulsion consists of emulsified polymer precursors stabilized by conductive nanoparticles in an ethanol carrier. Upon evaporation of the ethanol, the precursors are released, which then coalesce amid nanoparticle networks and spontaneously polymerize in contact with the atmospheric moisture. We printed the self-coagulating conductive Pickering emulsion onto a variety of soft polymeric systems, including all-soft actuators and conventional textiles, to sensitize these systems. The resulting compliant sensors exhibit high strain sensitivity with negligible hysteresis, making them suitable for wearable and robotic applications.

Experimental evaluation of the impact of packet length on wireless sensor networks subject to interference

Wireless sensor networks are nowadays considered an enabling technology for a wide spectrum of cyber-physical systems applications. However, in order to cope with stringent dependability and energy efficiency requirements, several research challenges have to be solved. Electromagnetic interference, for instance, adversely affects wireless communication, resulting into increased packet collisions and network congestion, and also increasing the energy consumption of devices. Highlighting the complex interplay between communication under interference and parameters of sensor networks is therefore mandatory for driving design choices and improving system performance. In this work we propose an experimental study of the reliability and energy efficiency of IEEE 802.15.4 compliant sensor networks under controlled interference, as a function of the packets length. The results of an extensive set of experiments on an ample range of low-power asynchronous, medium access protocols point out the trade-off between energy consumption and robustness to interference and also provide a comparative view of the protocols, thus indicating useful guidelines in the choice and in the design of several critical components.

Liquid metal-based resistive membranes for flow acoustics detection

Fluid flow over bodies results in vortex and turbulence-induced vibrations that reflect the boundary layer conditions and lead to drag and noise production. Many platforms would benefit from real-time monitoring of these acoustic signatures, providing feedback for control or increased efficiency. Of particular interest are compliant pressure sensors that can be wrapped around any surface. We present a silicone membrane integrated with a liquid phase gallium-indium coil. As the device deforms under pressure fluctuations, the resistance across the liquid metal can be monitored for real-time detection of flow features. The fabrication process is presented, including a masked spray deposition of gallium-indium coupled with layering of elastomer. The 18 mm-diameter sample demonstrates a point load sensitivity of 0.045 W/mm. To evaluate the performance of the device, it is tested in air with an acoustic tube, and its output is compared to impedance analysis at frequencies up to 10 kHz. Furthermore, the membrane vibrations are tracked with scanning laser Doppler vibrometer. These results are then compared to analytic models and COMSOL simulations. The sensor presented here is well suited to bulk fabrication in functional “skins” of active elements. [Work sponsored by the Office of Naval Research.]

A Novel Elastic Structure for Three-Axis Force/Torque Sensor: Kinematic Design and Feasibility Study

In this paper, we present a new elastic structure for a three-axis force/torque (F/T) sensor to enhance the compliant characteristics of an F/T sensor. Compliant sensor structure is desirable for safe human-robot interaction when the F/T sensor is used to measure the interaction force. The proposed structure is composed of a serial connection of three flexure hinges. While elastic structures of most F/T sensors are determined by experience and experiments, the proposed sensor structure is determined by analyzing the geometric structure of the desired compliance matrix. Observing the geometric characteristics of the desired compliance matrix reveals a unique form of the spring model, which consists of three serially connected torsional springs whose axes lie on the same plane. From this spring model, a new concept of a serial-based sensor structure is designed to sense the magnitude and point of action of an applied force. Feasibility of the proposed concept as an F/T sensor structure is confirmed by a finite-element analysis and experiments with a sensor test bed.

A Novel Model to Simulate Flexural Complements in Compliant Sensor Systems.

The main challenge in analyzing compliant sensor systems is how to calculate the large deformation of flexural complements. Our study proposes a new model that is called the spline pseudo-rigid-body model (spline PRBM). It combines dynamic spline and the pseudo-rigid-body model (PRBM) to simulate the flexural complements. The axial deformations of flexural complements are modeled by using dynamic spline. This makes it possible to consider the nonlinear compliance of the system using four control points. Three rigid rods connected by two revolute (R) pins with two torsion springs replace the three lines connecting the four control points. The kinematic behavior of the system is described using Lagrange equations. Both the optimization and the numerical fitting methods are used for resolving the characteristic parameters of the new model. An example is given of a compliant mechanism to modify the accuracy of the model. The spline PRBM is important in expanding the applications of the PRBM to the design and simulation of flexural force sensors.

Flexoelectric response in soft polyurethane films and their use for large curvature sensing

The flexoelectric effect is simply defined as the coupling between the strain gradient and polarization in solid dielectrics. It may be seen as an alternative transduction mechanism to the piezoelectric effect to directly sense the curvature of bent flexible thin structures. In the case of large curvatures, flexible and compliant sensors are required and soft polar elastomers may be suitable for curvature sensing. In this study, we report the flexoelectric characterization of soft semi-crystalline polyurethane (PU) films with thicknesses ranging from 1.7 μm to 350 μm. Dynamic bending experiments have been performed on PU films deposited onto rigid steel substrates in the vicinity of the mechanical resonance frequency of the cantilever beams. Quasi-static flexoelectric coefficients of PU films could be obtained by using a classical oscillating model. A global large increase of μ12′ with the decreasing film thickness was found, especially for thicknesses lower than 25 μm. The variation of μ12′ is explained by the presence of a Young's Modulus gradient through the thickness of PU films. Besides, a concomitant uncommon dramatic decrease in the dielectric constant is observed. The combination of these two effects contributes to enhancing the flexocoupling “F” constant with the decreasing thickness. At last, the potential use of a 6.6 μm-thick soft PU film as a large curvature sensor has been experimentally demonstrated by subjecting a flexible Aluminum foil/Polyethylene terephthalate bilayered cantilever to large deflections. A curvature of about 80 m−1 (radius of curvature of ∼1.2 cm) could be sensed under low frequency (3 Hz) bending motion. These results may pave the way for the development of low cost and easy to implement soft flexoelectric elastomer-based large curvature sensors on highly flexible metallic structures.

Targeted Feature Recognition Using Mechanical Spatial Filtering with a Low-Cost Compliant Strain Sensor

A tactile sensing architecture is presented for detection of surface features that have a particular target size, and the concept is demonstrated with a braille pattern. The approach is akin to an inverse of mechanical profilometry. The sensing structure is constructed by suspending a stretchable strain-sensing membrane over a cavity. The structure is moved over the surface, and a signal is generated through mechanical spatial filtering if a feature is small enough to penetrate into the cavity. This simple design is tailorable and can be realized by standard machining or 3D printing. Images of target features can be produced with even a low-cost compliant sensor. In this work a disposable elastomeric piezoresistive strain sensor was used over a cylindrical “finger” part with a groove having a width corresponding to the braille dot size. A model was developed to help understand the working principle and guide finger design, revealing amplification when the cavity matches the feature size. The new sensing concept has the advantages of being easily reconfigured for a variety of sensing problems and retrofitted to a wide range of robotic hands, as well as compatibility with many compliant sensor types.