Capacitive Strain Gauge Research Articles

Aerosol jet printing of advanced capacitive strain gauge for vibration monitoring of the human body

Aerosol jet printing of advanced capacitive strain gauge for vibration monitoring of the human body

Internal Rotation Measurement of the Knee with Polymer-Based Capacitive Strain Gauges versus Mechanical Rotation Measurement Taking Gender Differences into Account: A Comparative Analysis.

With the conventional mechanical rotation measurement of joints, only static measurements are possible with the patient at rest. In the future, it would be interesting to carry out dynamic rotation measurements, for example, when walking or participating in sports. Therefore, a measurement method with an elastic polymer-based capacitive measuring system was developed and validated. In our system, the measurement setup was comprised of a capacitive strain gauge made from a polymer, which was connected to a flexible printed circuit board. The electronics integrated into the printed circuit board allowed data acquisition and transmission. As the sensor strip was elongated, it caused a change in the spacing between the strain gauge's electrodes, leading to a modification in capacitance. Consequently, this alteration in capacitance enabled the measurement of strain. The measurement system was affixed to the knee by adhering the sensor to the skin in alignment with the anterolateral ligament (ALL), allowing the lower part of the sensor (made of silicone) and the circuit board to be in direct contact with the knee's surface. It is important to note that the sensor should be attached without any prior stretching. To validate the system, an in vivo test was conducted on 10 healthy volunteers. The dorsiflexion of the ankle was set at 2 Nm using a torque meter to eliminate any rotational laxity in the ankle. A strain gauge sensor was affixed to the Gerdii's tubercle along the course of the anterolateral ligament, just beneath the lateral epicondyle of the thigh. In three successive measurements, the internal rotation of the foot and, consequently, the lower leg was quantified with a 2 Nm torque. The alteration in the stretch mark's length was then compared to the measured internal rotation angle using the static measuring device. A statistically significant difference between genders emerged in the internal rotation range of the knee (p = 0.003), with female participants displaying a greater range of rotation compared to their male counterparts. The polymer-based capacitive strain gauge exhibited consistent linearity across all measurements, remaining within the sensor's initial 20% strain range. The comparison between length change and the knee's internal rotation angle revealed a positive correlation (r = 1, p < 0.01). The current study shows that elastic polymer-based capacitive strain gauges are a reliable instrument for the internal rotation measurement of the knee. This will allow dynamic measurements in the future under many different settings. In addition, significant gender differences in the internal rotation angle were seen.

Low-frequency laboratory measurements of the elastic properties of solids using a distributed acoustic sensing system

In recent decades, low-frequency (LF) experiments based on the forced-oscillation (FO) method have become common practice in many rock physics laboratories for measuring the elastic and anelastic properties of rocks. However, the use of the electronic displacement sensors in traditional acquisition systems of FO devices such as conventional capacitive transducers or strain gauges seriously limits both the efficiency and productivity of LF measurements, and, due to the limited contact area of the displacement sensors with a sample under test, increases the requirements for sample homogeneity. In this paper, we present the first results obtained in the development of a new laboratory method elaborated to measure the elastic properties of solids. The method is a further development of the FO method where traditional data acquisition is replaced by acquisition based on fiber-optic distributed acoustic sensing (DAS) technology. The new method was tested in a laboratory study using two FO setups designed for measurements under uniaxial and confining pressures. The study was carried out on a sample made from polymethyl methacrylate (PMMA) and an aluminium standard, first under uniaxial pressure at FO frequencies of 1, 10, 30, 60 and 100 Hz, and then under confining pressure at an FO frequency of 1 Hz. Both uniaxial and confining pressures were equal to 10 MPa, and the strain in the PMMA sample in all measurements did not exceed 4 × 10-8. The performance of DAS acquisition was compared with the measurements conducted at a strain of 1 × 10-6 using the traditional FO method based on the use of semiconductor strain gauges and the ultrasonic method. The results of the DAS measurements are in good agreement with the FO measurements carried out using semiconductor strain gauges and with the literature data.

Carbon Nanotube-Polyaniline Reinforced Flexible and Stretchable Strain Sensor with Enhanced Sensitivity for Smart Wearable and Skin-Mounted Applications

Flexible and stretchable strain sensors have emerged as promising components for integration into smart wearable devices and skin-mounted applications. These sensors enable accurate detection of physiological signals, thereby finding unique applications in diverse fields such as human health monitoring, soft robotics, human-machine interface, prosthetics, virtual reality, and professional sports. Two commonly utilized types of strain sensors are capacitive and resistive strain gauges, owing to their low production cost, simplified circuitry, and ease of construction. While resistive strain gauges exhibit high sensitivity, they are prone to nonlinearity and hysteresis. On the other hand, capacitive strain gauges demonstrate linear behavior with minimal hysteresis but offer lower sensitivity. In this study, we capitalize on the exceptional properties of carbon nanotubes, including high mechanical strength, electrical conductivity, and thermal stability, along with using polyaniline as an exemplary conductive polymer. These materials are employed as a reinforcing phase within the polymer matrix, while the dielectric layer is comprised of Ecoflex® 00-30. An interdigitated pattern is specifically designed for this strain gauge to enhance sensitivity. Through this research, we aim to develop a flexible and stretchable strain sensor with enhanced sensitivity and improved performance characteristics.

Modeling and development of an auxetic foam-based multimodal capacitive strain gauge

Auxetics are mechanical metamaterials with the unique properties of expanding their transversal section upon longitudinal positive strain, decoupling the deformations in normal and transversal directions. Such property can be exploited to develop soft sensors that can provide feedback to different mechanical stimuli, e.g. pressure and shear force. In this work, we propose for the first time a mathematical model to analytically simulate and design the auxetic behavior in a capacitive strain gauge, and show that, for a polyurethane (PU) auxetic foam, Poisson Ratio’s values can satisfy the negative gauge factor (GF) condition. We develop an innovative thermo-compressive process to obtain anisotropic auxetic PU sponges both in normal and normal/radial directions, and their mechanical properties are in agreement with the theoretical calculations validating our model. Then, we develop a capacitive strain gauge by integrating a normal auxetic PU foam with polydimethylsiloxane /carbon nanotubes electrodes. Results show that the capacitive change caused by an external force, is proportional to the induced deformation, but importantly it is also dependent on the direction of the applied force. A negative GF of GF = −2.8 is obtained for a longitudinal strain range up to 10%. This auxetic foam structure guarantees flexibility and paves the way for an improved design freedom for multimodal mechanical soft sensors providing new opportunities towards smart wearables and perceptive soft robots.

Additively manufactured strain sensors for in-pile applications

Accurate, real-time monitoring of strain in fuel, cladding, and structural components of nuclear reactors is critical to better understand radiation induced phenomena during reactor tests and operations. The data provided is crucial to verify physics-based, multiscale modeling and simulation efforts, which aim to shorten the timeline for the development of new nuclear materials. Resistive strain gauges have limited performance during in-pile experiments due to the harsh operating conditions and limited physical space between fuel and cladding components. In this work, aerosol jet printing using silver nanoparticle inks was used to fabricate interdigitated electrode capacitive strain gauges on aluminum alloy 6061 tensile specimens. To simulate the temperatures of a traditional light water reactor, the capacitive strain gauges were tested with a mechanical test frame up to 300 °C and compared to commercially available bondable resistive strain gauges. The printed capacitive strain gauges exhibited a gauge factor of 1.0 and showed higher reproducibility and predictability of strain sensing performance than the resistive strain gauge. The results demonstrate the potential of aerosol jet printing to fabricate strain sensors with predictable performance and reduced invasiveness for high-temperature applications with confined spacing.

Batch Fabrication of a Polydimethylsiloxane Based Stretchable Capacitive Strain Gauge Sensor for Orthopedics.

Polymer-based capacitive strain gauges are a novel and promising concept for measuring large displacements and strains in various applications. These novel sensors allow for high strain, well above the maximum values achieved with state-of-the-art strain gauges (Typ. 1%). In recent years, a lot of interest in this technology has existed in orthopedics, where the sensors have been used to measure knee laxity caused by a tear of the anterior cruciate ligament (ACL), and for other ligament injuries. The validation of this technology in the field has a very low level of maturity, as no fast, reproducible, and reliable manufacturing process which allows mass production of sensors with low cost exists. For this reason, in this paper, a new approach for the fabrication of polymer-based capacitive strain gauges is proposed, using polydimethylsiloxane (PDMS) as base material. It allows (1) the fast manufacturing of sensor batches with reproducible geometry, (2) includes a fabrication step for embedding rigid electrical contacts on the sensors, and (3) is designed to produce sensor batches in which the size, the number, and the position of the sensors can be adapted to the patient’s anatomy. In the paper, the process repeatability and the robustness of the design are successfully proven. After 1000 large-strain elongation cycles, in the form of accelerated testing caused much higher strains than in the above-mentioned clinical scenario, the sensor’s electrical contacts remained in place and the functionalities were unaltered. Moreover, the prototype of a patient customizable patch, embedding multiple sensors, was produced.

A sensitive and flexible interdigitated capacitive strain gauge based on carbon nanofiber/PANI/silicone rubber nanocomposite for body motion monitoring

Stretchable nanocomposites-based strain gauges have received much attention due to their adjustable properties in various applications, including soft robotics, human health monitoring, body motion detection, structural health monitoring, and artificial intelligence. Although low sensitivity (gauge factor) is one of the challenges of capacitive strain gauges, in this study, we design, manufacture, and illustrate characterizations of a stretchable interdigitated capacitive strain gauge based on carbon nanofiber/polyaniline/silicone rubber nanocomposite by an improvement in sensitivity with linearity, and low hysteresis. This strain gauge reaches a gauge factor of about 14 over an applied strain of 2% and about 2.8 over an applied strain of 20% and demonstrates linearity with negligible hysteresis. The sensitivity of the strain sensor is enhanced not only by the interdigitated design of electrodes but also by the electrodes’ outstanding electrical conductivity, even in a large strain. Due to its sensitivity, the proposed device is suitable for detecting small and large strains and can be used in wearable applications or straight on the skin for human motion detection.

Aerosol jet printed capacitive strain gauge for soft structural materials

Soft structural textiles, or softgoods, are used within the space industry for inflatable habitats, parachutes and decelerator systems. Evaluating the safety and structural integrity of these systems occurs through structural health monitoring systems (SHM), which integrate non-invasive/non-destructive testing methods to detect, diagnose, and locate damage. Strain/load monitoring of these systems is limited while utilizing traditional strain gauges as these gauges are typically stiff, operate at low temperatures, and fail when subjected to high strain that is a result of high loading classifying them as unsuitable for SHM of soft structural textiles. For this work, a capacitance based strain gauge (CSG) was fabricated via aerosol jet printing (AJP) using silver nanoparticle ink on a flexible polymer substrate. Printed strain gauges were then compared to a commercially available high elongation resistance-based strain gauge (HE-RSG) for their ability to monitor strained Kevlar straps having a 26.7 kN (6 klbf) load. Dynamic, static and cyclic loads were used to characterize both types of strain monitoring devices. Printed CSGs demonstrated superior performance for high elongation strain measurements when compared to commonly used HE-RSGs, and were observed to operate with a gauge factor of 5.2 when the electrode arrangement was perpendicular to the direction of strain.

Towards a Chipless and Wireless Passive System for Real‐Time Encoding of the Bladder Volume

AbstractNeurogenic bladder and other lower urinary tract dysfunctions represent a significant health hazard and life‐quality impairment in individuals suffering from neurological disorders. A few implantable and wearable technologies have been proposed to partially recover bladder functionality, mostly based on resistive and capacitive strain gauges designed to be surgically placed inside the pelvic cavity. In this work, an alternative proof‐of‐concept device for monitoring the volumetric changes of the bladder is presented, where the sensing element is based on a capacitive linear encoder integrated with a passive wireless radio‐frequency resonator, which can be remotely interrogated. The sliding mechanism at the core of the proposed system allows a wide sensing range without stringent requirements on materials properties and overall device stability.

A Soft Parallel Kinematic Mechanism.

In this article, we describe a novel holonomic soft robotic structure based on a parallel kinematic mechanism. The design is based on the Stewart platform, which uses six sensors and actuators to achieve full six-degree-of-freedom motion. Our design is much less complex than a traditional platform, since it replaces the 12 spherical and universal joints found in a traditional Stewart platform with a single highly deformable elastomer body and flexible actuators. This reduces the total number of parts in the system and simplifies the assembly process. Actuation is achieved through coiled-shape memory alloy actuators. State observation and feedback is accomplished through the use of capacitive elastomer strain gauges. The main structural element is an elastomer joint that provides antagonistic force. We report the response of the actuators and sensors individually, then report the response of the complete assembly. We show that the completed robotic system is able to achieve full position control, and we discuss the limitations associated with using responsive material actuators. We believe that control demonstrated on a single body in this work could be extended to chains of such bodies to create complex soft robots.

Application of Capacitive Strain Transducer in Geostress Survey

In order to meet the requirements of high precision and large measuring span in borehole strain measurement, RZB capacitive strain transducer is designed by establishing the relationship between capacitance and micrometric displacement. A preliminary analysis was conducted on the data observed through Sichuan Shimian Station RZB capacitive borehole strain gage by applying the transducer in the borehole strain gauge. The analysis results show that RZB capacitive borehole strain gauge is high in observation accuracy and can record clear solid tides and broadband stain seismic waves.

Investigation of strain gauges based on interdigitated Ba0.5Sr0.5TiO3 thin film capacitors

The demand for monitoring of strain is growing in high temperature and harsh environment systems. In this work, the thin film interdigitated capacitive strain gauges are fabricated using Ba0.5Sr0.5TiO3 (BST) as the dielectric and palladium-13wt% chromium (PdCr) alloy as the electrodes, and the strain sensing characteristics of the thin film devices are investigated. It was found that the sensors showed low values of dissipation factor and stable capacitance over the frequency range from 15kHz to 500kHz at different temperatures. The temperature coefficient of capacitance (TCC) has been measured for temperature ranging from 25°C to 480°C so that the effect of temperature on the devices is known. At low temperature less than 150°C the TCC is very small within 20ppm/°C. With the temperature increasing, the TCC increased rapidly, and at 480°C, the TCC was measured to be 3100ppm/°C while is obviously less than that of typical thick film capacitors. The gauge factor (GF) of the interdigitated capacitive strain gauge at temperatures ranging from 25°C to 400°C to be about 3. The gauge also exhibits good linearity and sensitivity at applied large strains.

Novel approach to dynamic knee laxity measurement using capacitive strain gauges.

Knee joint laxities are observed in patients after severe trauma to the joint, resulting in ligament tears. Specifically, injuries to the anterior cruciate ligament may cause a significant instability. The degree of these laxities is essential in diagnostics and may affect which treatment option is suggested. Polydimethylsiloxane (PDMS) strain gauges are proposed as a non-invasive, highly accurate and easy-to-use measurement method to quantify anterolateral and rotational laxities of the knee joint during active and passive motion. In this work, proof-of-concept measurements and a prototype of the proposed device are displayed. The measurements were taken using a knee test rig, which has specifically been designed for this purpose. This apparatus allows the simulation of isolated knee joint instabilities with a motor-controlled model of a human knee. The absolute sensitivity [Formula: see text] of an exemplary sensor was determined to be 2.038 [Formula: see text]; the relative sensitivity [Formula: see text] was 1.121 [Formula: see text]. Optimal positions of sensors to capture bone-to-bone displacement as projected displacement on the skin were identified. PDMS strain gauges are capable of measuring bone-to-bone displacements on the skin. We present an experimental in vitro study using an artificial knee test rig to simulate knee joint laxities and display the feasibility of our novel measurement approach.

Length Changes of the Anterolateral Ligament During Passive Knee Motion

Background: Persistent rotatory instability after anterior cruciate ligament (ACL) reconstruction may be a result of unaddressed insufficiency of the anterolateral structures. Recent publications about the anatomy of the anterolateral ligament (ALL) have led to a renewed interest in lateral extra-articular procedures, and several authors have proposed ALL reconstruction to supplement intra-articular ACL reconstruction. However, only limited knowledge about the biomechanical characteristics of the ALL exists. Purpose/Hypothesis: The purpose of this study was to analyze length changes of the ALL during passive knee motion. The study hypothesis was that the ALL lengthens with knee flexion and internal tibial rotation. Study Design: Controlled laboratory study. Methods: The ALL of 6 cadaveric knees was dissected. Specimens were mounted in a specifically designed test rig that allowed unconstrained passive flexion/extension movement between 0° and 90° as well as external/internal tibial rotation of 25° at various flexion angles. Highly elastic, capacitive polydimethylsiloxane strain gauges were attached to the insertion sites of the ALL. Length changes were recorded continuously at flexion angles between 0° and 90° and during internal/external tibial rotation at 0°, 15°, 30°, 45°, 60°, 75°, and 90°. All measurements were calculated as the relative length change (%) of the ALL compared with 0° of flexion and neutral rotation. Results: The mean relative length of the ALL significantly increased with increasing knee flexion (P < .001), with an estimated mean length change of +0.15% per degree. Both internal and external tibial rotation were independent determinants for length change; internal rotation significantly increased the length of the ALL (P < .001), whereas external rotation significantly decreased its length (P < .001). The mean length change with internal rotation increased with knee flexion, with a significantly greater length change at 90° compared with 0° (P = .048), 15° (P = .033), and 30° (P = .015). The maximum mean length change was +33.77% ± 9.62%, which was observed at 25° of internal rotation and 90° of flexion. Conclusion: The ALL is a nonisometric structure that tensions with knee flexion and internal tibial rotation. Length changes with internal rotation were greater at higher flexion angles, with the greatest length change of the ALL observed at 90° of flexion. Clinical Relevance: The ALL can be considered a stabilizer against internal tibial rotation, especially at deep flexion angles. With regard to ALL reconstruction procedures, tensioning and fixation of the graft should be performed near 90° of flexion because graft tensioning near extension may cause excessive ligament strain with increasing knee flexion.

Network of flexible capacitive strain gauges for the reconstruction of surface strain

Monitoring of surface strain on mesosurfaces is a difficult task, often impeded by the lack of scalability of conventional sensing systems. A solution is to deploy large networks of flexible strain gauges, a type of large area electronics. The authors have recently proposed a soft elastomeric capacitor (SEC) as an economical skin-type solution for large-scale deployment onto mesosurfaces. The sensing principle is based on a measurable change in the sensor’s capacitance upon strain. In this paper, we study the performance of the sensor at reconstructing surface strain map and deflection shapes. A particular feature of the sensor is that it measures surface strain additively, because it is not utilized within a Wheatstone bridge configuration. An algorithm is proposed to decompose the additive in-plane strain measurements from the SEC into principal components. The algorithm consists of assuming a polynomial shape function, and deriving the strain based on Kirchhoff plate theory. A least-squares estimator (LSE) is used to minimize the error between the assumed model and the SEC signals after the enforcement of boundary conditions. Numerical simulations are conducted on a symmetric rectangular cantilever thin plate under symmetric and asymmetric static loads to demonstrate the accuracy and real-time applicability of the algorithm. The performance of the algorithm is further examined on an asymmetric cantilever laminated thin plate constituted with orthotropic materials mimicking a wind turbine blade, and subjected to a non-stationary wind load. Results from simulations show good performance of the algorithm at reconstructing the surface strain maps for both in-plane principal strain components, and that it can be applied in real time. However, its performance can be improved by strengthening assumptions on boundary conditions. The algorithm exhibits robustness in performance with respect to load and noise in signals, except when most of the sensors’ signals are close to zero due to over-fitting form the LSE.

Capacitive strain gauges on flexible polymer substrates for wireless, intelligent systems

Abstract. This paper presents a novel capacitive strain gauge with interdigital electrodes, which was processed on polyimide and LCP (liquid crystal polymer) foil substrates. The metallization is deposited and patterned using thin-film technology with structure sizes down to 15 μm. We determined linear strain sensitivities for our sensor configuration and identified the most influencing parameters on the output signal by means of an analytical approach. Finite-element method (FEM) simulations of the strain gauge indicated the complex interaction of mechanical strains within the sensitive structure and their effect on the capacitance. The influence of geometry and material parameters on the strain sensitivity was investigated and optimized. We implemented thin films on 50 μm thick standard polymer foils by means of a temporary bonding process of the foils on carrier wafers. The characterization of the strain sensors after fabrication revealed the gauge factor as well as the cross sensitivities on temperatures up to 100 °C and relative humidity up to 100%. The gauge factor of a sensor with an electrode width of 45 μm and a clearance of 15 μm was −1.38 at a capacitance of 48 pF. Furthermore, we achieved a substantial reduction of the cross sensitivity against humidity from 1435 to 55 ppm %−1 RH when LCP was used for the sensor substrate and the encapsulation instead of polyimide. The gauge factor of a sensor half-bridge consisting of two orthogonal capacitors was 2.3 and the cross sensitivity on temperature was reduced to 240 ppm K−1. Finally, a sensor system was presented that utilizes a special instrumentation Integrated Circuit (IC). For this system, performance data comprising cross sensitivities and power consumption are given.

A Highly Elastic, Capacitive Strain Gauge Based on Percolating Nanotube Networks

We present a highly elastic strain gauge based on capacitive sensing of parallel, carbon nanotube-based percolation electrodes separated by a dielectric elastomer. The fabrication, relying on vacuum filtration of single-walled carbon nanotubes and hydrophobic patterning of silicone, is both rapid and inexpensive. We demonstrate reliable, linear performance over thousands of cycles at up to 100% strain with less than 3% variability and the highest reported gauge factor for a device of this class (0.99). We further demonstrate use of this sensor in a robotics context to transduce joint angles.

Highly flexible capacitive strain gauge for continuous long-term blood pressure monitoring

An innovative procedure for measuring blood pressure, with none of the disadvantages of current procedures, is proposed. A highly-flexible capacitive strain gauge has been designed to measure changes in the diameter of a blood vessel; such changes are indicative of blood pressure. The sensor is implanted and wrapped around an arterial blood vessel during the normal course of a surgical procedure. In vivo tests, demonstrating the feasibility of this concept, are reported, along with in vitro tests and notes on sensor design and fabrication. These continuous blood pressure monitoring sensors may be used for a continuous long-term monitoring of blood pressure and pulse. They may also be combined with a real-time nerve stimulation technique or a course of medication to create a closed-loop system for blood-pressure control.

Photolithographic structuring of stretchable conductors and sub-kPa pressure sensors

This paper presents a novel method to prepare stretchable conductors and pressure sensors based on the gold/polydimethylsiloxane (PDMS) system. The gold films were sputtered onto structured PDMS surfaces produced with a photolithographic surface treatment with the aim of reducing tensile strains in the gold film. Scanning electron microscopy (SEM) and atomic force microscopy analyses showed that these 3D patterns reduce cracks and delaminations in the gold film. Electrical measurements indicate that the patterns also protect the films against repeated tensile cycling, although the un-patterned samples remained conducting as well after the completion of 120 cycles. The extrapolated resistivity value of the patterned sample (4.5 × 10−5 Ωcm) compares well with previously published data. SEM micrographs indicate that the pattern features deflect the cracks and therefore toughen the gold film. However, x-ray photoelectron spectroscopy and contact angle analyses indicate that the patterning process also slightly modifies the surface chemistry. This patterning method was used to prepare capacitive strain gauges with pressure sensitivity (ΔZ/Z)/P of 0.14 kPa−1 in the sub-kPa regime. Such stretchable and potentially conformal low-pressure sensors have not been produced before and could prove advantageous for many smart fabric applications.