Metal Strain Gauges Research Articles

Development and study of low-cost VACNT/PDMS stretchable and resistive strain sensor

This study introduces fast manufactured (4−5 hours) vertical aligned carbon nanotubes/polydimethylsiloxane (VACNT/PDMS) stretchable sensor for aircraft structures and polymer composites. The VACNT growth method from thermal CVD with camphor and ferrocene precursors create a dense and homogeneous CNT structure with many ohmic conducting paths that surpass any more resistive tunneling phenomena. The extensive AC/DC piezoresistive performance investigation shows ohmic conduction. The VACNTs/PDMS sensor presented high linearity (r2 = 0.99) under ∼20 % strain with an average gauge factor of 3.28 at DC measurements and minor resistance variations (0.052 %) attributed to the copper terminals electrodeposition. An important feature is the resistance value compatible with conventional extensometry equipment. The DC gauge factor was ∼60 % higher than the conventional metallic strain gauge allowing measurements within a wider strain range.

Strain-Controlled Flexible Graphene/GaN/PDMS Sensors Based on the Piezotronic Effect.

Using simple graphene transfer and the laser lift-off process for a non-centrosymmetric GaN layer on a flexible polydimethylsiloxane (PDMS) substrate, the piezotronic effect by strain-induced current-voltage measurements at the two end points is studied. By inducing compressive strain on the flexible graphene/GaN/PDMS sensor, the Schottky barrier between the graphene and GaN/PDMS heterojunction can be electro-mechanically modulated by the piezotronic effect. It is observed that the flexible graphene/GaN/PDMS sensor is sensitive to various applied compressive and tensile strains in the positive/negative bias scans. The sensor is extremely sensitive to a compressive strain of -0.1% with a gauge factor of 13.48, which is 3.7 times higher than that of a standard metal strain gauge. Furthermore, the sharp response of the flexible graphene/GaN/PDMS sensor under the -0.1% compressive strain is also investigated. The results of this study herald the development of commercially viable large-scale flexible/wearable strain sensors based on the strain-controlled piezotronic effect in future investigations.

Development, Characterisation and High-Temperature Suitability of Thin-Film Strain Gauges Directly Deposited with a New Sputter Coating System.

New sensor and sensor manufacturing technologies are identified as a key factor for a successful digitalisation and are therefore economically important for manufacturers and industry. To address various requirements, a new sputter coating system has been invented at the Institute of Micro Production Technology. It enables the deposition of sensor systems directly onto technical surfaces. Compared to commercially available systems, it has no spatial limitations concerning the maximum coatable component size. Moreover, it enables a simultaneous structuring of deposited layers. Within this paper, characterisation techniques, results and challenges concerning directly deposited thin film strain gauges with the new sputter coating system are presented. Constantan (CuNiMn 54/45/1) and NiCr 80/20 are used as sensor materials. The initial resistance, temperature coefficient of resistance and gauge factor/k-factor of quarter-bridge strain gauges are characterised. The influence of a protective layer on sensor behaviour and layer adhesion is investigated as well. Moreover, the temperature compensation quality of directly deposited half-bridge strain gauges is evaluated, optimised with an external trimming technology and benchmarked against commercial strain gauges. Finally, the suitability for high-temperature strain measurement is investigated. Results show a maximum operation temperature of at least 400 °C, which is above the current state-of-the-art of commercial foil-based metal strain gauges.

Waterproof Flexible Polymer-Functionalized Graphene-Based Piezoresistive Strain Sensor for Structural Health Monitoring and Wearable Devices.

In recent times, flexible piezoresistive polymer nanocomposite-based strain sensors are in high demand in wearable devices and various new age applications. In the polymer nanocomposite-based strain sensor, the dispersion of conductive nanofiller remains challenging due to the competing requirements of homogenized dispersion of nanofillers in the polymer matrix and retaining of the inherent characteristics of nanofillers. In the present work, waterproof and flexible poly(vinylidene difluoride) (PVDF) with a polymer-functionalized hydrogen-exfoliated graphene (HEG)-based piezoresistive strain sensor is developed and demonstrated. The novelty of the work is the incorporation of polystyrene sulfonate sodium salt (PSS) polymer-functionalized HEG in a PVDF-based flexible piezoresistive strain sensor. The PSS-HEG provides stable dispersion in the hydrophobic PVDF polymer matrix without sacrificing its inherent characteristics. The electrical conductivity of the PVDF/PSS-HEG-based strain sensor is 0.3 S cm–1, which is two orders of magnitude higher than the PVDF/HEG-based strain sensor. Besides, near the percolation region, the PVDF/PSS-HEG shows a maximum gauge factor of 10, which is about two times higher than the PVDF/HEG-based flexible strain sensor and 5-fold higher than the commercially available metallic strain gauge. The enhancement in the gauge factor is due to the stable dispersion of PSS-HEG in the PVDF matrix and electron conjugation caused by the adherence of negatively charged sulfonate functional groups on the HEG. The developed waterproof flexible strain sensor is demonstrated using portable wireless interfacing device for various applications. This work shows that the waterproof flexible PVDF/PSS-HEG-based strain sensor can be a potential alternative to the commercially available metallic strain gauge.

A Flexible and Low-Cost Tactile Sensor Produced by Screen Printing of Carbon Black/PVA Composite on Cellulose Paper.

This study presents the design and fabrication of a flexible tactile sensor printed on a cellulose paper substrate using a carbon black (CB) – filled polyvinyl alcohol (PVA) polymer matrix as ink material. In the design, electrodes are obtained by screen printing of CB/PVA composite on dielectric cellulose paper. The screen-printing method is preferred for fabrication because of its simplicity and low manufacturing cost. The tactile sensor is formed by overlapping two ink-printed sheets. Electrical properties are investigated under compressive and tensile strains. The results indicate that the tactile sensor configuration and materials can be used for piezoresistive, capacitive, and also impedance sensors. The same tactile sensor structure is also examined using a commercial carbon-based ink for performance comparison. The comparative study indicates that CB/PVA ink screen-printed on paper demonstrates superior sensitivity for capacitive sensing with low hysteresis, as well as low response and recovery times. The piezoresistive-sensing properties of CB/PVA on cellulose paper show a gauge factor (GF) of 10.68, which is also very promising when conventional metal strain gauges are considered. CB/PVA screen-printed on cellulose paper features impedance-sensing properties and is also sensitive to the measurement frequency. Therefore, the response type of the sensor can be altered with the frequency.

Study and fabrication of a flexible Zr-based metallic glass thin film strain gauge

Metallic glass is an amorphous structure and has excellent mechanical properties such as high strength, hardness and elastic strain limitation. In this study, a Zr-based metallic glass thin film was deposited on a polymer substrate using the sputter technique for strain gauge applications. A copper thin film strain gauge was fabricated to compare with the properties of the Zr-based strain gauge. Different thicknesses of the thin films were fabricated for comparison. The gauge factor of the Zr-based strain gauge is 2.4 times higher than that of the copper strain gauge. The temperature coefficient of resistance of the Zr-based metallic glass thin film is extremely low, 8 × 10−6 °C−1, which is essential to reduce thermal drift for strain gauge applications.

Structural Health Monitoring System for Snow and Wind Load Measurement

This article presents a system for monitoring the load caused by strong winds and snow on buildings’ roofs. An estimation of the total load on the structure is obtained by measuring the strain on the main roof girders. The system is based on a wireless sensor network structure. The measurement node uses metal strain gauges and strain sensors based on conductive carbon polymers. The application of such sensors allowed us to achieve a measurement resolution of 5.5 ustrain. The node is managed by an Atmeg8A microcontroller. The use of energy saving modes allows for a battery life of 6 months.

μ-Coriolis Mass Flow Sensor with Resistive Readout.

This paper presents a -Coriolis mass flow sensor with resistive readout. Instead of measuring a net displacement such as in a capacitive readout, a resistive readout detects the deformation of the suspended micro-fluidic channel. It allows for actuation at much higher amplitudes than for a capacitive readout, resulting in correspondingly larger Coriolis forces in response to fluid flow. A resistive readout can be operated in two actuation vibrational modes. A capacitive readout can only be operated in one of these two modes, which is more sensitive to external disturbances. Three types of devices have been realized. We present measurement results for all three devices. One device clearly outperforms the other two, with a flow sensitivity of 2.22 °/(g·h−1) and a zero-flow stability of 0.02 g·h−1 over 30 min. Optimization of the metal strain gauges and/or implementation of poly-Silicon strain gauges could further improve performance.

Piezoresistive Multi-Walled Carbon Nanotube/Epoxy Strain Sensor with Pattern Design.

Carbon nanotube/polymer-based composites have led to studies that enable the realization of low-cost, high-sensitivity piezoresistive strain sensors. This study investigated the characteristics of piezoresistive multi-walled carbon nanotube (MWCNT)/epoxy composite strain sensors subjected to tensile and compressive loads in one direction at relatively small amounts of strain. A patterned sensor was designed to overcome the disadvantage of the load direction sensitivity differences in the existing sensors. The dispersion state of the MWCNTs in the epoxy polymer matrix with the proposed dispersion process was verified by scanning electron microscopy. An MWCNT/epoxy patterned strain sensor and a patch-type strain sensor were directly attached to an acrylic cantilever beam on the opposite side of a commercial metallic strain gauge. The proposed patterned sensor had gauge factors of 2.52 in the tension direction and 2.47 in the compression direction. The measured gauge factor difference for the patterned sensor was less than that for the conventional patch-type sensor. Moreover, the free-vibration frequency response characteristics were compared with those of metal strain gauges to verify the proposed patch-type sensor. The designed drive circuit compensated for the disadvantages due to the high drive voltage, and it was confirmed that the proposed sensor had higher sensitivity than the metallic strain gauge. In addition, the hysteresis of the temperature characteristics of the proposed sensor is presented to show its temperature range. It was verified that the patterned sensor developed through various studies could be applied as a strain sensor for structural health monitoring.

Strain sensitivity of steel-fiber-reinforced industrial smart concrete

Self-sensing cementitious composites can enable structures that are capable of carrying the loads applied on them while monitoring their condition. Most of earlier research has focused on the incorporation of nanofillers or microfibers into cement paste or mortar composites. However, there have been very limited number of studies on the development of steel-fiber-reinforced cementitious composites with self-sensing capabilities. This study explores strain sensitivity of concrete mixtures that include coarse aggregates up to 15 mm diameter and steel fibers with a length of 13 mm and a diameter of 0.25 mm. Five different concrete mixtures with steel fibers at 0%, 0.2%, 0.35%, 0.5%, and 0.8% volume ratios were fabricated. Compression tests with simultaneous measurement of strain and electrical resistance were conducted on the cubic specimens. Gauge factor and percent linearity that is a measure of error in strain sensing were calculated. Concrete mixtures with 0.5% steel fibers possess a strong linear relationship between applied strain and electrical resistance change with a gauge factor over 20 times larger than that of traditional metal strain gauges. Phenomenological models for different resistivity and gauge factors of cement paste/mortar with respect to concrete with large aggregates and short–long fiber cement composites were presented.

Giant piezoresistive effect by optoelectronic coupling in a heterojunction

Enhancing the piezoresistive effect is crucial for improving the sensitivity of mechanical sensors. Herein, we report that the piezoresistive effect in a semiconductor heterojunction can be enormously enhanced via optoelectronic coupling. A lateral photovoltage, which is generated in the top material layer of a heterojunction under non-uniform illumination, can be coupled with an optimally tuned electric current to modulate the magnitude of the piezoresistive effect. We demonstrate a tuneable giant piezoresistive effect in a cubic silicon carbide/silicon heterojunction, resulting in an extraordinarily high gauge factor of approximately 58,000, which is the highest gauge factor reported for semiconductor-based mechanical sensors to date. This gauge factor is approximately 30,000 times greater than that of commercial metal strain gauges and more than 2,000 times greater than that of cubic silicon carbide. The phenomenon discovered can pave the way for the development of ultra-sensitive sensor technology.

Metal and Polymeric Strain Gauges for Si-Based, Monolithically Fabricated Organs-on-Chips.

Organ-on-chip (OOC) is becoming the alternative tool to conventional in vitro screening. Heart-on-chip devices including microstructures for mechanical and electrical stimulation have been demonstrated to be advantageous to study structural organization and maturation of heart cells. This paper presents the development of metal and polymeric strain gauges for in situ monitoring of mechanical strain in the Cytostretch platform for heart-on-chip application. Specifically, the optimization of the fabrication process of metal titanium (Ti) strain gauges and the investigation on an alternative material to improve the robustness and performance of the devices are presented. The transduction behavior and functionality of the devices are successfully proven using a custom-made set-up. The devices showed resistance changes for the pressure range (0–3 kPa) used to stretch the membranes on which heart cells can be cultured. Relative resistance changes of approximately 0.008% and 1.2% for titanium and polymeric strain gauges are respectively reported for membrane deformations up to 5%. The results demonstrate that both conventional IC metals and polymeric materials can be implemented for sensing mechanical strain using robust microfabricated organ-on-chip devices.

Highly Sensitive Soft Foam Sensors to Empower Robotic Systems

AbstractSoft robots require seamless integration with sensors and actuators that are simple to manufacture at scale with low cost and minimum footprint. The sensor materials must be highly reliable, sensitive, and stable, and their mechanical features should match the sensing requirements of soft robots such as minimal response time and nonlinearity of hysteresis and relaxation. A resistive‐type sensor based on the synthesis of poly(glycerol secabate) (PGS) with a foam‐like structure and outstanding mechanical, electrical, and electromechanical properties is developed. These foam sensors present high sensitivity (gauge factor ≈ −9), very fast response (≤3 ms), negligible hysteresis, reliability, long lifetime (>1 200 000 cycles), and a pressure differential sensitivity of 34 Pa. They can accurately detect low and high frequency vibrations (up to 300 Hz) and small forces (200 mN) that cover the very low detection range of metallic strain gauges and the very large detection range of elastomeric strain sensors. These characteristics closely match those of the human fingertip, and hence pave the way toward tactile and compliant sensing elements embedded in prosthetic hands. Prospective applications for unexplored resistive‐type sensors to meet the sensory requirements of soft robotic systems are demonstrated.

A simple torque magnetometer for magnetic moment measurement of large samples: Application to permanent magnets and bulk superconductors.

The development of large size magnetic materials requires nondestructive measurement techniques to characterize their magnetic moment. In this work, we report the design and construction of a torque magnetometer able to accommodate sizable magnetic samples (>1 cm3) both at room temperature and cryogenic temperature. This device has an intermediate sensitivity between miniature torque magnetometers designed to work at cryogenic temperature and industrial torquemeters poorly adapted to extreme conditions. We show that torque sensing in the range 10-3-100 Nm can be achieved with piezoresistive metallic strain gages cemented on a cylindrical aluminum shaft with external temperature control. An absolute calibration of the device, carried out with a coil fed by a DC current, shows that magnetic moments down to 5 × 10-3 A m2 can be measured by this technique. The magnetometer is used to characterize a Nd-Fe-B permanent magnet and a permanently magnetized bulk, large grain superconductor at liquid nitrogen temperature (77 K). Results are in excellent agreement with data obtained with a flux extraction magnetometer for large samples. The device is able to measure magnetic moments in excess of 1.5 A m2, i.e., two orders of magnitude above the maximum magnetic moment of commercial magnetometers. The sample can be inserted in the air-gap of an electromagnet to measure the decrease in magnetic moment in the presence of a transverse applied field. The device was used to characterize the magnetic moment of "quasibulk" superconductors made of stacked coated conductor tapes (12 mm width) in such "crossed field" conditions.

Cracking effects in squashable and stretchable thin metal films on PDMS for flexible microsystems and electronics

Here, we study cracking of nanometre and sub-nanometre-thick metal lines (titanium, nickel, chromium, and gold) evaporated onto commercial polydimethylsiloxane (PDMS) substrates. Mechanical and electromechanical testing reveals potentially technologically useful effects by harnessing cracking. When the thin film metal lines are subjected to uniaxial longitudinal stretching, strain-induced cracks develop in the film. The regularity of the cracking is seen to depend on the applied longitudinal strain and film thickness—the findings suggest ordering and the possibility of creating metal mesas on flexible substrates without the necessity of lithography and etching. When the metal lines are aligned transversally to the direction of the applied strain, a Poisson effect-induced electrical ‘self-healing’ can be observed in the films. The Poisson effect causes process-induced cracks to short circuit, resulting in the lines being electrically conducting up to very high strains (~40%). Finally, cracking results in the observation of an enhanced transversal gauge factor which is ~50 times larger than the geometric gauge factor for continuous metal films—suggesting the possibility of high-sensitivity thin-film metal strain gauge flexible technology working up to high strains.

Measuring Dynamic Shear Force and Vibration With a Bioinspired Tactile Sensor Skin

There is a need to measure dynamic tactile information to enhance robotic hand performance during haptic exploration and object manipulation. Here, we report on the dynamic characterization of a flexible, bioinspired, and resistive microfluidic shear force sensor skin. When the skin is subjected to shear force, one side of the skin experiences tension while the other side experiences compression, both of which are measured by liquid metal strain gauges embedded in polydimethylsiloxane. The sensor can measure dynamic shear forces during stepwise unloading, incipient slip, and controlled vibration tests. The sensor can quantify vibration up to 800 Hz with an average minimum displacement of 0.43 μm, which is equivalent to or better than human fingertips. We demonstrate the utility of the sensor skin by performing experiments with a robotic hand and arm. The shear sensing skin senses tactile events while a robotic arm performs several manipulation tasks including pick and place, drop, and handover. The skin is robust when used on a robot manipulator and shows promise in providing rich and dynamic tactile information that is beneficial to robotic and prosthetic applications.

Sleep monitoring sensor using flexible metal strain gauge

This paper presents a sleep monitoring sensor based on a flexible metal strain gauge. As quality of life has improved, interest in sleep quality, and related products, has increased. In this study, unlike a conventional single sensor based on a piezoelectric material, a metal strain gauge-based array sensor based on polyimide and nickel chromium (NiCr) is applied to provide movement direction, respiration, and heartbeat data as well as contact-free use by the user during sleeping. Thin-film-type resistive strain gage sensors are fabricated through the conventional flexible printed circuit board (FPCB) process, which is very useful for commercialization. The measurement of movement direction and respiratory rate during sleep were evaluated, and the heart rate data were compared with concurrent electrocardiogram (ECG) data. An algorithm for analyzing sleep data was developed using MATLAB, and the error rate was 4.2% when compared with ECG for heart rate.

Foil Strain Gauges Using Piezoresistive Carbon Nanotube Yarn: Fabrication and Calibration.

Carbon nanotube yarns are micron-scale fibers comprised by tens of thousands of carbon nanotubes in their cross section and exhibiting piezoresistive characteristics that can be tapped to sense strain. This paper presents the details of novel foil strain gauge sensor configurations comprising carbon nanotube yarn as the piezoresistive sensing element. The foil strain gauge sensors are designed using the results of parametric studies that maximize the sensitivity of the sensors to mechanical loading. The fabrication details of the strain gauge sensors that exhibit the highest sensitivity, based on the modeling results, are described including the materials and procedures used in the first prototypes. Details of the calibration of the foil strain gauge sensors are also provided and discussed in the context of their electromechanical characterization when bonded to metallic specimens. This characterization included studying their response under monotonic and cyclic mechanical loading. It was shown that these foil strain gauge sensors comprising carbon nanotube yarn are sensitive enough to capture strain and can replicate the loading and unloading cycles. It was also observed that the loading rate affects their piezoresistive response and that the gauge factors were all above one order of magnitude higher than those of typical metallic foil strain gauges. Based on these calibration results on the initial sensor configurations, new foil strain gauge configurations will be designed and fabricated, to increase the strain gauge factors even more.

High resolution polymer coated strain sensors for in-liquid operation

Metallic strain gauges are well studied structures for their use in force sensing. Their fabrication is rather simple, easily compatible with many techniques are broadly scalable in both quantity and resolution. Here we present a fabrication process for high resolution strain sensors (down to the nN) that can also be operated in liquid, i.e. for life sciences applications. This fabrication method offers a very high yield and opens new possibilities for bioengineering, as for example single cell studies.

Feasibility of multi-walled carbon nanotube/epoxy thermoset-based strain sensors for sensing in structural applications

Embedded sensors are being explored to non-intrusively monitor the loading experienced by a part either during the manufacturing process or in subsequent service. With the advancement of additive manufacturing techniques, it may be possible to print sensors directly into a part fabricated from metals, polymers, or composites. There is even more benefit if the sensor is also an integral part of the assembly. Single-walled carbon nanotubes and multi-walled carbon nanotubes are used to improve the stiffness and strength of polymeric resins. But they also offer an additional benefit of reversible piezo-resistive behavior. The objective of this study was to explore the fidelity of a signal extracted from nanocomposite samples of multi-walled carbon nanotube/epoxy resin under uniaxial loading, as a function of increasing content of multi-walled carbon nanotube. Data obtained from the multi-walled carbon nanotube networks formed correlated with the strain as a function of the mechanical loading. The gauge factors calculated for the multi-walled carbon nanotube/epoxy samples ranged from 2.651 to 5.064 as compared to the metallic strain gauge factor of 2.125 used to calibrate the samples. Under repeated mechanical loading, the initial zero-strain condition was recovered within 5 min after unloading. In samples with 5 wt% of multi-walled carbon nanotubes, the electrical conductivity and gauge factor plateaued. Furthermore, increases in the w/w% of multi-walled carbon nanotube resulted in degradation of the mechanical properties of the multi-walled carbon nanotube/epoxy resin.