Resonant Strain Sensor Research Articles

Effect of bonding adhesive thickness on temperature characteristics and strain characteristics of SAW strain sensors

Accurate measurement of physical parameters based on sensing technology is an important basis for equipment structural health monitoring technology. In harsh environments, strain measurement techniques based on SAW sensors have attracted much attention. The bonding adhesive is a key step in the strain measurement process, and its effect on the accuracy of the measurement results cannot be ignored. In this paper, the one-port resonant SAW strain sensor is prepared using LiNbO3 as a piezoelectric substrate, Pt as an electrode and SiO2 as a protective layer. The strain characteristics and temperature characteristics of SAW strain sensors at the thickness of the bonding adhesive (Model: DOUBLE-BOND CHEMICAL®DB5016) are explored. The frequency–temperature curves of the SAW sensors show a quasi-linear decreasing trend with increasing temperature, and the temperature–frequency characteristics are similar for different bonding adhesive thicknesses. The strain sensitivity increases and then decreases with increasing temperature, and reaches a maximum at 150 °C. An increase in the thickness of the bonding adhesive leads to a decrease in the temperature linearity and an increase in the temperature sensitivity of the SAW strain sensor, which is maximized at a bonding adhesive thickness of 0.35 mm. The increase in the thickness of the bonding adhesive leads to a decrease in the strain linearity and strain sensitivity of the SAW sensor. The relationship between the strain transfer efficiency of the SAW sensor and the thickness of the bonding layer, the shear modulus of the bonding layer, the length of the sensor, the thickness of the sensor substrate and the shear modulus of the sensor is established through theoretical derivation. A theoretical foundation is provided for the accurate measurement of strain based on SAW technology.

Stress engineering of polycrystalline aluminum nitride thin films for strain sensing with resonant piezoelectric microbridges

For an optimized performance of micro electromechanical systems (MEMS) double-clamped bridge-type resonators for mechanical strain sensing, a modified sputter process was developed which exploits the influence of varying sputter pressure during deposition on the intrinsic stress component of functional thin films. In detail, four different, polycrystalline aluminum nitride layers, synthesized with different sputter parameter sets were characterized related to their microstructure with techniques, such as X-ray diffraction, scanning electron microscopy and atomic force microscopy, respectively. Furthermore, the intrinsic thin film stress and the longitudinal piezoelectric coefficient were evaluated. The best performing layers were integrated in the fabrication process of two MEMS resonant strain sensor devices to study the stress-related impact on the resonant device performance. The initial static buckling of the sensor devices was studied by white light interferometric measurements, whereas the frequency response as a function of externally applied as well as intrinsic strain was analyzed by laser Doppler vibrometry. The behavior of the sensor devices was compared to theoretical predications and the influence of intrinsic thin film stress on the resonance frequency as a function of strain was studied. With a precise tailoring of the intrinsic film stress, a responsivity of ∼17000 is measured, representing an improvement by a factor of ∼5 compared to state-of-the-art resonant strain sensors.

Resonant fiber-optic strain and temperature sensor achieving thermal-noise-limit resolution.

In the area of fiber-optic sensors (FOSs), the past decade witnessed great efforts to challenge the thermal-noise-level sensing resolution for passive FOS. Several attempts were reported claiming the arrival of thermal-noise-level resolution, while the realization of thermal-noise-level resolution for passive FOSs is still controversial and challenging. In this paper, an ultrahigh-resolution FOS system is presented with a sensing resolution better than existing high-resolution passive FOSs. A fiber Fabry-Perot interferometer as the sensing element is interrogated with an ultra-stable probe laser by using the Pound-Drever-Hall technique. Both strain and temperature measurements are carried out to validate the performance of the sensor. The measured noise floor agrees with the theoretical thermal noise level very well.

White-light-driven resonant fiber-optic strain sensor.

Recently, laser-driven resonant fiber-optic sensors (FOSs) have been reported to reach the ultimate resolution limit, set by the thermal noise in fiber. However, they are still far away from commercialization because of their dependence on expensive and ultra-narrow-linewidth lasers. In this Letter, a white-light-driven resonant FOS is proposed for the first time, to the best of our knowledge. By using the white-light multi-beam interferometry, the resonant peaks of a fiber Fabry-Perot interferometer sensor are obtained, and the resonance drifts are tracked for sensor readout by using the Pound-Drever-Hall technique. The proposed FOS is adopted for strain measurement, and the achieved strain resolution (0.9pε/√Hz at 100 Hz) is comparable with that of laser-driven FOSs.

Integrating coupled magnetoelastic sensors onto a flexible hernia mesh for high dynamic range strain measurements.

Despite better performance over primary repairs, tension-free ventral hernia repairs with mesh still suffer from a high recurrence rate. High stress gradients in the mesh are thought to contribute to hernia recurrence. We propose a postoperative monitoring system based on a coupled pair of magnetoelastic strain sensors to enable patients and physicians to non-invasively measure and track the strain distribution across the hernia mesh. Our design combines an encased resonator with a spring-loaded transducer to achieve high signal amplitude with a wide dynamic range. We also demonstrate a fabrication protocol to integrate the resonant strain sensors with a commercial polypropylene mesh. The packaged sensor is capable of detecting up to 37.5 millistrain, an order of magnitude greater than previously demonstrated.

Coupled π ‐shifted fibre Bragg grating ring resonant strain sensors

Resonant optical strain sensors, obtained by closing in loop coupled π-shifted Bragg gratings (π-FBGs), are reported. Despite of a conventional π-FBG, showing a transmission Lorentzian resonance in the reflection band, the coupled π-shifted Bragg grating structure is characterised by a spectral response formed by a split doublet of the primary transmission Lorentzian mode of π-FBG. By closing in loop this structure, the coupled π-shifted Bragg grating ring resonators show better performance than π-shifted Bragg grating ring resonators in terms of strain sensitivity (i.e. 2.86 pm/με) is demonstrated. Thus, the coupled π-FBGRR is suitable for enhanced sensitivity strain sensing applications.

Vacuum Packaged Low-Power Resonant MEMS Strain Sensor

This paper describes a technical approach toward the realization of a low-power temperature-compensated micromachined resonant strain sensor. The sensor design is based on two identical and orthogonally-oriented resonators where the differential frequency is utilized to provide an output proportional to the applied strain with temperature compensation achieved to first order. Interface circuits comprising of two front-end oscillators, a mixer, and low-pass filter are designed and fabricated in a standard 0.35- $\mu \text{m}$ CMOS process. The characterized devices demonstrate a scale factor of 2.8 Hz/ $\mu \varepsilon $ over a strain range of 1000 $\mu \varepsilon $ with excellent linearity over the measurement range. The compensated frequency drift due to temperature is reduced to 4% of the uncompensated value through this scheme. The total continuous power consumption of the strain sensor is 3 $\mu \text{W}$ from a 1.2 V supply. This low power implementation is essential to enable battery-powered or energy harvesting enabled monitoring applications. [2015-0328]

Design and Modeling of a Chevron MEMS Strain Sensor With High Linearity and Sensitivity

Two new physical designs for sensors that can be used for strain measurement in large structures, such as bridges, wind turbines, or airplanes, are presented. While the proposed sensor designs focus on high sensitivity, they are based on simple operating principle of comb-drive differential variable capacitances and chevron displacement amplification. The chevron beams convert small amount of applied strains to measurable changes in capacitance of comb fingers. The design of the structures enables simple fabrication methods for the realization of the sensors. Two designs are proposed with the first design can also be used as a sensitive resonant strain sensor. Device performances are validated both by analytical solutions and also by finite-element method simulations. The obtained nominal capacitance is 25 fF, with sensitivities of 13 and 2.7 aF per microstrain ( $\mu \varepsilon$ ) while demonstrating a maximum strain range of ±1000 and $\pm 1800~\mu \varepsilon $ , respectively, for the first and second designs. As a resonant strain sensor, the first design exhibits a sensitivity of $\sim 8.6$ Hz/ $\mu \varepsilon $ .

Passive Wireless Strain Sensors Using Microfabricated Magnetoelastic Beam Elements

This paper describes resonant wireless strain sensors fabricated from magnetoelastic alloys. The transduction mechanism is the $\Delta{\rm E}$ effect—the change in stiffness of magnetoelastic materials with applied strain or magnetic field. This is measured as a shift in the resonant frequency and is detected wirelessly using pick-up coils utilizing the magnetoelastic coupling of these materials. The sensors are fabricated from a 28- $\mu{\rm m}$ -thick foil of Metglas 2826 MB $({\rm Fe}_{40}{\rm Ni}_{\rm 38}{\rm Mo}_{\rm 4}{\rm B}_{18})$ , a ferromagnetic magnetoelastic alloy, using microelectrodischarge machining. Two sensor types are described—single and differential. The single sensor has an active area of 7 $\,\times\,$ 2 ${\rm mm}^{2}$ , excluding the anchors. At 23 $^{\circ}{\rm C}$ , it operates at a resonant frequency of 230.8 kHz and has a sensitivity of $13\times 10^{3}~{\rm ppm}/{\rm mstrain}$ ; the dynamic range is 0.05–1.05 mstrain. The differential sensor includes a strain independent reference resonator of area 2 $\,\times\,$ 0.5 ${\rm mm}^{2}$ in addition to a sensing element of area 2.5 $\,\times\,$ 0.5 ${\rm mm}^{2}$ that is divided into two segments. The sensor resonance is at 266.4 kHz and reference resonance is at 492.75 kHz. The differential sensor provides a dynamic range for 0–1.85 mstrain with a sensitivity of $12.5\times 10^{3}~{\rm ppm}/{\rm mstrain}$ at 23 $^{\circ}{\rm C}$ . The reference resonator of the differential sensor is used to compensate for the temperature dependence of the Young's modulus of Metglas 2826 MB, which is experimentally estimated to be ${-}{\rm 524}~{\rm ppm}/^{\circ}{\rm C}$ . For an increment of 35 $^{\circ}{\rm C}$ , uncompensated sensors exhibit a resonant frequency shift of up to 42% of the dynamic range for the single sensor and 30% of the dynamic range of the differential sensor, underscoring the necessity of temperature compensation. The geometry of both types of sensors can be modified to accommodate a variety of sensitivity and dynamic range requirements. $\hfill{[2013\hbox{-}0330]}$

Rapid Silicon-to-Steel Bonding by Induction Heating for MEMS Strain Sensors

A rapid and manufacturable technique to bond silicon to steel for microelectromechanical system (MEMS) sensor applications is presented. Using an induction heating system, the process is simple, clean, inexpensive, and fast, with bonding occurring in 3-5 s. Heat generation is isolated to within several micrometers below the steel surface, allowing for a controllable thermal budget that does not damage the surface treatment of steel. The process requires minimal surface conditioning and may be used with a variety of different metal systems. Bonds formed with adhesion layers of gold, silver, and nickel showed excellent mechanical characteristics, surviving surface strains exceeding +/- 2000 μe. The transferred strain was quantified by ANSYS simulation and measured experimentally for silicon substrates of varying thicknesses, with the 100 μm-thick substrates demonstrating the greatest strain sensitivity. A double-ended tuning fork MEMS resonant strain sensor, bonded to a steel parallel, demonstrated a repeatable 5.4-fold improvement in strain sensitivity compared to epoxy bonding. As such, this robust bonding process is well-suited for bonding MEMS sensors to monitor components in the automotive, manufacturing, and other harsh environment industries.

Strain sensing through the resonant properties of deformed metal nanowires

In this article, we study the potential of gold nanowires as resonant nanoscale strain sensors. The sensing ability of the nanowires is determined by calculating the variations in resonant frequency that occur due to applied uniaxial tensile and compressive strain. The resonant frequencies are obtained using the surface Cauchy–Born model, which captures surface stress effects on the nanowires through a nonlinear continuum mechanics framework; due to the continuum formulation, the strain-dependent nanowire resonant frequencies are calculated through the solution of a standard finite element eigenvalue problem, where the coupled effects of the applied uniaxial strain and surface stress are naturally included through the finite element stiffness matrix. The nanowires are found to be more sensitive to compressive than tensile strain, with resonant frequency shifts around 200–400 MHz with the application of 1% tensile and compressive strain. In general, the strain sensitivity of the nanowires is found to increase with decreasing cross-sectional size, with additional dependencies on their aspect ratio.

A SiC MEMS Resonant Strain Sensor for Harsh Environment Applications

In this paper, we present a silicon carbide MEMS resonant strain sensor for harsh environment applications. The sensor is a balanced-mass double-ended tuning fork (BDETF) fabricated from 3C-SiC deposited on a silicon substrate. The SiC was etched in a plasma etch chamber using a silicon oxide mask, achieving a selectivity of 5:1 and etch rate of 2500 Aring/min. The device resonates at atmospheric pressure and operates from room temperature to above 300degC. The device was also subjected to 10 000 g shock (out-of-plane) without damage or shift in resonant frequency. The BDETF exhibits a strain sensitivity of 66 Hz/muepsiv and achieves a strain resolution of 0.11 muepsiv in a bandwidth from 10 to 20 kHz, comparable to state-of-the-art silicon sensors

Sticking Prevention of Resonant Strain Sensors

Sticking is a fundamental phenomenon on micromachined devices or tructures. This paper describes about a practical and successful method of prevention for micromachined resonators from sticking and estimation of its sticking resistance based on experimental data.