Surface Acoustic Wave Resonator Sensors Research Articles

Wireless surface acoustic wave resonator sensors: fast Fourier transform, empirical mode decomposition or wavelets for the frequency estimation in one shot?

Abstract. Most applications which measure physical quantities, especially in harsh environments, rely on surface acoustic wave resonators (SAWRs). Measuring the variation of the resonance frequency is a fundamental step in such cases. This article presents a comparison between three techniques for best determining the resonance frequency in one shot from the point of accuracy and uncertainty: fast Fourier transform (FFT), discrete wavelet transform (DWT) and empirical mode decomposition (EMD). After proposing a model for the generation of synthetic SAW signals, the question of wavelet choice is answered. The three techniques are applied to synthetic signals with different central frequencies and signal-to-noise ratios (SNRs). They are also tested on experimental signals with different sampling rates, number of samples and SNRs. Results are discussed in terms of the accuracy of the estimated frequency and measurement uncertainty. This study is successfully extended to SAWR temperature sensors.

Extraction of Attenuation Parameter in Periodic Shorted Reflectors of One-Port SAW Resonator Using Scattering Matrix Approach

In this work, the well-known scattering matrix approach to the design of one-port surface acoustic wave (SAW) resonators is adapted toward extraction of the coupling-of-modes (COM) attenuation parameter, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> for Rayleigh SAW propagation on a periodic shorted aluminum grating. The attenuation close to resonance inside the reflectors of the one-port SAW resonator is extracted by matching the measured conductance of the resonator to its version calculated from the experimentally measured interdigital transducer (IDT) scattering coefficients, intrinsic substrate parameters, and geometry of the resonator. Based on the extracted value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> , it was observed that the proposed method could predict the unloaded <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor of the resonator at a minor discrepancy of ~7.02%. At the same time, since the proposed method utilizes the measured IDT scattering coefficients to consider the effect of IDT losses on the measured conductance of the resonator, it offers a convenient scheme to directly extract the attenuation of SAW contributed by the reflectors of the device through numerical means while operating purely in the frequency domain. The proposed method could be applied to quantify the increase in attenuation caused by viscous damping of SAW inside sensing film-coated reflectors of one-port SAW resonator sensors, and thereby, aid to optimize the sensing film thickness to deploy these devices for high-resolution frequency-based sensing applications.

Dual resonance modes in two-port SAW resonator sensors: an analysis through scattering matrix approach

We report dual resonance modes occurring in two-port surface acoustic wave (SAW) resonators attached with typical sensing film and investigated the significance in sensing applications. Two-port SAW resonators operating at 303 MHz frequency were modelled using the scattering matrix approach and resonance frequency characteristics of the resonators were studied for mass loading caused by an arbitrary film attached between the interdigital transducers of the resonator. Depending on the mass loading, symmetric and antisymmetric modes occur around the near resonance frequency of the SAW device. Either a single-mode or both modes are dominant depending on the phase shift caused by the mass loading of the film. Choosing arbitrary film thickness could result in complexity to track the resonance frequency shift and degrade sensing performance. The approach used for analysis can aid to determine the optimum sensing film thickness for two port SAW resonator-based sensors that can exhibit better linearity.

Scattering Matrix Approach to Design of One-Port Surface Acoustic Wave Resonator Sensors Utilizing Reflectors as Sensing Element.

It has been recently demonstrated that one-port surface acoustic wave (SAW) resonators known for their high Q value and relatively small device footprint could be utilized for in-liquid mass loading sensing applications where only the reflectors of the device are coated with the sensing film, while the interdigital transducer (IDT) is isolated from the sensing environment. The sensor relies on changes induced in reflectivity and phase velocity of SAW in the region of the reflectors upon detection of the measurand and is particularly advantageous for SAW resonator-type sensors as any contact of the sensing film with the IDT could change its static capacitance during sensing and thereby introduce serious instability in the sensor response. Accordingly, in the present work, the existing scattering matrix approach to the design of one-port SAW resonator filters, which does not cater to the integration of sensing film on the resonator surface, is adapted to develop a method to design one-port SAW resonator sensors utilizing reflectors as sensing element. The reflector block of the one-port SAW resonator is readily split into sensing-active and sensing-inactive parts using the SAW grating transmission matrix in order to study the changes introduced in input admittance of the device for varying level of coverage of the sensing film. The theoretical design approach presented in this work could be used to fabricate high-performance one-port SAW resonator sensors operating at its point of highest sensitivity while utilizing one of the device reflectors as sensing element, without the use of additional impedance matching circuit elements.

A novel high frequency SAWR based sensor combined with living cells for shellfish toxin quantitative determination

Shellfish toxin can be easily found in seafood, and bring potential harm to human health. Nowadays some analytical methods are used for shellfish toxin quantitative detection. However, there are some disadvantages in these analytical methods such as time consuming, high cost, etc. Therefore, a more suitable method is in demand. In this study, a novel method using high frequency surface acoustic wave resonator (SAWR) based sensor combined with living cells was developed for shellfish toxin continuous monitoring. ECA9706 cell lines were used as the sensing elements to establish the sensor device for DSP toxin okadaic acid (OA) quantitative monitoring. The sensor device is in serial with SAWR. SAWR output frequency was recorded for detecting characterization. The OA concentration of prepared solutions could be characterized by using SAWR sensor, and good linear relationship between sensor responses and OA concentration is achieved. Liquid chromatography tandem mass spectrometry (LC–MS/MS) is used for characterization of OA content in shellfish extract. Good linear relationship between OA content and the concentration determination provided by SAWR sensor. HEK293 cells were fixed on electrode and utilized as negative control. The results showed that ECA9706 cell-based sensor had selective responses to OA. This method is promising in shellfish toxins rapid quantitative determination.

A Pigeonhole Principle-Based Method for Estimating the Resonant Frequency of SAWR Sensors

Surface acoustic wave resonator (SAWR) sensors are passive and wireless devices whose resonant frequency is dependent to a measurand. The nominal value of the resonant frequency is about several hundred megahertz which slightly varies with the measurand to about several hundred megahertz. Due to the passive structure of an SAWR sensor, it is stimulated using a relevant signal. In response, the SAWR sensor resends a decaying signal toward the interrogation unit whose carrier frequency is equal to the sensor’s resonant frequency. In order to determine the value of the measurand, the carrier frequency of the response signal should be extracted. In this paper, a pigeonhole principle-based method is proposed which is used for accurate estimation of the carrier frequency of the short decaying response signal of the SAWR device. This method employs two signal paths of dc output voltages which are strongly related to the response signal frequency. The combination of these two voltages for each values of the input signal frequency is unique. So by accurate measuring these dc voltages and comparing their combination with the values stored in a lookup table, the signal carrier frequency is detected. We have designed and implemented a case study of the proposed scheme which can measure about 250-kHz variations of the carrier frequency around 434 MHz in less than some microseconds. The best and worst average resolutions of the implemented scheme are 216 Hz and 6.5 kHz, respectively.

SAWR dynamic strain sensor detection mechanism for high-temperature harsh-environment wireless applications

High-temperature harsh-environment dynamic strain sensors are needed in multiple contemporary industrial and defense monitoring and control applications, in particular in power plants, metal manufacturing, and aerospace industries. Surface acoustic wave resonator (SAWR) technology stands out as an ideal sensor platform due to attractive technological features such as: small size, capability of wireless operation, battery-free operation, and resilience to high-temperatures. The SAWR dynamic strain sensor detection mechanism discussed in this paper reveals that both frequency and magnitude of the dynamic strain signal can be directly measured. Moreover, in-phase and quadrature component analyses of the measured free-resonating SAWR signal exposed to the dynamic strain perturbation show that both frequency and amplitude modulation are present. The results confirm the appropriateness of the SAWR sensor to detect both the frequency and magnitude of dynamic strain, making this technology very attractive for dynamic strain sensor applications, including situations that require wireless operation in high-temperature harsh-environments.

Ratiometric Decoding of Pheromones for a Biomimetic Infochemical Communication System.

Biosynthetic infochemical communication is an emerging scientific field employing molecular compounds for information transmission, labelling, and biochemical interfacing; having potential application in diverse areas ranging from pest management to group coordination of swarming robots. Our communication system comprises a chemoemitter module that encodes information by producing volatile pheromone components and a chemoreceiver module that decodes the transmitted ratiometric information via polymer-coated piezoelectric Surface Acoustic Wave Resonator (SAWR) sensors. The inspiration for such a system is based on the pheromone-based communication between insects. Ten features are extracted from the SAWR sensor response and analysed using multi-variate classification techniques, i.e., Linear Discriminant Analysis (LDA), Probabilistic Neural Network (PNN), and Multilayer Perception Neural Network (MLPNN) methods, and an optimal feature subset is identified. A combination of steady state and transient features of the sensor signals showed superior performances with LDA and MLPNN. Although MLPNN gave excellent results reaching 100% recognition rate at 400 s, over all time stations PNN gave the best performance based on an expanded data-set with adjacent neighbours. In this case, 100% of the pheromone mixtures were successfully identified just 200 s after they were first injected into the wind tunnel. We believe that this approach can be used for future chemical communication employing simple mixtures of airborne molecules.

A context-aware frequency estimation method for wireless passive resonant SAW sensors

PurposeThis paper aims to present an context evaluation and frequency measurement method for surface acoustic wave (SAW) resonant sensor.Design/methodology/approachThis method is based on a signal subspace construction, which, along with assembling optional value set, provides the results.FindingsThe method can assess the application context and improve the resolution and accuracy of the passive wireless SAW resonator sensor system.Originality/valuePassive wireless SAW resonators have been used as sensor elements for different physical parameters such as temperature, pressure and force in a number of industrial and medical applications. Various wireless channel environments introduce different application contexts.

High-temperature static strain langasite SAWR sensor: Temperature compensation and numerical calibration for direct strain reading

Strain sensing for extreme environments such as aerospace, power generation, and industrial applications require robust sensors capable of sustaining the targeted high-temperatures, while maintaining a stable sensor response. Current technologies face challenges regarding device or system size, complexity, operational temperature, or dynamic range. Surface acoustic wave resonator (SAWR) sensors fabricated on langasite (LGS) using Pt-alloy based electrodes are capable of withstanding temperatures up to 1000°C, have a small footprint, are battery-free, and can be wirelessly interrogated, thus presenting themselves as a promising technology for harsh-environment strain sensing. This paper presents direct SAWR strain sensor readings up to 114με on a constant stress beam between 24°C and 400°C. In order to obtain the direct strain reading at high-temperature, a finite element method (FEM) numerical model was developed. The FEM model was compared to measurements performed using a room temperature commercial sensor, and revealed correlation between the measured and modeled strain data close to unity. At elevated temperatures, the SAWR frequency measurements were calibrated using the developed FEM model adapted for operation at high-temperatures, due to the absence of a reliable high-temperature static strain sensor. A temperature compensation scheme was employed to mitigate temperature-strain cross-sensitivity, and Savitzky-Golay signal processing filtering employed to improve signal detection. Ceramic Al2O3-based epoxy was used to attach the sensor to the targeted constant stress beam fixture, and −41Hz/με sensitivity was obtained at 400°C. The sensor fabrication, calibration and measurements reported in this work illustrates the capability of LGS based SAW resonators to perform as high-temperature harsh-environment static strain sensors.

Cell suspension concentration monitoring by using a miniaturized serial high frequency SAWR sensor

In this paper, a miniaturized cell suspension concentration monitoring method was investigated. The sensing unit was a carbon screen-printed electrode (CSPE) in serial with a 433MHz vacuum-packaged surface acoustic wave resonator (SAWR). SAWR provided a stable and high operating frequency, which helps to keep the stability and sensitivity of the monitoring system. Living cells suspensions in different concentrations were prepared and dropcast on CSPE. Frequency responses of the sensor were recorded. Cell quantity variation within the same culture media volume changed the dielectric properties of CSPE and finally affected the SAWR frequency. SAWR frequency declined with the decrease of cell concentration. The proposed sensor provided high sensitivity and remarkable stability for the cell suspensions.

Characterization of Differentially Measured Strain Using Passive Wireless Surface Acoustic Wave (SAW) Strain Sensors

This paper presents characterization results from surface acoustic wave resonator (SAWR) strain sensors used to carry out wireless passive differential strain measurement. An elevated temperature SAWR strain sensor characterization data set is presented and the results used to demonstrate temperature compensation strategies and overall measurement channel signal sensitivity improvements with a differential SAW sensing element pair. The results are generated by a new wide bandwidth wireless interrogator capable of interrogating two SAWR sensors at high sample rates. Cross axis sensitivity caused by loading conditions generating strains in different directions to the desired strain condition is shown and a compensation strategy presented. The compensation method has reduced the cross axis sensitivity from 24% to 4% of full scale output (FSO). The elevated temperature characterization performance between 20 °C and 100 °C demonstrated a maximum hysteresis level of 0.49% of FSO between 400 με in compression and tension and an overall reduction in the apparent strain due to temperature from approximately 5.5% to 2.7% of FSO.

Graphene-coated Rayleigh SAW Resonators for NO2 Detection

This paper describes the development of a novel low-cost Rayleigh Surface Acoustic Wave Resonator (SAWR) device coated with a graphene layer that is capable of detecting PPM levels of NO2 in air. The sensor comprises two 262 MHz ST-cut quartz based Rayleigh SAWRs arranged in a dual oscillator configuration; where one resonator is coated with gas-sensitive graphene, and the other left uncoated to act as a reference. An array of NMP-dispersed exfoliated reduced graphene oxide dots was deposited in the active area inside the SAWR IDTs by a non-contacting, micro ink-jet printing system. An automated Mass Flow Controller system has been developed that delivers gases to the SAWR sensors with circuitry for excitation, amplification, buffering and signal read-out. This SAW-based graphene sensor has sensitivity to NO2 of ca. 25 Hz/ppm and could be implemented in a low-power low-cost gas sensor.

Effects of the Set Phase Position on the Response of a SAW Resonator Oscillator Based Chemical Sensor

Theoretical calculation results are described to illustrate the experimental results of the dependencies of surface acoustic wave resonator sensor's sensitivity on the set phase position within an oscillator circuit. The experiment highlights that the dependencies are insignificant for thin coatings but quite significant for thick coatings. The calculations were performed using an equivalent circuit model for polymer loading that incorporates a sensitivity factor or phase dependent factor. The datasheet parameters of the device used in the experiment are utilized to simulate the electrical behavior of the sensor. Based on a ratio $R$ of the coating film strain magnitudes induced by cross-film to in-plane gradients, the thickness regime of operations are defined such that if $R\ll 1$ , the film is acoustically thin and if $R\geq 1$ , the film is thick. It is shown that, with film thickness as a parameter, while the sensitivity factor curves are unaffected for $R$ in the range from 0.46 to 0.91, the curves exhibit a prominent spike shape with varying magnitude around resonance peak for $R$ in range from 1.37 to 3.65. With shear modulus as parameter, varying $R$ from 0.65 to 8.23 also results in a spike shape sensitivity factor curves with its peak magnitude varies from 1.026 to 1.92. These results show that in the thick regimes, depending on the magnitude of the sensitivity factor at a point of circuit oscillation, the sensitivity of the sensor may not only increase or decrease but also change its direction. The calculation results thus show consistency with the experimental result.

A novel serial high frequency surface acoustic wave humidity sensor

A novel topology of surface acoustic wave (SAW) humidity sensor was proposed. An interdigital electrode (IDE) was added in serial with a SAW resonator (SAWR) as the sensitive component. Silicon-containing polyelectrolyte humidity sensitive film was deposited on the surface of IDE instead of SAWR. By this way, the stringent technical requirements for film coating on SAW surface could be avoided, which was helpful to bring down the processing cost of the sensor. At the same time, vacuum packaged SAWR could provide a more stable and high operating frequency (433 MHz) for the sensor, which was profitable for both stability and sensitivity of the sensor. Sensitivity, stability, response-time, temperature coefficient of the novel sensor were tested and compared with the traditional SAWR sensor in our previous work. The results showed that both sensitivity (0.4 kHz/%RH at low humidity level; >1 kHz at high humidity level) and stability (noise level <10 Hz) of the sensor have got remarkable improvement for the new design.

Detecting and Evaluating the Signals of Wirelessly Interrogational Passive SAW Resonator Sensors

The wireless sensing signal of a passive surface acoustic wave (SAW) resonator sensor is the response of the SAW resonator in a passive circuit to wireless radio frequency interrogation. The response is produced only in the case that the interrogation covers the operational frequency band of the resonator. The wireless response is transient and can only be detectable in a proximity after switching off the interrogation. Due to the fact that, while used as a sensor, the resonant frequency of the resonator is related to and varying with the measurand, the interrogation to a passive SAW resonator sensor has to trace and follow the correspondent variation of the frequency band of the device. The energy evaluation of the response is applied to detect the availability of the sensing response and is used as a feedback argument to roughly localize the operational frequency range of the sensor. A modified frequency estimation is employed to estimate the sensing characteristic frequency in the transient wireless sensing signal with a low signal-to-noise ratio. The estimation is used to further adjust the interrogation frequency to follow the frequency variation of the sensor until the response becomes optimal. The evaluation of signal energy along with the statistical quantity of frequency estimation gives a reference for the confidence of the estimated frequency.

Determination of dipyridamole by modified extraction-gravimetry with a surface acoustic wave resonator sensor

A simple and sensitive extraction-gravimetric method for the determination of dipyridamole is presented. The method is based on the extraction of free dipyridamole with chloroform, after neutralization with a basic agent, followed by measurement of the frequency shift response of the specially designed surface acoustic wave resonator sensor after evaporation of the extractant from the surface of the resonator. The frequency shift response was proportional to the amount of dipyridamole in the range 0.065–1.12 μg. Experimental parameters and the effect of interfering substances on the assay of dipyridamole were also examined in this study. The method was applied to the determination of dipyridamole in tablets.