Santina Rocchi

Surface State Model for Conductance Responses During Thermal-Modulation of SnO

Metal oxide gas sensors (MOXs) are widely used in olfactory electronic systems for their high sensitivity and low-cost. These sensors modify their conductivity in presence of oxidizing and reducing gases, and their performance is strictly dependent on the measurement technique adopted. In particular, it was already established by many works that a noticeable improvement in selectivity can be obtained by operating MOXs with a variable temperature. The temperature profile, however, must be tailored to the specific application, and the shape of the optimum profile for a given application depends both on the specific sensor and on the tested chemicals. In this context, there exists a strong interest in developing simplified models able to predict the sensor response, and aiming at a better comprehension of the mechanisms involved in sensing operations. In this paper, three simple gray-box models able to predict the behavior of some commercial thick film SnO2-based sensors in presence of oxygen and a reducing gas (CO) are proposed and discussed, whereas in a second paper the experimental validation of the model is presented

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In this paper, an innovative measurement system for odor classification, based on quartz crystal microbalances (QCMs), is presented. The application proposed in this paper is the detection of typical wine aroma compounds in mixtures containing ethanol. In QCM sensors, the sensitive layer is, e.g., a polymeric layer deposited on a quartz surface. Chemical mixtures are sorbed in the sensitive layer, inducing a change in the polymer mass and, therefore, in the quartz resonance frequency. In this paper, the frequency shift is measured by a dedicated, fully digital front-end hardware implementing a technique that allows reducing the measurement time while maintaining a high-frequency resolution . The developed system allows, therefore, measuring variations of the QCM resonance frequency shifts during chemical transients obtained with abrupt changes in odor concentration. Hence, the reaction kinetics can be exploited to enhance the sensor selectivity. In this paper, some measurements obtained with an array of four sensors with different polymeric sensitive layers are presented. An exponential fitting of the transient responses is used for feature extraction. Finally, to reduce data dimensionality, principal component analysis is used.

-Based Thick Film Sensors: Part I—Model Derivation

This paper and its companion (Part I) are devoted to the development and to the experimental verification of three simple gray-box models able to predict the behavior of some commercial thick film SnO2-based sensors in presence of oxygen and a reducing gas (CO). In this paper the models developed in Part I are applied to different commercial tin oxide sensors, and experimental results are discussed in order to gain a deeper insight into the sensor behavior

A measurement system for odor classification based on the dynamic response of QCM sensors

Airborne polyvinylidenefluoride transducers have been designed for robotic applications in air. Characteristics of transducer prototypes are: working frequencies from 61 kHz to 86 kHz, quality factor Q from 4 to 6, and two-way insertion loss of about 90 dB. The small dimension, the lightness, and the low-cost fabrication technology allow the development of arrays or matrices for ultrasonic imaging systems in air. In this work two different image reconstruction algorithms are proposed: the first carries out a combined spectral and aperture synthesis for detecting isolated scatterers with a spatial resolution of about 2 mm; the second is based on an accurate ranging algorithm with sub-millimeter resolution at distances up to 50 cm. Finally, this works application to the reconstruction of three-dimensional object profiles is discussed.

Surface State Model for Conductance Responses During Thermal-Modulation of SnO

The short-time Fourier transform provides a picture of the spectral components temporal location in time-varying signals, but its performance is limited by the intrinsic trade-off between time and frequency resolutions. In the present study, this problem is addressed using a spectral estimator based on a combination of the autoregressive (AR) modeling technique and a new automatic model order selection method. The order estimation is achieved by means of the singular value decomposition (SVD) of an appropriate data matrix in conjunction with a new criterion (dynamic mean evaluation, DME). The latter is used to decide which singular values correspond to the signal and which to the noise subspaces, avoiding an a priori threshold definition, thus giving the variable AR model order on consecutive short-time segments. Combination of the AR high frequency resolution capabilities and the SVD plus DME robustness and simplicity make the overall method reliable in many practical applications, mainly in the analysis of time-varying signals corrupted by noise. The proposed procedure has been applied to benchmark as well as to Doppler signal analysis. Some examples are reported confirming the above-mentioned properties.

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A new multisensor headspace and electronics for gas detection are presented. The system is PC-based and fully software controllable and reconfigurable. It was designed to allow for a very versatile use with different metal oxide sensor elements. In particular, the system supports different measuring techniques such as sensor impedance measurement in a frequency range spanning from DC to 15 MHz, and sensor gas sensitivity measurement when the temperature follows an arbitrary-defined trend. Some preliminary results concerning wine classification are also presented.

-Based Thick Film Sensors: Part II—Experimental Verification

In this paper a comparison among different parameter extraction techniques, applied to the dynamical responses of QCM (quartz crystal microbalance) sensors, is presented. Some results concerning the application of the studied technique to the wine aroma recognition are presented.

Piezo-polymer transducers for ultrasonic imaging in air

In this paper an innovative measurement system for odor classification, based on Quartz Crystal Microbalances (QCHs), is presented. In particular, the problem of detecting typical wine aroma components in mixtures where ethanol is present is taken into account. In QCM sensors the sensitive layer is a polymeric layer deposited on a quartz surface. Chemical mixtures are adsorbed by the polymeric layer, inducing a change in the quantz mass and therefore in its resonance frequency. The frequency shift is measured by a dedicated fully digital front-end hardware implementing a technique proposed by Cantoni (2000). This approach allows reducing the measurement time while maintaining a high frequency resolution. The developed system allows measuring variations of the QCM resonance frequency shifts during chemical transients obtained with abrupt changes in odor concentration. Hence the reaction kinetics can be exploited to differentiate among different compounds. In this study some measurements obtained with an array of 4 sensors with different sensitive layers are presented. An exponential fitting of the transient responses is used for feature extraction. Principal Component Analysis (PCA) to reduce data dimensionality is used.

Adaptive SVD-based AR model order determination for time-frequency analysis of Doppler ultrasound signals

In this paper an ultrasonic multisensor acquisition and processing system with up to 32 air-ultrasound transducers in the frequency range of 40-200 kHz is presented. The system was developed for an obstacle avoidance applications in the robotics field. The acquisition and processing capability of the equipment (based on the digital signal processor TMS320C25 and can sample up to five parallel channels) allows the robot to perform tasks of different difficulty levels, from target-ranging to pattern recognition. As an example of application, a neural approach to a typical problem in the robotic navigation field is presented, which highlights the system-flexibility features.

Versatile headspace and electronics for measurements with gas sensor arrays

The advantages of ferroelectric polymer technology when building hemicylindrical ultrasonic transducers for airborne applications, have been already broadly investigated by the authors. The theory, experimentally confirmed, showed that the resonance frequency is inversely proportional to the bending radius. Based on this unique property a double frequency array sensor was built up in order to have a large bandwidth otherwise obtainable with more sophisticated and expensive fabrication technologies. In this work, the design is described of two interlaced series of transducers with the resonance frequency equal to 60 kHz and 86 kHz respectively. Experimental characteristics of the array elements have been derived and an imaging system based on the reflection tomography technique was gathered and tested in a laboratory environment. The overall system performance is reported via the point spread function image evaluated by the response to a thin steel wire. The spatial resolution is about 3 mm along both the lateral and axial direction