Ensuring desirable performance for piezoelectric microcantilever sensors constitutes a crucial research subject particularly for the applications such as detection of biochemical entities, virus particles or human biomarkers. However, these sensors' performance may be affected by the environmental conditions such as temperature variation, and/or the uncertainty in the material properties. The objective of this study is to explore Young modulus uncertainty of microcantilever's structural layer, thermomechanical and geometrical temperature dependency effects, on the natural frequency, bias and sensitivity of microcantilever mass sensors. These effects have been investigated for different sensor lengths and resonant modes. Also, a temperature compensation method which omits the need for bulky non-contact thermometers or fabrication of built-in temperature sensor has been proposed. As theoretical model, Euler-Bernoulli beam theory has been employed and solved by Galerkin expansion procedure. Using this model, it is demonstrated that the sensitivity of microcantilever sensor decreases with increasing the added mass. The microcantilever sensor sensitivity operating at the second resonant mode has been improved almost five times comparing to the first mode sensitivity regardless of microcantilever length. The simulation results show that temperature variation causes thermal frequency shift which in turn introduces a significant mass bias far beyond the sensors' minimum detectable mass. This mass bias is constant for a given microcantilever in its first and second resonant mode. Additionally, the effect of temperature variation on the sensitivity of the given mass sensors is negligible. However, it has been shown that the variations in sensors sensitivity due to uncertainty of Young modulus remain constant for different lengths and two resonant modes of the microcantilever sensor.
For ever-increasing applications of resonant piezoelectric-excited millimeter-sized cantilever (PEMC) such as biosensors, viscosity, and density sensors, the need for design and implementation of a portable circuit for measuring the resonant frequency shift and/or the variation in the quality factor of PEMC becomes crucial. In this article, active and passive selfsensing bridge circuits are designed, fabricated, and implemented for a PEMC. The performances of these circuits are examined for the resonant frequency and quality factor measurements for vibration of PEMC in two different environments. For this purpose, a parameter-tuning procedure for the passive bridge based on experimental identification of Van Dyke model parameters is proposed and applied for the vibration of PEMC in air and 98% glycerol solution. Also, a compensation method for potential instability of active bridge circuit is proposed and developed experimentally for the vibration of PEMC in these environments. To increase the quality factor, the fabricated passive bridge is used in a designed control circuit, which is based on positive feedback signal proportional to the vibration velocity. The experiments show that with the proposed and implemented control circuit, the quality factor will increase by about 80% in air and 25% in glycerol.
Capability of piezoelectric microcantilever mass sensors to detect very small amount of mass, such as detection of a single virus has attracted many researchers' interests. In many cases, detection of mass is performed by adsorbing of materials in porous matrixes on the sensor surface, in which the adsorbed material does not form a solid layer on the cantilever surface. On the other hand, there are materials such as gold and mercury that constitute a solid layer on cantilever surface. The thickness and the young modulus of this layer, increase the cantilever's stiffness. Consequently, both the mass and the stiffness changes, alter the cantilever's natural frequency, which should be considered in modeling. Two lumped parameter modeling approach for resonant microcantilevers mass sensors has been investigated. Model I neglects the stiffness change of the cantilever sensor after mass loading. In model II the added mass constitutes a solid layer on the cantilever bottom surface. This additional solid layer affects the mass and also the stiffness of microcantilever. The results shown that model II represents a better modeling comparing to model I with respect to experimental data. For cantilevers of the same thickness in this study, the stiffness variation percentage due to added mass layer remains constant. Furthermore, the deviation between model I and model II is the same regardless the cantilevers dimensions.
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