Issue 29

V. Zega et alii, Frattura ed Integrità Strutturale, 29 (2014) 334-342; DOI: 10.3221/IGF-ESIS.29.29 335 consequently able to displace with respect to the underlying silicon substrate. The proposed four-axis sensor makes use of differential resonant detection for both in-plane and out-of plane acceleration components and roll and yaw velocity components. Resonant detection, as compared to other measuring techniques, has the advantage of affording a direct frequency output, of a quasi-digital type, a high sensitivity and a wide dynamic range. While several resonant accelerometers have been proposed in the literature [7-10] there exist a few examples of micro-gyroscopes with resonant detection [11-12]. The proposed device makes simultaneous use of two different operating principles typical of resonant MEMS: the frequency shift due to the presence of axial stresses in slender resonating beams and the frequency shift due to variations of the so-called electrical stiffness in torsional resonators. In particular, external in-plane acceleration or yaw velocity make the proof masses translate and induce axial forces in bending resonators, while external out-of-plane acceleration or roll velocity make the proof masses tilt and change the electrical stiffness of torsional resonators. The paper is organized as follows. The first section presents the device design, discusses the mechanical behavior of the resonating parts and provides an analytical estimate of the sensitivity. The second section focuses on the fabricated device and presents the results of numerical modal simulations. Experimental characterization of the integrated sensor is currently under development. D EVICE DESIGN Device structure he proposed structure is schematically shown in Fig. 1. The device is constituted by two moving proof masses attached to the substrate by means of folded springs which allow their translation in the plane and their tilting around the axes a-a . Four bending resonators (labelled I, II, III and IV in Fig. 1) are located beside the masses and attached to them through the external springs and four torsional resonators (labelled 1, 2, 3 and 4 in Fig. 1) are directly attached to the masses. The variation of the resonance frequency in the flexural resonators is induced by the presence, upon translation of the inertial mass, of axial stresses while in the torsional resonators it is induced by variations of the so-called “electrical stiffness” to which the resonator mass is subjected. The simultaneous use of these two different type of resonators allows realizing of a four-axis sensor with reduced dimensions. By means of the flexural resonator elements, the integrated detection structure enables differential detection of an angular velocity acting about an out-of-plane direction, the yaw angular velocity Ω z , and of a linear acceleration a y along the y -axis. In addition, by means of the torsional resonator elements, the integrated detection structure enables differential detection of an angular velocity acting about the y -axis, the roll angular velocity Ω y , and of a linear out-of-plane acceleration a z . Figure 1 : Schematic plan view of the structure, for detection of acceleration and angular velocity. T