Design and Performances of Two Quartz Monolithic Vibrating Inertial Microsensors

For several years, ONERA has been developing inertial vibrating microsensors, for military and civil applications, such as dynamic control and navigation of vehicles (coupled with radio-electric positioning system). The object of these works is to define original and accurate MEMS devices compatible with collective micromachining technologies, in order to satisfy industrial needs. This paper presents the design and the performances of two sensors : a Vibrating Beam Accelerometer (VBA), called VIA (Vibrating Inertial Accelerometer), and a Coriolis Vibrating Gyrometer (CVG), called VIG (Vibrating Integrated Gyrometer). These devices are based on monolithic structures, which are micromachined by wet etching of crystalline α-quartz substrates. The piezoelectric properties of quartz are used to both drive and detect the mechanical vibrations. The main originality of the presented designs is their specific decoupling frames, which have been developed in order to insulate the useful vibrations and to preserve the quality factor of the resonance modes. Hence energy losses represent less than 10⁻⁸ of the total vibrating energy. Thanks to these frames, it is then possible to take full advantages of the excellent mechanical properties of quartz and to achieve high accuracies. The VIA accelerometer principle is based on the frequency shift of a vibrating microcantilever, due to the axial stress generated by a proof mass submitted to an acceleration. The working frequency is around 60 kHz and the sensitivity is 24 Hz/g. Upon a measuring range of 100 g, the scale factor error is about 10 ppm and the bias error is a few hundreds of μg. The VIG uses a tuning-fork as sensitive element, which is excited at its in-plane flexural mode (driving mode). When a rotation speed is applied to this system, a transversal vibration (sensing mode) is induced by the Coriolis forces. The amplitude of this induced vibration is proportional to the rotation speed. Piezoelectricity is used for both the excitation of the driving mode and the detection of the sensing mode. The VIG measuring range is 1000 °/s, with a resolution around 0.01 °/s/√Hz and a bandwidth of 100 Hz.