1. Field of the Invention
The present invention relates to a resonator, to a vibrating sensor including such a resonator, and to a method of fabricating the resonator. Such a resonator is used for example in vibrating sensors of the gyro type.
2. Brief Discussion of the Related Art
The general principle of a vibrating sensor is to subject a resonator to vibration and to detect a physical magnitude that is representative of the influence of an acceleration on the vibration.
Vibrating sensors exist that include an electrode-carrying plate with a resonator mounted thereon. The resonator comprises a body having a substantially hemispherical resonant part with a pole that is connected to the electrode-carrying plate by a sensor stem. The resonant part comprises a hemispherical web defined by an outside surface and an inside surface, which surfaces have free edges that are connected to each other by a plane annular surface that extends facing the electrodes secured to the electrode-carrying plate. The resonant part and the stem are covered in an electrical conduction layer. The electrical conduction layer and the electrodes are connected to different potentials so as to subject the web to periodic elliptical deformation and so as to detect the orientation of the ellipse, e.g. as a function of capacitances of values that depend on the gap formed between the electrodes and the plane annular surface.
The body is made of silica because of its isotropic properties and its very low mechanical damping.
In an embodiment that used to be widespread, the resonant part and the stem were completely covered in a layer of chromium forming the conduction layer (see for example document US-A-2003/0019296).
Nevertheless, it was subsequently found that the layer of chromium contributed non-negligible mechanical damping.
Resonators were thus considered in which the layer of chromium did not extend over all of the resonant part: the layer of chromium then covered the stem and the annular surface, and was provided with branches extending over the inside surface from the stem to the annular surface. That enabled damping to be reduced significantly, thereby giving rise to a large increase in the performance of vibrating sensors incorporating such resonators.
Subsequently, replacing the chromium in those resonators with platinum has enabled the performance of vibrating sensors to be further improved.
The document EP-A-1445580 discloses a resonator having an inside surface fully covered by a conductive metal coating and an outside surface left uncovered.
Nevertheless, silica is a material with a very active surface that tends to establish bonds with its surroundings. Silica becomes covered on its surface in groups of silanol Si—OH or silane Si—H that are strongly polar and that can combine with examples in the surroundings. In vibrating sensors, the resonator is in a vacuum so that such contamination of the silica does not occur. Nevertheless, the research that has led to the invention has shown that such contamination of the silica that is left uncovered by the conduction layer gives rise to unstable and anisotropic modification of the geometrical properties and of the mechanical damping of the resonator, even when the level of contamination is low: such modification is likely to lead to drift that cannot be compensated electronically because it is unstable.