This application claims priority from European Patent Application No. EP 04104267.2 filed 3 Sep. 2004, the entire disclosure of which is incorporated herein by reference.
The present invention concerns, generally, piezoelectric resonators and more specifically, resonators of very small dimensions, which are usually used for making frequency generators, particularly for portable electronic equipment, in numerous fields, such as watchmaking, information technology, telecommunications and medicine.
Tuning fork piezoelectric resonators are known in which the electrical excitation field is created in a direction parallel to one of their faces by electrodes, some of which are arranged on said face. Such a tuning fork resonator 1, shown in
Although of small dimensions, of the order of a millimeter, such piezoelectric generators are still too bulky in light of the ever increasing demand for miniaturisation of electronic equipment, such as portable telephones or watches.
One solution recommended in the prior art, shown in
However, such a solutions has certain drawbacks. An analysis of this prior art solution has demonstrated that these notches 5a and 5b do not enable the tuning fork to be uncoupled from thermal stress generated by the lack of thermal tuning of the thermal expansion coefficients, thus resulting in the propagation of a static mechanical stress in a median zone 8 of base 2 located between the two arms 3a and 3b. Moreover, dynamic elastic stress is maximal in this median zone 8. The results of this combination of static mechanical stress varying with temperature and the dynamic elastic stress is an alteration in the features of resonator 1.
One solution for improving the resistance of the resonator to these types of stress is the use of a flexible adhesive agent, like for example conductive silicon adhesives, for mounting the resonator in its case. Nonetheless, such flexible adhesive agents exhibit problems of adherence and resistance to shocks, such a solution is thus undesirable.
Moreover, notches 5a and 5b weaken the mechanical structure of resonator 1, which raises a problem in the event of shocks, and more specifically in the event of lateral shocks. Indeed, when there is a lateral shock, the forces exerted on the whole of resonator 1, i.e. on arms 3a and 3b and on base 2, result in maximum stress at the point of intersection between each notch 5a and 5b and base fixing zone 6.
It is one of the main objects of the invention to overcome the aforementioned drawbacks by making a piezoelectric resonator having, on the one hand, a reduced size as well as an equally reduced energy consumption, and on the other hand, good resistance to the various stresses that the resonator may undergo, as well as good shock resistance.
Within the scope of the present invention, it has been demonstrated that replacing the notches formed in the base by a hole made in the fixing zone of the resonator and the vibrating arms allows the static thermal stresses to be perfectly uncoupled from the median zone of the base located between the arms, and thus to have no coupling in the dynamic elastic stress zone. This hole also allows the effective length of the arms to be artificially elongated, thus reducing the total size of the resonator for the same resonance frequency. Moreover, lateral shock resistance is reinforced, since the resonator no longer has a weak point in proximity to the fixing zone, while maintaining the same resistance as a solution with notches in the direction perpendicular to the resonator plane.
Thus, according to a preferred embodiment of the invention, the resonator includes a base formed of a first fixing zone, a second central mechanical uncoupling zone and a third zone, opposite the first fixing zone, from which at least two vibrating arms extend, on each of which at least one groove is formed on at least one of the top or bottom faces of the arms, characterised in that the base is provided with a hole through the second central mechanical uncoupling zone.
This type of resonator is generally intended to be packaged in a case. Owing to the advantages obtained by the use of a hole instead of notches, it is possible to further improve shock resistance by selecting a rigid mounting, like for example an epoxy conductive adhesive, for fixing the resonator to its case.
It will also be noted, again for the purpose of improving the resistance of the resonator, that the hole advantageously has a polygonal shape the sides of which are cut so as to minimise visualization of the crystalline planes of the piezoelectric resonator.
Finally, the present invention also concerns a vibrator including a case in which a piezoelectric generator according to any of the embodiments of the invention is rigidly fixed.
Other features and advantages of the present invention will appear more clearly upon reading the following detailed description of embodiments of the invention, given solely by way of non-limiting examples and illustrated by the annexed drawings, in which:
As previously mentioned, the present invention concerns a piezoelectric resonator having, on the one hand, reduced power consumption owing to excitation by a more homogenous electrical field and, on the other hand, proper mechanical uncoupling between the base and the arms of the resonator as well as greater shock resistance, particularly to lateral shocks.
The use of two small grooves optimises the piezoelectric coupling while maximising the dynamic capacity of the resonator and reducing to a minimum the section of the arms.
For the purpose of having the most resistant structure possible for the arms, a first variant, shown in
According to a second simpler variant having satisfactory resistance for the arms, shown in
By way of additional variants, it will be noted that it is also possible to have only one groove on one face or both faces of each of the arms. Nonetheless, in such case, the spacing between the grooves at the base will preferably be reduced, if one wishes to prevent weakening the mechanical structure of the arms too much.
Still examining
Such resonators are of very small dimensions. Within the scope of the present invention, certain advantageous dimension ratios have been demonstrated. The following dimensions are given by way of example:
Thus the resonator preferably has the following dimension ratios:
As mentioned previously, the effect of using two grooves having a constant spacing between them (
Advantageously, the ratio of the width of a groove I5 to the width of an arm I4 is comprised between 10 and 30%, and the ratio of the distance between grooves d2 to the width of an arm I4 is comprised between 20 and 30%.
With a view to further improving the shock resistance of resonator 10, the third zone 123 of base 12 has a cut out portion where arms 13a and 13b are fixed, minimising the visualization of the crystalline planes of the piezoelectric resonator. In the example of a quartz resonator, the cut out portion forms angles of approximately 60° (
These two variants have been shown with grooves 14a and 14b having a constant spacing. However, it is clear that all of the variants of the different groove structures able to be used and presented in relation to
According to the example shown in
The resonator is mounted by welding or bonding with a hard conductive adhesive the connection pads, corresponding to electrodes 19 and 20 (in
It will be understood that various modifications and/or improvements evident to those skilled in the art can be made to the various embodiments of the invention described in the present description, particularly by combining the advantages presented in these various embodiments, without departing from the scope of the invention defined by the annexed claims.
Number | Date | Country | Kind |
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04104267 | Sep 2004 | EP | regional |
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20060049722 A1 | Mar 2006 | US |