EP2078207A2 - Accelerometer using resonant tuning-fork with balancing masses - Google Patents

Abstract

The invention relates to a resonant accelerometer (10) capable of measuring the component an acceleration along a sensitive axis (Z) and comprising a tuning-fork shaped resonator (12) that comprises two longitudinal flat and identical arms (18) which are parallel to each other in a reference plane (P) orthogonal to the sensitive axis (Z). The two arms (18) are capable of oscillating in a first planar-flexion vibration mode (M1) and in a first torsion vibration mode (M2) about their longitudinal axis, both vibration modes being coupled together and defining a coupled vibration mode (M12) having its own vibration frequency (F12), characterised in that the coupling is obtained kinetically by mounting on each arm (18) an unbalancing mass (14), the gravity centre (G) of which is offset relative to the reference plane (P) so as to induce a torsion of the arms (18) during the transverse oscillations thereof.

Description

 Resonant accelerometer with a pod-like resonator equipped with unbalance

TECHNICAL FIELD [001] The invention relates to a resonant accelerometer which comprises a resonator in the form of a tuning fork.

The invention more particularly relates to a resonant accelerometer which is capable of measuring the intensity of an acceleration along a sensitive axis and which comprises: a tuning fork resonator comprising two identical flat longitudinal arms parallel to each other; extend from a recessed end to a free end in a reference plane orthogonal to the sensitive axis, the two arms being capable of oscillating according to a first vibration mode in planar flexion and in a first mode of torsional vibration about their longitudinal axis, these two modes of vibration being coupled to each other and defining a coupled mode of vibration having a natural frequency of vibration; and

actuation means that excite the oscillation of the resonator arms at the natural frequency of vibration.

[003] The component of an acceleration along the sensitive axis causes a quasistatic joint bending of the arms out of the reference plane, the natural frequency of the resonator varying linearly with respect to the amplitude of the arrow of the arms. [004] A resonant accelerometer comprises a resonator having one or more natural frequencies of vibration. When the resonator is subjected to an acceleration along a sensitive measurement axis, the structure of the resonator is deformed or constrained so that its natural frequency varies proportionally with the intensity of the measured acceleration. In other words, the resonant accelerometer responds to a determined stimulus, here acceleration, by a strong anisochronism of frequency. Thus, by measuring the shift of the resonator's natural frequency with respect to a reference natural frequency, it is possible to deduce the component of the acceleration along the sensitive axis, its orientation and its intensity being able to be determined by the combination of two or three accelerometers at most.

State of the art

Already known [Eernisse E. O. et al., "Survey of Quartz BuIk Resonator Sensors Technologies", IEEE Transactions on Ultrasonics, Ferroelectronics and Frequency Control, 35, No. 3, May 1988, pp.

1994-2003] accelerometers comprising a resonator in the form of a double tuning fork. The arms of the resonator undergo longitudinal tensile or compressive stresses which are induced by the measured acceleration. The natural frequency of the bending vibration mode of the arms varies proportionally to the intensity of these induced stresses, and therefore proportionally to the intensity of the measured acceleration.

[006] However, the natural frequency varies non-linearly with respect to the intensity of the acceleration, which makes the use of such a sensor inconvenient.

[007] It is also known to equip some quartz watches with a diaphragm-shaped resonator made of quartz.

[008] This resonator is made of quartz cutting "X + α °", that is to say that the substrate of the resonator is rotated by an angle "α" (typically 2 to 5 °) around the X. electric axis Watches equipped with such resonators have, due to the elastic anisotropy of this quartz cut (via its coefficient of convenience S24), a position effect due to gravitation that can be troublesome. [009] In the watch, the quartz is subjected to an electric field so as to excite by piezoelectric effect the first natural mode of vibration in planar flexion of the arms.

[010] It is shown that, because of the anisotropic structure of the resonator, the planar flexion displacements of an arm elastically induce torsion of the arms about their longitudinal axis, defining a first mode of vibration in torsion. Thus, the planar bending oscillation movements are slightly coupled to the first torsional vibration mode of the arms, the frequency of which is higher than that of the first bending mode. In other words, the torsional vibration mode is coupled elastically and quasistatically to the first planar flexural vibration mode.

[011] Now, when the resonator is exposed to an acceleration along the sensitive axis orthogonal to the plane of the resonator, the arms flex so that their free end is out of the plane of the resonator. The natural frequency of the combined vibration mode in flexion and torsion is then slightly modified of the order of 1 ppm

(parts per million or 10 ^"6 ) for an acceleration of intensity equivalent to Earth's gravity" 1 g ", this unit will be noted later" ppm / g. "It is shown that the variation of the natural frequency is linearly proportional the acceleration suffered.

[012] This property is a major disadvantage for use as a resonator for a watch, but it makes the resonator sensitive to acceleration.

[013] However, such a resonator is too insensitive to acceleration for use as an accelerometer.

Disclosure of the invention

[014] To solve these problems, the invention proposes an accelerometer of the type described above, characterized in that the coupling is obtained kinetically by the arrangement on each arm of an unbalance whose center of gravity is offset from the reference plane, so as to induce torsion of the arms during their transverse oscillations. The coupling obtained is thus much larger and the sensitivity of the resonator to acceleration is improved. [015] According to other features of the invention:

an unbalance is arranged under the free end of each arm;

- the imbalances are realized come from matter with the arms; the imbalances form seismic masses which favor the bending of the arms out of the reference plane when the accelerometer is subjected to acceleration along its sensitive axis;

the resonator is made of a monocrystalline material;

- The resonator is made of silicon and oriented along the axes <100> or <110> of the crystal lattice, so as to have orthotropic elastic properties (that is to say that the elastic quadric is double axial symmetry); and

the actuation means comprise a piezoelectric layer of aluminum nitride on one face of each arm and excite the resonator by an inverse piezoelectric effect.

Brief description of the drawings

Other features and advantages of the invention will emerge during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

FIG. 1 is a perspective view showing a tuning fork resonator made according to the teachings of the invention; FIG. 2 is a perspective view showing the resonator of FIG. 1 in oscillation according to a first planar flexural vibration mode not coupled to a torsional vibration mode;

FIG. 3 is a view similar to that of FIG. 1, which shows the resonator of FIG. 1 in oscillation according to a first mode of vibration in axial torsion not coupled to a mode of vibration in bending;

FIG. 4 is a perspective view showing the resonator of FIG. 1 subjected to acceleration along its sensitive axis; and

- Figure 5 is an end view along a longitudinal axis which shows the resonator of Figure 1 in oscillation according to the first planar flexural vibration mode coupled with the first mode of vibration in torsion. Mode (s) of realization of the invention

[017] For the remainder of the description will be adopted without limitation longitudinal orientations "L" and transverse "T" and a sensitive axis "Z" perpendicular to the longitudinal and transverse orientations. These orientations are indicated by the trihedron "L, Z, T" of FIG.

[018] Thereafter, identical or similar elements will be indicated by the same reference numbers.

[019] FIG. 1 shows a resonant accelerometer 10 which comprises a resonator 12 which is vibrated by means of actuation means 14. The accelerometer 10 is able to measure an acceleration along the sensitive axis " Z "which is represented in a nonlimiting manner along a vertical axis oriented from bottom to top in the figures.

[020] The resonator 12 is in the form of a tuning fork which has a transverse base 16 in the form of transverse longitudinal plate since which extend longitudinally forward two parallel arms 18. The arms 18 have a shape of longitudinal main axis blade "A". They extend in the same plane as the base 16 and have a rear end 22 embedded in the base 16 and a free front end 20, supporting masses of inertia or unbalance

24, whose role will appear later. The resonator 12 thus extends in a transverse longitudinal plane of reference "P" which is orthogonal to the sensitive axis "Z" with the exception of inertia masses which are off-center. The arms 18 have identical dimensions. [021] The arms 18 and the base 16 are made of a piece of material, for example silicon, quartz or metal. [022] The resonator 12 is able to vibrate according to at least two modes of vibration. [023] A first mode of vibration "M1" uncoupled planar flexion, having a natural frequency "F1", is shown in Figure 2.

When the resonator 12 is excited at the natural frequency "F1", the arms 18 flexively oscillate in the reference plane "P" in the manner of beams embedded in the base 16. Thus, the free ends 20 of the arms 18 move. transversely to their initial position as indicated by the arrows 19.

[024] This first mode of vibration "M1" is evanescent because the arms 18 oscillate in phase opposition with respect to one another so that the moments induced by the oscillations of the arms 18 cancel each other out on a weak depth of penetration in the base 16.

[025] A first coupling mode of vibration "M2" without coupling, having a natural frequency "F2", is represented in FIG.

When the resonator 12 is excited at the natural frequency "F2", the arms 18 oscillate in torsion around their longitudinal axis "A" as indicated by the arrows in an arc. [026] This second mode of vibration "M2" is also evanescent because the arms 18 oscillate in phase opposition with respect to each other so that the couples induced by the oscillations of the arms 18 cancel each other out on a low depth of penetration into the base 16.

[027] In known manner, the actuation means of the resonator 12 may be piezoelectric type or electrostatic type. [028] According to an advantageous variant of the invention, the resonator is excited by piezoelectric effect, for example, by means of a layer of aluminum nitride, also known under the acronym "AIN", between two electrodes. As indicated in FIG. 1, representing the resonator 12, the piezoelectric coupling is achieved thanks to a deposition of aluminum nitride in the central region of the arm 18, at the location where the elongation deformations induced by the planar flexion of the arms 18 are the most important, either towards the inside of the tuning fork as shown in Figure 1, or towards the outside. This rectangular zone extends backwards along the arms 18 via a thin strip to a connection area on which can be welded an electrical connection wire. The aluminum nitride layer is covered with a conductive layer (platinum, aluminum, AISi or other conductive alloy), which layer is also deposited directly on the substrate to form electrical connection pads to the latter.

[029] In the case where the substrate silicon is not doped, it would be necessary to provide a second electrode between the substrate and the aluminum nitride layer. This second electrode is preferably made of platinum, a material which is particularly suitable for the growth of aluminum nitride. A molybdenum electrode could also be suitable. [030] The substrate is for example a silicon plate 100 whose lower face is silicon oxide. Such plates are called "SOI" of English Silicium On Insulator, which means silicon on insulator. In practice, the SOIs have a thick silicon base comprising a thin layer of SiO 2 on which a thin layer of silicon has been made. [031] According to another embodiment of the invention, the actuation means 14 may also be of the electrostatic type. In this case, they will comprise two electrodes, one of which may be made on the transverse or longitudinal face of the free end of the arms and the other on a support disposed opposite the first. By applying an alternating voltage between the two electrodes, it is possible to oscillate the arms. The required voltages are, however, high, which makes this type of actuation less attractive compared to the piezoelectric variant.

[032] Because of their high resistance to fracture and through known machining processes avoiding sharp angles and microcracking primers, the use of silicon resonators is, particularly with respect to quartz tuning forks, particularly suitable for so-called shock accelerometers or "high shock", which are exposed to accelerations of very high intensity. [033] According to the teachings of the invention, the two planar flexion vibration modes "M1" and torsion "M2" are coupled kinetically (in other words by inertia) so that the excitation of the vibration mode in planar flexion "M1" shows torsional content by coupling with the first mode of torsional vibration "M2". The resonator 12 thus comprises a coupled vibration mode "M 12", close to the planar flexion, but at a frequency "F12" lower than the frequency "F1" of the uncoupled bending mode, the frequency "F2" of the mode of unmated twist being greater than "F1".

[034] For this purpose, each unbalance 24 is arranged under the lower face of the free end 20 of the associated arm 18. More precisely, the unbalances

24 form flywheels for torsional content. [035] The center of gravity "G" of each unbalance is arranged at the same distance from the reference plane "P", here to the right of the longitudinal axis of each arm 18 so that the torsional movements induced by the movements of the ends in the planar flexural vibration mode "M1" are identical for the two arms 18. In the example shown in the figures, the unbalances 24 are identical in mass and dimensions, and they are arranged identically in each arm 18.

[036] In general, the two arms equipped with their unbalance 24 have moments of inertia identical at least along their main axis

"AT".

[037] When the resonator 12 is not subject to any acceleration, its arms 18 are included in the reference plane "P". Its natural frequency "F12" is then equal to a reference natural frequency "F12ref".

[038] Thus, as shown in FIG. 5A, when the arms 18 oscillate in planar flexion, the free ends 20 of the arms 18 move transversely in the reference plane. The transverse displacement of the unbalance 24 is delayed by the force of inertia which applies to its center of gravity "G". The center of gravity "G" of the unbalance 24 being shifted downwardly relative to the line of displacement of the free ends 20, the inertial force which is opposite to the displacement of the arms 18 retains the unbalance 24 and induces a torque on the arm 18, thus causing rotation of the free end 20 of the arm 18.

[039] Thus, the free end 20 of each arm 18 is driven by transverse oscillations along the axis T and rotations about the axis L, in phase opposition relative to the two arms of the tuning fork.

[040] When the resonator 12 is subjected to a negative acceleration directed up and down along the sensitive axis "Z", as shown in FIG. 4, the arms 18 jointly bend downwards out of the reference plane in the manner of embedded beams, the arrows of the arms 18 being identical. The free end 20 of the arms 18 is then arranged below the reference plane.

[041] As shown in FIG. 5C, it is shown that the natural frequency "F12" of the torsionally coupled planar flexion mode "M12" increases with respect to the reference natural frequency "F12ref". Indeed, the bending down of the arms, due to the negative component of the acceleration due to the torsion of the oscillating arms in phase opposition of one arm with respect to the other one, a decrease of the transverse excursion along the T axis, both arms

18 masses, and thereby a decrease in the kinetic energy of the assembly, without modifying the elastic energy and the free energy of the system (sum of elastic and kinetic energies), which translates by a frequency increase proportionally to the inverse of half the kinetic energy variation, the latter being proportional to the square of the frequency.

[042] On the contrary, when the resonator 12 is subjected to a positive acceleration directed from bottom to top along the sensitive axis "Z", as illustrated in FIG. 5B, the arms 18 bend together upwards out of the reference plane. in the manner of embedded beams, the arrows of the arms 18 being identical. The free end 20 of the arms 18 is then arranged above the reference plane.

[043] It is shown that the natural frequency "F12" of the combined vibration mode "M12" in planar flexion coupled in torsion decreases with respect to the reference natural frequency "F12ref". Indeed, the upward bending of the arms, due to the positive component of the acceleration due to the torsion of the oscillating arms in phase opposition of one arm relative to the other, causes an increase in the transverse excursion. along the axis T, both arms 18 and unbalances 24, and thereby an increase in the kinetic energy of overall, without modifying the elastic energy and the free energy of the system (sum of the elastic and kinetic energies), which results in a decrease of frequency proportionally to the inverse of half of the variation of kinetic energy, the latter being proportional to the square of the frequency.

[044] The natural frequency "F12" of the resonator 12 is thus linearly proportional to the amplitude of the deflection of the free end 20 of the arms 18. The natural frequency "F12" of the resonator 12 thus varies linearly with the intensity of the the acceleration, the intensity being positive or negative depending on the direction in the direction of the sensitive axis "Z".

[045] Advantageously, the unbalances 24 are made in one piece integral with the resonator 12. For example, the resonator 12 is manufactured by deep etching of a silicon wafer according to already known machining methods.

[046] The resonator 12 is made of a monocrystalline material such as monocrystalline silicon, the main axis of the tuning fork being oriented in the <100> or <110> direction of a plate (001), so that that the planar elasticity is orthotropic (that is to say that the elastic quadric is with double axial symmetry).

[047] According to a variant of the invention, the resonator 12 is made of anisotropic monocrystalline material such as quartz or silicon. The coupling between the first planar flexural vibration mode "M1" and the first torsional vibration mode "M2" is then performed both kinetically and elastically.

[048] The unbalances 24 advantageously form seismic masses which promote the flexion of the arms 18 perpendicularly to the reference plane "P" to further increase the sensitivity of the accelerometer 10. [049] It is shown that, as a first approximation, the sensitivity of the natural frequency "F12" to the acceleration is independent of the transverse width of the arms 18.

[050] The accelerometer 10 equipped with a resonator 12 according to the invention has a high sensitivity over a wide range of acceleration intensities.

[051] Indeed, the sensitivity to the acceleration of the resonator 12 in flexion is greatly amplified by replacing or improving the elastic coupling of the two modes of vibration "M1, M2" by a kinetic coupling (in other words, by inertia) equivalent .

[052] Such an accelerometer 10 allows for example to reach a sensitivity that can exceed 3500 ppm / g in a frequency range "F12" which extends from 10 kHz to 100 kHz.

[053] Moreover, the variation of the natural frequency "F12" being linearly proportional to the intensity of the acceleration measured, the use of the accelerometer is not limited to a restricted range of measurements.

[054] The natural frequency of such a resonator 12 is likely to vary slightly depending on the temperature. However, the resonator

12 realized according to the teachings of the invention is sufficiently sensitive to acceleration so that the temperature does not substantially alter the measurement of the intensity of the acceleration. However, according to the teaching of patent application PCT / EP2007 / 060641 a tuning fork cut in an SOI plate comprises outer layers of silicon oxide which make it possible to compensate for all or part of the thermal drift.

[055] In addition, the proposed resonator 12 has a very shock resistant structure, which can for example support an acceleration along its sensitive axis "Z" of an intensity of more than 10000 g and / or an acceleration of the rotation of an intensity of more than 7000 g. mm without the resonator 12 being damaged. In addition, the resonator 12 is highly resilient so that the resonator 12 can recover its original properties after an impact.

Claims

claims 1. A resonant accelerometer (10) which is capable of measuring the component of an acceleration along a sensitive axis (Z) and which comprises: - a tuning fork resonator (12) comprising two identical parallel flat longitudinal arms (1 8) extending from a recessed end (22) to a free end (20) in a reference plane (P); ) orthogonal to the sensitive axis (Z), the two arms (18) being able to oscillate according to a first planar flexural vibration mode (M1) and according to a first mode of torsional vibration (M2) around their axis longitudinal, these two modes of vibration being coupled to each other and defining a coupled vibration mode (M12) having a natural frequency of vibration (F12); and actuation means (14) which excite the oscillation of the arms (18) of the resonator (12) at the natural vibration frequency (F12); an acceleration along the sensitive axis (Z) causing a joint bending of the arms (18) out of the reference plane (P), the natural frequency (F12) of the resonator (12) varying linearly with respect to the amplitude of the deflection arms (18), characterized in that the coupling is obtained kinetically by the arrangement on each arm (18) of an unbalance (24) whose center of gravity (G) is offset relative to the reference plane (P) so as to induce torsion of the arms (18) during their transverse oscillations. 2. Accelerometer (10) according to the preceding claim, characterized in that an unbalance (24) is arranged under the free end (20) of each arm (18). 3. Accelerometer (10) according to the preceding claim, characterized in that the imbalances (24) are made integrally with the arms (18). 4. Accelerometer (10) according to any one of the preceding claims, characterized in that the unbalances (24) form masses seismic devices which promote bending of the arms (18) out of the reference plane when the accelerometer (10) is subjected to acceleration along its sensitive axis (Z). 5. Accelerometer (10) according to any one of the preceding claims, characterized in that the resonator (12) is made of a monocrystalline material. 6. Accelerometer (10) according to the preceding claim, characterized in that the resonator (12) is made of silicon. 7. Accelerometer (10) according to any one of the preceding claims, characterized in that the resonator (12) has an orthotropic elasticity. 8. Accelerometer (10) according to any one of the preceding claims, characterized in that the actuating means (14) are piezoelectric type and comprise a layer of aluminum nitride deposited on one side of each arm (18) . 9. Accelerometer (10) according to any one of claims 1 to 7, characterized in that the actuating means (14) are of the electrostatic type. 10. Accelerometer (10) according to the preceding claim, characterized in that the actuation means of the electrostatic type (14) are arranged in front of the free end (20) of the arms (18).