Source: http://www.google.com/patents/US6386035?dq=6175559
Timestamp: 2016-05-06 07:46:40
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Patent US6386035 - Monolithic miniature accelerometer - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA monolithic miniature accelerometer machined in a plate of material, comprising a fixed part, two first mobile mass parts referred to as inertial masses, two hinge blades each having one end fastened to one of the two mobile mass parts, and two resonators each having one end fastened to one of the two...http://www.google.com/patents/US6386035?utm_source=gb-gplus-sharePatent US6386035 - Monolithic miniature accelerometerAdvanced Patent SearchPublication numberUS6386035 B2Publication typeGrantApplication numberUS 09/828,925Publication dateMay 14, 2002Filing dateApr 10, 2001Priority dateOct 20, 1998Fee statusPaidAlso published asDE69930652D1, DE69930652T2, EP1123511A1, EP1123511B1, US20010015102, WO2000023808A1Publication number09828925, 828925, US 6386035 B2, US 6386035B2, US-B2-6386035, US6386035 B2, US6386035B2InventorsDenis Janiaud, Olivier Le Traon, Serge MullerOriginal AssigneeOffice National D'etudes Et De Recherche Aerospatiales (Onera)Export CitationBiBTeX, EndNote, RefManPatent Citations (7), Referenced by (11), Classifications (7), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetMonolithic miniature accelerometer
FIG. 1 shows an accelerometer of the above type disclosed in U.S. Pat. No. 4,945,765. The body of this accelerometer 14 is monolithic and is obtained by chemically machining a silicon plate. The body includes a fixed part 18, two inertial masses 20 and 22, two resonators 28 and 30 and two hinges 24 and 26. The resonators 28 and 30 vibrate in torsion and are excited electrostatically by means of a device (not shown) at whose output their resonant frequencies are delivered. The direction of sensitivity of the accelerometer is close to perpendicular to the faces of the plate. Acceleration applied in this direction causes a tension force to one resonator and a compression force to the other resonator, and the output signal of the accelerometer is the difference between the frequencies of the two resonators. The mechanical design of the accelerometer 14 nevertheless has a drawback associated with the vibration of the two resonators 28 and 30. The alternating mechanical forces generated by the vibrations of the two resonators where they are “built into” the fixed part 18 lead to dissipation of vibratory mechanical energy in the fixed part. This reduces the Q quality factor of the vibration of each of the resonators 28 and 30. This affects the precision of the measurement of the differential frequency and therefore the value of the acceleration deduced therefrom.
The operation of the accelerometer is now described with reference to FIG. 4, which shows the advantages of the particular disposition of the third mobile part 4 and the flexible stem 5. FIG. 4 is a front view of the accelerometer CA shown in FIG. 3 with the resonator 3 1 vibrating in flexion in the fundamental mode, parallel to the faces of the plate. The deformations of the resonator 3 1 and the flexible stem 5 correspond to vibration amplitudes and are exaggerated to make it easier to understand the drawing. When the resonator 3 1 is vibrating in flexion at its resonant frequency F1 it applies to each of its ends fastened to the mobile parts 2 1 and 4 a shear force R alternating with a bending moment C. The mobile parts are therefore subjected to alternating displacements whose main component δ is a translation parallel to the faces of the plate and perpendicular to the central longitudinal axis Z′Z. This alternating displacement δ is also imposed on the mobile part 2 2, primarily through the intermediary of the blade 6 2. The three mobile mass parts 2 1, 2 2 and 4 thus constitute a “mobile assembly” whose mass is much greater than that of the vibrating resonator 3 1. The alternating displacement δ of said mobile assembly is therefore much smaller than the vibration amplitude Δ of the resonator 3 1. This small alternating displacement δ creates flexional vibration of low amplitude of the flexible stem 5. Said stem being flexionally flexible, the fixed part 1 receives only very small alternating forces, principally a force r and a torque c whose magnitudes are very much less respectively than those of the shear force R and the bending moment C applied by the resonator 3 1 to each of the mobile parts 2 1 and 4. For example, forces r and c one hundred times smaller respectively than the forces R and C can be obtained. Thus the flexibility of the stem 5 and the total mass of the three mobile parts 2 1, 2 2 and 4 characterize a mechanical filter between the resonator 3 1 and the fixed part 1 of the accelerometer, said fixed part being affected very little by the vibrations of the resonator. The Q quality factor of the resonator is therefore hardly degraded at all. Remember that the alternating displacements and deformations whose amplitudes are shown in FIG. 4 constitute the main vibratory phenomena operating in the accelerometer. In reality there coexist other vibratory phenomena, of lower amplitude, such as alternating rotation of the mobile assembly consisting of the three mobile parts 2 1, 2 2 and 4 about the central longitudinal axis Z′Z, for example. This alternating rotation of the mobile assembly is caused by the flexional vibrations of the resonator 3 1 and is due to the fact that the mid-plane in which these vibrations occur does not pass through the axis Z′Z, which is substantially a main axis of inertia of the mobile assembly. The alternating rotation of the mobile assembly generates alternating torsion of the flexible stem 5 about the axis Z′Z, the torsional flexibility of the stem transmitting to the fixed part 1 only a very low alternating torque about the axis Z′Z. This very low torque has only a negligible influence on the effectiveness of the mechanical filtering of the vibrations of the resonator. More generally, because of the flexibility of the stem 5, the latter effectively filters most of the alternating mechanical loads imparted by the vibrations to the resonator 3 1. However, for the mechanical filtering to be as efficient as possible, it is preferable for the central longitudinal axis Z′Z to be substantially an axis of symmetry of the accelerometer CA, as shown in FIG. 3. In a variant where is substantially no such symmetry, the alternating mechanical loads transmitted to the fixed part 1 have magnitudes greater than those corresponding to the substantially symmetrical embodiment, but in general significantly lower than the magnitudes of the alternating forces R and C applied by the resonator 3 1 to each of the mobile parts 2 1 and 4.
For applications in which simplicity of use is important, it is beneficial to compensate the inclination ε. FIG. 6B shows one example of such compensation and is a view of the accelerometer CA shown in FIG. 6A in cross section taken in a plane Π orthogonal to the axis Z′Z and located in the fixed part 1. Said fixed part is fixed to a base BA, for example glued to it. In the embodiment shown, the base BA is a plate whose face FC stuck to the fixed part 1 and whose opposite face PP are not parallel to each other; the face PP is referred to as the “setting plane” of the accelerometer and is intended to be fixed to the structure of a device (not shown). The lines representing the faces FC and PP in the cross section plane Π are inclined to each other at an angle equal to the angle ε of inclination of the vector K relative to the axis I, so that the vector K is perpendicular to the face PP of the base BA. The accelerometer according to the invention is therefore sensitive to the component of the acceleration orthogonal to the face PP. This configuration is generally appreciated by accelerometer users because it is simple to use.
FIG. 7B shows another embodiment of an accelerometer according to the invention. The monolithic body of the accelerometer Cab is made of quartz. The means for imparting vibration to the resonators 3 1b and 3 2b in the accelerometer CAb are similar to those described in the French patent N� 2,685,964 (page 11, line 13-page 12, line 13) and therefore particularly suitable for vibrations in flexion parallel to the faces of the plate of material. The accelerometer CAb differs from the accelerometer CA shown in FIG. 3 principally in its disk-like general shape having a diameter D and a thickness Eb, and in the U-shape of the fixed part 1 b. The fixed part has a base section 10 b in the form of discoid segment fastened to the flexible stem 5 b, two branches 11 b and 12 b in the form of discoid segments extending substantially along the resonators 3 1b and 3 2b, respectively, and two sections 13 b and 14 b in the form of ring portions joining the base section 10 b to the two branches 11 b and 12 b, respectively. The inertial masses 2 1b and 2 2b, the resonators 3 1b and 3 2b, the third mobile mass part 4 b, the flexible stem 5 b and the two blades 6 1b and 6 2b are therefore inside the U-shape of the fixed part 1 b. The branches 11 b and 12 b are fixed, for example glued, to the base Bab of a cylindrical case. With reference to the filtering previously explained and shown in FIG. 4, which consisted of ensuring that the fixed part was hardly loaded at all by the vibrations of the resonators, the effectiveness of the accelerometer CAb is substantially equivalent to that of the accelerometer CA because the flexibility of the flexible stem 5 b is substantially equal to the flexibility of the flexible stem 5 and the total mass of the three mobile parts 2 1b, 2 2b and 4 b is substantially equal to the total mass of the three mobile parts 2 1, 2 2 and 4. With regard to miniaturization and fabrication yield, and therefore manufacturing cost, the accelerometers CA and CAb are also substantially equivalent. As shown in FIG. 7B, the means for imparting vibration to each of the resonators, for example the resonator 3 1b, are in the form of two metal electrodes 33 1b and 34 1b having opposite polarities and exciting flexional vibrations of the resonator 3 1b by piezoelectric effect. The electrodes 33 1b and 34 1b are disposed on the face of the resonator 3 1b facing towards the exterior of the body of the accelerometer, and their “three track” configuration is described in the French patent No. 2,685,964 already cited. Electrical connections between the electrodes 33 1b and 34 1b and sealed feed-throughs (not shown) in the base are made in the fixed branch 11 b by welding to respective metal contact pads 35 1b and 36 1b of substantially rectangular shape. As shown in FIG. 7B, the metal pads 35 1b and 36 1b are connected to the respective electrodes 33 1b and 34 1b by respective metal conductive strips 37 1b and 38 1b supported by the visible face of the mobile part 4 b, the flexible stem 5 b, the base section 10 b and the section 13 b in the form of a ring portion. The electrodes, the conductive strips and the contact pads can be obtained simultaneously by etching a metal layer adhering to the visible face of the quartz plate using conventional photolithographic processes. This adherent metal layer may advantageously be that previously used as a protective mask for machining the monolithic body of the accelerometer. The sealed feed-throughs of the base connected to the electrodes 33 1b and 34 1b are electrically connected to the two terminals of an oscillator circuit 7 1 at the output of which there is produced an alternating current signal having the resonant frequency F1 of the resonator 3 1b. An identical disposition of electrodes, conductive strips and contact pads is provided on the resonator 3 2b and the opposite face of the mobile part 4 b, of the flexible stem 5 b, the base section 10 b, the section 14 b in the form of a ring portion and of the fixed branch 12 b, connected to a second oscillator circuit 7 2 at the output of which there is produced an alternating current signal having the resonant frequency F2 of the resonator 3 2b. The outputs of the two oscillator circuits 7 1 and 7 2 are connected to a differential frequency measuring device including a frequency subtractor circuit 8 and a frequency meter 9, the frequency (F1−F2) measured by the frequency meter 9 being representative of the acceleration to be measured. The values of the resonant frequencies F1 and F2 of the resonators are preferably similar but sufficiently different for the differential frequency (F1−F2) to be significantly greater than the upper limit of the bandwidth of the accelerometer, regardless of the intensity and the direction of the acceleration within the measurement range provided. This enables the frequency meter 9 to measure the differential frequency (F1−F2) under good conditions and means that measurement precision is not degraded. For example, it may be advantageous to produce an accelerometer in accordance with the invention in which the frequencies F1 and F2 of the resonators are respectively 55 000 Hz and 50 000 Hz in the absence of acceleration, the differential frequency variation is 25 Hz/g (where g is the acceleration due to gravity), and the measurement range runs from −100 g to +100 g; the differential frequency (F1−F2) is therefore equal to 5 000 Hz in the absence of acceleration and varies from 2 500 Hz to 7 500 Hz when the acceleration varies from −100 g to +100 g; the minimum value of 2 500 Hz of the differential frequency means that the bandwidth of the accelerometer runs from 0 to 500 Hz. The dimensions of the monolithic body of the accelerometer shown in FIG. 7B are D=6 mm and Eb=0.4 mm. The fact that the resonant frequencies F1 and F2 differ by not less than approximately five percent reduces sufficiently the effects of mechanical coupling between the two resonators so that the precision of measurement is not degraded. In the embodiment still shown in FIG. 7B a conductive metal strip 39 1b is situated between the conductive strips 37 1b and 38 1b on the visible face of the mobile part 4 b, the flexible stem 5 b, the base section 10 b and the section 13 b in the form of a ring portion, and between the contact pads 35 1b and 36 1b on the visible face of the fixed branch 11 b. Another conductive metal strip 39 2b (not visible in FIG. 7B) is disposed identically on the opposite face of the monolithic body of the accelerometer, between the conductive strips and the contact pads connected to the second oscillator circuit 7 2. The conductive metal strips 39 1b and 39 2b are connected to the electrical ground common to the oscillator circuits 7 1 and 7 2, which reduces electrical coupling between the alternating current signals produced at the outputs of the two oscillators 7 1 and 7 2. The differential frequency alternating current signal produced at the output of the frequency subtractor circuit 8 can therefore have an improved signal-to-noise ratio.
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