Acceleration measurement sensor assembly, comprising a three-axis measurement accelerometer sub-assembly, with a seismic mass moving in a straight line about a principal axis A, mounted in a housing, configured to determine an acceleration about a measurement axis Y.

Abstract

Acceleration measurement sensor assembly, comprising a three-axis accelerometer sub-assembly (40), mounted in a housing (41) with fixing elements (42), configured to determine an acceleration about a principal axis A, comprising: - a single-axis primary accelerometer (40a) with a seismic mass moving in a straight line about a principal axis A, making an acceleration measurement about the principal axis A misaligned with a reference axis Y by a maximum of 50 mrad, - a secondary accelerometer (40b) with at least two measurement axes making measurements respectively about two axes X and Z forming with the reference axis Y a direct orthonormal trihedron (O, X, Y, Z), the measurement accuracy of the two-axis accelerometer about each of its axes being at least ten times less than the measurement accuracy of the single-axis accelerometer, and - an electronic computing unit configured to calculate a compensated acceleration S. Figure for the abridged version: [Fig.4]

Description

Title of the invention: Acceleration measurement sensor assembly, comprising a three-axis measurement accelerometer sub-assembly, with a seismic mass moving in a straight line about a principal axis A, mounted in a housing, configured to determine an acceleration about a measurement axis Y.

[0001] The invention relates to an acceleration measurement sensor assembly, comprising a three-axis measurement accelerometer, with a seismic mass moving in a straight line along a principal axis A, mounted in a housing equipped with fixing elements, configured to determine an acceleration along a measurement axis Y.

[0002] The field of application of the present invention covers all applications requiring an accelerometer that is not very sensitive to transverse accelerations, i.e. in the two axes orthogonal to its sensitive axis.

[0003] Such an accelerometer is described in document EP 1 570 275 Bl, in which several embodiments of such an accelerometer equipped with a seismic mass guided by guide arms or beams are presented.

[0004] Fig. 1 represents an example of an embodiment of such an accelerometer, in which the seismic mass is located on one side of the amplification means, the pairs of arms being arranged in a butterfly shape, in a non-limiting manner.

[0005] The example of a resonator shown comprises two beams forming a tuning fork, which are vibrated in opposite phase by means of two electrodes. This tuning fork configuration is shown in [Fig. 1]. A vibrating beam, several vibrating beams, or a torsion bar could equally well be used as a resonator.

[0006] The vibrating beam accelerometer described preferably comprises two cells of measurements that can be carried out by machining a silicon-on-insulator (SOI) substrate or quartz or other material, but other methods are also possible.

[0007] A silicon-on-insulator substrate consists of a fixed monolithic silicon substrate a few hundred micrometers thick (e.g., 450 pm) forming the base of the accelerometer. This substrate has a thin layer of silicon oxide a few micrometers thick (e.g., 2 pm) on its front face, which is itself covered by a layer of monocrystalline silicon a few tens of micrometers thick (e.g., 60 pm). Machining consists of attacking the monocrystalline silicon from its front face until the oxide layer is reached, using a product of Selective etching attacks the silicon without significantly damaging the oxide layer. The etching process stops when the oxide layer is exposed. This oxide layer can itself be removed by selective etching with another material, leaving only the surface layer of silicon. This silicon can then be etched with the desired surface patterns using photogravure techniques or other techniques used in microelectronics to obtain the desired planar, movable structure.

[0008] The frame O,x,y,z represented on [Fig. 1] is that in which the plane of [Fig. 1] is the plane O,x,y, the axis Oz representing the direction perpendicular to this plane.

[0009] The mobile planar structure 10 of an accelerometer measurement cell, schematically represented in [Fig. 1], comprises a movable seismic mass 1 capable of translational movement along the sensitive axis A of the accelerometer, parallel to the Oy axis, which is the axis of the acceleration y to be measured, and means for amplifying 2 the force generating this translation, measured by means of two vibrating beams 30 placed along an Ox axis perpendicular to the Oy axis, which are subjected to tension or compression depending on the direction of the acceleration. The beams are arranged symmetrically with respect to an axis of symmetry A of the structure, this axis of symmetry being parallel to the Oy axis at the chip production output and passing through the center of gravity of the mass: the geometry of the seismic mass therefore does not affect the performance of the accelerometer.

[0010] The vibrating beams 30 are fixed at each end to a rigid termination 4. Each of these terminations 4 has a pair of micro-machined arms. The two pairs are symmetrical about the axis of symmetry A. A first micro-machined arm 5 connects the termination 4 to the seismic mass 1. To prevent the termination 4 from floating relative to the base, i.e., relative to the fixed monolithic silicon substrate, a second micro-machined arm 6, symmetrical to the first arm about the beam axis, connects the termination 4 to an anchor foot 7 fixed to the base. These arms 5 and 6 are respectively connected to the seismic mass 1, the termination 4, and the anchor foot 7 by attachment points. The thickness of an arm 5 or 6 can vary along its length.

[0011] Figure 1 also schematically represents a close-up of part of the amplification means. The first arm 5 is articulated on the termination 4 by an attachment point B. A portion of electrode E is also shown. The two vibrating beams 30 are embedded in the termination 4, as they are formed, for example, by etching the same layer of material. The hatching represents the material, for example, monocrystalline silicon in the case of a cell produced by machining a SOI. As previously stated, the surface features such as the arms 5, attachment point B, termination 4, beams 30, and electrode E were obtained by etching of monocrystalline silicon, then by attacking the oxide layer.

[0012] The angle 0 formed by the axis Ox and the line joining the attachment points B and C of the first arm 5, which, due to the symmetry of the arms 5 and 6 with respect to the axis connecting the terminations by their midpoint, is symmetric to the angle formed by the axis Ox and the line joining the attachment points of the second arm 6. This angle 0 is sufficiently small so that the force exerted in tension or compression on the beam 30 is greater than the acceleration force exerted on the seismic mass 1.

[0013] These amplification means 2 also make it possible to free up space around the vibrating beams 30, particularly for placing the electrodes in the case of electrostatic excitation. It should be noted that the vibrating beams are set into vibration at their resonant frequency using electrodes arranged opposite these beams, or directly on the beams, depending on whether the excitation is electrostatic or piezoelectric.

[0014] The seismic mass 1 is intrinsically guided in translation along the Oy axis due to the symmetry of the structure. To retain only this degree of freedom along Oy, the structure can optionally be further stiffened along Ox and Oz by means of guide arms 8 oriented along the Ox axis, one end of which is fixed to the seismic mass 1 and the other to a part 9 fixed to the base.

[0015] The force generating the translation of the seismic mass 1 along the Oy axis is transmitted by the first arms 5 to each of the terminations 4, which, depending on the direction of translation, move apart or closer together along the Ox axis, thus causing tension or compression of the vibrating beams 30. Since the structure 10 is symmetrical with respect to the A axis and, with respect to the arms, with respect to the axis of the beams, the displacements of the seismic mass 1, the terminations 4, and the beam 30 are perfectly axial. Thus, when the beams 30 form a tuning fork shape, the tension or compression is exerted in the same way on each of the beams of the tuning fork.

[0016] The performance of this structure, that is to say the amplification coefficient obtained, is simply determined by this angle 0.

[0017] When the seismic mass 1 is subjected to an inertial force My along Oy, this force is amplified and transmitted by the amplification means 2 to the vibrating beams 30. The amplification ratio is substantially equal to 1 / tan 0.

[0018] Figure 1 represents a preferred embodiment, in which the mass seismic 1 surrounds the amplification means 2 with vibrating beams as shown in figures 2 to 5 of document EP 1 570 275 Bl. Such a configuration makes it possible to obtain a more compact structure.

[0019] The arms 5, 6 can be arranged in different configurations. They can be arranged in a butterfly (or X) configuration as shown in Figures 1, 2 and 4 of document EP 1 570 275 Bl, this arrangement meaning that the first attachment point B of the first arm 5 at termination 4 is located closer to the axis of symmetry A than its second attachment point C to the seismic mass 1. In this case, a translation of the seismic mass 1 towards the beams 30 then causes a compression of the beams.

[0020] As mentioned in paragraph

[0011] of document EP 1 570 275 Bl, due to the symmetry of this structure, the displacements of the seismic mass, the fixed terminations, and the resonator are perfectly axial; in other words, there is axial displacement along a single axis, in this case axis A parallel to Oy of [Fig. 1], but no sensitivity along the two other transverse axes, in this case axes Ox and Oz of [Fig. 1]. This property is fundamental to this invention because it contributes to the detection of assembly defects.

[0021] The measurement axis OY of an acceleration measurement sensor assembly is always given with respect to a reference, generally its method of attachment, one or more centering pins, a reference plane or surface ... as illustrated in [Fig.4]

[0022] The assembly or embodiment of the accelerometer always leads to a certain level of calibration error of the sensitive axis Oy relative to the measurement axis OY to be used.

[0023] As illustrated in [Fig.2] the sensitive or principal axis A, parallel to the Oy axis is vertical but offset from the measurement axis OY by an angle α in the XOY plane, and by an angle [3 in the YOZ plane, after respective orthogonal projections AXY, AYZ onto these planes of the sensitive axis A.

[0024] The angles are oriented angles, measured either clockwise or trigonometrically.

[0025] For each of the transverse axes, thanks to the transverse insensitivity of the sensors as described in document EP 1 570 275 Bl, the acceleration supplied at the output of the sensor is mathematically tainted by an error solely due to misalignment during mounting in the case of transverse acceleration.

[0026] This error is given by the following relation: Error = yx.sin(a) - yz.sin([3]) in which: yx represents the acceleration along the transverse axis X, in m.s2, and yz represents the acceleration along the transverse axis Z, in m.s2.

[0027] Due to the transverse insensitivity of the main accelerometer, the equation does not include an intrinsic transverse error component related to the defects of the main accelerometer, as is normally the case with accelerometer technologies other than those described in EP 1 570 275 BL.

[0028] This error, which is physically unavoidable, quickly proves prohibitive for an application requiring a low measurement error under transverse acceleration; typically, this error must be less than Img, i.e., 102 m / s², for applications certified civil aeronautics.

[0029] The g is a unit of acceleration corresponding approximately to the acceleration due to gravity at the Earth's surface. Its conventional value, defined by the Third General Conference on Weights and Measures in 1911, is 9.80665 m / s²

[0030] The present problem relates in particular to sensor housings for flight controls. Such an application requires a calibration accuracy of the sensitive axis of less than 3mrad, i.e. of the main sensitive axis A with respect to the measurement axis OY.

[0031] This low level of axis alignment error, when the accelerometer uses a technology with non-zero transverse sensitivity, is not achieved naturally during the manufacturing and assembly process, regardless of assembly precautions, and requires manual adjustment by operators. This manual operation proves costly relative to the product's manufacturing cost.

[0032] More generally, maintaining the alignment of the main sensitive axis A, below one or several tens of mrad, is complicated to obtain without mechanical adjustment operations, which are costly.

[0033] This problem is usually addressed by: - strict assembly / manufacturing constraints to guarantee the stability of the shaft alignment - sorting and waste; - from mechanical adjustment to sensor mounting, to obtain precise alignment of the sensor's main sensitive axis with the product's measurement axis; or - surfacing of a reference face, such as by sanding part of the sensor casing, to compensate for the error.

[0034] For example, for an accelerometer based on a pendulum system, the adjustment operation requires a mechanical adjustment device, with rotation around the two transverse axes (ball joint system for example), with an amplitude dependent on the initial misalignment during assembly.

[0035] Such a device is expensive (moving parts, adjustment range and locking device once the adjustment has been made) and requires a specific adjustment / locking operation before calibration of the product.

[0036] State-of-the-art solutions are expensive.

[0037] One aim of the invention is to overcome the problems mentioned above, and in particular to stay within a budget or error rate under transverse acceleration of less than 1 mg' without respecting the axis alignment that this constraint would geometrically require.

[0038] According to one aspect of the invention, an acceleration measurement sensor assembly is proposed, comprising a three-axis measurement accelerometer sub-assembly, mounted in a housing equipped with mounting elements, configured to determine acceleration along a measurement axis OY, comprising: - a single-axis main accelerometer with seismic mass moving in rectilinear motion about a main sensitive axis A, insensitive to transverse accelerations, performing an acceleration measurement about the main sensitive axis A misaligned with the measurement axis OY by a maximum of 50 mrad, - a secondary accelerometer with at least two measurement axes, performing measurements respectively along two axes X and Z, forming with the measurement axis OY a direct orthonormal trihedron (O, X, Y, Z), the measurement accuracy of the biaxial accelerometer along each of its axes being at least ten times less than the measurement accuracy of the primary single-axis accelerometer, and - an electronic computing unit configured to calculate and output an acceleration S, compensated by means of the following relationship: Y SprA - '^secX PfaîX ' $secZ in which: - SprA represents the acceleration measurement along the main sensitive axis A; - ^Caix5 represent the dimensionless compensation parameters for the X and Z axis alignments of the sensor assembly, measured after assembly of the sensor assembly; and - SsecX' SsecZ represent the compensation acceleration respectively along the transverse axis X and along the transverse axis Z.

[0039] According to one embodiment, the electronic control unit includes a microcontroller or a digital signal processing unit (or DSP for acronym of "Digital Signal Processor" in English).

[0040] In one embodiment, the electronic control unit includes a programmable FPGA integrated circuit.

[0041] According to one embodiment, the biaxial accelerometer is class 20 mg.

[0042] According to one embodiment, the main accelerometer is of class Img.

[0043] In one embodiment, the biaxial accelerometer is misaligned by a maximum of 50 mrad with respect to the X and / or Z axes.

[0044] The invention will be better understood upon examination of some embodiments described by way of non-limiting examples and illustrated by the accompanying drawings, in which: - Figure [1] schematically illustrates a single-axis seismic mass accelerometer (1) with rectilinear motion about a principal axis, according to the prior art - Figure 2 schematically illustrates the axes of an acceleration sensor assembly configured to determine acceleration along a principal axis, according to an aspect of the invention; - Figure 3 schematically illustrates a sensor assembly according to one aspect of the invention; and - Fig. 4 schematically illustrates a sensor assembly according to one aspect of the invention mounted in a housing with an angular deviation of the main axis from a desired axis.

[0045] Fig. 2 represents the axes of an acceleration measurement sensor assembly configured to determine an acceleration along a principal axis A.

[0046] Figure 3 represents a sensor assembly according to one aspect of the invention.

[0047] As shown in [Fig.4], the sensor assembly comprises a high-performance single-axis main accelerometer 40a with seismic mass 1 moving in a straight line about a main axis A, mounted in a housing 41 with fixing elements 42 constituting the reference points of the XYZ axes, and making an acceleration measurement about the main sensitive axis A misaligned with the measurement axis Y by a maximum of 50 mrad, and an accelerometer 40b making measurements respectively about two axes almost aligned with the X and Z axes, the X and Z axes forming with the reference axis Y a direct orthonormal trihedron (O, X, Y, Z).

[0048] In this example, four fixing elements 42 have been shown, aligned two by two horizontally along axes parallel to the X axis, and aligned two by two vertically along axes parallel to the Y axis. This arrangement facilitates assembly.

[0049] High performance means class 1 mg.

[0050] Quasi-aligned means axes misaligned by a maximum of 50 mrad with respect to the X and / or Z axes.

[0051] The measurement accuracy of the biaxial accelerometer along each of its axes is at least ten times less than the measurement accuracy of the single-axis accelerometer. - The sensor assembly also includes an electronic processing unit configured to calculate a compensated acceleration S using the following relationship: SprA " '^secX "F l^CalX ' in which: ■ SprA represents the acceleration measurement along the principal axis A; - ^Caix' ^Caiz represent the dimensionless compensation parameters for the X and Z axis alignments of the sensor, measured after sensor assembly; and SsecX' and SsecZ represent the compensation acceleration respectively placed along the transverse axis X and along the transverse axis Z.

[0052] This equation is simplified compared to that known to a person skilled in the art of due to the insensitivity to transverse accelerations of the main accelerometer.

[0053] The present invention makes it possible to stay within a desired budget or error rate depending on the intended application, in the presence of transverse acceleration, without having to respect the axis alignment that this constraint would geometrically require.

[0054] For these purposes, the acceleration measurement sensor integrates, in addition to the single-axis sensor sensitive only along the main axis A, necessary to measure the acceleration on the desired measurement axis Y, a secondary accelerometer of lower performance, at a minimum biaxial, assembled so that the secondary axes X and Z form an orthonormal trihedron with the sensitive axis Y of the main single-axis sensor.

[0055] The sensor assembly is then carried out without any specific operation to adjust the axis alignments, contrary to what is known from the state of the art and which is costly.

[0056] Axis alignment errors are measured during a standard factory calibration carried out during the manufacture of the sensor according to known state-of-the-art methods.

[0057] In operation, a sensor assembly according to one aspect of the invention makes it possible to perform compensation of the effect of transverse accelerations by calculation (digital component of microcontroller type, digital signal processing processor, or FPGA) or in an analog manner.

[0058] The desired reference axes of the sensor are denoted X, Z (secondary transverses) and Y (measurement).

[0059] The advantage is to allow the use of compensation accelerometers which are much less efficient on bias than the one placed on the measurement axis of the sensor (factor greater than or equal to 10), and above all very inexpensive.

[0060] This is possible when the misalignment is sufficiently small (control of the production process), typically less than 50 mrad, so that the error projected onto the desired measurement axis Y is, to a first approximation, only that of the projections (sin α; sin θ) of the error of the compensating dual-axis accelerometer. For example, with a misalignment of 10 mrad, the projection onto the sensitive axis is only 10 thousandths of the transverse measurement error.

[0061] Assuming identical bias performance for the compensation accelerometer along the X and Z axes (^caix ^CaJZ = ^Cai), the bias of the KO sensor, in mg, is then bounded by (in absolute value) the following relation: KO <K0Y+ 2.^^0^ in which: K0Y represents the sensor bias along the Y-axis, in mg; and K0comp represents the sensor bias along each secondary X, Z compensation axis, in mg.

[0062] An example of a numerical application follows: if one wishes to maintain an error rate of Image with a primary accelerometer (A-axis) of class 0.5mg, for example, you would need: - a secondary accelerometer for compensation along the X and Z axes, class 25 mg with 10 mrad of misalignment, - a secondary accelerometer for compensation along the X and Z axes of class 12.5 mg with 20 mrad of misalignment.

[0063] These results are particularly interesting insofar as 20 mg class accelerometers can be found for a few euros, which is not at all the case for performances of class 1 mg or less.

Claims

Claims

1. Acceleration measurement sensor assembly, comprising an accelerometer subassembly (40) with three measurement axes, mounted in a housing (41) provided with fixing elements (42), configured to determine an acceleration along a measurement axis OY, comprising: - a single-axis main accelerometer (40a) with seismic mass (1) with rectilinear movement along a main sensitive axis A, insensitive to transverse accelerations, carrying out an acceleration measurement along the main sensitive axis A misaligned with the measurement axis OY by a maximum of 50 mrad, - a secondary accelerometer (40b) with at least two measurement axes carrying out measurements respectively along two axes X and Z forming with the measurement axis Y a direct orthonormal trihedron (O, X, Y, Z), the measurement accuracy of the dual-axis accelerometer along each of its axes being at least ten times less than the measurement accuracy of the single-axis main accelerometer,and - an electronic calculation unit configured to calculate and output a compensated acceleration S by means of the following relationship: $prA - '^secX ' ^secZ in which: - Sp-A represents the acceleration measurement along the main sensitive axis A; - ^Caix' ^Caiz represent the compensation parameters of the X, Z axis settings respectively of the sensor assembly, dimensionless, measured after assembly of the sensor assembly; and - ^secX> SsecZ represent the compensation acceleration respectively placed along the transverse axis X and along the transverse axis Z.,

2. A sensor assembly according to claim 1, wherein the electronic control unit comprises a microcontroller or a digital signal processing processor.

3. Sensor assembly according to claim 1, wherein the electronic control unit comprises an FPGA programmable integrated circuit.

4. Sensor assembly according to one of the preceding claims, in which the dual-axis accelerometer is of class 20 mg.

5. Sensor assembly according to one of the preceding claims, in which the main accelerometer is of the Img class.

6. A sensor assembly according to any preceding claim, wherein the dual-axis accelerometer is misaligned by at most 50 mrad relative to the X and / or Z axes.

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