Acceleration measuring instrument with a three-axis accelerator measuring unit arranged in a housing and arranged in one axis, with a linearly movable seismic mass

Description

[0001] The invention relates to an acceleration measurement sensor assembly, comprising an accelerometer with three measurement axes, with a seismic mass with rectilinear movement along a main axis A, mounted in a housing provided 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 with low sensitivity 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 B1, in which several embodiments of such an accelerometer are presented, provided with a seismic mass guided by guide arms or beams.

[0004] There [ 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 resonator taken comprises two beams forming a tuning fork, which are made to vibrate in phase opposition by means of two electrodes. It is this tuning fork configuration which is represented on the [ Fig.1 ]. One could just as easily use a vibrating beam or several vibrating beams or a torsion bar as a resonator.

[0006] The described vibrating beam accelerometer preferably comprises two measuring cells which can be made by machining a silicon-on-insulator (SOI) or quartz substrate or another 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 (450 µm for example) constituting the base of the accelerometer, which carries on its front face a thin layer of silicon oxide a few micrometers thick (2 µm for example) itself covered by a layer of monocrystalline silicon a few tens of micrometers thick (60 µm for example). Machining consists of attacking the monocrystalline silicon from its front face until reaching the oxide layer, with a selective etching product which attacks the silicon without significantly attacking the oxide. The etching is stopped when the oxide layer is exposed. This oxide layer can itself be removed by selective etching with another product so as to preserve only the surface layer of silicon.This can then be etched according to the desired surface patterns using photoengraving techniques or another technique used in microelectronics to obtain the desired mobile planar structure.

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

[0009] The mobile planar structure 10 of an accelerometer measuring cell, schematically represented on the [ Fig.1 ], comprises a mobile seismic mass 1 capable of moving in translation along the sensitive axis A of the accelerometer parallel to the axis Oy, which is the axis of the acceleration γ to be measured, and means 2 for amplifying the force generating this translation, measured by means of two vibrating beams 30 placed along an axis Ox perpendicular to the axis Oy, which undergo traction 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 axis Oy at the production output of the chip and passing through the center of gravity of the mass: the geometry of the seismic mass then does not affect the performance of the accelerometer.

[0010] The vibrating beams 30 are embedded at each end in a rigid termination 4. Each of these terminations 4 comprises a pair of micro-machined arms. The two pairs are symmetrical with respect to the axis of symmetry A. A first micro-machined arm 5 connects the termination 4 to the seismic mass 1. So that the termination 4 does not float with respect to the base, that is to say with respect to the fixed monolithic silicon substrate, a second micro-machined arm 6, symmetrical with the first arm with respect to the axis of the beam, connects the termination 4 to an anchoring foot 7 fixed to the base. These arms 5 and 6 are respectively connected to the seismic mass 1, to the termination 4 and to the anchoring foot 7, by attachment points. The thickness of an arm 5 or 6 can vary along its length.

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

[0012] The angle θ 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 middle, is symmetrical to the angle formed by the axis Ox and the line joining the attachment points of the second arm 6. This angle θ is sufficiently small so that the force exerted in traction 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, in particular for placing the electrodes in the case of electrostatic excitation. It is recalled that the vibrating beams are made to vibrate at their resonance frequency using electrodes arranged opposite these beams, or directly on the beams, depending on whether it is an electrostatic or piezoelectric excitation.

[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 possibly be further stiffened along Ox and Oz via 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 axis Oy is transmitted by the first arms 5 to each of the terminations 4 which, depending on the direction of translation, move away from or towards each other along the axis Ox, thus causing traction or compression of the vibrating beams 30. The structure 10 being symmetrical with respect to the axis A and with regard 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, the traction 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 θ.

[0017] When the seismic mass 1 is subjected to an inertial force M.γ according to 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 / tg θ .

[0018] There [ Fig.1 ] represents a preferred embodiment, in which the seismic mass 1 surrounds the amplification means 2 with vibrating beams as shown in figures 2 to 5 of document EP 1 570 275 B1. Such a configuration makes it possible to obtain a more compact structure.

[0019] The arms 5, 6 can be arranged in different variations. They can be arranged in a butterfly (or X) shape as shown figures 1 , 2 And 4of document EP 1 570 275 B1, this arrangement meaning that the first point of attachment B of the first arm 5 at the termination 4 is located closer to the axis of symmetry A than its second point of attachment 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 B1, due to the symmetry of this structure, the displacements of the seismic mass, the embedding terminations and the resonator are perfectly axial, in other words, there is an axial displacement along a single axis, in this case the axis A parallel to Oy of the [ Fig.1 ], but no sensitivity along the other two transverse axes, in this case the Ox and Oz axes of the [ Fig.1 ]. This property is fundamental in the context of this invention because it contributes to the detection of assembly defects.

[0021] The OY measurement axis of an acceleration measurement sensor assembly is always given relative to a reference, generally its fixing method, one or more centering pins, a reference plane or surface... as illustrated in the [ 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 measuring axis OY to be used.

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

[0024] Angles are oriented angles, measured either clockwise or counterclockwise.

[0025] For each of the transverse axes, thanks to the transverse insensitivity of the sensors as described in document EP 1 570 275 B1, the acceleration provided at the sensor output is mathematically affected by an error solely due to misalignment during assembly in the event of transverse acceleration.

[0026] This error is given by the following relation: Erreur = γ x . sin α − γ Z . sin β in which: γ x represents the acceleration along the transverse axis X, in ms -2< , and γ Z represents the acceleration along the transverse axis Z, in ms -2< .

[0027] Due to the transverse insensitivity of the main accelerometer, in the equation, there is no intrinsic transverse error component to the defects of the main accelerometer as is normally the case with accelerometer technologies other than the accelerometers according to document EP 1 570 275 B1.

[0028] This error, physically unavoidable, quickly proves prohibitive for an application requiring a low measurement error under transverse acceleration, typically this error must be less than 1 mg, or 10 -2< m / s 2< for certified civil aeronautical applications.

[0029] The g is a unit of acceleration corresponding approximately to the acceleration of gravity at the Earth's surface. Its conventional value, defined by the Third General Conference on Weights and Measures of 1911, is 9.806 65 ms -2<

[0030] This problem particularly concerns sensor boxes for flight controls. Such an application requires a calibration accuracy of the sensitive axis of less than 3 mrad, i.e. of the main sensitive axis A relative to the measuring axis OY.

[0031] This low level of axis calibration error, when the accelerometer is of a technology with non-zero transverse sensitivity, is not obtained naturally at the end of the production and assembly process, regardless of the assembly precautions, and requires manual adjustment by operators. This manual operation proves costly in relation to the manufacturing cost of the product.

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

[0033] This problem is usually addressed by: high assembly / production constraints to ensure that the axis alignment is maintained; sorting and scrapping; mechanical adjustment when mounting the sensor, to obtain precise alignment of the main sensitive axis of the sensor with the product 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 adjustment is made) and requires a specific adjustment / locking operation before calibrating the product.

[0036] State-of-the-art solutions come at a high cost.

[0037] Other known acceleration measurement sensor assemblies are described in "Measurement of sensor axis misalignment in fiber-optic accelerometers", by Freitas JM et al, Measurement Science and Technology, IOP, Bristol, GB, vol. 17, no. 7, July 2006, pages 1819-1825, and "Analysis and Compensation of Cross-Axis Sensitivity in Low-Cost MEMS Inertial Sensors", Hiller T. et al, 2021 IEEE International Symposium on inertial sensors and systems, March 22, 2021, pages 1-4.

[0038] One aim of the invention is to overcome the problems mentioned above, and in particular to remain within a budget or error rate under transverse acceleration of less than 1 mg' without, however, respecting the axis setting that this constraint would geometrically require.

[0039] According to one aspect of the invention, there is provided an acceleration measurement sensor assembly comprising an accelerometer subassembly with three measurement axes, mounted in a housing provided with fixing elements, configured to determine an acceleration along a measurement axis OY, comprising: a single-axis main accelerometer with seismic mass 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 with at least two measurement axes carrying out 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 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 an acceleration S, compensated by means of the following relationship: S = S prA − μ CalZ . S secX + μ CalX . S secZ in which: S prA represents the acceleration measurement along the main sensitive axis A; µ CalX , µ CalZ represent the compensation parameters of the X, Z axis settings respectively of the sensor assembly, dimensionless, measured after assembly of the sensor assembly; and S secX , S secZ represent the compensation acceleration respectively along the transverse axis X and along the transverse axis Z.

[0040] According to one embodiment, the electronic control unit comprises a microcontroller or a digital signal processing processor (or DSP for "Digital Signal Processor" in English).

[0041] In one embodiment, the electronic control unit comprises an FPGA programmable integrated circuit.

[0042] According to one embodiment, the dual-axis accelerometer is of class 20 mg.

[0043] According to one embodiment, the main accelerometer is of the 1mg class.

[0044] In one embodiment, the dual-axis accelerometer is misaligned by up to 50 mrad relative to the X and / or Z axes.

[0045] The invention will be better understood by studying a few embodiments described as non-limiting examples and illustrated by the appended drawings in which: there [ Fig.1 ] schematically illustrates a single-axis accelerometer with seismic mass (1) with rectilinear movement along a main axis, according to the state of the art; the [ Fig.2 ] schematically illustrates the axes of an acceleration measurement sensor assembly configured to determine an acceleration along a main axis, according to one aspect of the invention; the [ Fig.3 ] schematically illustrates a sensor assembly according to one aspect of the invention; and the [ 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.

[0046] There [ Fig.2 ] represents the axes of an acceleration measurement sensor assembly configured to determine an acceleration along a main axis A.

[0047] There [ Fig.3 ] represents a sensor assembly according to one aspect of the invention.

[0048] As shown in the [ Fig.4], the sensor assembly comprises a high-performance single-axis main accelerometer 40a with seismic mass 1 with rectilinear movement along a main axis A, mounted in a housing 41 provided with fixing elements 42 constituting the reference points of the XYZ axes, and carrying out an acceleration measurement along the main sensitive axis A misaligned with the measurement axis Y by a maximum of 50 mrad, and an accelerometer 40b carrying out measurements respectively along two axes quasi-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).

[0049] 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.

[0050] High performance is defined as class 1 mg.

[0051] Quasi-aligned means axes misaligned by a maximum of 50 mrad with respect to the X and / or Z axes.

[0052] The measurement accuracy of the dual-axis accelerometer along each of its axes is at least ten times lower than the measurement accuracy of the single-axis accelerometer. The sensor assembly also includes an electronic computing unit configured to calculate a compensated acceleration S using the following relationship: S = S prA − μ CalZ . S secX + μ CalX . S secZ in which: S prA represents the acceleration measurement along the main axis A; µ CalX, µ CalZ represent the compensation parameters of the calibrations of the X, Z axes of the sensor, dimensionless, measured after assembly of the sensor; and S secX , S secZ represent the compensation acceleration respectively placed along the transverse X axis and along the transverse Z axis.

[0053] This equation is simplified compared to that known to those skilled in the art due to the insensitivity to transverse accelerations of the main accelerometer.

[0054] The present invention makes it possible to remain within a desired budget or error rate depending on the intended application, in the presence of transverse acceleration, without however respecting the axis setting that this constraint would geometrically require.

[0055] For these purposes, the acceleration measurement sensor integrates, in addition to the single-axis sensor sensitive only along the main axis A, necessary for measuring the acceleration on the desired measurement axis Y, a secondary accelerometer of lower performance, at least biaxial, assembled in such a way that the secondary axes X and Z form an orthonormal trihedron with the sensitive axis Y of the main single-axis sensor.

[0056] The sensor assembly is then carried out without any specific operation to adjust the axis settings, contrary to what is known from the state of the art and which is expensive.

[0057] Axis calibration errors are measured during a standard factory calibration carried out during sensor manufacturing using known state-of-the-art methods.

[0058] In operation, a sensor assembly according to one aspect of the invention makes it possible to compensate for the effect of transverse accelerations by calculation (digital component of the microcontroller type, digital signal processing processor, or FPGA) or in an analog manner.

[0059] The desired reference axes of the sensor are noted X, Z (secondary transverses) and Y (measurement).

[0060] The interest is to allow the use of compensation accelerometers which are much less efficient on the bias than those placed on the sensor measurement axis (factor greater than or equal to 10), and above all very inexpensive.

[0061] This is possible when the misalignment or 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 the first order, only that of the projections (sin α; sin β) 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.

[0062] Assuming identical bias performance for the compensating accelerometer along the X and Z axes (µ CalX = µ CalZ = µ Cal ) the sensor bias K 0, in mg is then bounded by (in absolute value) the following relation: K 0 ≤ K 0 Y + 2 . μ Cal . K 0 Comp in which: K 0 Y represents the sensor bias along the Y axis, in mg; and K0 Comp represents the sensor bias along each secondary X, Z compensation axis, in mg.

[0063] An example of a numerical application follows: if we wish to maintain an error rate of 1mg with a main accelerometer (A axis) of class 0.5mg, we must for example: a secondary X and Z axis compensation accelerometer of class 25 mg with 10 mrad of misalignment, a secondary X and Z axis compensation accelerometer of class 12.5 mg with 20 mrad of misalignment.

[0064] These results are particularly interesting since 20 mg class accelerometers can be found for a few euros, which is not at all the case for 1 mg class performance or less.

Claims

1. An acceleration-measuring sensor assembly comprising an accelerometer subassembly (40) with three measurement axes, mounted in a housing (41) equipped with securing elements (42), configured to determine an acceleration along a measurement axis OY, comprising: - a single-axis main accelerometer (40a) with a seismic mass (1) moving in a straight line along a main sensitive axis A, insensitive to transverse accelerations, measuring acceleration along the main sensitive axis A which is misaligned with respect to the measurement axis OY by 50 mrad at most, the acceleration-measuring sensor assembly being characterised in that it also comprises: - a secondary accelerometer (40b) having at least two measurement axes and measuring, respectively, along two axes X and Z which form with the measurement axis Y a direct orthonormal trihedron (O, X, Y, Z), the measurement precision of the two-axis accelerometer along each of its axes being at least ten times inferior to the measurement precision of the main single-axis accelerometer, and - an electronic computation unit configured to compute and output an acceleration S compensated for by means of the following relation: S = S prA − μ CalZ ⋅ S secX + μ CalX ⋅ S secZ in which: - SprA represents the acceleration measurement along the main sensitive axis A; - µCalX, µCalZ represent the parameters compensating respectively for the alignments of axes X, Z of the sensor assembly, these being dimensionless quantities, measured after assembly of the sensor assembly; and - SsecX, SsecZ represent the compensation acceleration placed along the transverse axis X and along the transverse axis Z, respectively.

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

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

4. The sensor assembly according to one of the preceding claims, wherein the two-axis accelerometer is of the 20 mg class.

5. The sensor assembly according to one of the preceding claims, wherein the principal accelerometer is of the 1 mg class.

6. The sensor assembly according to one of the preceding claims, wherein the two-axis accelerometer is misaligned from the axes X and / or Z by 50 mrad at most.