Sensor for measuring acceleration forces along three axes

SENSOR FOR MEASURING ACCELERATION FORCES

ACCORDING TO THREE AXES

0003The present invention relates to the technical field of sensors measuring acceleration forces along three axes and more precisely to piezoelectric triaxial accelerometers.

0004The operating principle of an accelerometer is well known. A seismic mass exerts, under the effect of the acceleration, compressive or shearing forces on a piezoelectric material which then generates an electric charge proportional to the force applied to it, in this case, proportional to the acceleration .

0005In general, such a sensor comprises a piezoelectric element having an axis of symmetry parallel to one of the measurement axes. This piezoelectric element has a polarization axis parallel to this measurement axis. In the case of a compression type sensor, this piezoelectric element is mounted in compression between a seismic mass and a support or a base, using a screw ensuring the compression of the piezoelectric element and the assembly between the seismic mass and the support. An electrode is arranged on the face of the piezoelectric element in contact with the support while a counter-electrode is arranged on the face of the piezoelectric element in contact with the seismic mass.

0006The electrode and the counter-electrode are connected to a processing circuit for processing the electrical signals delivered by the electrode and against the electrode. Indeed, when an acceleration occurs along this measurement axis in the base-earth seismic direction, the piezoelectric element which is subjected to compression produces positive charges on the counter-electrode. In the case where the acceleration occurs along this axis of measurement but in the opposite sense seismic mass-base, the piezoelectric element which is subjected to a relaxation produces negative charges on the counter-electrode.

0007In the state of the art, it is known to many accelerometer solutions implementing such a principle and allowing the measurement of acceleration forces along three axes. A first category of solutions consists in using at least three piezoelectric elements mounted to be sensitive to the acceleration forces along three axes. For example, US Patent 6,038,924 discloses a triaxial accelerometer having three angularly shifted sensing elements and a logic circuit canceling transverse forces in each direction from the signals from the three directions. This category of accelerometers has a complexity of implementation related in particular to the geometric conditions of the assembly of the elements to be respected.

0008A second category of solutions consists in using a single piezoelectric element comprising a plurality of differently polarized parts as proposed by US Pat. No. 5,052,226 or comprising several electrodes and counter-electrodes to reduce external electrical or electromagnetic influences as proposed by US Pat. No. 5,117 696. If the use of a single piezoelectric element makes it possible to limit the cost of such a sensor, such a sensor does not make it possible to measure the acceleration forces along three axes and does not make it possible to eliminate parasitic transverse forces.

0009US 2002/014126 relates to a force and acceleration sensor for simultaneously measuring forces along a first axis, a second axis and a third axis, perpendicular to each other in pairs. This acceleration sensor comprises a series of electrodes arranged on one side of a sensitive element te! a piezoelectric element and a counter electrode arranged on an opposite face of this piezoelectric element. These electrodes and this counter electrode are connected to a processing circuit making it possible to obtain sensitive signals along the three axes from the combination of the measurement signals. As is clear from FIGS. 6 and 7, the processing circuit takes into account the analog signals delivered by making exclusively sums and differences,

0010Such a document which provides only an analog signal processing does not compensate for orientation defects and transverse effects and potentially all imperfections of realization of the sensor.

0011Document US 2004/027033 also relates to a measurement sensor having the same disadvantages as the sensor described in document US 2002/014126.

0012The present invention aims to overcome the drawbacks of the state of the art by proposing a sensor of simple design, able to measure acceleration forces along three axes, this sensor implementing a piezoelectric element while allowing to remove including the parasitic transverse forces.

0013The object of the invention is to propose an acceleration sensor that does not require a precise geometry of the electrodes and counterelectrodes, nor even a perfect homogeneity of the piezoelectric material.

0014To achieve such an objective, the object of the invention relates to a triaxial acceleration sensor for simultaneously measuring acceleration forces along a first axis, a second axis and a third axis perpendicular to each other in pairs, the sensor comprising :

0015a piezoelectric element having an axis of revolution parallel to the third axis, the piezoelectric element being mounted between a support and a seismic mass, and having a polarization axis parallel to the axis of revolution;

0016a series of electrodes arranged on one face of the piezoelectric element;

0017a counter electrode arranged on one side of the piezoelectric element opposite to that provided with the electrodes;

0018the electrodes and the counter-electrode being arranged on the one hand in a circular manner around the axis of revolution and on the other hand, facing each other to constitute at least three pairs each formed of an electrode and counter-electrode located opposite, and each delivering a measurement signal;

0019a processing circuit connected to the electrodes and against the electrode for processing the electrical measurement signals delivered by the electrodes and the counter-electrode.

0020According to the invention, the processing circuit ensures the digitization of the measurement signals and the processing of the digitized measurement signals to deliver:

0021a sensitive signal along the first axis, from the combination of the digitized measurement signals;

0022a sensitive signal along the second axis, from the combination of the digitized measurement signals;

0023a sensitive signal according to the third axis, from the combination of measurement signals such as: <img id="imgf000006_0001" he="22" wi="66" file="WO2018178564A1_D0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

0024with T, a set of transfer coefficients assigned to each measurement signal;

0025- The processing circuit determines in a calibration phase the transfer coefficients T to compensate for defects in the realization of the sensor.

0026In addition, the sensor according to the invention may additionally have in combination at least one and / or the other of the following additional characteristics:

0027the piezoelectric element is mounted in compression between the support and the seismic mass so that the measured accelerations are converted into measurement signals by virtue of its longitudinal deformation mode and in that the piezoelectric element is produced in the form of a disc having two opposite planar faces on one of which the electrodes are arranged and on the other of which is arranged against the electrode. the piezoelectric element is mounted between the support and the seismic mass whose center of mass is offset from the plane of symmetry of the piezoelectric element normal to its axis of revolution so that the measured accelerations are converted into measurement signals by means of its mode of deformation in shear and in that the

0028the series of electrodes comprises a first and a second electrode arranged diametrically opposite with respect to the axis of revolution for recovering charges from the element along the first axis, and a third and a fourth electrode arranged diametrically opposite with respect to the axis of revolution for recovering loads from the element along the second axis;

0029- An annular electrode arranged symmetrically about the axis of revolution and vis-à-vis a counter-electrode for delivering a sensitive signal along the third axis;

0030the processing circuit comprises, at the input, at least one circuit for digitizing the signals delivered by the electrodes and against the electrode;

0031the piezoelectric element comprises a stack of piezoelectric materials;

0032the piezoelectric element comprises a stack of two piezoelectric materials having axes of polarization of opposite direction;

0033- The base and the seismic mass are interconnected by a device ensuring the compression of the piezoelectric element.

0034Various other characteristics appear from the description given below with reference to the accompanying drawings which show, by way of non-limiting examples, embodiments of the object of the invention.

0035Figure 1 is an elevational sectional view of an exemplary embodiment of a triaxiai acceleration sensor according to the invention, operating in a compression deformation mode and having 4 pairs of electrodes.

0036FIG. 2 is a view of a preferred embodiment of the electrodes of the triaxial acceleration sensor according to the invention illustrated in FIG. 1.

0037FIG. 3 is a view of another exemplary embodiment of the electrodes of the triaxial acceleration sensor according to the invention illustrated in FIG. 1.

0038Figure 4 is a sectional elevation of an embodiment of a triaxial acceleration sensor according to the invention, operating in a shear deformation mode and still in the example of 4 pairs of electrodes.

0039Figure 5 is a perspective view of the piezoelectric element implemented for the sensor shown in FIG. 4.

0040Figure 6 is a block diagram illustrating the processing of the signals of the triaxial acceleration sensor according to the invention.

0041As is more specifically apparent from the Figures, the object of the invention relates to an acceleration sensor 1 adapted to simultaneously measure acceleration forces along a first axis X, a second axis Y and a third axis Z perpendicular to each other two by two. The sensor 1 comprises a piezoelectric element 2 having a polarization axis P parallel to the third axis Z. This piezoelectric element 2 comprises an axis of revolution S parallel to the third axis Z. In the example illustrated in FIG. 1, the piezoelectric element 2 is in the form of a disk having two opposite planar faces 2a, 2b extending parallel to one another in the X, Y plane. The piezoelectric element 2 is provided at its center, a through passage 3 centered on the axis of revolution S.

0042According to a first embodiment illustrated by FIGS. 1 and 2, this piezoelectric element 2 is mounted in compression between a support or a base 5 and a seismic mass 6 according to the longitudinal deformation mode d33. In the example illustrated in the drawings, the base 5 and the seismic mass 6 are interconnected by a device 7 ensuring the compression of the piezoelectric element 2 along the third axis Z. For example, the compression device is a screw 7 which is supported by its head 8 on the seismic mass 6, being screwed into a threaded hole 9 made in the base 5, through the piezoelectric element 2 through the passage 3. Of course, the compression fitting of the piezoelectric element 2 can be produced differently, for example by means of

0043Conventionally, the piezoelectric element 2 is electrically isolated from the base 5 and from the seismic mass 6. This piezoelectric element 2 is provided on its flat face 2a in contact with the support 5, with a series of electrodes A, B , C, D, ... and on its opposite planar face 2b in contact with the seismic mass 6, a counter-electrode K. These electrodes A, B, C, D, ... and this counter-electrode K are connected to a processing circuit 10 shown in FIG. 6, adapted to acquire and process the electrical signals Sa, Sb, Se, Sd, ... delivered by the electrodes and against the electrode, as will be explained in the following description.

0044According to the invention, the electrodes A, B, C, D, ... and the counter-electrode K are arranged in a circular manner around the axis of revolution S and facing each other or facing each other. others to constitute at least three, and in the example illustrated in FIG. 2, four pairs each formed of an electrode and the counter-electrode. For each of the pairs, an electrode and the counter-electrode are located opposite one another.

0045In the example illustrated in FIG. 2, each electrode A, B, C, D has the shape of a circular sector extending in a range of the order of 90 °. Thus, the electrodes A, B, C, D are distributed in a circular manner about the axis of revolution S. In the example illustrated, the electrodes A and B are symmetrically opposite with respect to the axis of revolution S parallel to the third axis Z, being centered substantially along the first axis X to be mainly responsive to an acceleration along the first axis X while the electrodes C and D are symmetrically opposite with respect to the axis of revolution S parallel to the third axis Z, being centered substantially along the second axis Y to be mainly responsive to an acceleration along the second axis Y.

0046Each electrode A, B, C, D is positioned facing the counter-electrode K made in a circular manner around the axis of revolution S. Thus, each electrode A, B, C, D is located opposite in accordance with FIG. direction of the third axis Z, against the electrode K, there is thus formed four pairs of electrodes and against electrode AK, BK, CK, DK each delivering a measurement signal respectively Sa, Sb, Se, Sd in the illustrated example. Typically, the counter-electrode is a common reference, like mass.

0047Of course, the number of electrodes distributed in a circular manner around the axis of revolution S may be different from four.

0048Fig. 3 illustrates another embodiment in which the series of electrodes further comprises electrodes A, B, C, D, of FIG. 2, an annular electrode E centered around the third axis Z and more precisely sensitive to compression along the third axis Z. According to this variant embodiment, the counterelectrode K further comprises an annular part centered around the third axis Z and located in front of the annular electrode E. In the illustrated example, the annular electrode E is located in the center but it can also be located at the periphery of the electrodes.

0049The Fîg. 4 and 5 illustrate a second embodiment of the sensor for which the seismic mass 6 exerts, under the effect of acceleration, shear forces on the piezoelectric element 2. As explained above, this piezoelectric element 2 presents a polarization axis P parallel to the third axis Z. According to this embodiment, the piezoelectric element 2 is mounted between the support 5 and the seismic mass 6 whose center of mass is offset relative to the plane of symmetry of the element piezoelectric 2 which is normal to its axis of revolution S. Given the offset of the center of mass, the measured accelerations are converted into measurement signals Sa, Sb, Se, Sd, ... thanks to its mode of deformation in shear ! 5. The piezoelectric element 2 is made in the form of

0050Each electrode A, B, C, D is positioned facing the counter-electrode K made in a circular manner around the axis of revolution S. Thus, each electrode A, B, C, D is located opposite the counter-electrode K in a direction normal to the direction of the third axis Z. There are thus formed four pairs of electrodes and against electrode AK, BK, CK, DK each delivering a measurement signal respectively Sa, Sb, Se, Sd in the example shown.

0051In the example illustrated in FIG. 5, the electrodes A and B are symmetrically opposite with respect to the axis of revolution S parallel to the third axis Z, centered substantially along the first axis X to be sensitive mainly to an acceleration along the first axis X while the electrodes C and D are symmetrically opposite to the axis of revolution S parallel to the third axis Z, centered substantially along the second axis Y to be sensitive mainly to an acceleration along the second axis Y. It should be noted that the electrodes A, B, C, D may be made unsymmetrically.

0052The measurement signals Sa, Sb, Se, Sd are recovered by the processing circuit 10 which provides the digitization of these measurement signals and the processing of the digitized measurement signals to deliver:

0053a sensitive signal Sx along the first axis X, from the combination of the digitized measurement signals;

0054a sensitive signal Sy along the second axis Y, from the combination of the digitized measurement signals; a sensitive signal Sz along the third axis Z, from the combination of the digitized measurement signals.

0055The processing circuit thus performs a numerical calculation using a choice of measurement signals to obtain a sensitive signal on each of the three axes X, Y, Z.

0056Thus, for the example illustrated in FIG. 2, the electrodes A and B are assigned to the recovery of the electrical charges of the sensitive piezoelectric element along the first axis X. The electrode A recovers the negative compression charges for an acceleration along the first axis X and the incoming direction represented by the arrow in FIG. 2. The electrode B recovers the positive relaxation charges for an acceleration along the first axis X and the incoming direction represented by the arrow in FIG. 2.

0057Similarly, the electrodes C and D are assigned to the recovery of the electrical charges of the sensitive piezoelectric element along the second axis Y. The electrode C recovers the negative compression charges for an acceleration along the second axis Y and the direction incoming represented by the arrow in FIG. 2. The electrode D recovers the positive expansion charges for an acceleration along the second Y axis and the incoming direction represented by the arrow in FIG. 2. As explained above, these four electrodes A, B, C, D, are combined with the counter-electrode K.

0058In a perfect embodiment, for an acceleration purely along the first axis X and zero along the second axis Y, each pair CK or DK electrode and against electrode receives a portion of compressive force and a portion of stress relaxation which cause a cancellation of the electric charges appearing on these pairs CK or DK of electrode and against electrode. The sensor 1 according to the invention is therefore only sensitive along the first axis X, being insensitive in the transverse direction, namely the second axis Y.

0059Similarly, for an acceleration purely along the second axis Y and zero along the first axis X, each pair AK or B ~ K of electrode and against electrode receives a part of compressive force and a part of effort in trigger which cause a cancellation of the electrical charges appearing on these pairs AK or BK of electrodes and against electrode. The sensor 1 according to the invention is therefore only sensitive along the second axis Y, being insensitive in the transverse direction, namely the first axis X,

0060For an acceleration along any axis in the plane formed by the first axis X and the second axis Y, the electrical charges appear on the pairs of electrodes and counter electrodes, assigned to the first axis X and the second axis Y, proportionally to the vector decomposition of the axis of application of the acceleration, according to the first and second axes X, Y.

0061In general, it seems advantageous to assign to each measurement signal, a correction coefficient for suppressing influences of parasitic transverse forces or orientation defects of the piezoelectric element, electrodes or counter-electrode. This correction is performed during a calibration phase of the sensor for each of the three axes.

0062Advantageously, during this calibration phase, it may be envisaged to adjust the sensitive signals along each axis to a normed value.

0063For a good understanding of the principle, in the example of a perfect embodiment of the sensors, without parasitic transverse effects or orientation defects of the piezoelectric element, electrodes or counter-electrode, the processing circuit 10 illustrated in FIG. .6 digitizes the measurement signals Sa, Sb, Se, Sd and ensures, for example, the processing of these digitized measurement signals Sa, Sb, Se, Sd to deliver:

0064a sensitive signal Sx along the first axis X, obtained for example from the difference between the first Sa and second Sb measurement signals;

0065a sensitive signal Sy along the second axis Y, obtained for example from the difference between the third and fourth Sd measurement signals; a sensitive signal Sz along the third axis 2, obtained from the sum of, for example, the first Sa, second Sb, third Se and fourth Sd measurement signals.

0066Thus the sensitive signals become: <img id="imgf000014_0001" he="19" wi="57" file="WO2018178564A1_D0002.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

0067In this case of perfect symmetry and geometry, simple additions and subtractions of the signals are enough to restore axis-by-axis accelerations, which can be done by simple analog circuits. Everything is based on the geometry of the electrodes and the state of the art offers many special configurations.

0068In the case of the present invention, a set of correction coefficients H is advantageously determined during the calibration and is then assigned to each measurement signal Sa, Sb, Se, ... Sd, to compensate for orientation defects and the transverse effects as well as potentially all imperfections of realization of the sensor or its constituent piezoelectric material.

0069Advantageously, during this calibration phase, the processing circuit 10 adjusts the sensitive signals Sx, Sy and Sz to a normed value by multiplying the values ​​by a correction gain G specific to each axis X, Y, 2.

0070Thus, more generally, the sensitive signals become for the example of four electrodes taken as an example: <img id="imgf000014_0002" he="19" wi="97" file="WO2018178564A1_D0003.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

0071The above two-matrix decomposition is actually presented only for good understanding and the notion of gain (or sensitivity) can be more simply generalized to a 3-dimensional matrix by n, where n is the number of electrodes. In the general case n can be any greater than or equal to 3, ie:<img id="imgf000015_0001" he="19" wi="64" file="WO2018178564A1_D0004.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

0072with T, transfer coefficients resulting from the combination of the correcting coefficients H and the values ​​of the correction gain G. These transfer coefficients T are thus determined during a calibration phase to compensate for defects in the realization of the sensor aimed primarily at the geometry defects of the electrodes and counter-electrode and the lack of homogeneity of the material of the piezoelectric element.

0073As is apparent from the foregoing description, the sensor 1 according to the invention makes it possible to measure accelerations along three axes X, Y, Z, with a single piezoelectric element 2 used in longitudinal mode d33 or in shear mode d15, with possibility compensation for transverse effects, orientation defects of the piezoelectric element 2 or electrodes A, B, C, ... or other defects in the embodiment of the sensor, and with possibility of adjustment of the final sensitivities to normalized values. This design is particularly interesting to guarantee excellent metrological quality while reducing the cost of the sensor by avoiding precise mechanical assembly.

0074In the examples illustrated in Fîg. 2 and 5, the electrodes and the counterelectrode have a circular sector profile. Of course, the shape of the electrodes and the counter-electrode may be different while being made circular.

0075According to a preferred embodiment, the piezoelectric element 2 is made of a single ceramic. According to another exemplary embodiment, the piezoelectric element 2 comprises a stack of piezoelectric materials. In the case of use with large thermal transients, the piezoelectric element 2 comprises a stack of two piezoelectric materials having axes of polarization in the opposite direction.

0076The invention is not limited to the examples described and shown because various modifications can be made without departing from its scope.