Accelerometer device
Field of the invention
The invention generally relates to measurement systems and, in particular, to an
5 accelerometer device for measuring the acceleration along three axes of an object
mechanically connected to a measurement system.
Prior art
Surface and submarine vessels are generally equipped with acoustic antennas for
transmitting and/or receiving acoustic signals being propagated through the sea.
10 An acoustic antenna is formed from a set of sensors (hydrophones) that can be
accessed separately, i.e. the outputs of which may be combined in various way~.
The acoustic antennas thus comprise a set of hydrophones which may be arranged
in the acoustic modules of the antenna in various combinations depending
on the configuration of the antenna.
15 For example, a towed linear acoustic antenna (ALR or flute antenna) may be of a
substantial length, measuring several hundreds of meters, and of small diameter.
Such an antenna is towed by a towing cable of many hundreds of meters and is
intended to be submerged up to quite substantial sea depths (the pressure
increasing by one bar for every 10 m in depth). Owing to its length, such an
20 antenna comprises a substantial number of hydrophones which are regularly
distributed along the antenna in acoustic modules. It is known practice, for
example, to arrange an elementary acoustic multisensor along the flute antenna in
a separate acoustic module in order to remove the left/right ambiguity found in
submarine acoustics. Each acoustic module is protected by a protective module,
2 5 which is of small dimensions owing to the small diameter of the flute antenna.
Each independent acoustic multisensor is mounted on a printed circuit board
(concentrator system) at one end thereof and perpendicularly to the plane of the
board. The acoustic module may additionally comprise multiple auxiliary pieces of
equipment, such as depth and direction sensors, and pieces of electronic equipfl
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ment which are mounted on the concentrator system, in the small space delimited
by the protective enclosure of the acoustic module.
The acoustic multisensors are, in this case, designed to deliver electrical signals
which are proportional to the dynamic pressures exerted thereon. However, their
5 accelerometric sensitivity must be known so that the delivered signals are referenced
according to terrestrial acceleration forces. This acceleration reference is
obtained through the use of an acceleration sensor (also referred to as an accelerometer)
mounted on the concentrator in combination with each acoustic multisensor
and delivering a signal representative of the acceleration of the flute along
1 o two axes, and through compensation for the effect of this acceleration in a
differential manner over the signals delivered by each acoustic multisensor. Such
an accelerometer makes it possible to determine the biaxial acceleration of the
object to which it is fixed for a horizontal antenna.
15 Such accelerometers are expensive. In particular, in the case of a flute antenna,
the accelerometers are generally arranged in a known manner along the antenna,
generally every 2 m for interpolation. Furthermore, for each acoustic multisensor,
an accelerometer is provided on the same concentrator along with other electronic
components of the acoustic module so as to avoid interpolation. Taking the length
20 of the antenna into account, the number of accelerometers required and, consequently,
the cost incurred by the set of these accelerometers may thus be very
substantial.
Another drawback of these accelerometers is their bulk with respect to the volume
25 of the enclosure of the acoustic module (itself limited by the small diameter of the
flute antenna). In particular, it may be useful to stack multiple printed circuit boards
and to integrate them within each acoustic module by connecting the acoustic
multisensor to each of the stacked boards, thereby making it possible to ensure
the continuous operation of the multisensor in the event of failure. However, the
30 current bulk of conventional accelerometers (in the direction perpendicular to the
axis of the antenna) and their number would not allow such a stack.
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General definition of the invention
The invention improves the situation. To this end, an accelerometer device is
proposed for determining the acceleration of an object to which the accelerometer
5 device is mechanically connected, along three axes X, Y and Z of a main orthonormal
reference system (X, Y, Z) comprising three axes X, Y, Z, said object
being subject to a surrounding pressure. The device comprises a number N of
accelerometer sensors of MEMS type, the number N of accelerometer sensors
being at least equal to two, each sensor being defined by construction in an
10 auxiliary reference system comprising three orthonormal axes (Xi, Yi, Zi), the set
of accelerometer sensors comprising at least one pair of sensors mounted so that
they face in opposite directions and are substantially parallel to one another, and:
- for each of the pairs of sensors, the sensors have, in twos, components of
opposite sign along two axes of the main reference system; and
15 - the axes of the reference system along which the components of the accelerometer
sensors oppose the set of pairs of sensors in twos comprise at least two of
the three axes X, Y and Z of the reference system, thereby making it possible to
compensate for the effect of the pressure on at least two axes of the reference
system.
20
According to another feature, the device may comprise a processing unit for
calculating the mean of the accelerations along each axis X, Y and Z, the mean of
the accelerations along a given axis being determined from the accelerations
determined by each accelerometer sensor along the given axis.
25 The calculation of the mean of the accelerations along a given axis may additionally
account for the gain and/or the offset of each sensor predetermined in a unit
calibration phase implemented separately for each sensor before mounting.
The calculation of the means of the accelerations along a given axis also accounts
for a predefined gain compensation factor.
30 In one embodiment, the set of sensors may comprise two accelerometer sensors
connected to at least one printed circuit board, while the axes of the reference
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system along which the components of the sensors oppose the set of sensors in
twos comprise two of the three axes X, Y and Z of the reference system, thereby
making it possible to correct for the effect of the pressure which is exerted on the
object along the two axes.
5 In another embodiment, the set of accelerometer sensors may comprise three
sensors, while the axes of the reference system along which the components of
the sensors are opposed in twos over the set of sensors comprise the three axes
X, Y and Z of the reference system, thereby making it possible to correct for the
effect of the pressure which is exerted on the object along the three axes.
10 According to one feature of the invention, the object may be an acoustic antenna
comprising a set of acoustic multisensors, each acoustic multisensor being
mounted on at least one printed circuit board, while the accelerations determined
by the accelerometer device along each axis X, Y and Z are used to compensate
for the effect of the acceleration of the acoustic antenna on the position of the
15 acoustic multisensor.
The sensitivity of the sensors to pressure may vary.
The invention additionally proposes an acoustic module for an acoustic antenna
extending along a main axis, comprising an accelerometer device according to
one of the previous features, the accelerations determined by the accelerometer
2 o device along each axis X, Y and Z being used to compensate for the effect of the
acceleration of the acoustic antenna on the position of the acoustic multisensor.
The acoustic sensor may be an acoustic multisensor with an axis that is substantially
parallel to the axis of the antenna.
The acoustic module may comprise at least one printed circuit board on which the
25 accelerometer sensors are mounted.
The module may comprise a plurality of printed circuit boards, stacked:so as to be
substantially perpendicular to the axis of the antenna, on which the accelerometer
sensors are mounted.
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The invention additionally proposes an acoustic antenna, comprising a set of
acoustic modules according to one of the preceding features.
The invention thus provides a low-bulk accelerometer device. The use of such a
device in an acoustic module of an antenna thus makes it possible to stack
5 multiple printed circuit boards in one and the same module.
10
Another advantage of the accelerometer device according to the invention is its
low cost. Such a device is thus particularly suitable for use in an antenna employing
a substantial number of acoustic modules.
The accelerometer device also allows the system to be made more reliable
through self-referencing, i.e. comparison with respect to a duel reference and
calibration with respect to the acceleration due to gravity.
15 Description of the figures
Other features and advantages of the invention will become apparent with the aid
of the description which follows and the figures of the appended drawings in
which:
2 o - figure 1 shows an exemplary environment in which certain embodiments of the
invention may be implemented;
-figure 2 is a schematic representation of the acoustic modules of a flute antenna,
according to certain embodiments of the invention;
- figure 3 is a schematic view of the accelerometer device according to one
25 embodiment of the invention;
- figure 4 shows the arrangement of the accelerometer sensors facing in opposite
directions in a main reference system, according to one embodiment of the
invention;
- figure 5 shows an accelerometer device with three accelerometer sensors in a
'
3 o main reference system, according to one embodiment of the invention;
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- figure 6 is a schematic representation of the acoustic modules of a flute antenna
comprising accelerometer devices with three accelerometer sensors, according to
a certain embodiment of the invention;
- figure 7 is a top view of the accelerometer sensors facing in opposite directions in
5 a main reference system, according to one embodiment of the invention;
- figure 8 is a diagram illustrating the processing of the signals measured by the
accelerometer sensors facing in opposite directions, according to one embodiment
of the invention; and
- figure 9 is a flowchart showing the steps of calculating references along each
10 axis of the principal reference system, according to one embodiment of the
invention.
The drawings and the annexes to the description will be able not only to aid in
better understanding the description, but also to contribute to the definition of the
15 invention, as appropriate.
Detailed description
Figure 1 shows an exemplary environment in which certain embodiments of the
2 o invention may be implemented.
Surface and submarine vessels 2 are generally equipped with a set of acoustic
antennas for transmitting and/or receiving acoustic signals through the sea, such
as for example flute antennas 10 which benefit from freedom of movement,
25 distributed antennas or flank antennas. In order to facilitate the description of
embodiments of the invention, the rest of the description will be given with reference
to a flute antenna 10 by way of non-limiting example.
Figure 2 shows a segment of a flute antenna 10 comprising a set of acoustic
30 modules ;100. The flute antenna 10 additionally comprises a set of acoustic
sensors 101 (for example of hydrophone type) distributed regularly along the
antenna in the acoustic modules 100. In the embodiment of figure 2, an acoustic
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sensor 101 is arranged at regular intervals along the flute antenna in a separate
acoustic module 100.
In each acoustic module 100, comprising one or more acoustic sensors 101, at
5 least one printed circuit board 102 (also referred to as a "concentrator system"),
for example generally rectangular in shape, is arranged along the axis of the
antenna. The acoustic sensors 101 may be, for example, acoustic multisensors. In
one particular embodiment, the sensors 101 of acoustic multisensor type 101 may
comprise N hydrophones, which are located in a plane perpendicular to the axis L'.
1 o of the linear antenna and are distributed on a circle centered on the axis of the
antenna, and have a diameter that is smaller than the diameter of the antenna.
Each acoustic sensor 101 may be mechanically or electrically connected to at
least one board 102 for its electrical interface. In a variant, each acoustic module
15 100 may comprise a plurality of printed circuit boards stacked in the direction
perpendicular to the axis of the antenna and mechanically and electrically connected
to the acoustic sensor 101 such that, in the event of the failure of one of
the boards, the other boards may continue to interface with the acoustic sensor.
The mechanical position of each board 102 is rigidly connected to the acoustic
2 o sensor 101. The rest of the description will be given with reference to a single
printed circuit board 102 and to an acoustic sensor 101 of multisensor type, by
way of non-limiting example.
Each acoustic module 100 may be protected by an enclosure 103 (also referred to
2 5 as an "acoustic centering device") of small dimensions. The height of the enclosure
103 may be, for example, limited by the small diameter of the antenna 10, in
the case of a flute antenna. Each acoustic sensor 101 may be mounted on the
concentrator system 102. In figure 2, each acoustic multisensor 101 is mounted at
one end of the concentrator system and perpendicularly to the plane of the
30 concentrator in order to optimize the way in which the space delimited by the
enclosure 103 of the acoustic module is occupied. The acoustic multisensors are
configured to deliver electrical signals which are proportional to the dynamic
pressures exerted thereon.
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Each acoustic module 100 may additionally comprise multiple pieces of auxiliary
electronic equipment.
5 The flute antenna 10 may, in particular, be equipped with a protective envelope
104 intended to protect the pieces of electronic equipment. It may contain, in
particular, a liquid of lower density than water and of very high resistivity (such as
oil, for example) in order to prevent the envelope and its contents being crushed.
Such a liquid allows, in particular, the buoyancy of the antenna to be substantially
1 o zero and the pressure inside the antenna to be equalized with the exterior.
According to one aspect of the invention, each acoustic module 100 may comprise
an accelerometer device 1 for determining the acceleration of the antenna 10
along three axes X, Y and Z defining an orthonormal main reference system and in
15 order to compensate for the effect of the acceleration on the signal delivered by
each acoustic multisensor 101. When applying the invention to acoustic antennas,
the X axis corresponds to the roll axis, the Z axis corresponds to the pitch axis and
the Y axis corresponds to the yaw axis.
20 Figure 3 is a schematic view of the accelerometer device 1 according to one
embodiment of the invention showing the main reference system (X, Y, Z). The
accelerometer device 1 comprises a set of digital accelerometer sensors 11, 12 of
electronic microsystem (also referred to as MEMS) type. The MEMS accelerometer
sensors 11 and 12 may be mounted on at least one printed circuit board 102
2 5 and so as to be substantially parallel to one another. The accelerometer sensors
may be, for example, generally rectangular in shape and of very low height. In one
embodiment, the accelerometer device may be rigidly mounted on the same
printed circuit boards 102 as the acoustic sensor 101. In the following description
of certain embodiments, reference will be made to a printed circuit board 102
30 ; commonly connected to the acoustic sensor 101: and to the accelerometer device
1 by way of illustrative example.
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According to one aspect of the invention, the accelerometer device 102 comprises
at least one pair of accelerometer sensors 11 and 12 arranged so that they face in
opposite directions with respect to one another. The sensitivity of the sensors 11
and 12 to pressure may vary. In one embodiment, the pair of sensors facing in
5 opposite directions comprises two sensors 11 and 12 arranged on either side of
the printed circuit board 102 (also referred to as the "upper sensor" and "lower
sensor", respectively) as shown in figure 3.
Each accelerometer sensor 11 and 12 is associated by construction with a sensor
10 reference system (Xi, Yi, Zi) which allows the positioning of the accelerometer
sensor to be defined. The plane (Xi, Yi) defines the plane of each accelerometer
sensor. Each accelerometer sensor is arranged in the device 1 such that its plane
(Xi, Yi) is parallel to the plane (X, Z) of the reference system.
15 The accelerometer sensors 11 and 12 mounted so as to face in opposite directions
may be opposite each other with respect to the axis of the antenna or
arranged at a chosen distance d away from one another with respect to the axis of
the antenna, as shown in figure 4. The distance d may be chosen depending on
the orientation of the support plane (for example, it may be larger or smaller
20 depending on whether or not the support plane is parallel). Furthermore, the
accelerometers may be fixed to the board 102 (fixed by means of welding, for
example) so as to limit errors.
In the examples shown in figures _3 and 4, the reference system of the upper
25 sensor 11 is denoted by (X1, Y1, Z1) while the reference system of the lower
sensor 12 is denoted by (X2, Y2, Z2). The positioning of the two sensors 11 and
12 so as to face in opposite directions relative to one another is such that the
respective axes Z1 and Z2 of the first sensor 11 and of the second sensor 12 are
parallel and of opposite signs.
;30
As used here with respect to an accelerometer device with N sensors, the term
"facing in opposite directions" refers to the mounting of the accelerometer sensors
from among the N sensors such that:
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- for each pair of sensors considered, the sensors have, in twos, components of
opposite sign along two axes of the main reference system (for example, the axis
Z1 of the upper accelerometer sensor 11 corresponds to the axis -Z2 of the lower
accelerometer sensor 12 and the axis Y1 of the upper accelerometer sensor 11
5 corresponds to the axis -Y2 of the lower accelerometer sensor 12); and
- the axes of the main reference system along which the components oppose the
set of pairs of sensors comprise at least two of the three axes X, Y and Z of the
reference system.
1 o Positioning the sensors in such a way makes it possible to correct for the effect of
the pressure exerted on the antenna on at least two axes.
With reference to figure 4, the positioning of the accelerometer sensors 11 and 12
with respect to the main reference system (X, Y, Z) is such that:
15 the axis X1 of the first accelerometer sensor 11 coincides with the axis -Z
of the main reference system;
the axis Y1 of the first accelerometer sensor 11 coincides with the axis X of
the main reference system;
the axis Z1 of the first accelerometer sensor 11 coincides with the axis -Y
2 o of the main reference system;
the axis X2 of the second accelerometer sensor 12 coincides with the axisZ
of the main reference system;
the axis Y2 of the second accelerometer sensor 12 coincides with the axis -
X of the main reference system;
25 the axis Z2 of the second accelerometer sensor 12 coincides with the axis
Y of the main reference system.
Thus, when the accelerometer device comprises two sensors 11 and 12 (20
assembly), the mounting of the two accelerometer sensors so as to face in
30 opposite directions is such that the two sensors have components of opposite sign
along two axes of the main reference system (for example, the axis +Z1 of the
upper accelerometer sensor 11 corresponds to the axis -Z2 of the lower accelerometer
sensor 12 and the axis +Y1 of the upper accelerometer sensor 11 correII
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spends to the axis -Y2 of the lower accelerometer sensor 12). Thus, two axes out
of the three axes of the main reference system face in opposite directions (Y
corresponding to the direction of the axes Z1/Z2 and X corresponding to the
direction of the axes Y1/Y2), thereby making it possible to compensate for the
5 effect of pressure along these two axes.
Figure 5 shows an accelerometer device with three sensors. The positioning of the
accelerometer sensors 11, 12 and 13 with respect to the main reference system
(X, Y, Z) is such that:
1 o the axis X 1 of the first accelerometer sensor 11 coincides with the axis -Z
of the main reference system;
the axis Y1 of the first accelerometer sensor 11 coincides with the axis X of
the main reference system;
the axis Z1 of the first accelerometer sensor 11 coincides with the axis -Y
15 of the main reference system;
the axis X2 of the second accelerometer sensor 12 coincides with the axis -
Z of the main reference system;
the axis Y2 of the second accelerometer sensor 12 coincides with the axis -
X of the main reference system;
20 the axis Z2 of the second accelerometer sensor 12 coincides with the axis
Y of the main reference system;
the axis X3 of the third accelerometer sensor 13 coincides with the axis Z of
the main reference system;
the axis Y3 of the third accelerometer sensor 13 coincides with the axis -X
2 5 of the main reference system;
the axis Z3 of the third accelerometer sensor 13 coincides with the axis -Y
of the main reference system.
The components of each sensor pair thus have components of opposite sign.
30 Thus, for the pair{11, 12}:
-the axes Y1, Y2 of the first and of the second sensor 11 and 12 face in opposite
directions (corresponding to the direction of the axis X of the reference system);
and
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- the axes Z1, Z2 of the first and of the second sensor 11 and 12 face in opposite
directions (corresponding to the direction of the axis Y of the reference system).
For the pair {11, 13}:
5 - the axes X 1, X3 of the first and of the third sensor 11 and 13 face in opposite
directions (corresponding to the direction of the axis Z of the reference system);
and
10
- the axes Y1, Y3 of the first and of the third sensor 11 and 13 face in opposite
directions (corresponding to the direction of the axis Y of the reference system).
For the pair {12, 13}:
-the axes X2, X3 of the second and of the third sensor 12 and 13 face in opposite
directions (corresponding to the direction of the axis Z of the reference system);
and
15 - the axes Z2, Z3 of the second and of the third sensor 12 and 13 face in opposite
directions (corresponding to the direction of the axis Y of the reference system).
In a variant, the third sensor 13 may be mounted so as to face in the opposite
direction along the axis Y with respect to the first sensor 11: the axes Z2 and Z3 of
20 the second accelerometer sensor 12 and of the third accelerometer sensor 13,
respectively, are then parallel and of opposite sign to the axis Z1 of the first
accelerometer sensor 11, which corresponds to 2 pairs of accelerometer sensors
mounted so as to face in opposite directions: 11/12 and 11/13. In a similar
manner, the sensor 13 may be positioned at the level of the sensor 12.
25
Thus, when the accelerometer device comprises three sensors 11, 12 and 13
(30}, as illustrated in the examples of figures 5 and 6, the mounting of the three
accelerometer sensors so as to face in opposite directions is such that:
- for each pair of sensors considered, for example {12, 13} and {11, 12}, the
30 sensors have, in twos, components of opposite sign along two axes of the main
reference system (for example, Z3/Z2 and X3/X2 for the pair {12, 13} or Z1/Z2 and
Y1/Y2 for the pair {11, 12}); and
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- the axes of the main reference system along which the components oppose the
set of pairs of sensors comprise the three axes X, Y and Z of the reference
system.
5 Thus, the components along the three axes X, Y, Z oppose the set of pairs,
thereby making it possible to compensate for the effect of pressure along the three
axes.
In one embodiment, each sensor 11, 12, 13 is an accelerometer sensor with three
10 axes, of capacitive type, with high resolution and ultra-low power (such as described
in the technical specifications of the ultra-low power digital accelerometer
component ADXL346 by Analog Devices).
It should be noted that although a number of sensors equal to three is sufficient to
15 compensate for the effect of pressure on the three axes, the device may comprise
a greater number of accelerometer devices.
Figure 7 shows a schematic top view of the three accelerometer sensors 11, 12
and ·13 of figure 5. In the example shown, each accelerometer sensor is a sensor
20 of ADXL346 type in LGA (land grid array) format and comprises 16 connection
pins denoted by 1 to 16. MEMS of LGA format are asymmetrically encapsulated in
three dimensions such that the stresses which are exerted along the axis Z (which
corresponds, in certain embodiments, to the vertical of the circuit) are higher than
on the axes X, Y, which disrupts the capability of the sensors. MEMS of LGA type
25 are therefore very sensitive to pressure. In particular, substantial drifts are observed
on the axis Z of each of these sensors, for example of the order of 20% at
100 bars, and therefore also on the standard of the sensor (representing the value
of the acceleration due to gravity) when the housing is subject to this pressure.
30 As MEMS are very sensitive to pressure, when submersed to substantial depths
they may therefore deliver a highly inaccurate estimate of the acceleration owing
to the high pressure exerted thereon. The pressure in the sheath is applied to the
incompressible oil and to the MEMS.
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The use of the accelerometer device according to the embodiments of the invention
makes it possible not only to compensate for the effect of acceleration on the
signals delivered by the acoustic multisensors but also to make the accelerometer
5 sensors of MEM8 type 11, 12, 13 insensitive to the pressure P which is exerted
thereon, without information on the value of the pressure P.
By mounting the accelerometer sensors of MEM8 type in such a configuration that
they face in opposite directions, for each pair such as, for example, the pair {11,
1 o 12}, the difference in referencing due to the pressure exerted on the first sensor 11
(high sensor) is of opposite sign to the difference in referencing due to the pressure
exerted on the second sensor 12. This makes it possible to compensate for
the effect of the pressure exerted on the acoustic module on at least two axes. ·
15 Figure 8 shows a schematic view of the antenna segment 10 comprising three
acoustic modules M1, M2, M3 (100) showing the architecture for processing the
signals measured by the sensors, according to one embodiment of the invention.
In the example shown, only the module M2 is equipped with an accelerometer
device 1 with two sensors 81 and 82 delivering accelerations along three axes
20 and an associated acoustic sensor 101 (the elements 1 and 101 are not shown in
figure 8).
The accelerometer device 1 may comprise a processing unit 15 (for example of
FPGA, field programmable gate array, type) configured to calculate a mean (also
2 5 referred to as a "reference") on each of the axes X, Y and Z of the accelerations
measured by each pair of accelerometer sensors facing in opposite directions (11,
12) of the device 1 (mean of the components of opposite signs). For example, in
the embodiments in which the device comprises more than N accelerometer
sensors, one of which is a pair facing in opposite directions, mounted such that N-
30 1 accelerometer sensors (upper sensors) are arranged on the upper part of the
board while one sensor is mounted on the lower part (lower sensor), (N-1) pairs
may potentially be considered for calculating the references per axis, each pair
comprising one of the (N-1) upper sensors and the lower sensor. The references
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thus calculated make it possible to decrease the difference in acceleration due to
the static pressure P which is exerted along each axis on the acoustic antenna.
In certain embodiments, the processing unit 15 may be used to calibrate the
5 values of the accelerations along the three axes X, Y and Z prior to mounting the
MEMS accelerometer sensors 11 and 12 and in order to improve the precision of
the final estimate of the acceleration along three axes. The calibration makes it
possible, in particular, to determine the positioning error with respect to the coding
of each MEMS accelerometer sensor 11 and 12 (mechanical calibration). Fur-
10 thermore, from the electrical standpoint, the accelerometer sensors may not be
identical and locally in a stable position such that the measurements may be
altered. The electrical calibration of the accelerometer sensors makes it possible
to compensate for the architecture in order to obtain the expected values in terms
of gain and offset. For example, by choosing the support of the acoustic sensor as
15 a reference, the expected value for 1g is 256 and for -1g is -256, for each coordinate
of the reference.
In the example of figure 8, the processing unit 15 of the accelerometer device of
the module M2 (FPGA) allows the accelerometer sensors 11, 12 to be calibrated
2 o by finding the mean of the respective accelerations measured along the three axes
S1 = (X1, Y1, Z1) and S2 = (X2, Y2, Z2) on the basis of the previously calculated
calibration parameters.
In one embodiment, each accelerometer sensor may be calibrated individually
25 (separately).
The calibration method may comprise the following steps:
- before mounting the accelerometer device 1, for each accelerometer sensor 11
and 12 of the device 1, the calibration parameters (gains and offset) are estimated
30 for each axis X, Y, Z on the basis of a series of measurements. In particular, an
acceleration of 1*g along each axis of the accelerometer sensor is encoded as a
known output value at the output of the sensor in the direction in question (the
expected output value may be "256", for example). The measurements taken .
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along each axis for each accelerometer sensor 11, 12 of the device and the
comparison of the measurements with the expected value (e.g. "256") then allows
the offset and the gain of each sensor along each axis to be estimated. The gain
and the offset estimated for each sensor and for each accelerometer sensor axis
5 11, 12 of the device 1 (also referred to as "calibration parameters") may then be
stored in memory among the constants of the FPGA device 15.
- under real conditions, for example in the sea, the FPGA processing device 15
may process the input acceleration values which comprise three values per
10 accelerometer sensor 11, 12 of the accelerometer device 1, these three values
corresponding to one acceleration value per axis X, Y or Z. Thus, for an accelerometer
device 1 equipped with two MEMS accelerometer sensors, six values will
be processed. This processing method consists of calibrating the set of these
acceleration values (at least six values) using the calibration values predetermined
15 in the calibration phase during the manufacture of the device 1 (stored in the
FPGA device 15). More specifically, for each axis X, Y and Z, the mean of the
pairs of calibrated values is calculated for each pair of accelerometer sensors 11
and 12 comprising two sensors facing in opposite directions.
2 o The prior calibration phase allows precision to be increased and the error offset to
be decreased. It also allows the installed function to be tested, in particular during
integration and in the maintenance phase for various types of MEMS accelerometer
sensors, thereby decreasing risks.
2 5 It should be noted that the initial calibration phase is an optional phase which may
or may not be implemented depending on the level of precision sought.
In addition, the acoustic antenna may comprise an adapter module 16 for applying
a gain compensation factor G (also referred to as a scale factor) which may be
30 applied in order to rescale the calibrated acceleration< values (mean calculated for
each axis). The scale factor G allows the dynamics of the numbers to be adapted
in order to be able to carry out the calculations using a simpler computer (for
example using integers and not numbers with decimal points).
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Figure 9 is a flowchart representing the method for calculating the acceleration
values calibrated for an accelerometer device 1 with three accelerometer sensors
comprising at least two sensors facing in opposite directions (k pairs of acceler-
5 ometer sensors).
The method may be implemented with the chosen periodicity (for example every
50 ms). The method is, in particular, implemented for each pair of MEMS accelerometer
sensors (arranged so as to face in opposite directions) of the accelerome-
1 o ter device 1.
In order to aid in understanding the description which follows, the following
notation has been defined for each of the k pairs of accelerometer sensors of the
accelerometer device 1 (one pair is composed of two sensors arranged so as to
15 face in opposite directions comprising an upper sensor, e.g. 11, and a lower
sensor, e.g. 12):
- bx,k,sup. by,k,sup. bz,k,sup denote the offsets of the upper accelerometer sensor facing
in opposite directions (e.g. 11) along each axis X, Y, Z, respectively;
- bx,k,inf, by,k,inf, bz,k,inf denote the offsets of the lower accelerometer sensor facing in
20 opposite directions (e.g. 12) along each axis X, Y, Z, respectively;
- gx,k,sup. gy,k,sup, gz,k,sup denote the gains of the upper accelerometer sensor facing
in opposite directions (e.g. 11) along each axis X, Y, Z, respectively;
- gx,k,inf, gy,k,inf, gz,k,inf denote the gains of the lower accelerometer sensor facing in
opposite directions (e.g. 12) along each axis X, Y, Z, respectively; and
25 - G denotes the scale factor for the gain compensation.
In step 900, each accelerometer sensor of the accelerometer device 1 is calibrated
before mounting the sensors in order to determine the gain and the offset of each
accelerometer sensor along each axis of the sensor Xi, Yi, Zi (calibration parame-
30 1 ters). In step 901, the calibration parameters are subsequently stored in the
processing unit 15.
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Once the device 1 has been mounted on an acoustic antenna, in step 902, the
accelerations may be measured by each sensor along each axis X, Y and Z.
In step 903, for each pair k comprising two sensors facing in opposite directions
5 (for example 11, 12 and 12, 13 in the example of figure 5), the acceleration
components Xk, Yk and Zk calibrated along each of the axes X, Y or Z are determined
by finding the mean of the values measured by the various sensors along
the corresponding axis. The calculation of the mean on each axis may account for
the calibration parameters such as the gain and/or the offset of each sensor along
10 the axis and/or the G factor. In one embodiment, the mean acceleration components
Xk, Yk and Zk may be obtained via the following equations:
15
20
(y -b )*--1 ~ -(Y -b )•-
1-
' k,l y,k,l k,2 y,k,2
Xk= G * g,._,_, g,.·'·'
2
-(z -b )•-
1-+(z -b )•-
1-
k,l :",k,l k,2 z,k,l
Yk= G * g=.<.I g=.k.2
2
-(x -b )•-1 ~ -(x -b )•-
1-
k,l x,k,l k,l x,k,2
Zk=G * gx.k,l gx,k,2
2
Those skilled in the art will understand that steps 900 and 901 are carried out prior
to steps 902 and 903 without time limitation. Furthermore those skilled in the art
will understand that the values obtained in step 901 may be used for multiple
measurements in steps 902 and 903.
The accelerometer device 1 based on MEMS sensors according to the embodiments
of the invention thus makes it possible to compensate for the effect of the
pressure P exerted on the antenna. The inventors have, in particular, carried out
pressure tests from 0 to 60 bars over 35 trials with MEMS accelerometer sensors
' 25 of eight different types. Within the context bf such tests, a referencing offset of +-1
lsb (standard deviation of 0.3) has been observed on the X and Y axes and a
referencing offset of +-2 lsb (standard deviation of 1) on the Z axis.
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In the embodiments in which the sensitivity to pressure of the sensors of the
accelerometer device varies, it is possible to account for this varying sensitivity
using the error law due to pressure and the positioning of the sensors.
5 The accelerometer device 1 thus provides a low-cost solution for measuring the
acceleration of an acoustic antenna and compensating for the effect of pressure.
In certain acoustic applications of the invention, it has been observed, in particular,
that the accelerometer device may be used up to 100 bars (1000 m) with a good
level of resistance up to 500 bars (5000 m).
10
The accelerometer device 1 is also less bulky, thereby allowing a plurality of
boards 102 to be stacked in order to ensure continu. .o us 'o peration in 't he ev. ent of
failure.
15 The accelerometer device additionally allows the error rate of 20% observed with
the conventional accelerometer devices to be brought down to a rate equal to 1%.
It also allows redundancy in the values and therefore reliability to be improved.
The accelerometer device 1 also has a good level of endurance and reliability over
20 time. It has been observed in particular that the pressure sensitivity of an MEMS
sensor is constant over time for the entire life of the device.
The invention is not limited to the embodiments described hereinabove by way of
non-limiting example. It encompasses all the variant embodiments that may be
25 envisaged by the person skilled in the art. In particular, the invention is not limited
to an accelerometer sensor of MEMS type (11, 12). Nor is it limited to a particular
number of accelerometer sensors or to a particular arrangement of the accelerometer
device in an associated acoustic module. Furthermore, the calibration
phase in steps 900 and 901 may be omitted in certain embodiments or imple-
3 o mented differently.
More generally, the invention is not limited to one application of the accelerometer
device with an acoustic antenna of flute type and may be used to measure the
acceleration along three axes of any object to which it is mechanically connected.
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The invention may be used, for example, to remove the left/right ambiguity found
in submarine acoustic detection or else to determine the direction of an object in
space. In particular, the accelerometer device 1 may be used, for example, upon
stopping to measure the terrestrial gravity in a mode referred to as "autotest". The
5 output of the processing unit 15 may, in particular, be used by a position adjustment
mechanism (not shown) in order to adjust the position of the object on the
basis of references calculated by the processing unit 15 along each axis, according
to a control loop.

Claims
1. An accelerometer device for determining the acceleration of an object to
which the accelerometer device is mechanically connected, along three
axes X, Y and Z of a main orthonormal reference system (X, Y, Z) compris-
5 ing three axes X, Y, Z, said object being subject to a surrounding pressure,
characterized in that it comprises a number N of accelerometer sensors of
MEMS type (11, 12, 13), said number N of accelerometer sensors being at
least equal to two, each sensor (11, 12, 13) being defined by construction in
an auxiliary reference system comprising three orthonormal axes (Xi, Yi,
10 Zi), said set of accelerometer sensors comprising at least one pair of sensors
mounted so that they face in opposite directions and are substantially
parallel to one another, and in that:
- for each of said pairs of accelerometer sensors, the sensors have, in twos,
components of opposite sign along two axes of the main reference system;
15 and
- the axes of the reference system along which the components of the accelerometer
sensors oppose the set of pairs of sensors in twos comprise at
least two of the three axes X, Y and Z of the reference system, thereby
making it possible to compensate for the effect of the pressure on at least
2 o two axes of the reference system.
2. The accelerometer device as claimed in claim 1, characterized in that it
comprises a processing unit (15) for calculating the mean of the accelerations
along each axis X, Y and Z, the mean of the accelerations along a
given axis being determined from the accelerations determined by each ac-
25 celerometer sensor along said given axis.
3. The accelerometer device as claimed in claim 2, characterized in that the
calculation of the mean of the accelerations along a given axis additionally
accounts for the gain and/or the offset of each sensor predetermined in a
unit calibration phase implemented separately for each sensor before
30 mounting.
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4. The accelerometer device as claimed in either of claims 1 and 2, characterized
in that the calculation of the means of the accelerations along a given
axis additionally accounts for a predefined gain compensation factor.
5. The accelerometer device as claimed in one of the preceding claims, char-
5 acterized in that said set of sensors comprises two accelerometer sensors
(11, 12) connected to at least one printed circuit board (102), and in that the
axes of the reference system along which the components of the sensors
oppose the set of sensors in twos comprise two of the three axes X, Y and
Z of the reference system, thereby making it possible to correct for the ef-
1 o feet of the pressure which is exerted on the object along the two axes.
6. The accelerometer device as claimed in one of claims 1 to 4, cbaracterized
in that said set of accelerometer sensors comprises three sensors (11, 12,
13), and in that the axes of the reference system along which the components
of the accelerometer sensors oppose the set of pairs of sensors in
15 twos comprise the three axes X, Y and Z of the reference system, thereby
making it possible to correct for the effect of the pressure which is exerted
on the object along the three axes.
7. The accelerometer device as claimed in one of the preceding claims, characterized
in that said object is an acoustic antenna (1 0) comprising a set of
20 acoustic multisensors (101), each acoustic multisensor (101) being
mounted on at least one printed circuit board (1 02), and in that the accelerations
determined by the accelerometer device along each axis X, Y and
Z are used to compensate for the effect of the acceleration of the acoustic
antenna (1 0) on the position of the acoustic multisensor.
25 8. The accelerometer device as claimed in one of the preceding claims, characterized
in that the sensitivity of the sensors to pressure varies.
9. An acoustic module for an acoustic antenna (10) extending along a main
axis, characterized in that it comprises an accelerometer device (1) as
claimed in one of claims 1 to 8, and in that the accelerations determined by
30 the accelerometer device along each axis X, Y and Z are used to compenwo
2016/008782 23 PCT/EP2015/065589
sate for the effect of the acceleration of the acoustic antenna (1 0) on the
position of the acoustic multisensor.
10. The acoustic module as claimed in claim 9, characterized in that the acoustic
sensor (1 01) is an acoustic multisensor with an axis that is substantially
5 parallel to the axis of the antenna.
11. The acoustic module as claimed in either of claims 9 and 10, characterized
in that it comprises at least one printed circuit board on which the accelerometer
sensors are mounted,
12. The acoustic module as claimed in claim 11, characterized in that it com-
10 prises a plurality of printed circuit boards (102), stacked so as to be sub-
15
-.. -·
stantially perpendicular to the axis of the antenna, on which the accelerometer
sensors are mounted.
13. An acoustic antenna, characterized in that it comprises a set of acoustic
modules as claimed in one of claims 9 to 12.