Inertial core for inertial measurement unit and corresponding inertial measurement unit

Abstract

The present invention relates to an inertial core (10) of an inertial measurement unit comprising a set of inertial sensors (12) including at least one accelerometer (16) and at least one gyrometer (18), in which a first sensor (12) of the set of sensors (12) defines at least one receiving surface (20). At least a second sensor (12) of the sensor assembly (12) is attached to the first sensor (12) at the receiving surface (20), the first sensor (12) forming a receiving structure for the second sensor (12).

Description

[0001] TITLE: Inertial Core for Inertial Measurement Unit and Associated Inertial Measurement Unit

[0002] The present invention relates to the field of inertial measurements and more particularly, but not exclusively, inertial navigation.

[0003] The present invention relates in particular to inertial cores for inertial measurement units and to associated inertial measurement units.

[0004] It applies particularly to inertial systems designed to determine the location and navigation of a moving object, such as an aircraft. These systems include at least one inertial measurement unit, as well as a set of external sensors that produce instantaneous measurements characterizing the location and / or navigation of the moving object. An application of such a system is, for example, aircraft navigation.

[0005] As is known, an inertial measurement unit commonly referred to as "IMU" according to the acronym corresponding to the English terminology "Inertial Measurement Unit", includes an inertial core, also referred to as "ISA" for "Inertial Sensors Assembly", made up of several inertial sensors which deliver one or more measurements describing the movement of the mobile.

[0006] In a typical use case, inertial sensors are gyroscopes and / or accelerometers, commonly three of each type, arranged along three orthogonal axes.

[0007] Each accelerometer delivers an acceleration signal representative of the acceleration experienced by the inertial core it equips, along a given axis (linked to this inertial core).

[0008] Each gyroscope delivers an angular velocity signal, representative of an angular velocity of rotation of the inertial core it equips around a given axis (linked to this inertial core).

[0009] These sensors are typically supported by a rigid mechanical structure, which is itself mounted in the mobile device.

[0010] Navigation calculations performed using the elementary data measured by inertial sensors require preliminary processing to express the output quantities of the different sensors in a single orthogonal coordinate system. To achieve this, compensations for axis misalignment are applied to account for machining errors during the sensor mounting on the rigid mechanical structure.

[0011] These axis misalignment errors are typically compensated for with temperature using a model that may introduce modeling errors. These modeling errors are smaller when the initial axis misalignment errors are small and exhibit low temperature sensitivity.

[0012] One way to reduce these axis misalignment errors is to mount the sensors on a mechanical structure with the highest possible rigidity. Such a mechanical structure is made, for example, of steel or aluminum alloy. However, some errors may still occur.

[0013] One aim of the invention is therefore to offer an inertial core allowing better control of axis alignments.

[0014] To this end, the invention relates to an inertial core of an inertial measurement unit comprising a set of inertial sensors including at least one accelerometer and at least one gyroscope, in which a first sensor of the set of sensors defines at least one receiving surface, at least a second sensor of the set of sensors being fixed to said first sensor at the level of said receiving surface, the first sensor forming a receiving structure for said second sensor.

[0015] Fixing several sensors together reduces the number of axis transfers, thus improving the control and stability of axis alignments.

[0016] According to other advantageous aspects of the invention, the inertial core comprises one or more of the following features, taken individually or in all technically possible combinations:

[0017] - all sensors of the sensor set other than the first sensor are fixed to said first sensor at the level of a reception surface of said first sensor, said first sensor forming a reception structure for said sensors;

[0018] - the first sensor comprises a monolithic block, the second sensor(s) being fixed to said monolithic block;

[0019] - the monolithic block includes at least one cavity, the second sensor(s) being received at least partially in said cavity(es);

[0020] - the monolithic block is made of a material with a thermal expansion of less than 10 ppm / °C, preferably 5 ppm / °C;

[0021] - the monolithic block is made of a glass-ceramic material;

[0022] - the first sensor is a tri-axial gyroscope forming a support structure for three mono-axial accelerometers;

[0023] - the second sensor(s) are attached to the first sensor by gluing, welding or by means of mechanical fasteners;

[0024] - the inertial core being devoid of an additional intermediate mechanical structure intended to serve as a support for the sensors. The invention also relates to an inertial measuring unit comprising an inertial core of the aforementioned type and a signal processing unit for signals from the inertial sensors.

[0025] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0026] [Fig. 1] Figure 1 is a schematic representation of an inertial measurement unit according to the invention, comprising an inertial core; and

[0027] [Fig.2] Figure 2 is a perspective view of the inertial core of Figure 1.

[0028] Figure 1 schematically represents an embodiment of an inertial measurement unit 8 according to the present invention.

[0029] The inertial measurement unit 8 is configured to be carried in a mobile device, for example an aircraft, and is suitable for providing navigation measurements for said mobile device.

[0030] The inertial measurement unit 8 is advantageously rigidly linked to the moving part, for example to a structural element of the moving part, either directly or by means of a support.

[0031] The inertial unit 8 comprises an inertial core 10 including a set 11 of inertial sensors 12, and a signal processing unit 14 from the inertial sensors 12.

[0032] Figure 2 represents an inertial core 10 according to one embodiment of the invention.

[0033] The set 11 of inertial sensors 12 includes at least one accelerometer 16 and at least one gyroscope 18. Each sensor 12 is configured to deliver an inertial signal representative of an acceleration or angular velocity of rotation of the inertial measuring unit 8.

[0034] Each accelerometer 16 delivers an acceleration signal representative of the acceleration undergone by the inertial measurement unit 8 that it equips, along one or more orthogonal axes of a first given orthogonal frame (linked to the inertial measurement unit 8).

[0035] Each gyroscope 18 delivers an angular velocity signal, representative of an angular velocity of rotation of the inertial measurement unit 8 that it equips around one or more orthogonal axes of a second given orthogonal frame (linked to the inertial measurement unit 8).

[0036] These acceleration and angular velocity signals constitute inertial signals, providing information on the dynamics of the inertial measurement unit 8.

[0037] The acceleration and angular velocity signals delivered by the accelerometers and gyroscopes equipping the inertial core 10 make it possible to fully determine the three components of the acceleration vector of the inertial measurement unit 8 comprising said inertial core 10, as well as the three components of the angular velocity vector of this measurement unit 8.

[0038] The 12 sensors are chosen from single-axis, dual-axis and tri-axis sensors. This means that the said sensors are capable of measuring acceleration or angular velocity along one, two or three axes respectively.

[0039] For example, as in the embodiment shown in Figure 2, the inertial core 10 includes a tri-axial gyroscope 18 and three mono-axial accelerometers 16 (only two accelerometers are visible).

[0040] Alternatively, the inertial core 10 comprises three single-axis gyroscopes 18 or one single-axis gyroscope and one bi-axial gyroscope.

[0041] Alternatively or in addition, the inertial core 10 includes a mono-axial accelerometer 16 and a bi-axial accelerometer 16, or a tri-axial accelerometer 16.

[0042] A first sensor 12 from the set 11 of sensors 12 is configured to form a host structure for at least one second sensor 12 from the sensor set. The first sensor 12 is subsequently designated as the host sensor and each subsequent second sensor 12 is subsequently designated as the received sensor.

[0043] The receiving sensor 12 is typically larger than the sensor(s) received.

[0044] Preferably, the inertial core 10 includes a host sensor 12 configured to form a host structure for all sensors 12 other than said host sensor 12, of the inertial core 10.

[0045] Advantageously, the inertial core 10 is devoid of an additional mechanical support for the sensors 12, so that the receiving sensor 12 serves as a receiving structure for all the other sensors 12. In other words, the receiving sensor 12 acts as a rigid mechanical support.

[0046] The receiving sensor 12 has at least one receiving surface 20 intended to receive at least one second sensor 12 from the set of sensors 12. Each received sensor 12 is fixed to the receiving sensor 12 at the level of a receiving surface 20.

[0047] The received sensor(s) 12 are, for example, attached to the receiving sensor 12 by gluing, welding, or by means of mechanical fasteners.

[0048] In the embodiment shown in Figure 2, the inertial core 10 comprises a tri-axial gyroscope 18 and three mono-axial accelerometers 16, only two of which are visible. The gyroscope 18 acts here as the first sensor, or receiving sensor, and the accelerometers 16 act as the second sensors, or receiving sensors.

[0049] Thus, the gyroscope 18 has a plurality of mounting surfaces 20 on which the accelerometers 16 are fixed. Preferably, the first sensor 12 comprises a monolithic block 22 to which the second sensor(s) are fixed. The monolithic block 22 thus acts as a support for the second sensor(s) 12.

[0050] Advantageously, the monolithic block 22 includes at least one receiving cavity 24, the second sensor(s) 12 being received at least in part in said cavity(es) 24.

[0051] For example, in the embodiment shown in Figure 2, the gyroscope 18 is a tri-axial laser gyroscope, comprising a monolithic block 22 in which three optical paths are cut, arranged orthogonally in pairs, and six mirrors 26 (four of which are visible in Figure 2) connected in pairs by the optical paths. WO 2009 / 101105 describes an example of a tri-axial laser gyroscope.

[0052] In the embodiment shown in Figure 2, the monolithic block 22 has the general shape of a cube with cut corners. However, the monolithic block 22 is capable of having other shapes.

[0053] The monolithic block 22 has three receiving cavities 24, each cavity 24 being delimited by internal walls 25 and intended to receive at least part of a given accelerometer 16.

[0054] The said cavities 24 are here provided between the optical paths.

[0055] The accelerometers 16 are received in the said cavities 24 and fixed at the level of the said internal walls 25.

[0056] The arrangement of sensors 12 in cavities 24 facilitates the alignment of said sensors 12 and improves their accuracy. It is indeed possible to automate the creation of said cavities in order to adjust the surfaces intended to receive the second set of sensors 12.

[0057] This also makes it possible to reduce the size of the inertial core 10, by nesting the sensors 12 forming said core together.

[0058] Preferably, as shown in Figure 2, the receiving sensor 12 has a symmetrical structure and the received sensors 12 are positioned symmetrically on the receiving sensor 12, which improves the control of axis alignments.

[0059] According to an advantageous configuration shown in Figure 2, the monolithic block 22 also has one or more additional cavities 28, which are not intended to house sensors. These additional cavities 28 allow for a reduction in the mass of the inertial core 10.

[0060] Preferably, the monolithic block 22 is made of a material with a thermal expansion of less than 10 ppm / °C, preferably 5 ppm / °C. Low thermal expansion thus allows for greater long-term stability of the alignment axes of the inertial core 10. In particular, this helps to avoid, or at least reduce, the deviation of the axes of the second sensors 12 relative to the receiving sensor 12.

[0061] For example, the monolithic block 22 is made of a glass-ceramic material.

[0062] The processing unit 14 comprises an electronic processing circuit connected to the inertial sensors 12 and is designed to retrieve the signals from the sensors 12 and process them by implementing at least one localization algorithm (more commonly called an inertial navigation algorithm) which is configured to calculate in real time position information (latitude, longitude, altitude), speed and attitude (heading, roll and pitch angles of the inertial core) of the inertial core 10 under consideration. Such a unit 14 is known per se.

[0063] By eliminating the intermediate mechanical structure, the inertial core 10 according to the invention makes it possible to remove an axis transfer and thus improve the accuracy and stability of the axis alignment. Each second sensor 12 is directly transferred to the receiving sensor 12, or even directly integrated into it, taking advantage of the available space in the receiving sensor 12. This eliminates the need for two transfers (accelerometers on the intermediate part and gyroscope on the same intermediate part), leaving only one to consider.

[0064] Furthermore, the transfer of the received sensor(s) 12 to the receiving sensor 12 is capable of being automated, further improving the accuracy of axis alignment, as well as the repeatability of operations.

[0065] The inertial core 10 according to the invention also takes advantage of the non-functional surfaces and spaces of the receiving sensor 12 to accommodate the received sensor(s) 12, thus significantly reducing its compactness compared to an inertial core with an intermediate mechanical structure. This compactness is accompanied by an increase in the frequencies of the inertial core's natural modes, thereby avoiding interactions that are normally difficult to handle.

[0066] Removing the intermediate mechanical structure also allows for a reduction in the mass of the inertial core 10, as well as a reduction in costs.

[0067] Several variations are possible.

[0068] For example, according to one variant, the host sensor 12 is a triaxial accelerometer 16 forming a host structure for gyroscopes 18.

[0069] In one variant, the host sensor 12 is a single-axis or dual-axis sensor, forming a host structure for all the other sensors. For example, the host sensor 12 is a single-axis laser gyroscope or a single-axis fiber gyroscope.

Claims

7 DEMANDS 1. Inertial core (10) of an inertial measuring unit (8) comprising an array (11) of inertial sensors (12) including at least one accelerometer (16) and at least one gyroscope (18), in which a first sensor (12) of the array (11) of sensors (12) defines at least one receiving surface (20), at least a second sensor (12) of the array (11) of sensors (12) being fixed to said first sensor (12) at the level of said receiving surface (20), the first sensor (12) forming a receiving structure for said second sensor (12).

2. Inertial core (10) according to claim 1, wherein all the sensors (12) of the set (11) of sensors (12) other than the first sensor (12) are fixed to said first sensor (12) at the level of a surface (20) for receiving said first sensor (12), said first sensor (12) forming a structure for receiving said sensors (12).

3. Inertial core (10) according to claim 1 or 2, wherein the first sensor (12) comprises a monolithic block (22), the second sensor(s) (12) being fixed to said monolithic block (22).

4. Inertial core (10) according to claim 3, in which the monolithic block (22) comprises at least one cavity (24), the second sensor(s) (12) being received at least in part in said cavity(s) (24).

5. Inertial core (10) according to claim 3 or 4, wherein the monolithic block (22) is made of a material having a thermal expansion of less than 10 ppm / °C, preferably 5 ppm / °C.

6. Inertial core (10) according to any one of claims 3 to 5, wherein the monolithic block (22) is made of a glass-ceramic material.

7. Inertial core (10) according to any one of the preceding claims, wherein the first sensor (12) is a tri-axial gyroscope (18) forming a host structure for three mono-axial accelerometers (16).

8. Inertial core (10) according to any one of the preceding claims, wherein the second sensor(s) (12) are fixed to the first sensor (12) by gluing, welding or by means of mechanical fasteners.

9. Inertial core (10) according to any one of the preceding claims, said inertial core (10) being devoid of an additional intermediate mechanical structure intended to serve as a support for the sensors (12).

10. Inertial measuring unit (8) comprising an inertial core (10) according to any one of the preceding claims; and a signal processing unit (14) from the inertial sensors (12).

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