Method and device for inertial navigation alignment of a vehicle

Abstract

The invention relates to a device for inertial navigation alignment of a vehicle, the device comprising: - an inertial navigation system (INS1) intended to be carried on board the vehicle, the system being provided with a first target (MIR1); - a reference inertial navigation system (INS_REF) provided with a reference target (MIR_REF) having an accuracy that is, on the one hand, greater than that of the first system and, on the other hand, suitable for measuring the direction of the Earth's axis of rotation; - an alignment arm (BRA) provided with cameras (CAM1, CAM_REF) at the ends thereof, the arm being configured to be installed partly inside the vehicle and partly outside the vehicle, the targets being placed in the field of view of the cameras; - a computing unit (UC) connected to the inertial measurement units and to the cameras in order to align the navigation of the first system (INS1) using the navigation of the reference system (INS_REF).

Description

Description TITLE: Method and device for inertial navigation alignment of a vehicle. Technical field of the invention

[0001] The present invention relates to the field of inertial navigation of a vehicle, in particular for a space launcher. Technological background

[0002] At launch, a space launcher must have an onboard navigation solution for its piloting and / or localization.

[0003] For this purpose, an inertial navigation system is generally used, which includes an inertial navigation unit (this unit includes accelerometers and gyroscopes).

[0004] Indeed, the inertial navigation system must integrate information on the attitude of the launcher during its mission (the effective horizontality and the azimuth heading which gives the orientation relative to geographic North).

[0005] The inertial navigation system must be initialized on the launch pad while the launcher is stationary. This improves the navigation accuracy calculated by the system during the mission.

[0006] The phase during which the inertial navigation system is initialized is called the alignment phase. The inertial navigation system must be accurate enough to measure the Earth's rotation in order to deduce the azimuth heading for the launch vehicle and to measure the direction of gravity in order to deduce the horizontality (or verticality) of the system.

[0007] This requires an inertial navigation system with sufficient accuracy to measure the direction of the Earth's axis of rotation: 360° in 24h or about 0.004° / s), which is therefore capable of performing autonomous alignment and is consequently expensive.

[0008] In the current context where we see the emergence of low-cost launchers (micro-launchers), it is necessary to reduce the cost of an inertial navigation system and therefore to use less precise systems (particularly with regard to gyroscopes) which no longer allow for autonomous alignment.

[0009] It is therefore necessary to insert a heading into the inertial navigation system.

[0010] To this end, various techniques exist, such as estimating the heading based on the environment. However, heading estimation can quickly become significantly flawed.

[0011] Documents CN-A-108375383, CN-A-107728182, CN-A-109540173, and CN-A-1024117 describe methods and devices for inertial navigation alignment. Publications by Ogata Kunihiro et al., "A Robust Position and Posture Measurement System Using Visual Markers and an Inertia Measurement Unit," 2019 IEEE / RSJ International Conference on Intelligent Robots and Systems (IROS), pp. 7497–7502, and by Kim Anthony et al., also provide relevant information. : “Initial Calibration of an Inertial Measurement Unit Using an Optical Position Tracking System”, PLANS 2004: Position Location and Navigation Symposium, 2004 and Mu Rongjun et al.: “Suitability of two INS transalignment methods in powered phase of launch vehicle”, 2014 International Conference on Mechatronics and Control (ICMC), 2014 also describe inertial navigation alignment methods and devices.

[0012] Beyond space launchers, similar problems can be encountered for other types of vehicles when that vehicle needs to know its static alignment.

[0013] One objective of the invention is to propose a way of performing inertial navigation alignment that does not present at least one of the aforementioned disadvantages. Summary of the invention

[0014] To this end, an inertial navigation alignment device for a vehicle is proposed, the device comprising: - a first inertial navigation system intended to be carried in the vehicle, said first system being equipped, on one of its external faces, with a first target; - a second inertial navigation system, called the reference system, having on the one hand a greater accuracy than the first inertial navigation system and adapted on the other hand to measure the direction of the Earth's axis of rotation, said reference inertial navigation system being equipped, on one of its external faces, with a target called the reference target; - an alignment arm equipped with a first camera at one end and a second camera at the other end, said alignment arm being configured to be able to be installed by placing the first target in the field of vision of a first camera and placing the reference target in the field of vision of a second camera; and - a computing unit connected to the first inertial navigation system, the reference inertial navigation system and the cameras so that said computing unit can align the navigation of the first inertial navigation system with the navigation of the reference inertial navigation system.

[0015] The invention also relates to a method for aligning the inertial navigation of a vehicle implemented with a device according to the invention, said method comprising the following steps: a) placing one end of the alignment arm equipped with the first camera inside the vehicle so that the first target of the first inertial navigation system is in the field of vision of the first camera; b) positioning the reference inertial navigation system so that the reference target is in the field of vision of the second camera; c) connecting the first inertial navigation system, the reference inertial navigation system and the cameras to the computing unit; d) performing an autonomous alignment of the reference inertial navigation system; e) aligning the inertial navigation of the first inertial navigation system using the inertial navigation provided by the reference inertial navigation system.

[0016] Thus, thanks to the device and method according to the invention, it is possible to perform inertial navigation alignment, without the disadvantages of known techniques, and at a reduced cost.

[0017] The process according to the invention may comprise one or more of the steps below, taken individually or in combination with each other: - step e) includes the following sub-steps: ei) measure a distance and angular offsets between the first target of the first inertial navigation system and the first camera; 62) measure a distance and angular offsets between the reference target of the reference inertial navigation system and the second camera; es) from the information from substeps ei) and 62), deduce a distance and angular offsets between the first navigation system inertial and the reference inertial navigation system; then 64) insert the inertial navigation data from the reference inertial navigation system into the first inertial navigation system, by compensating for the distance and angular offsets determined in sub-step es); - during step e), we also define an uncertainty rate of the inertial navigation alignment of the first inertial navigation system.

[0018] The invention also relates to a method for calibrating a device according to the invention, comprising the following steps: A) measure a distance and angular offsets between the first target and gyroscopes of the first inertial navigation system; B) measure a distance and angular offsets between the reference target and gyroscopes of the reference inertial navigation system; C) measure a distance and angular offsets between the two cameras located at the respective ends of the alignment arm (BRA).

[0019] The calibration procedure may further include the following steps: any one of step A) or step B) is carried out on a marble slab comprising a marble target and positioning means for an inertial navigation system whose position relative to the marble target is known, the procedure comprising the following substeps: - install the inertial navigation system equipped with its target on the marble positioning means, the gyroscopes of the inertial navigation system then presenting a known position relative to the positioning means; - position a camera so that the marble target and the inertial navigation system target are in the camera's field of vision; - provide a computing unit and connect it to the inertial navigation system and the camera; - determine, using the calculation unit, a distance and angular offsets between the marble target and the target of the inertial navigation system; - determine, using the calculation unit, a distance and angular offsets between the target of the inertial navigation system and the gyroscopes of the inertial navigation system, knowing a position of the positioning means relative to a position of the target on the marble, a position of the gyroscopes of the inertial navigation system relative to the positioning means, and a distance and angular discrepancies between the marble target and the target of the inertial navigation system.

[0020] Finally, during calibration, step c) may be carried out on a marble (MRB*) comprising a first target whose position on the marble is known and a second target whose position on the marble is also known, said process comprising the following sub-steps: - place the alignment arm equipped with its cameras on a support placed on the marble, positioning the first target in the field of vision of the first camera and the second target in the field of vision of the second camera; - provide a computing unit and connect it to the two cameras; - determine, using the calculation unit, a distance and angular offsets between each camera and the target that is in its field of vision, then - determine, using the calculation unit, a distance and angular offsets between the two cameras. Brief description of the figures The invention will be better understood with the aid of the following description, given solely by way of example and made with reference to the accompanying drawings in which: - Figure 1 represents an inertial navigation alignment device for a vehicle according to the invention; - Figure 2 is a diagram representing the main steps of an inertial navigation alignment process for a vehicle, implemented with the device in Figure 1; - Figure 3 is a diagram representing the main steps of a calibration process; - Figure 4 represents a calibration installation that can be used to implement certain steps of the calibration process - Figure 5 represents another installation that could be used to implement other steps in the calibration process. Detailed description of the invention

[0021] The invention proposes an inertial navigation alignment device D for an HCV.

[0022] We can refer to figure 1.

[0023] The inertial navigation alignment device D of an HCV includes: - a first inertial navigation system INS1 intended to be carried in the vehicle, said first system being equipped, on one of its external faces, with a first target MIR1; - a second inertial navigation system INS_REF, called the reference system, which can for example be installed outside the vehicle, having a precision which is on the one hand greater than that of the first inertial navigation system INS1 and on the other hand adapted to measure the direction of the Earth's axis of rotation, said reference inertial navigation system being equipped, on one of its external faces, with a reference target MIR REF; - an alignment arm (BRA) equipped with a first camera (CAM1) at one end and a second camera (CAM REF) at the other end, said alignment arm being configured so that it can be installed by placing the first target (MIR1) in the field of view of the first camera (CAM1) and placing the reference target (MIR REF) in the field of view of the second camera (CAM REF); and - a computing unit CU connected to the first inertial measurement unit INS1, to the reference inertial measurement unit INS_REF and to the cameras CAM1, CAM REF so that said computing unit can align the navigation of the first inertial navigation system INS1 with the reference inertial navigation system INS-REF.

[0024] To pass an arm inside the vehicle, for example a space launcher as illustrated in Figure 1, an existing TRH manhole in the launcher's fuselage must be used. This is generally an access hatch to equipment allowing a personnel member to pass through.

[0025] An inertial navigation system typically includes accelerometers and gyroscopes. The difference between the first inertial navigation system INS1 and the reference inertial navigation system INS_REF lies, at least, in the accuracy provided by the gyroscopes. The accuracy of the gyroscopes in the first inertial navigation system INS1 does not allow for autonomous alignment, unlike the reference inertial navigation system INS_REF.

[0026] With such a device D, one can implement an inertial navigation alignment process for a vehicle, the main steps of which are shown schematically in Figure 2.

[0027] The inertial navigation alignment procedure includes the following steps: a) placing one end of the alignment arm BRA equipped with the first camera CAM1 inside the VHC vehicle so that the first target MIR1 of the first inertial navigation system INS1 is in the field of view of the first camera; b) positioning the reference inertial navigation system INS_REF so that the reference target MIR REF is in the field of view of the second camera CAM REF; c) connecting the first inertial navigation system INS1, the reference inertial navigation system INS_REF and the cameras CAM1, CAM REF to the computing unit UC; d) performing an autonomous alignment of the reference inertial navigation system INS_REF; e) align the inertial navigation of the first inertial navigation system INS1 using the inertial navigation provided by the reference inertial navigation system INS-REF.

[0028] More specifically, step e) may include the following substeps: ei) measure the distance and angular offsets between the first target MIR1 of the first inertial navigation system INS1 and the first camera CAM1; 62) measure the distance and angular offsets between the reference target MIR REF of the reference inertial navigation system INS_REF and the second camera CAM REF; es) from the information obtained in substeps ei) and 62), deduce the distance and angular offsets between the first inertial navigation system INS1 and the reference inertial navigation system INS_REF; then 64) insert the inertial navigation of the reference inertial navigation system INS_REF into the first inertial navigation system INS1, by compensating for the distance and angular offsets determined in substep es).

[0029] Steps ei) to 64) are implemented using pre-calibrated equipment (CAM1 and CAM REF cameras, the BRA arm, and the INS1 and INS_REF inertial navigation systems). Specifically, for each inertial navigation system, the compensation between the target and the gyroscopes is already integrated through prior calibration of the device. This calibration will be discussed later.

[0030] Furthermore, during step e), we can also define an uncertainty rate of the inertial navigation alignment of the first inertial navigation system INS1 with respect to the inertial navigation provided by the reference inertial navigation system INS_REF.

[0031] Before implementing the method according to the invention, for example on a launch pad for a space launcher, calibration of various elements is necessary.

[0032] Indeed, while the previously described alignment device and procedure allow, during their implementation, the determination of a distance and angular offsets between a camera and a target within its field of view, the quality of the alignment requires calibration between the gyroscopes of each inertial navigation system and the target it carries, as well as prior calibration of the BRA alignment arm. This ensures that the measurements taken will indeed allow for alignment (with a known and controlled risk of error) of the navigation of the first inertial navigation system INS1 with that of the reference inertial navigation system INS_REF.

[0033] In the following, we therefore describe the different calibrations implemented before the device according to the invention can be used.

[0034] To calibrate device D of the invention, the following main steps should be taken: A) measure distances and angular offsets between the first target MIR1 and the gyroscopes of the first inertial navigation system INS1; B) measure distances and angular offsets between the reference target MIR REF and the gyroscopes of the reference inertial navigation system INS_REF; C) measure distances and angular offsets between the two cameras CAM1, and CAM REF located at the respective ends of the BRA alignment arm.

[0035] See Figure 3.

[0036] To implement step A) of the calibration process, an MRB marble, shown in Figure 4, can be used.

[0037] The MRB marble includes an MIR MRB target as well as M POS positioning means for an inertial navigation system (in this case, we have represented in Figure 4 the first inertial navigation system INS1 for step A). ​​The position of the positioning means M POS relative to the target of the marble MIR MRB is known.

[0038] Step A) of the process may include the following sub-steps: - install the INS1 inertial navigation system equipped with its MIR1 target on the M POS positioning means of the marble, the gyroscopes of the INS1 inertial navigation system then presenting a known position relative to the positioning means; - position a CAM MRB camera so that the target of the MIR MRB marble and the MIR1 target of the INS1 inertial navigation system are in the camera's field of vision; - provide a computing unit UC and connect it to the INS1 inertial navigation system and the CAM MRB camera; - determine, using the calculation unit UC, the distance and angular offsets between the MIR MRB target of the marble and the MIR1 target of the INS1 inertial navigation system; - determine, using the computing unit UC, the distance and angular offsets between the MIR1 target of the INS1 inertial navigation system and the gyroscopes of the INS1 inertial navigation system.

[0039] Step A) according to these substeps can be carried out provided that the following are known: i) the position of the M POS positioning means relative to the MIR MRB target on the marble slab, ii) the position of the gyroscopes of the inertial measurement system relative to the M POS positioning means. The distance and angular offsets between the MIR MRB target on the marble slab and the MIR1 target of the inertial navigation system are determined during calibration.

[0040] Step B) of the calibration process can be carried out using sub-steps identical to those described above for step A). ​​The reference inertial navigation system INS_REF then undergoes the same calibration steps as the first inertial navigation system.

[0041] As for step C) of the calibration process, it can be carried out on an MRB* marble containing a first target MIR*1 whose position on the marble is known and a second target MIR*2 whose position on the marble is also known.

[0042] See Figure 5.

[0043] Step C) may therefore include the following sub-steps: - place the BRA alignment arm equipped with its CAM1, CAM REF cameras on an SPT support placed on the MRB* marble by placing the first target MIR*1 in the field of vision of the first CAM1 camera and the second target MIR*2 in the field of vision of the second CAM REF camera; - provide a computing unit UC and connect it to the two cameras CAM1, CAM REF; - determine, using the computing unit CU, the distance and angular offsets between each camera and the target that is in its field of vision, then - determine, with the computing unit UC, the distance and angular offsets between the two cameras CAM1, CAM REF.

[0044] Step C) according to these sub-steps can be carried out insofar as the position of the marble target MIR*1 relative to the marble target MIR*2 is known.

Claims

Demands [1] Inertial navigation alignment device (D) for a vehicle, the device comprising: - a first inertial navigation system (INS1) intended to be carried in the vehicle, said first system being equipped, on one of its external faces, with a first target (MIR1); - a second inertial navigation system (INS_REF), called the reference system, which has a higher accuracy than the first inertial navigation system (INS1) and is also adapted to measure the direction of the Earth's axis of rotation, said reference inertial navigation system being equipped, on one of its external faces, with a target called the reference target (MIR_REF); - an alignment arm (BRA) equipped with a first camera (CAM1) at one end and a second camera (CAM REF) at the other end, said alignment arm being configured so that it can be installed by placing the first target (MIR1) in the field of view of a first camera (CAM1) and placing the reference target (MIR REF) in the field of view of a second camera (CAM REF); and - a computing unit (CU) connected to the first inertial navigation system (INS1), to the reference inertial navigation system (INS_REF) and to the cameras (CAM1, CAM REF) so that said computing unit can align the navigation of the first inertial navigation system (INS1) with the navigation of the reference inertial navigation system (INS_REF). [2] Method for aligning inertial navigation of a vehicle implemented with a device according to claim 1, said method comprising the following steps: a) placing one end of the alignment arm (BRA) equipped with the first camera (CAM1) inside the vehicle so that the first target (MIR1) of the first inertial navigation system (INS1) is in the field of vision of the first camera (CAM1); b) position the reference inertial navigation system (INS_REF) so that the reference target (MIR REF) is in the field of view of the second camera (CAM REF); c) connect the first inertial navigation system (INS1), the reference inertial navigation system (INS_REF) and the cameras (CAM1, CAM REF) to the computing unit (CU); d) perform an autonomous alignment of the reference inertial navigation system (INS_REF); e) align the inertial navigation of the first inertial navigation system (INS1) using the inertial navigation provided by the reference inertial navigation system (INS_REF). [3] Method according to the preceding claim, wherein step e) comprises the following substeps: ei) measuring a distance and angular offsets between the first target (MIR1) of the first inertial navigation system (INS1) and the first camera (CAM1); 62) measure a distance and angular offsets between the reference target (MIR REF) of the reference inertial navigation system (INS_REF) and the second camera (CAM REF); es) from the information from substeps ei) and 62), deduce a distance and angular offsets between the first inertial navigation system (INS1) and the reference inertial navigation system (INS_REF); then 64) insert the inertial navigation data from the reference inertial navigation system (INS_REF) into the first inertial navigation system (INS1), by compensating for the distance and angular offsets determined in substep e3). [4] Method according to any one of claims 2 or 3, wherein in step e) an uncertainty rate of the inertial navigation alignment of the first inertial navigation system (INS1) is also defined. [5] A method for calibrating a device according to claim 1, comprising the following steps: A) measure a distance and angular offsets between the first target (MIR1) and gyroscopes of the first inertial navigation system (INS1); B) measure a distance and angular offsets between the reference target (MIR REF) and gyroscopes of the reference inertial navigation system (INS_REF); C) measure a distance and angular offsets between the two cameras (CAM1, CAM REF) located at the respective ends of the alignment arm (BRA). [6] Calibration method according to the preceding claim, wherein any one of step A) or step B) is carried out on a marble slab (MRB) comprising a target (MIR MRB) of the marble and positioning means (M POS) for an inertial navigation system (INS1, INS_REF) whose position relative to the target of the marble is known, the method comprising the following substeps: - install the inertial navigation system (INS1, INS_REF) equipped with its target (MIR1, MIR REF) on the positioning means (M POS) of the marble (MRB), the gyroscopes of the inertial navigation system then presenting a known position relative to the positioning means; - position a camera so that the marble target (MIR MRB) and the inertial navigation system target (MIR1, MIR REF) are in the camera's field of vision; - provide a computing unit (CU) and connect it to the inertial navigation system and the camera; - determine, using the calculation unit (CU), a distance and angular offsets between the target (MIR MRB) of the marble and the target (MIR1 , MIR REF) of the inertial navigation system; - determine, using the computing unit (CU), a distance and angular offsets between the target (MIR1, MIR REF) of the inertial navigation system (INS1 , INS_REF) and the gyroscopes of the inertial navigation system knowing a position of the positioning means (M POS) relative to a position of the marble target, a position of the gyroscopes of the inertial navigation system relative to the positioning means and, a distance and angular offsets between the marble target (MIR MRB) and the target (MIR1 , MIR REF) of the inertial navigation system. [7] Calibration method according to claim 5 or 6, wherein step c) is carried out on a marble (MRB*) comprising a first target (MIR*1) whose position on the marble is known and a second target (MIR*2) whose position on the marble is also known, said method comprising the following substeps: - place the alignment arm (BRA) equipped with its cameras (CAM1, CAM REF) on a support (SPT) placed on the marble (MRB*) by placing the first target (MIR*1) in the field of vision of the first camera (CAM1) and the second target (MIR*2) in the field of vision of the second camera (CAM REF); - provide a computing unit (CU) and connect it to the two cameras; - determine, using the computing unit (CU), a distance and angular offsets between each camera and the target that is in its field of vision, then - determine, with the computing unit (CU), a distance and angular offsets between the two cameras (CAM1 , CAM REF).

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