Method for controlling an onboard optronic device and corresponding onboard optronic device

Abstract

The invention relates to a method for controlling an optronic device (1) on board a vehicle (V), the optronic device (1) including a sighting head (5) and at least one optronic sensor (9) for capturing images of stars, the method implementing a harmonisation phase which comprises the steps of selecting a number (M) of stars in a star catalogue, and for each of the selected stars: • - pointing the sighting head (5) towards each of the stars (k) on the basis of inertial measurements, moving the sighting head (5) as required in order to centre the star in question on the optronic sensor (9) and reading the estimation of the current attitude of the sighting head (5) provided by the inertial measurement unit (10); • - defining for each star (k) a matrix bk of the estimation of the current attitude of the sighting head and a matrix ak of the coordinates of the star (k) in the frame of reference of the line of sight such that formula (I); • - setting a cost function formula (II) and searching for the optimal values of the rotation matrix T to minimise the cost function; and • - recording the values of the rotation matrix thus defined. The invention also relates to an optronic device and vehicle used to carry out this method.

Description

[0001] METHOD FOR CONTROLLING AN ON-BOARD OPTRONIC DEVICE AND CORRESPONDING ON-BOARD OPTRONIC DEVICE

[0002] The present invention relates to the field of optronic devices on board vehicles, such as for example observation sights.

[0003] BACKGROUND OF THE INVENTION

[0004] A vehicle-mounted observation sight generally comprises a turret movable in bearing and a sighting head mounted on the turret to be movable in elevation so as to be able to orient the sighting head, at least one optronic sensor mounted in the sighting head to capture images along an optical line of sight normal to the plane of the optronic sensor, an inertial measurement unit mounted in the sighting head to detect movements of the sighting head, and an electronic processing unit connected to the inertial measurement unit and to motors for driving the turret about the bearing axis and the sighting head about the elevation axis to bring the sighting head in a sighting direction and stabilize the sighting direction independently of the movements of the vehicle on which the sighting device is mounted.The inertial measurement unit comprises: linear inertial sensors, namely accelerometers, arranged along the axes of a measurement frame linked to a frame of the inertial measurement unit to provide measurement increments of components of a specific force vector; and angular inertial sensors, namely gyroscopes or gyrometers, arranged to provide measurement increments of angular movements of the inertial frame relative to an inertial frame. These measurements are used to determine an attitude in the inertial frame and a position in the local geographic frame. This attitude and this position are used to orient and maintain the aiming head towards targets (the aiming head is said to be gyrostabilized around two axes).

[0005] However, the inertial measurement unit provides measurements relative to a reference frame linked to its installation plane which is different from the reference frame of the optronic sensor and defines a so-called inertial line of sight which may be different from the optical line of sight of the optronic sensor if the two reference frames are offset from each other. To obtain a precise orientation of the optronic sensor contained in the aiming head, it is therefore necessary to estimate the difference between these two reference frames, a difference commonly called the harmonization matrix.

[0006] SUBJECT OF THE INVENTION

[0007] The invention aims in particular to provide a new method for estimating the difference between the two reference systems.

[0008] SUMMARY OF THE INVENTION

[0009] To this end, the invention provides a method for controlling an optronic device on board a vehicle, the optronic device comprising a sighting head movable in elevation and bearing, at least one optronic sensor mounted in the sighting head to provide star images, and an inertial measurement unit mounted in the sighting head to provide an estimate of the current attitude of the sighting head in an inertial measurement frame ([TMU]) and an estimate of the current position of the sighting head in a local geographical frame.

[0010] ([g]), and an electronic processing unit which is connected to a motor drive of the sighting head in elevation and bearing, to the optronic sensor and to the inertial measurement unit to orient the sighting head. The method implements a harmonization phase, prior to a nominal operating phase, which comprises the steps, implemented by the electronic processing unit, of selecting a number (M) of stars from a star catalog which is stored in a memory accessible by the electronic processing unit and which contains equatorial coordinates of the stars as a function of an observation position, and for each of the selected stars: pointing the sighting head towards the star (k) using horizontal coordinates of the star (k) determined from the equatorial coordinates of the star and the current position estimate;moving the aiming head as needed to center the star on an axis of an optical line of sight ([LoS]) reference frame normal to the optronic sensor; reading the current attitude estimate of the aiming head provided by the inertial measurement unit; defining a matrix bk of the current attitude estimate of the aiming head for the star (k) and a matrix ak of the coordinates of the star (k) in the line of sight reference frame such that b; k = with

[0011] -[fan / ]

[0012] J [LrfV| the rotation matrix between the inertial measurement frame ([IMU]) and the optical line of sight frame {[LdV]);

[0013] The harmonization phase then includes the steps of establishing a cost function and searching for optimal values ​​of the rotation matrix T to minimize said cost function; recording the values ​​of the rotation matrix thus defined to allow their use in the nominal operating phase.

[0014] Thus, the method of the invention is based on the use of stars to estimate the pointing errors between an inertial reference given by the inertial measurement unit and the actual line of sight of the optronic sensor of the device. Indeed, the directions of the stars from n r any observation point are known and documented in star catalogs and can therefore serve as a supposedly perfect reference for estimating the pointing errors of the sighting head.

[0015] According to optional features, used individually or in whole or in part in combination:

[0016] - the search for the values ​​of the rotation matrix T is carried out by setting B = Sk=ibk a k = USV T in which USV T is a singular value decomposition of B and the optimal values ​​of the rotation matrix are U diag(l,l,det(Ù)detfF))^ ;

[0017] - the number of stars selected is about 10,-

[0018] - the selected stars have an elevation greater than approximately 15°.

[0019] The invention also relates to an optronic device comprising a sighting head movable in elevation and bearing, at least one optronic sensor mounted in the sighting head to capture images of stars, and an inertial measurement unit mounted in the sighting head to provide a current attitude of the sighting head in an inertial measurement frame of reference and a current position of the sighting head in a local geographical frame of reference, and an electronic processing unit which is connected to a motor drive of the sighting head in elevation and bearing, to the optronic sensor and to the inertial measurement unit and which is arranged to access at least one memory containing a catalog of stars and a computer program comprising instructions arranged to implement this method.

[0020] Other characteristics and advantages of the invention will emerge from reading the following description of a particular and non-limiting embodiment of the invention.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS Reference will be made to the attached drawings, among which:

[0022] [Fig. 1] Figure 1 is a schematic view provided with an optronic device according to the invention equipping a vehicle;

[0023] [Fig. 2] Figure 2 is a schematic view of the contents of the aiming head of the optronic device according to the invention;

[0024] [Fig. 3] llaa ffiigguurree 33 eesstt a view showing the different reference points used and the rotation matrices between said reference points;

[0025] [Fig. 4] Figure 4 is a flowchart illustrating the method of the invention.

[0026] DETAILED DESCRIPTION OF THE INVENTION

[0027] With reference to figures 1 and 2, the invention is described here in application to an observation sight, generally designated 1, on board a vehicle V. The observation sight 1 comprises a turret 2 mounted on a base 3 to pivot around a bearing axis.

[0028] 4 and a sighting head 5 mounted on the turret 2 to pivot about a bearing axis 6. A motor 7 rotates the turret 2 about the bearing axis 4 and a motor 8 rotates the sighting head 5 about the bearing axis 6 so as to be able to orient the sighting head 5 in a sighting direction.

[0029] At least one optronic sensor 9 is mounted in the sighting head 5 and positioned behind an optical system to capture images along a line of sight normal to the plane of the optronic sensor 9. The optronic sensor 9 is known per se and has characteristics (sensitivity, definition, resolution, integration time) suitable for capturing images of the vehicle's environment and the stars present in the sky. An inertial measurement unit 10 (1MU) is mounted in the sighting head 5 to detect movements of the sighting head 5. The inertial measurement unit 10 comprises, in a manner known per se: linear inertial sensors rnamely accelerometers, arranged along the axes of a measurement frame linked to a frame of the inertial measurement unit 10 to provide measurement increments of components of a specific force vector; and angular inertial sensors, namely gyroscopes or gyrometers, arranged to provide measurement increments of angular movements of the measurement frame relative to an inertial frame. These measurements make it possible to calculate a current estimate of attitude A(t), h(t), D(t) of the aiming head and a current estimate of position L(t), (t) of the aiming head (and therefore of the vehicle V) at each current instant t.

[0030] The observation sight 1 comprises an electronic processing unit 11 connected to the optronic sensor 9, to the inertial measurement unit 10, and to the motors 7, 8. The electronic processing unit 11 here comprises, in a manner known per se, at least one processor and a memory containing computer programs executable by the processor. Conventionally in the envisaged application, the programs are arranged for the processing and fusion of data by implementing, for example, navigation calculation algorithms from quaternions and rotation matrices from one reference frame to another, one or more Kalman filter(s), image processing algorithms, target tracking algorithms, for example by deviation measurement, servo-control algorithms for controlling the motors 7, 8, etc.

[0031] The electronic processing unit 11 is arranged to ensure several conventional functions: - control the motors 77 and 88 to orient the aiming head 5 in a aiming direction and to target the optronic sensor 9 towards a target or a landmark;

[0032] - control the motors 7 and 8 from the attitude measurements of the inertial measurement unit 10 to maintain the aiming head 5 in the aiming direction independently of the movements of the vehicle V;

[0033] - determine from the attitude measurements of the inertial measurement unit 10 a bearing of the target or the landmark relative to the vehicle.

[0034] The memory of the processing unit 11 also includes a star catalog or ephemeris containing a list of stars with their equatorial coordinates in the celestial reference system (IICCRRSS) (International Celestial Reference System)er January 2000 at 12:00 (J2000.00). The use of such star catalogs and their updating by the electronic processing unit 11 to find the coordinates of stars at a given date and location on Earth are known in themselves and will not be detailed here.

[0035] The electronic processing unit 11 is arranged to provide several functions:

[0036] - from the current calculated position estimate and from the measurements of the inertial measurement unit 10, select from the star catalog at least one star visible from said current position; calculate the horizontal coordinates A and h of the selected star from the equatorial coordinates appearing in the star catalog for the selected star; control the motors 7 and 8 to orient the aiming head 55 in a direction of aim corresponding to said horizontal coordinates so as to point the optronic sensor 9 towards the selected star using the attitude measurements of the inertial measurement unit 10; drive motors 7 and 8 to correct the aiming direction by bringing the selected star to the center of the optronic sensor 5 (more precisely, to the center of the observation field of the optronic sensor 5) and memorize the attitude measurements of the inertial measurement unit 10.

[0037] As visible in figures 1 and 2 and more particularly in figure 3, we note: ~ [IMU]=(M, XIMUZ YIMU, ZIMU) the measurement frame of reference of the inertial measurement unit 10 or inertial measurement frame;

[0038] [LdV]= (C, XLCIV, Yimv, ZLCIV) the line of sight reference of the optronic sensor 9 (X L dv normal to the plane of the optronic sensor 9; Yi.dv and ZLJV in the plane of the optronic sensor 9);

[0039] [g]=(O, Xg, YY g g,, Z g ) the local geographic reference, here of the NED type; the rotation matrix (or transition matrix) between the inertial measurement frame [IMU] and the local geographic frame [g] with the attitude angle measurements (J4;,h;,D() provided by the inertial measurement unit 10; the rotation matrix between the line-of-sight frame [LdV] and the local geographic frame [g] with A and h the real horizontal coordinates of the selected star obtained by the star catalog (D is not used and does not appear in the star catalog); the rotation matrix between the inertial measurement frame [IMU] and the line-of-sight frame [l>dv] aavveecc V'y-V'x) J- esangular deviations between the two reference frames. The electronic unit ddee ttrraaiitteemmeenntt 1111 eesstt arranged to perform a harmonization phase (see Figure 4 in particular) to estimate the rotation matrix (also called harmonization matrix) between the inertial measurement frame [IMU] and the line of sight [LdV] reference frame. This makes it possible to compensate or correct the deviation between the optical line of sight (defined by the optronic sensor 9 and extending along the XLCÎV axis) and the inertial line of sight (defined by the inertial measurement unit 10 and extending along the Xi axis W). In this phase, the electronic processing unit 11 selects a number M of stars from the star catalog (step 100). The stars selected here preferably have an elevation greater than approximately 15°. This avoids problems with modeling atmospheric refraction. For an elevation greater than 15°, the refraction compensation error is negligible. For an elevation less than 15°C, to limit the impact of the refraction compensation error, it is necessary to know precisely the temperature and pressure outside the sighting head.

[0040] The number M of selected stars is at least 5 and preferably greater than or equal to approximately 10. This improves the accuracy of the harmonization matrix. The electronic processing unit then executes a loop for each of the selected stars i (each time incrementing by 1 an index i initially at zero: i=i+l). In this loop, the electronic processing unit 11:

[0041] - calculates (step 110) the horizontal coordinates A(t) and h(t) of star i determined from the equatorial coordinates «(J2000.00) and 6(12000.00) of star i and the current position estimate

[0042] L(t), <|>(t) at the current time t (defined according to the UTC standard); controls the motors 7, 88 to point the aiming head 5 towards the star i using the horizontal coordinates A(t) and h(t) of the star i and the current attitude estimate provided by the inertial measurement unit 10 to servo-control the motors 7, 8 in position (step 120); checks (step 130) whether the star in question is visible in the field of the optronic sensor 9 (the electronic processing unit 11 can recognize the star i from the magnitude and color of the star i which appear in the star catalogs); if not, returns to the beginning of the loop to move on to the star i+1; if so, commands the motors 7, 8 to move the tracking head as needed to center the star i on the line of sight of the optronic sensor 9 and performs a deviation measurement from the images provided by the optronic sensor 9 (star tracking or tracking - step 140);records the current attitude estimate Ai(t), hi(t), Di(t) of the aiming head 5 provided by the inertial measurement unit 10 at the current time t (step;

[0043] 150).

[0044] The electronic processing unit 11 therefore has the horizontal coordinates A(t) and h(t) of the star i at

[0045] at time t, of the current attitude estimate Ai(t), h £ (t), Di(t) of the aiming head 5 at time t and of the current position estimate L(t), <M t) à l'instant t.

[0046] The electronic processing unit 11 restarts this loop as long as the index 1 is less than M.

[0047] At the end of the loop, the electronic processing unit 11 has for each star i observed: the measurement angles of the current attitude estimate (Ai,ht,Dt) of the sighting head 5 and therefore We therefore know perfectly the pointing direction of the inertial line of sight at the time of the measurement, given by: [ " horizontal coordinates ÇA,K) and the position in the [WGS84] reference frame from the star catalog. We note that the theoretical slope, noted D, is not known, but we nevertheless know perfectly the theoretical direction of the star in [g], given by

[0048] " of the current position estimate L, é of the aiming head 5.

[0049] In the above and in the following, notation denotes a measure. We recall that the star is placed in the center of the optical sensor 5 so that its coordinates in the reference frame

[0050] We can pose the following equality:

[0051] Please note that in the preceding formula and those that follow, the notation () T or ai or V 3"denotes the transpose of the matrix (} OoUu or V (the notation T has nothing to do with rotation matrices T).

[0052] It is understood that the number N of stars visible among the M selected stars can be either equal to M or less than M. Preferably, oonn chooses M eenn according to the observation conditions in such a way that the number of stars N is at least equal to 10.

[0053] For all visible stars k among the N visible stars, the electronic processing unit IL calculates

[0054] We then have:

[0055] The harmonization phase then comprises the step (170) of posing a cost function and to search for T values optof the optimal rotation T to minimize the said cost function. The minimization of this function is commonly called the "WAHBA Problem" in the literature and several solutions to this problem have been proposed. wi represents weighting coefficients possibly used to give more or less weight to a measure depending on its credibility for example (they can be given the value 1 if we do not wish to use them).

[0056] One of the resolution methods is to perform a singular value decomposition of

[0057] We then note in which USV T is the singular value decomposition of B (see e.g. Markley, F. L., Equivalence of two solutions of Wahba's Problem, Journal of the Astronautical Science, 2015).

[0058] The optimal solution is then

[0059] We then deduce the values ​​of the rotation matrix 301 The electronic unit of ttrraaiitteemmeenntt 11 then records the values ​​of the rotation matrix thus defined to take into account (by correction or compensation) the deviation between the inertial line of sight and the optical line of sight during nominal operation of the observation sight.

[0060] Of course, the invention is not limited to the method of embodiment described but includes any variant falling within the scope of the invention as defined by the claims.

[0061] In particular, the optronic device may have a structure different from that described.

[0062] The optronic device may comprise more than one optronic sensor and / or more than one inertial measurement unit.

[0063] It is possible to avoid using a singular value decomposition to find the values ​​of the rotation matrix T. For example, it is possible to use a quaternion estimation algorithm such as the QUEST (or Quaternion Estimator) algorithm developed by Malcolm D. Shuster (see MD Shuster and SD OOhh,, Three-axis attitude determination from vector observations, J. Guidance and

[0064] Control, Vol. 4, No. 1, 1981). The processing functions may be grouped in a single processing unit or several. Thus, instead of a single electronic processing unit, the device may include an electronic image processing unit for identifying stars and determining the polar coordinates of the stars, an electronic processing unit in the inertial unit for providing inertial attitude and position, and an electronic processing unit calculating navigation from all the data provided by the other electronic units. The memory containing the star catalog may be incorporated in the electronic processing unit or be transferred to a server connected to the electronic processing unit by a computer network.

[0065] L r This invention can be used on any type of vehicle, land, air, space, water, etc.

Claims

CLAIMS 1. Method for controlling an optronic device: (1) on board a vehicle (V), the optronic device (1) comprising a sighting head (5) movable in elevation and bearing, at least one optronic sensor (9) mounted in the sighting head (5) to provide star images, and an inertial measurement unit: (10) mounted in the sighting head (5) to provide an estimate of the current attitude of the sighting head in an inertial measurement frame ([IMO]) and an estimate of the current position of the;aiming head in a local geographical reference ([g]), and an electronic processing unit (11) which is connected to a motorization (7 8) for driving the aiming head (5) in elevation and bearing, to the optronic sensor (9) and to the inertial measurement unit (10) to orient the aiming head (5), characterized in that the method implements a harmonization phase, prior to a nominal operating phase, which comprises the steps, implemented by the electronic processing unit (11), of selecting a number (M) of stars in a star catalog which is stored in a memory accessible by the electronic processing unit (11) and which contains equatorial coordinates of the stars as a function of an observation position, and for each of the selected stars:; - point the aiming head (5) towards 1 / star (k) using horizontal coordinates of the star (k) determined from the equatorial coordinates of the star and the current position estimate; - move the aiming head (5) as needed to center the star on an axis of an optical line of sight ([LoS]) reference mark normal to the optronic sensor (5); - record the current attitude estimate of the aiming head (5) provided by the inertial measurement unit (10); - define a matrix b* of the current attitude estimate of the aiming head for the star (k) and a matrix ar of the coordinates of the star (k) in the r•pct b ligu- the aiming head IHP with the rotation matrix between the measurement mark inertia ([IMQ]) and the optical line of sight ([LdV]); and in that the harmonization phase then includes the steps of - pose a cost function search for optimal values ​​of the rotation matrix T to minimize said cost function record the values ​​of the rotation matrix thus defined to allow their use in the nominal operating phase.

2. Method according to La T->^ncliu,t ian 1, in which the search for the values ​​of the rotation matrix T is carried out by posing in which is a lecjn.p-^i'i in singular values ​​of B and the optimal values ​​of the rotation matrix are 3. Method according to claim 1 or 2, wherein the number of stars selected is at least 5 and preferably greater than 10.

4. A method according to any preceding claim, wherein the selected stars have an elevation greater than 15 e .

5. Optronic device comprising a sighting head (5) movable in position and bearing, on at least one optical sensor (9) mounted in the sighting head (5) for capturing images of the object, and an inertial measurement unit. (10) mounted in the aiming head (5) to provide a current attitude of the aiming head (5) in an inertial measurement frame ([IMJ]) and a current position of the aiming head ((55)) in a local geographic frame {[g]), and an electronic processing unit (11) which is _<11-- a mu m"'-ru 1~H ni i , 8) for driving the aiming head (5) in elevation and bearing, to the optronic sensor (9) and to the inertial measurement unit (10) and which tï: 'jgeîjceLL poui ÛLCH-V L to ,TIiiuiiiiti a m-noit- C'U œ nar.t a star catalog and a computer piLqiarnne comprising instructions arranged to implement the control method^according to any one of the preceding claims.

6. Vehicle comprising a device according to claim 5.

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