TARGET TRACKING DEVICE COMPRISING A PHOTODETECTOR WITH QUADRANTS

FIELD OF THE INVENTION

The present invention relates to a device for tracking a target using a quadrant photodetector.

STATE OF THE ART

To track a target at a distance, it is known to illuminate the target with a laser beam, so that this beam is reflected off the target and the laser echo resulting from this reflection is projected onto a four quadrant photodetector, plus simply referred to as a “four quadrant detector” or “4Q detector”.

This type of detector is conventionally on board an aircraft such as a helicopter.

FIG. 1 shows the four quadrants Q1 to Q4 of a conventional detector 4Q. The detector estimates a current position of a spot by weighting the light energies received by the quadrants Q1 to Q4, which extend around a center C. From this current position, we can deduce how the target is oriented with respect to to the aircraft 1.

However, in certain applications, in order to be able to achieve a very high deviation measurement precision (typically less than 500 microradians), the optronic assembly made up of the optical system and the 4Q detector must be designed so that the laser echo is projected on the quadrants in a very small spot T, to the point that the spot extends only in one of the quadrants of the detector 4Q (the quadrant Q4 in the example shown in FIG. 1).

In such a situation, no weighting can take place since only one of the quadrants receives light energy. We know then that the spot is generally inside the only quadrant which receives light energy, but the exact position of the spot in this quadrant cannot be precisely estimated.

So that a weighting can again be implemented, a known solution is to move the spot T so that it simultaneously covers several quadrants of the detector 4Q. Such a displacement is typically obtained by reorienting the device comprising the optical system and the detector 4Q relative to the target.

However, it is not possible to know in advance the orientation command necessary so that after application of the command at least two of the four quadrants are illuminated. Consequently, the reorientation of the device comprising the optical system and the detector 4Q is controlled manually by a user.

Generally, such a manual control imparts a rotational movement to the device comprising the optical system and the detector 4Q such that the spot follows a spiral path towards the center of the detector 4Q, as shown in Figure 1. This method is however long and tedious. In addition, this method depends on the dexterity and experience of the user.

It has been proposed in document US 2013/0070239 and in document US 3,954,340 to include in a target tracking device comprising a quadrant photodetector an optical device configured to enlarge a spot entirely contained in only one of the quadrants of a quadrant. 4Q photodetector. This optical device is in fact a defocuser which enlarges the spot by defocusing the light beam. The defocusing device comprises a defocusing lens movable in translation with respect to the photodetector 4Q.

Enlarging the projected spot causes the edge of the spot to move closer to a border between two quadrants.

If the approximation caused by the magnification is such that the spot simultaneously covers at least two of the quadrants, then a weighting can directly be implemented, and the position of the spot can be estimated accordingly. When this condition is not directly satisfied at the end of the enlargement step, a displacement of the spot must be implemented. Nevertheless, the displacement that must be implemented so that the spot simultaneously covers at least two of the quadrants becomes shorter after the enlargement step. The command that should be used to ensure this movement of the spot is therefore much simpler after the enlargement. For example, assuming that the spot can be moved on the photodetector in increments, the number of

However, enlarging the spot using a defocus has drawbacks.

A defocus is very sensitive to vibrations and to variations in temperature, which has adverse consequences on a possible harmonization between the tracking device used and an illuminator emitting the laser beam. The movement of the defocusing lens also consumes energy, and is not easy to control. Finally, a defocus has an improved transmission quality.

DISCLOSURE OF THE INVENTION

An object pursued by the invention is to be able to find more quickly the position of a spot projected onto a quadrant detector, when this spot is found confined to a single quadrant, by means of a device more robust to vibrations or to thermal less consuming electrical energy, easier to control, and having a higher quality of transmission.

There is therefore proposed, according to a first aspect of the invention, a device for tracking a target, the device comprising an optical system and a quadrant photodetector, in which the optical system is configured to project a light beam coming from of the target into a spot on at least one of the quadrants, wherein the photodetector is configured to estimate a current position of the spot by weighting light energies received by the quadrants, and wherein the optical system includes an optical device configured to , when the spot is entirely contained in only one of the quadrants, enlarge the spot. The optical device comprises a polyhedron intended to be crossed by the light beam and having several optical axes,

The polyhedron used is more robust to vibrations and to heat than a defocusing lens, and presents a better quality of transmissions.

The tracking device according to the first aspect of the invention can further include the following optional features, taken alone or in combination.

The spot can be enlarged until the spot simultaneously covers at least two of the quadrants. Thus, it is not necessary to generate a command to move the enlarged spot so that the position of this spot can be estimated by weighting the light energies received by the dials.

On the other hand, the spot can be enlarged until the spot has a predetermined diameter greater than or equal to the length of one side of a quadrant. In this case, the enlarged spot may not immediately cover several quadrants. A control for moving the spot adapted to satisfy this condition is nevertheless singularly simplified by the fact of having enlarged the spot to such a diameter.

The polyhedron may be a hexahedron, for example a cube, having three optical axes comprising respectively three diagonals of the hexahedron, the axis of rotation comprising another diagonal of the hexahedron. As a variant, the polyhedron may have two optical axes and comprise a face in the form of a quadrilateral, for example square, intended to be crossed by the light beam, the two optical axes respectively comprising two diagonals of the face and the axis of rotation being perpendicular to the face.

The tracking device may further include a multi-stable actuator configured to place the polyhedron in different angular positions, in which the projected spot has different sizes.

In addition, the following characteristics can be provided:

• the photodetector has a center having a predetermined central position,

• the quadrants are arranged around the center,

• the target tracking device is suitable for being mounted mobile on an aircraft,

The target tracking device comprises a control module configured to generate, from the estimated current position and the predetermined central position, at least one command for reorienting the tracking device relative to the aircraft, the command being adapted so that the spot moves towards the center of the photodetector along a substantially rectilinear path.

According to a second aspect of the invention, an aircraft is proposed comprising a target tracking device according to the first aspect of the invention.

According to a third aspect of the invention, there is proposed a method of tracking a target implemented by a device comprising an optical system configured to project a light beam coming from the target in a spot on at least one quadrant of the invention. 'a quadrant photodetector configured to estimate a current position of the spot by weighting light energies received by the quadrants covered by the spot, the method comprising, when the spot is entirely contained in only one of the quadrants, an enlargement of the spot by an optical device of the optical system.

DESCRIPTION OF FIGURES

Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which should be read with reference to the appended drawings in which:

• Figure 1 schematically illustrates a four quadrant photodetector onto which a light beam is projected;

• Figures 2 and 3 schematically illustrate a device for tracking a target according to one embodiment of the invention;

• Figure 4 is a perspective view of an optical device according to a first embodiment of the invention,

• Figure 5 is a perspective view of an optical device according to a second embodiment of the invention,

FIG. 6 is a flowchart of steps of a method for tracking a target according to one embodiment of the invention,

• Figures 7 and 8 each schematically illustrate a four-quadrant photodetector and two spots projected onto this photodetector during the implementation of the monitoring method according to Figure 6.

In all of the figures, similar elements bear identical references.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 2, an aircraft 1 comprises a device 2 for tracking a target. The tracking device 2 is rotatably mounted on the aircraft 1, for example by means of a ball joint or a pivot link.

The tracking device 2 comprises internal means for rotating the tracking device 2 relative to the aircraft 1. These internal means typically comprise at least one motor and a control module for the or each motor. Each motor makes it possible to put the device 2 in rotation around an axis associated with this motor. Therefore, when the internal means include several motors, the tracking device can be rotated around two different axes.

Furthermore, the device 2 for tracking a target comprises an objective 3, an optical system 4 and a photodetector 6.

A light beam coming from a target to be tracked can enter the tracking device 2 through this objective 3.

Referring to Figure 3, the optical system 4 is configured to project a light beam received by the objective 3 on the photodetector 6 in a spot T.

The photodetector 6, known in itself, comprises four quadrants Q1, Q2, Q3 and Q4 like those shown schematically in FIG. 1. The four quadrants extend around a center C of photodetector 6, the position of which, called the central position, is predetermined. Each Qi quadrant is adapted to generate an electrical signal depending on the quantity of light energy it receives per unit area.

The four quadrants can each have a quarter-circle shape, as in the example shown in Figure 1. The set formed by the four quadrants is thus in the shape of a circle, and the center C is the center of this circle. In this case, we consider by convention that the side of a quadrant is equal to the radius of this circle.

Alternatively, the four quadrants can each have a square shape. In this case, the set formed by the four quadrants is also square, and the center C is at the center of this square.

The photodetector 6 also comprises (or is coupled to) a weighter 8 configured to estimate a current position of the spot T by weighting the electrical signals generated by the various quadrants Q1 -Q4 according to a known weighting method of the state of the technical.

The optical system 4 furthermore comprises an optical device 10 configured to modify a light beam coming from the objective 3 so as to be able to modify the size of the spot constituting the projection of the light beam on the photodetector 6.

The tracking device 1 further comprises a control unit for generating and sending commands to the optical device 10, these commands being are adapted to thereby modify the light beam. The control unit is for example included in the weighter 8, or else coupled to the latter.

The optical device 10 comprises a translucent polyhedron intended to be crossed by a light beam received by the objective 3. The polyhedron has several optical axes, and is mobile in rotation with respect to the photodetector 6 about an axis different from each of its optical axes.

Such a polyhedron for modifying the beam has several advantages over a defocuser: it is more robust to vibrations or to thermal (sensitivity of the harmonization between the tracking device 1 and the illuminator), consumes less electrical energy. , and is easier to fly. In addition, the polyhedron exhibits better transmission quality than a defocusing lens.

The tracking device 1 furthermore comprises an actuator for rotating the polyhedron with respect to the photodetector 6.

The actuator is configured to place the polyhedron in different angular positions, in which the spot projected on the photodetector 6 has different sizes.

The actuator is preferably multi-stable, which allows additional energy savings.

The polyhedron can be declined in several variants.

Referring to Figure 4, an optical device 10 according to a first embodiment comprises a polyhedron 16 in accordance with the above, having three optical axes X2, Y2, 22. In this case, the polyhedron is a hexahedron: it can then be a cube, as shown in Figure 5, or a parallelepiped.

The hexahedron has eight vertices, including: two first opposite vertices passing through a first diagonal of the hexahedron, two second opposite vertices passing through a second diagonal of the hexahedron, two third opposite vertices passing through a third diagonal of the hexahedron , and two fourth opposite vertices passing through a fourth diagonal of the hexahedron.

The three optical axes X2, Y2, Z2 respectively comprise the first, second and third diagonals of the hexahedron.

The hexahedron is moreover mobile in rotation with respect to the photodetector 6 about an axis of rotation R comprising the fourth diagonal of the hexahedron.

An advantage provided by a hexahedron with three optical axes makes it possible to obtain three different sizes of spots; this provides flexibility in the implementation of the method which will be described below.

With reference to FIG. 5, a second embodiment of the optical device 10 comprises a polyhedron 18 having only two optical axes X3, Y3.

The polyhedron has in particular a face 20 intended to be crossed by a light beam received by the objective 3. The face 20 is a quadrilateral, for example a square, which has four vertices, including: two first opposite vertices passing through a first diagonal of the face, two second opposite vertices passing through the second diagonal of the face.

The two optical axes X3, Y3 of the polyhedron 18 respectively comprise the first and second diagonals of the face 20.

The polyhedron 18 is moreover mobile in rotation with respect to the photodetector 6 about an axis of rotation R3 orthogonal to the plane of the face 20. The axis of rotation R3 passes for example through the point of intersection of the diagonals of the face. .

In this variant, the polyhedron typically has the shape of a thin plate measured perpendicular to the face.

The polyhedron 18 is simpler in design and more compact than the polyhedron 16. With reference to FIG. 6, a method implemented by means of the tracking device 2 1 comprises the following steps, to follow a target.

It is assumed that the target T is illuminated by an illuminator, for example on board the aircraft 1. This illuminator is for example a laser.

A light beam generated by the illuminator is reflected on the target T, and enters the tracking device 1 through its objective 3.

The light beam received by the objective 3 is projected by the optical system 4 onto the quadrant photodetector 6 in a spot T.

At this point, the optical device 10 operates in a first mode of operation, in which the spot T formed by the projection of the light beam on the photodetector 6 is smaller than the side of a quadrant.

Of course, when the target moves relative to the tracking device 2, the spot moves on the photodetector 6 (the aircraft 1 is indeed mobile, and the target can obviously also be).

During a step 100, the photodetector 6 detects that the spot T is contained in only one of its quadrants, for example the quadrant Q1, as shown in FIG. 7. Such detection is typically carried out by the unit. weighting, by comparing the intensity of the electrical signals generated by the different quadrants of the photodetector 6 with a predetermined threshold. Such a detection occurs more particularly by noting that the three electrical signals generated by three of the quadrants (here the quadrants Q2, Q3, Q4) have intensities lower than the predetermined threshold, meaning that these three quadrants have not received energy. light, while the electrical signal generated by the Q1 quadrant has an intensity greater than the threshold, which means that the Q1 quadrant has

When such a detection 100 occurs, the photodetector 6 is not able to determine with precision where the spot is located, and in particular risks leaving the field of view of the objective 3 of the tracking device 2.

Also, when such a detection 100 occurs, the optical device 10 is reconfigured (step 102) so as to enlarge the spot located at this stage only on the quadrant Q1, and this until the spot covers not only the quadrant Q1, but also at least one of the three quadrants Q2, Q3, Q4.

In the example shown in FIG. 5, the spot T becomes after enlarging a spot referenced T ′ covering at least partially the quadrants Q1 and Q4 simultaneously.

The reconfiguration 102 of the optical device 10 comprises, for example, a generation, by the weighter, of a command for reconfiguring the optical device 10s and the transmission of this command to the optical device 10 to cause the enlargement of the spot T to the spot T '.

The enlargement caused by the reconfiguration 102 is for example stopped as soon as the weighting unit detects that at least two of the four electrical signals that it receives has an intensity greater than the predetermined threshold (i.e. two electrical signals among the four signals, either three electrical signals among the four signals generated, or the four signals). This in fact means that the enlarged spot T 'covers several quadrants simultaneously.

It is formally considered that the optical device 10 is in a second operating mode once this condition is verified.

Moreover, once this condition is verified, the weighting unit can weight the electrical signals that it receives so as to estimate the current position of the center of the spot on the detector.

Then, the control module generates, from the estimated current position and the predetermined central position, at least one reorientation command of the tracking device 2 adapted so that the spot moves towards the center of photodetector 6 along a substantially trajectory. straight.

The generated command is transmitted to the engine (s), which causes a rotation of the tracking device 2 relative to the aircraft 1. During this rotation, the spot moves towards the center of photodetector 6 along a substantially rectilinear path.

Provision can be made for the enlarging step to be carried out until the spot has a predetermined diameter. In fact, enlarging the spot too much could lead to a loss of light energy (a large part of the light beam would be projected outside the photodetector 6). This predetermined diameter is preferably greater than or equal to the length of one side of a quadrant.

Of course, it is possible that, when the predetermined diameter has been reached, the enlarged spot does indeed cover several quadrants of the photodetector 6 as assumed previously, thus making it possible to again implement a weighting of light energies received by the quadrants.

However, it is also possible that this condition is not satisfied when the predetermined diameter has been reached. With reference to FIG. 8, there is shown an example of a spot T located in the quadrant Q1. The spot T is distant from the neighboring quadrant Q4 by a distance L. At the end of the enlargement step, the spot T has become the enlarged spot T '(shown in dotted lines) having a diameter equal to the length d' one side of a quadrant. Although the enlarged spot T 'remains at a distance from the quadrants Q2, Q3 and Q4, the fact remains that this distance has been reduced by the enlargement. For example, the enlarged spot is distant from the quadrant Q4 by a distance L 'less than the distance L.

When it is detected that only one quadrant receives light energy from the beam even after enlargement (Q1 in the example of Fig. 8), a displacement of the spot towards the opposite quadrant is commanded. As the distance L ′ is less than the distance L, the command to be used remains simpler than the command which would have had to be used without the enlargement step.

Ultimately, two events can trigger the end of the enlargement step: coverage of the spot on several quadrants, or the attack

with a spot diameter of predetermined value (greater than or equal to the length of one side of a quadrant).

CLAIMS

1. A device for tracking (2) a target, the device comprising an optical system (4) and a quadrant photodetector (6) (Q1 -Q4), in which

"The optical system (4) is configured to project a light beam coming from the target in a spot on at least one of the quadrants (Q1 -Q4),

• the photodetector (6) is configured to estimate a current position of the spot by weighting the light energies received by the quadrants (Q1 -Q4),

• the optical system (4) comprises an optical device (10) configured for, when the spot is entirely contained in only one of the quadrants (Q1 -Q4), to enlarge the spot,

the tracking device (2) being characterized in that the optical device (10) comprises a polyhedron (16, 18) intended to be crossed by the light beam and having several optical axes (X2, Y2, Z2, X3, Y3) , the polyhedron (16, 18) being mobile in rotation with respect to the photodetector (6) about an axis of rotation (R2, R3) different from each of the optical axes.

2. Target tracking device (2) according to the preceding claim, wherein the spot is enlarged until the spot simultaneously covers at least two of the quadrants (Q1-04).

3. Target tracking device (2) according to one of the preceding claims, wherein the spot is enlarged until the spot has a predetermined diameter greater than or equal to the length of one side of a quadrant ( Q1 -Q4).

4. Target tracking device (2) according to one of claims 1 to 3, wherein the polyhedron is a hexahedron (16) having three optical axes (X2, Y2, Z2) respectively comprising three diagonals of the hexahedron ( 16), the axis of rotation (R2) comprising another diagonal of the hexahedron (16).

5. Target tracking device (2) according to the preceding claim, in which the polyhedron is a cube.

6. Target tracking device (2) according to one of claims 1 to 3, wherein the polyhedron (18) has two optical axes (X3, Y3) and comprises a face (20) in the form of

quadrilateral intended to be crossed by the light beam, the two optical axes (X3, Y3) respectively comprising two diagonals of the face (20) and the axis of rotation (R3) being perpendicular to the face (20).

7. Target tracking device (2) according to the preceding claim, wherein the face (20) in the form of a quadrilateral is square.

8. A target tracking device (2) according to one of the preceding claims, wherein:

• the photodetector (6) has a center having a predetermined central position, “the quadrants (Q1 -Q4) are arranged around the center,

• the target tracking device (2) is suitable for being mounted mobile on an aircraft (1),

• the target tracking device (2) comprises a control module configured to generate, from the estimated current position and the predetermined central position, at least one command for reorienting the tracking device (2) with respect to the aircraft (1), the control being adapted so that the spot moves towards the center of the photodetector (6) along a substantially rectilinear path.

9. Target tracking device (2) according to one of the preceding claims, further comprising a multi-stable actuator configured to place the polyhedron in different angular positions, in which the projected spot has different sizes.

10. Aircraft (1) comprising a target tracking device (2) according to one of the preceding claims.

1 1. Method of tracking a target implemented by a device comprising an optical system (4) configured to project a light beam from the target in a spot on at least one quadrant of a quadrant photodetector (6) (Q1 -Q4) configured to estimate (104) a current position of the spot by weighting light energies received by the quadrants (Q1 -Q4), the method being characterized in that it comprises, when the spot is entirely contained in a only one of the quadrants (Q1 -Q4), an enlargement of the spot by an optical device (10) of the optical system (4), the method being characterized in that the optical device (10) comprises a polyhedron (16, 18) intended to be crossed by the light beam and having several optical axes (X2, Y2, Z2, X3, Y3), the polyhedron (16,

12. The method of claim 1 1, wherein the polyhedron is a hexahedron (16) having three optical axes (X2, Y2, Z2) comprising respectively three diagonals of the hexahedron (16), the axis of rotation (R2) comprising another diagonal of the hexahedron (16).

13. Method according to the preceding claim, in which the polyhedron is a cube.

14. The method of claim 1 1, wherein the polyhedron (18) has two optical axes (X3, Y3) and comprises a face (20) in the form of a quadrilateral intended to be traversed by the light beam, the two optical axes ( X3, Y3) comprising respectively two diagonals of the face (20) and the axis of rotation (R3) being perpendicular to the face (20).

15. The method of the preceding claim, wherein the face (20) in the form of a quadrilateral is square.