GB2312300A

GB2312300A - Line of sight stabilising system

Info

Publication number: GB2312300A
Application number: GB9707519A
Authority: GB
Inventors: Jacques Rioland; Christian Pepin; Denis Rabault
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1996-04-16
Filing date: 1997-04-14
Publication date: 1997-10-22
Anticipated expiration: 2017-04-14
Also published as: GB2312300B; FR2747481A1; GB9707519D0; FR2747481B1

Abstract

In a system to stabilise the line of sight of an optronic device on a target, movement of the target causes the image thereof to be displaced at 16. Rotation of one of a pair of mirrors 11,12 that are parallel in their nominal position is controlled according to a user's instruction and the measured position 15 of the moveable mirror 12 so that the optical beam is brought back into alignment with the line of sight of the device. The alignment of the optronic assembly is then controlled using the angular position of the mobile mirror 12 as an input to bring its optical axis back into alignment with the line of sight to the target.

Description

LINE-OF-SIGHT STABILISING DEVICE The present invention relates to a method and device to stabilise a line of sight on a target.

It can be used, in particular, to servo-control the line of sight of sensors such as heat cameras with weak fields and high resolution, in such a way that the quality of stabilisation is compatible with the resolution of said sensors.

There are known direct stabilisation and indirect stabilisation devices used to stabilise a line of sight on a target.

A direct stabilisation device comprises a platform articulated along an elevation angle axis and a relative bearing axis. This platform supports an optronic sensor.

The assembly has an inertial configuration, i.e. a deflection by an angle e of the beam of incident light rays corresponds to the same deflection e of the platform to stabilise the optical axis along the beam of incident light rays. The drawback of this standard device is that it is limited by the disturbances introduced by the friction torque into the suspension shafts and by residual imbalances. Usually, an improvement is provided by the addition of so-called a fine stabilisation mirrors which deflect line of sight by angles corresponding to aiming error measured. This method is used to reduce the errors of primary stabilisation, but its performance characteristics remain limited by the quality of the primary stabilisation, the passband of the gyroscopes that measure the primary stabilisation error, and the passband of the feedback control loops that control the so-called fine stabilisation mirrors.

In the case of an indirect stabilisation device, the platform is fixed, linked to the carrier, and a mirror provides for the deflection of the line of sight. This device makes it possible to obtain greater precision of aim but requires the implementation of an inertia compensator.

This is a difficult and costly technique. The performance characteristics are, in this case, limited by phenomena of mechanical resonance.

The invention is aimed at overcoming the abovementioned drawbacks.

According to one asyçt of this invention there is provided a method for the stabilisation of the line of sight of an optronic device on a target formed by an optronic assembly comprising an afocal system that is swivelling and servocontrolled along an elevation angle axis, mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis, characterised in that it consists in deflecting the beam of light rays that have crossed the afocal system by two mirrors that are parallel in nominal position, one of which is fixed while the other is mobile about an axis perpendicular to the axis of the beam of the light rays going through the afocal system to be deflected, bringing the optical beam back along the line of sight, by making the mobile mirror rotate on its axis by means of a feedback control loop regulated by an instructed value to be attained and controlled by a measurement made by a gyrometer fixedly joined to the mobile mirror, and bringing the optical axis of the optronic assembly along the line of sight by a feedback control loop system regulated by the angular position of the mobile mirror.

According to another aspect of this invention there is pmvided a device for the inplsxrentation of the abave-rentiorred method.

The invention will be understood more clearly and its advantages and other features shall appear from the following description, given as a non-exhaustive example and made with reference to the appended figures, of which: - Figure 1 shows a platform for the stabilisation of a line of sight of an optronic assembly which embodies the invention; - Figure 2 shows an embodiment of a simplified optical block forming the optronic assembly of figure 1; - Figure 3 shows embodiments of devices to control the shifts of the mobile mirror and of the stabilisation platform of a line of sight according to the invention; - Figures 4 and 5 show embodiments of simplified optical blocks forming components of the optronic assemblies of variants of a device for the stabilisation of a line of sight according to the invention.

Figure 1 shows the general structure of a line of sight stabilisation device comprising an optronic assembly 1 that can be swivelled on a elevation angle axis 2 mounted on a structure 3 that can itself be swivelled on a relative bearing axis 4. The whole unit is supported by a platform 5. Torque motors 6 and 7, which are fixedly joined respectively to the structure 3 and to the platform 5, provide for the rotational shifts firstly of the optronic assembly 1 about the elevation angle axis 2 and secondly of the structure about the relative bearing axis 4. The optronic assembly 1 has an objective 8, a gyrometer-mirror unit 9 and a unit 10 consisting of an output optical system and a sensor 10.

In the embodiment of figure 2, where the elements homologous to those of figure 1 are represented by the same references, the objective 8 consists of an afocal optical system comprising a convergent group of front lenses Li and a divergent group of output lenses L2. The gyrometer-mirror unit 9 has a first plane mirror 11 and a second plane mirror 12 juxtaposed one on top of the other and positioned between the divergent group of output lenses L2 and the unit 10 consisting of the output optical system and sensor.

The direction of the plane of the first mirror 11 is inclined to the optical axis xx' of the objective 8 by a determined angle a equal to 45" for example, while that of the second mirror 12 is mobile in rotation about an axis 13 perpendicular to the axis xx'. The placing of the second mirror 12 in rotation about the axis 13 is obtained by means of a torque motor 14 and is controlled by a gyrometer 15. The unit 10 formed by the output optical system and sensor consists of a lens L3 placed before an imaging detector 16. According to this arrangement, a beam 17 of parallel light rays at input of the objective 8 emerges with the rays parallel at output of the afocal system.

After reflection, twice on the mobile mirror 12 for example and twice on the fixed mirror 11 for example, the light rays of the beam 18 remain parallel to one another. After having crossed the lens L3, the beam is focused on the imaging detector 16. In this configuration, the axis of the line of sight is accurately aimed at the target when its image appears at the centre of the imaging detector 16.

When there is a shift in aim 6, during a relative motion of the objective 8 with respect to the target, the beam 19 of light rays at output of the afocal system with a ratio n is deflected by an angle nO, with n = 4 for example. This deflection 48 is kept after reflection on the mirrors 11 and 12 when these mirrors are, in their nominal configuration, parallel to each other. After the beam has been focused by the lens L3, the image of the target on the imaging detector 16 appears to be offset with respect to the centre of the imaging detector 16. This offset corresponds to a direction of arrival of the beam offset by an angle 40 in relation to the optical axis of the lens L3.

Assuming that the mobile mirror 12 rotates about its axis 13 by an angle o in the same direction as the beam 19 of light rays, the image of the target on the imaging detector 16 is shifted by an angle 2m6, for example 40 if m, the number of reflections on the mobile mirror, is equal to 2.

This arrangement makes it possible, by a rotation in an appropriate direction of the mobile mirror 12 by an angle o to bring the target back to the centre of the imaging detector 16, this centre being located on the axis of the line of sight.

In the embodiment of figure 3, where the elements homologous to those of figures 1 and 2 are shown with the same references, the commands for shifting the mobile mirror 12 on the one hand and for shifting the optronic assembly 1 on the other hand, come from feedback loops referenced 20 and 26 within boxes formed by dashes. The feedback loop 20 of the mobile mirror 12 is controlled by an instructed value of speed 21. This instructed value comes from a remote control system, given by means of a joystick type device for example, actuated by the operator.

The imaging detector 16 sends the image aimed at by the objective 8 to a screen, not shown, that is available to the operator. When the image of the target shifts from the centre of the screen which corresponds to the axis of the line of sight, the operator communicates a motion to the joystick in the direction opposite that of the shift. There follows a signal that is converted by the remote control system into an instructed value of speed 21 designed for the feedback control loop 20 of the mobile mirror 12. The instructed value of speed 21 is compared in the feedback control loop 20 with the measurement 22 of the speed of rotation of the mirror 12 carried out by a gyrometer 15 that is fixedly joined to the mobile mirror 12. The result of the comparison is filtered by a corrector 23. After filtering, the resultant signal is amplified by an amplifier 24. The amplified signal excites a torque motor 14. This torque motor 14 drives the mobile mirror 12 in rotation. The feedback control loop 20 of the mobile mirror 12 stabilises the line of sight on the target with respect to the motions of the optronic assembly 1. To also bring the optical axis of the optronic assembly 1 back to the direction of the target, the feedback control loop 26 of the optronic assembly 1 is driven by an instructed angular value 27. This angular value is the result of an integration of the speed of rotation of the mobile mirror 12 by an integration device 25, such as a potentiometer for example, which measures the angular shift of the mirror 12 and is placed at output of the feedback control loop 20.

In the feedback control loop 26, the instructed angular value 27 is compared with the angular position 28 of the optronic assembly 1. The result of the comparison is filtered by a corrector 29. After filtering, the signal is amplified by an amplifier 30. The amplified signal excites a torque motor 6. This torque motor 6 drives the optronic assembly 1 in rotation. The current angular position 28 sets up a counter-reaction against the action of the instructed angular value 27.

In a system for stabilising a line of sight on a target along two axes, the device described here above through figures 1, 2 and 3 is complemented firstly by a second gyrometer-mirror unit identical to the gyrometer-mirror unit 9. It is inserted after the first unit 9 and before the unit 10 formed by the output optical system and the sensor. The rotational axis of the mobile mirror is perpendicular to the axis 13 and to the line of sight xx'. The device is complemented secondly by a second control device identical to the one described according to figure 3. The feedback control loop of the second mobile mirror stabilises the line of sight on the target with respect to the motions of the structure 3.

The feedback control loop 26 of the optronic assembly 1 and the feedback control loop of the same type belonging to the structure 3 are aimed at forcing the optical axis of the input afocal system 8 to follow the line of sight stabilised by the feedback control loops such as 20 of each of the mobile mirrors.

Given that this function is only a function of following, the performance characteristics in terms of stabilising the line of sight on the target are entirely defined by the internal device with mirrors, and more specifically by the gyrometrical servo-control of the internal mobile mirrors.

Variants of this line of sight stabilising device can be imagined.

A first variant, not shown, consists of the use of an inverter afocal system. The first reflection on the gyrometer-mirror unit will then be done on the fixed mirror and not on the mobile mirror. Indeed, during a shift in aim o produced by a relative motion of the objective with respect to the target, the beam of light rays at input to the afocal system rotates by an angle 0, in the clockwise direction for example. At output of the afocal system, the rotation is in the reverse, hence the anticlockwise, direction. After going through the afocal system, the first reflection on a mirror again reverses the direction of rotation of the beam of light rays. The beam of light rays, after the first reflection, thus rotates in the clockwise direction, in the same direction as at input to the afocal system. The use of the fixed mirror to obtain the first reflection makes it possible to rotate the mobile mirror, which comes into play only from the 2nd reflection onwards, by an angle o in the same direction as the beam of light rays at input to the afocal system to bring the target back into the line of sight.

A second variant, shown in figure 4, can be used for two optical channels that are separate and in reverse to each other, one inverter reception channel 31 and one non-inverter laser transmission channel 32 for example. As shown iS figure 4, it is characterised by a configuration associating two fixed mirrors 33 and 34 and a mobile mirror 35 used on both faces and positioned between the two fixed mirrors 33 and 34. As described in the foregoing variant, the first reflection of the inverter reception channel 31 is done on the fixed mirror 33.

Another of these variants, shown in figure 5, is characterised by a configuration of stabilisation for two axes 36 and 37, using only one stabilisation device internal to the gyrometer-mirror block shown in 38 within a box of dashes. The stabilisation of the second axis 37 is then obtained by a mirror 39 in an inertial configuration located upline with respect to the afocal system 40.

It goes without saying that the invention is not limited to the exemplary embodiments that have just been described, but is on the contrary capable of being made in other equivalent forms using the same principle.

Claims

1. A method for the stabilisation of the line of sight of an optronic device on a target formed by an optronic assembly comprising an afocal system , swivelling and servo-controlled along an elevation angle axis mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis wherein the method comprises deflecting the beam of light rays having crossed the afocal system by two mirrors that are parallel in nominal position, one of which is fixed while the other is mobile about an axis perpendicular to the axis of the beam of the light rays going through the afocal system to be deflected, bringing the optical beam back along the line of sight, by making the mobile mirror rotate on its axis by means of a feedback control loop regulated by an instructed value to be attained and controlled by a measurement made by a gyrometer fixedly joined to the mobile mirror , and bringing the optical axis of the optronic assembly along the line of sight by a feedback control loop regulated by the angular position of the mobile mirror

2. A method according to claim 1, wherein the deflection of the beam obtained by the two parallel mirrors of the gyrometer-mirror unit is in the ratio of magnification of the afocal system.

3. A method according to claim 1 or claim 2, wherein characterised in that the afocal system is an inverter system and in that the first reflection on the gyrometer-mirror unit is done on the fixed mirror.

4. A method according to claim 3, wherein the mobile mirror is reflective on both its faces and is common to a second gyrometer-mirror unit positioned at output of a non-inverter channel and at input of an unit formed by an output optical system unit and a sensor.

5. A method according to claim 1 or claim 2, wherein the gyrometer-mirror unit is one with a first reflection on the mobile mirror and with a last reflection on this same mirror.

6. A method according to one of claims 1 to 5, wherein the angular stepping down is done along two axes perpendicular to one another by the addition of a second gyrometer-mirror unit that is identical to the first one and has the axis of its mobile mirror perpendicular to the first mobile mirror and to the axis of the line of sight.

7. A device for the stabilisation of the line of sight of an optronic device on a target, comprising an optronic assembly swivelling and servo-controlled along an elevation angle axis , mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis , wherein the optronic assembly comprises two gyrometer-mirror units inserted in a cascade-connection between the input optical system and the objective of the sensor , each block comprises two parallel mirrors and , of which one is mobile about an axis perpendicular to the direction of the optical beam to be deflected, the two axes being perpendicular to each other, each mobile mirror being fixedly joined to a gyrometer and regulated in rotation by a feedback control loop to servo-control the axis of aim on the target.

8. A device according to claim 7, wherein one of the two gyrometer-mirror units is replaced by a mobile mirror placed upline with respect to the afocal system.

9. A method for the stabilisation of the line of sight of an optronic device on a target formed by an optronic assembly comprising an afocal system, swivelling and servo-controlled along an elevation angle axis, mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis, substantially as described hereinbefore with reference to the accompanying drawings.

10. A device for the stablisation of the line of sight of an optronic device on a target, comprising an optronic assembly swivelling and servo-controlled along an elevation angle axis, mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis, substantially as described hereinbefore with reference to and as shown in the accompanying drawings.

Amendments to the claims have been filed as follows

1. A method for the stabilisation of the line of sight of an optronic device on a target formed by an optronic assembly comprising an afocal system I swivelling and servo-controlled along an elevation angle axis mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis wherein the method comprises deflecting the beam of light rays having crossed the afocal system by two mirrors that are parallel in nominal position, one of which is fixed while the other is mobile about an axis perpendicular to the axis of the beam of the light rays going through the afocal system to be deflected, bringing the optical beam back along the line of sight, by making the mobile mirror rotate on its axis by means of a feedback control loop regulated by an instructed value to be attained and controlled by a measurement made by a gyrometer fixedly joined to the mobile mirror , and bringing the optical axis of the optronic assembly along the line of sight by a feedback control loop regulated by the angular position of the mobile mirror.

2. A method according to claim 1, wherein the deflection of the beam obtained by the two parallel mirrors of the gyrometer-mirror unit is in the ratio of magnification of the afocal system,

3. A method according to claim 1 or claim 2, wherein the afocal system is an inverter system and in that the first reflection on the gyrometer-mirror unit is done on the fixed mirror.

7. A device for the stabilisation of the line of sight of an optronic device on a target to implement a method according to claim 6, said device including an optronic assembly swivelling and servo-controlled along an elevation angle axis, mounted on a structure that is itself swivelling and servo-controlled along a relative bearing axis wherein the optronic assembly includes an input optical system, a unit consisting of an output optical system and a sensor, and two gyrometer-mirror units inserted in a cascade-connection between the input optical system and the objective of the sensor, each gyrometer-mirror includes two parallel mirrors of which one is mobile about an axis perpendicular to the direction of the optical beam to be deflected, said two axes being perpendicular to each other, each mobile mirror being fixedly joined to a gyrometer and regulated in rotation by a feedback control loop to servo-control the axis of aim on the target.

8. A device according to claim 7, wherein said input optical system is an afocal system and wherein one of the two gyrometer-mirror units is replaced by a mobile mirror placed upline with respect to the afocal system.