Method and device for local stabilization of a radiation spot on a remote target object

Abstract

A method for local stabilization of a radiation spot formed by a high energy laser beam includes receiving radiation reflected by the target object, where the radiation reflected by the target object passes through the same optical path as the high energy laser beam. An image processing is performed by analyzing and comparing the image of the illuminated target object or part of the illuminated target object to at least one image of the illuminated target object or part of the illuminated target object produced at a prior point in time. A correction signal is computed, with which an optical correction device is actuated. A filter correction signal is produced by a filter device, while a controller correction signal is produced by a fine tracking controller. Finally, the correction signal is formed from the filter correction signal and the controller correction signal.

Claims

1. A method for local stabilization of a radiation spot on a remote target object, wherein the radiation spot is formed by a high energy laser beam that is aimed at the target object by a high energy radiation emitter, the method comprising: illuminating the target object by an illumination beam that is aimed at the target object by an illumination device; receiving, by an image acquisition device, radiation reflected by the target object that is illuminated by the illumination beam, wherein the radiation reflected by the target object to the image acquisition device passes through the same optical path as the high energy laser beam; performing an image processing by analyzing and comparing an image of the illuminated target object or part of the illuminated target object acquired by the image acquisition device to at least one image of the illuminated target object or part of the illuminated target object produced at a prior point in time or to an image stored in an object database; determining a correction signal, based on said comparing, with which an optical correction device arranged in the optical path passed through by both the high energy laser beam and the reflected radiation is actuated; providing a result of said comparing to a filter device and to a fine tracking controller; producing a filter correction signal by the filter device; producing a controller correction signal by the fine tracking controller; and forming the correction signal from the filter correction signal and the controller correction signal.

2. The method in accordance with claim 1, wherein producing the filter correction signal in the filter device further comprising calculating a displacement of the radiation spot from a hold point for a next time interval using at the result of the image processing.

3. The method in accordance claim 1, wherein producing the filter correction signal comprises producing a filter correction signal by the filter device in a faster cycle than a cycle of the image processing, thereby reducing contour errors due to dead times.

4. The method in accordance with claim 1, wherein, in addition to the result of said image processing, at least one of measurement values of a radar measurement of the target object and measurement values of a rough tracking image processing are provided to the filter as additional measured values.

5. The method in accordance with claim 1, wherein the fine tracking regulator is called up in a cycle of the image processing, and wherein the filter device is called up with a cycle that is greater than the cycle of the image processing.

6. A device configured to locally stabilize a radiation spot on a distant target object according to the method of claim 1, wherein the device comprises: an illumination device for emitting the illumination beam onto the target object; a high energy laser for emitting the high energy laser beam onto the target object; an optical correction device that is arranged in the optical path of the high energy laser beam and is configured to be actuated by a control device; a radiation decoupling device arranged in the optical path of the high energy laser beam and that is configured to decouple radiation from this optical path that is received as a reflection of the illumination beam and to guide said radiation onto the image acquisition device; and an image processing device, connected to the image acquisition device and comprising the control device, configured to transmit an image signal therefrom, wherein the control device is configured to produce the correction signal and to transmit the correction signal to the optical correction device, wherein control device comprises the filter device and the fine tracking controller arranged parallel thereto.

7. The method in accordance with claim 2, wherein the filter model of the movement of the radiation spot has portions based on a movement of the target object and based on turbulence.

8. The method in accordance claim 2, wherein producing the filter correction signal comprises producing a filter correction signal by the filter device in a faster cycle than a cycle of the image processing, thereby reducing contour errors due to dead times.

9. The method in accordance with claim 7, wherein the correction signal is determined by summing the filter correction signal and a regulator correction signal.

10. The method in accordance with claim 9, wherein the regulator correction signal is calculated in the fine tracking regulator based on the result of the image processing.

11. The method in accordance with claim 10, wherein the regulator algorithm is adapted to the current dead time using a time stamp.

12. The method in accordance with claim 1, wherein the correction signal is determined by summing the filter correction signal and a regulator correction signal.

13. The method in accordance with claim 12, wherein the regulator correction signal is calculated in the fine tracking regulator based on the result of the image processing.

14. The method in accordance with claim 13, wherein the regulator algorithm is adapted to the current dead time using a time stamp.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic structure of an inventive device for local stabilization of a radiation spot on a remote target object; and,

(2) FIG. 2 is a block diagram of a fine tracking control circuit.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

(3) FIG. 1 schematically depicts an inventive device for local stabilization of a radiation spot on a remote target object. This device has a high energy radiation emitter 1 that emits a high energy laser beam L. This high energy laser beam L strikes the first tilted mirror 10 and from there is relayed to a second tilted mirror 12. The first tilted mirror 10 is embodied as a dichroic mirror in order to decouple the high energy laser beam from an image received on the same optical path. The second mirror 12 is a tip/tilt mirror, the angle of which is variable and commanded by a control device.

(4) The high energy laser beam L reflected on the second tilted mirror then passes through a focusing device 14 embodied, for instance, as a telescope, and strikes the outer skin of the very remotely situated and/or moving target object Z, which in the illustrated example is formed by an aircraft. In this manner the high energy laser beam L produces a radiation spot S on the outer skin of the target object Z. Using the effect of the high energy laser beam L, the outer skin of the target object Z is heated at this radiation spot S such that the structure at this location is weakened and the target object Z is destroyed or damaged thereby. One typical use is the engagement of aircraft weapons.

(5) On its path from the device V to the target object Z, the high energy laser beam L passes through the atmosphere A, in which turbulences T occur; the latter are represented schematically in FIG. 1 as a wavy line. The effect of such turbulences is that the radiation spot S on the target object Z is not location-fast, but instead deviates slightly with respect to the hold point H, sighted by a target device, on the target object Z. The result is that the radiation energy applied locally to the outer skin of the target object Z does not remain constant during the irradiation, so that the effect of the irradiation is sub-optimal. The temperature required on the outer skin of the target object Z to soften or melt the outer skin is therefore only attained after a longer irradiation period and/or only when using greater radiation energy. To attain this goal even with lower radiation energy and a shorter irradiation period, the radiation spot S on the outer skin of the target object Z must be stabilized locally. In addition to the conventional methods for target tracking that follow the radiation spot S extremely precisely, even for a moving target object Z, the problem of target point displacement of the high energy laser beam L due to turbulences must be solved.

(6) To this end, the target object Z is illuminated from essentially just as great a distance as the distance between the device V and the target object Z by means of an illumination device 2 that is formed, for instance, by an illumination laser 20. The illumination beam B emitted by the illumination laser 20 normally has a wavelength that deviates from the high energy laser beam L. The illumination laser 20 is divergent and illuminates the entire target object or at least extensive areas of the target object Z.

(7) The illumination beam B also travels a great distance from the illumination device 2 to the target Z and strikes the target object Z, specifically also on the outer skin at least in the area of the radiation spot S. From there the illumination beam B′ reflected on the outer skin of the target object Z is guided on the same optical path P to the device V that the high energy laser beam L takes from the device V to the target Z. This means that the reflected illumination beam B′ also passes through the atmosphere A and its turbulences T and therefore experiences the same optical deviations as the high energy laser beam L passing through the turbulences T at the same point in time.

(8) The reflected illumination beam B′ coming from the target object Z passes through the focusing device 14 and strikes the second tilted mirror 12, which deflects it towards the first tilted mirror 10.

(9) This first tilted mirror 10 is permeable for the wavelength of the illumination beam B so that it forms a radiation decoupling device that does not deflect the illumination beam B′ reflected by the target object Z, but instead lets it pass through. The reflected illumination beam B′ passing through the first tilted mirror 10 then strikes an image acquisition device 3 that is formed, for instance, by a high speed camera 30. The image acquisition device 3 acquires an image of an area of the target object Z or even the entire target object Z.

(10) The image signal obtained in the image acquisition device 3 is guided via a signal line 32 to an image processing device 34 that analyzes the image represented by the image signal and compares it to an image produced previously. This previously produced image may be an image acquired at a prior or earlier point in time or it may be a synthetically produced image. From a series of such comparisons of images produced successively or images acquired by the image acquisition device 3 of the area illuminated by the illumination beam B on the outer skin of the target object Z, which area is perceived via the reflected radiation illumination radiation B′, the image processing device 34 can provide a prediction about which optical influences the high energy laser beam L being radiated at this point in time is subjected to on its path through the atmosphere A. The image processing device determines from this prediction a correction signal that is sent by a control device 36, which is provided in the image processing device 34 or is connected thereto—symbolized by the open arrow K—as a control signal to a control device for the second tilted mirror 12 embodied as a tip/tilt mirror.

(11) Naturally, with minor concessions to accuracy, it is also possible to determine the correction signal directly from a comparison of the image signals of the most recently received images of the illumination point without a prediction being provided and then being used as the foundation for the correction.

(12) The second tilted mirror 12 thus forms an optical correction device and compensates the optical effects that are expected on the path between the device V and the target object Z that are caused essentially by the turbulences T. In this manner a fine tracking circuit is formed that compensates the damaging effects of the turbulences T on the high energy laser beam L (and naturally also on the reflected illumination beam B′) so that the radiation spot S formed by the laser beam L on the outer skin of the target object Z remains nearly locationally constant.

(13) In the device depicted in FIG. 1, light returned from the target is received and evaluated in a high speed camera in the same optical canal or on the same optical path that the high energy laser passes through. On the basis of this a correction of the high energy laser beam is made using, for instance, a tip/tilt mirror embodied as a piezo mirror as the optical correction device. Although the target object in the illustrated example is illuminated by means of an illumination laser, other forms of illumination may also be used, including for evaluating the back reflection due to solar irradiation.

(14) The core point of the described inventive method, illustrated as an example, is the comparison of a so-called template image of an object (the target object or part of the target object) with the image currently being acquired by the image acquisition device 2 (called the fine tracking image in the following). The goal is to determine the parameters of a transformation that reproduces the template image optimally, in the context of a certain quality, on the current fine tracking image. The minimum quadratic error amount of the pixel deviations may be used for this, for instance. At this point it is also possible to weight the influence of certain parameters of the transformation in the error amount differently or to give more or less weight to certain pixel deviations. In addition, a hierarchical optimization in which the components of a transformation (e.g. rotation, translation) are successively optimized is also possible. The transformation itself is called “warping” and the parameters of the transformation are known as warping parameters.

(15) The transformation can permit various degrees of freedom. As a rule, rotation, translation, extension or compression, and shearing of the template image are permitted. In this case, six warping parameters must be determined. However, projective or other transformations are also possible. In this case the number of warping parameters may vary. For the application, however, the path of reducing the warping parameters proved useful. Depending on the encounter geometry and target trajectory, certain effects such as shearing or compression do not occur, so that the use of fewer warping parameters (for instance three or five) may be useful.

(16) If the hold point (the sighted point on the target object Z that the high energy laser beam L is supposed to strike) is defined in the template image, the hold point may be transformed into the current image using the determined transformation instructions and the displacement that the control device 36 uses to calculate the correction signal may be determined directly.

(17) The warping parameters are also preferably used for calculating correction signals for actuating a deformable mirror for compensating higher modes (corresponding to a deformation of the radiation spot formed by the high energy laser).

(18) The template image itself may be a generic image of the target object Z that is produced in advance and stored in a memory of the image processing device 34. Alternatively, the template image may be an image generated using an image sequence initially recorded by the image acquisition device 3 (prior to initiating the engagement by activating the high energy laser). Finally, the template image may alternatively also be formed using a selection, made by an operator, of an excerpt from the fine tracking image. Which of these alternative methods is employed depends, for instance, on the type of target object. For rapidly moving target objects, the selection must be made automatically; for slow target objects it is reasonable for the operator to decide. The same applies to the selection of the hold point, which may be provided automatically or by the operator.

(19) During the course of the engagement, the view of the object may change profoundly, for instance due to a flight maneuver of the target object, so that the underlying template image is no longer appropriate to the situation. To counter this effect, the template image may be modified using information from the fine tracking image—the template is then designed to be adaptive. The portion of information from the fine tracking image that is used to modify the template may be limited (“learning rate”).

(20) Since, in general, the template image contains fewer pixels than the image acquired by the image acquisition device, and since the calculation complexity and thus also the required computing time increases with the number of pixels, the template is preferably “warped,” that is, subjected to a transformation. Since the warping must occur during the course of optimization in every iteration step, this significantly reduces the calculation complexity. In principle, however, the reverse is also possible. In addition, complexity can be spared in that the pixel resolution of the template image is reduced.

(21) Under the aspect of correcting the influence of turbulence, a spatially limited area (characterized by the Fried radius r.sub.0) around the hold point on the target object is critical. If the target object is large compared to this area, it may be reasonable to use a so-called sub-template that contains only this area of the target object. In this case, the template image is used for determining a rough orientation (primarily for correcting the movement of the target). Then another detailed orientation is determined using the sub-template (primarily for correcting the movement due to turbulences). Like the selection of the template, the selection of the sub-template may also be made in different ways. Automatic selection of an area around the pre-specified hold point, the size of which is selected as a function of r.sub.0, seems particularly reasonable.

(22) The described device produces photographic images of the target object that, using the warping method, are the basis for determining a hold point. Simultaneously, a high energy laser spot is radiated onto the target in the same optical canal or on the same optical path through which the light for the photographic images is received.

(23) The signal processing in a control circuit formed with the control device 36 in the device is depicted as a block diagram in FIG. 2. The control device is downstream of the image processing device 34 has a filter device 38 and a fine tracking controller 39 as controller device. As may be seen from FIG. 2, the filter device 38 and the fine tracking controller are switched in parallel, not in series.

(24) Both the filter device 38 and the fine tracking controller 39 obtain signals from the image processing device 34 via corresponding signal lines. A filter result signal E is conducted from the filter device 38 back to the image processing device 34. The output signals of the filter device 38 and of the fine tracking controller 39 are conducted to the control device 11 of the second tilted mirror 12 as correction signals K.

(25) The manner in which the controller device 36 works is described in the following.

(26) Once the image has been acquired using the image acquisition device 3, the hold point H in the current fine tracking image is determined in the image processing device 34 using a suitable image processing method. Iterative methods in particular that calculate an affine transformation using template images appear suitable for this (for instance the warping methods described in the foregoing).

(27) The results of the image processing, specifically the hold point position in the current fine tracking image and other parameters of an affine transformation, for instance, and, if warping methods are used, the warping parameters, form the input variables for both the filter device 38 and the fine tracking controller 39. Using a filter provided in the filter device, the image information is combined with the movement information J supplied to the filter device 38. During this combination, information for instance about the target object that is obtained using measurements by a radar observing the target object or that is obtained as results of rough tracking image processing (not shown) flow in as movement information J. In addition, depending on the target object type and on the maneuver that the target object is performing, different filters may be used and calculated in parallel, and the best filter for the current situation may be selected using a suitable strategy.

(28) The filter results are used to pre-control the piezo-mirror corresponding to the estimated target movement in order to minimize contouring errors in this manner. In the (theoretically) ideal case, when the filter model exactly matches to the movement of the high energy spot on the target, the disturbances are thus compensated completely, without the control algorithm becoming active.

(29) In practice, however, deviations occur due to modelling errors and unmodeled dynamics. Suppressing these deviations is the task of the fine tracking controller 39. Since the filter device 38 in the structure illustrated in FIG. 2 is used in parallel to the fine tracking controller 39, the control algorithm may continue to react to higher frequency portions.

(30) In addition to the current hold point position in the image, the control algorithm also obtains the information about the dead time caused by the image processing method, such as for instance image integration, so that the control algorithm may be adapted appropriately.

(31) The filter results are also used to support the calculation of the image processing in that the image processing is initiated in a suitable manner. This is especially advantageous for iterative methods (for instance, for warping methods). This increases the robustness relative to a target loss and minimizes the number of necessary iterations for the image processing method.

(32) Reference numbers in the claims, description, and drawings are merely intended to facilitate better understanding of the invention and shall not limit the protective scope.

REFERENCE LIST

(33) 1 High energy laser 2 Illumination device 3 Image acquisition device 10 First tilted mirror (radiation decoupling device) 11 Control device 12 Second tilted mirror (optical correction device) 14 Focusing device 20 Illumination laser 30 High speed camera 32 Signal line 34 Image processing device 36 Controller device 38 Filter device 39 Fine tracking controller A Atmosphere B Illumination beam B′ Reflected illumination beam E Filter results H Hold point J Movement information K Correction signal L High energy laser beam P Optical path S Radiation spot T Turbulences V Device Z Target object