FR2739461A1 - Video image collection system for passive IR detection - Google Patents

Abstract

The mechanism includes an input objective lens (1) which is carried by a frame (2) which is rotated by a circular motor (10). A second frame supports the lens and is driven by an elevation motor (11). An image capture lens (3) solidly attached to the frame maintains image centring in the plane of the detector (4). The detector is solidly attached to the frame (5), as are the drive motors for the mechanism. In the case of the elevation motor, the linkage is provided by a lever mechanism (17-16) with adjustable angle couplings at the ends of the lever. A circular optical scanning device (7) may be provided and combined with an optical sight axis control system.

Description

VIDEO IMAGING DEVICE,
USEFUL FOR PASSIVE INFRARED SELF-DIRECTING
The present invention relates to a video imaging device, the application of which is more particularly envisaged for producing a passive infrared seeker.

 The solution generally used for this device consists in placing the detector and its optics on the same support orientable along two orthogonal axes. The support thus equipped is most often stabilized by gyroscopic effect, either directly by the gyroscope router placed on the support formed by a gimbal, or indirectly by mechanical or electrical connection with a stabilized platform. Under these conditions the optical axis, constituting the aiming axis of the device, can move with respect to a reference axis which is constituted by the longitudinal axis of the machine in the case of a seeker , this axis being linked to the body of the machine which supports the entire imaging device. This assembly requires connections by electrical connection between the mobile part and the body of the machine, in particular to the circuits for processing and processing the detected signals. These arrangements therefore have drawbacks due to the parasitic torques introduced by the links, without also taking into account the requirements due to the installation of a cooling device for the detector.

 I1 is known from French patent document 2492516 a solution which makes it possible to mount the detector 'fixed on the frame which supports the assembly, the frame corresponding to the body of the machine in a self-directing embodiment. Since the detector device is no longer carried by the structure that can be oriented in elevation (or elevation) and in deposit (or circular), this results in a great simplification of the equipment and the improvement of its performance.

 According to this solution, the video imaging device also comprises means for image transfer making it possible to maintain the position of the center of the image invariable in the detector plane in the presence of elevation and circular rotations. These image shifting means are made up with plane reflecting mirrors or assemblies equivalent to prisms, or else using ordered optical fiber bundles. This image offset optic, although ensuring the stability of the optical axis at output, introduces on the other hand a rotation of the image which is a function of the rotations printed along the axes of the orientable assembly, so-called rotations in elevation and in circular of the optical axis. To compensate for this image rotation, it is necessary to equip the device with angular sensors measuring the elevation and circular rotations and with compensation means performing the necessary correction on the basis of the detected angular values. These compensation means can be of various kinds, the solution adopted can be electronic or optical.

 The aforementioned solutions have limits with regard to the cumulative preservation of various interesting operational characteristics which are, a large angular movement in elevation and in circular without altering the pupil of the device, the latter having to be as large as possible, and obtaining low inertia of the steerable crew.

 The purpose of the present invention is to use the aforementioned techniques by adapting them to a solution which makes it possible to hold all of these characteristics together for high values, in particular the angular deflections can reach 60 to 70 "and the pupil n is practically unaffected by elements such as fixing, motors, angular sensors, or others, during these deflections. In addition, by a low inertia the crew allows a very fast response and the assembly can be used to perform a target search phase.

 According to the invention, it is proposed to produce a video imaging device, comprising an optical assembly for producing a field image observed in the photodetection plane of a detector device, this optical assembly grouping an input objective carried by a mounting orientable to at least two degrees of freedom with means for driving in rotation around two perpendicular mechanical axes so as to orient the optical sighting axis around a center of rotation, and an image shift optic for preserving the centering of the image in the detection plane, the detector being integral with a frame which also supports this optical assembly by means of the orientable mounting, and means for compensating for the image rotation due to the offset optics, these compensation means comprising angular sensors of said perpendicular rotations, characterized in that the drive means comprise two motors rs, these motors as well as the angular sensors being available outside the assembly and mounted fixedly attached to the frame, so as to produce an orientable head with low inertia and having large angular deflections.

 In the infrared domain and in particular in the bands 3 to 5 microns and 8 to 12 microns, there are currently no matrix detectors or suitable tubes, it is therefore necessary to use a bar detector and to scroll the field image in front of the linear detector array, an optical deflection device must be provided. Some solutions scroll the image linearly in a direction perpendicular to the bar, thus producing a Cartesian scan in X and Y. A solution of this kind is described in French patent document 2477349, it uses a crown of reflective dihedrons. According to other solutions, a circular scan is produced by rotating the image around a center, the bar being arranged radially from this center. On this subject, we can consult the French patent document 2492616 which describes a solution. with cylindrical lenses, or French patent document 2528981 which uses a rotating reflector dihedral.

 According to another aspect of the invention, the video imaging device used in the infrared field will also be equipped with an optical deflection system to produce the desired linear or circular scanning at the level of the detector bar.

The features and advantages of the invention will emerge from the description which follows, given by way of example with the aid of the appended figures which represent:
- fig. 1 - a general diagram of a video imaging device according to the present invention;
- fig. 2 - the reference triedes of the frame and the optical sight;
- figs. 3 to 6 - detailed diagrams of the circular and elevation drive mechanisms of the swiveling assembly which supports the receiving optics;
- fig. 7 - a simplified diagram showing the incorporation of circular image scanning means;
- fig. 8 - an interesting search scanning mode for the imaging device;
- fig. 9 - a general block diagram of the orientable head and of the associated processing and control circuits.

 Referring to the general diagram of FIG. 1, the video imaging device comprises an input objective 1 carried by an orientable assembly with two degrees of freedom 2, an image offset optics 3 and a detector 4. The assembly orientable is a gimbal assembly which comprises a first frame 21 which can be oriented around an axis Z called the circular axis, and a second frame 22 which rotates around an axis perpendicular to the axis Z. This second frame supports the optics input 1 and part of the image shift optics. We have considered the image offset optics constituted by a catadioptric formula using five plane reflecting mirrors. The mirrors 31, 32, 33 are integral with the frame 22 and used to reflect the optical input axis in the direction of rotation of the frame 22. A fourth mirror 34 positioned at the center O of the gimbal allows the return in the other direction of rotation. Finally, a last mirror 35 secured to the fixed frame 5, or body of the missile, returns the optical axis in its terminal direction towards the detector 4 itself fixed to the frame 5.

 Block 6 represents all of the supply circuits, of processing of the detected signals and of exploitation of these signals.

In the case of a detector strip, the optical deflection means are represented by the optics 7 which performs the field scanning, this optics being driven by a motor 70 to which an angular sensor 71 is coupled.

 In order to ensure large angular deflections and preserve the pupil of the device, the driving of the frames 21 and 22 is carried out by external drive members. With regard to the circular rotation about the Z axis, this rotation is symbolized in the diagram by a motor 10 mounted on the axis and whose outer cage, that is to say the stator, is mounted fixed, integral with the frame 5. I1 is the same for the rotation in elevation around the axis Y which is made using a motor 11 with a fixed outer cage and which drives the frame 22 using a particular assembly with rod and crank which will be described later. The other elements indicated comprise the angular detectors 12 and 13 of the elevation and circular rotations, these detectors being shown directly coupled to the motors 10 and 11. The detected angular values are used in particular to produce the image rotation corrections due to the device. offset 3. The corresponding means are not shown and can be included in the set 7 and 70.

Figure 2 is used to define the axes of this equipment. We consider X, Y and Z as the normalized reference trihedron of the machine and the trihedron X1, Y1 and Z1 that integral with the orientable input optics, the direction X1 representing the optical axis, the line of sight of the equipment. On the configuration shown, we considered a first circular rotation e I around the axis Z, according to which the aiming axis came in the intermediate position Xi, and an elevation rotation of value g2 2 around the axis Y1 according to which the final configuration X1, Y1 is obtained,
Z1 of the gear trihedron. It goes without saying that the order of rotation can be reversed.

 Thus the receiving optic I and the catadioptric head optic constituted by the mirrors 31, 32 and 33 being integral with the frame 22 undergo the two rotations 1 and 2. The mirror 34 which is integral with the frame 21 only undergoes the rotation 1 The mirror 35 is fixed, mounted integral with the frame 5.

 The detector 4 is shown associated with its cooling system 40, the assembly being fixed to the structure 5.

 The optical system of the assembly further comprises field or additional lenses contributing to the correction and the formation of a good quality image on the detector, these additional elements are not shown in this simplified diagram.

 The receiving optics are preferably carried out according to a Cassegrain assembly with a concave main mirror and a flat or convex secondary mirror. As shown in FIG. 2, the optical receiving axis passes through the center O of the gimbal which constitutes the instantaneous center of rotation of the orientable mobile assembly.

 FIG. 3 shows the arrangement of the drive motors for the frames 21 and 22 of the gimbal assembly. This assembly was preferred because it ensures a natural decoupling of the small movements of the machine around the axes Y and Z. The motors controlling the two movements are with a fixed external cage secured to the frame 5 supporting the assembly, or body of the in the case of a seeker. The motor 10 directly controls the Z axis by means of a belt 13 and end pulleys 14 and 15. This drive can be carried out in the ratio 1/1 shown. The second movement is accomplished by means of the motor 11 which controls the axis Y1 of the mobile assembly to perform the elevation rotation e2 around this axis by means of a crank-type assembly. The crank 16 is driven by the rotation of the motor 11 and it drives the frame 22 by means of the link 17.

This mounting is only possible thanks to coupling systems with two degrees of freedom and 19 at the ends of the link on the frame side 22 on the one hand, and crank side 16 on the other hand.

 The rotation 63 of the motor 11 is not equal to the rotation e2 to be obtained around the axis Y1 but it is a known function which involves the parameters of rotation 9 1 and 62 as well as the mechanical parameters L length of the link 17 and R distance from the link to the center O of the universal joint on the attachment side to the frame 22, as well as R ′ distance from the link on the attachment side 18 to the axis of rotation of the motor 11. This function is determined by application of the analytical geometry. The position control not shown in this representation comprises additional circuits with a secondary loop which is provided in such a way that when the rotation e 1 has been controlled the crank turns so as to obtain and keep the desired value | 2 2 constant.

Angular position sensors not shown are mounted on the axes of the motors so as to measure the rotations e 1 and 03 and to know the angles of rotation e 1 around Z and e 2 around
Y2. The rotation e 2 being a function of the two rotations e 1 and 63, and of the values L, R and R 'mentioned above.

 As can be seen in FIG. 3, the arrangement of the motors is not arbitrary. If we consider the plane P1 passing through the axes OX and OZ, median plane of the assembly for the values 61 and -3 equal to zero, the axis of the motor 10 is included in this plane and the axis of the motor 11 has a direction parallel to that of Y in which it is arranged so that the drive by the link 17 is in this reference position substantially in the median plane.

 This arrangement of the mechanical orientation assembly as well as its operation with the articulations of the link will be highlighted in the following and with the aid of FIGS. 4 and 5.

The joints 18 and 19 at the ends of the link 17 are constituted by two-axis assemblies of the gimbal type, or equivalent, and an axis A1 is considered on the side of the frame 22 axis along which the articulation 19 can rotate, this articulation comprising a second axis A10 perpendicular to the previous one according to which it can rotate in a plane perpendicular to Al. Joint side 18 A2 is also called a first axis of the joint which corresponds to a small mechanical axis at the end of the crank pin 16, the joint 18 comprising a second axis of rotation A20 perpendicular to A2. Note that the axis A2 which is integral with the crankpin has a fixed direction parallel to that of the axis of rotation
Y2 of motor 11 and its direction is therefore parallel to that Y of the reference trihedron. For zero initial values e 1 and e 2, the frames 21 and 22 are in the position indicated in FIG. 3, the axes Y1 and Y2 are parallel as well as the axis Al and the axis A2. When then the frames undergo circular rotations el and in elevation e 2 the relative positions of the axes Al and A2 change, these axes no longer being parallel which is made possible by the cardan end assemblies. FIG. 5 shows a traditional embodiment of a two-axis gimbal. Figure 6 shows another embodiment with a gimbal with pivots at one end. It is also possible to adopt other types of embodiments, for example simple ball-joint or ball-joint devices according to known techniques.

Considering a use in the infrared field by means of a linear detector, the device comprises between the output of the image offset optics and the detector an optical assembly 7 which performs a rotary scan. This scanning can be carried out as it was said previously using cylindrical lenses or by means of optical systems, Wollaston prism or equivalent, such as a formula with three mirrors recalled in FIG. 7 where an optic has been considered. Cassegrain type input 1, a complementary convergent objective IC to reproduce the radiation in the form of a beam parallel to the input of the image transfer device 3 and the rotary scanning assembly 70 constituted by the three plane mirrors 72 , 73 and 74. The assembly is rotated about the optical axis of the image transfer device output by the motor 71 which prints a constant speed of rotation w whereby the image rotates in the plane of detection at speed 2 w and so that all of its points are successively analyzed by the strip 4, each photodetector element analyzing the points located at a corresponding distance from the center
C of the image.

The combination of the rotary scan which has just been recalled and the orientable optical assembly which makes it possible to move the line of sight relative to the reference axis X of the system, makes it possible to produce interesting search scans. Figure 8 shows an example of a scan that can be obtained. The research is carried out around a direction and with defined angular amplitudes. The two drive motors in circular and elevation are controlled for this purpose so as to describe each of the sinusoidal oscillations offset by 90 to produce figures of
Lissajoux. With amplitudes in circular and in elevation which are slowly increasing then slowly decreasing as a function of time so that the center of the field describes an expanding Archimedes spiral then in contraction we obtain a slow scanning diagram of the center of the field as indicated in Figure 8. In addition, depending on the requested search pattern, this scan can be circular as shown or elliptical; in the latter case the circular and elevation amplitudes are different.

 The rapid scanning of the instantaneous field (field scanning at speed 2 superimposed on the preceding movement leads to a figure of the type represented. On this one is represented at a point the exploration density produced by the circular scanning 70 at point M In fact, with the spiral expanding and then decreasing, the exploration density is satisfactory for the device. With this type of scanning, the research exploration is obtained in low times, for example in a little more than one second per spiral for a high angular field of view of up to 200 on 3oxo.

The opto-mechanical head described has a number of advantages which are mainly: - the decoupling of small rapid movements of the machine around the axes Y and Z; - obtaining a circular surface as large as possible which ensures large spans, since the suspension drive systems around the Z and Y axes are offset to the outside and mounted integral with the fixed structure constituted by the body of the machine; - the use of a fixed detector also secured to the body of the machine with its cooling system without causing parasitic torques which would be due to electrical wires or cooling pipes; - Obtaining very high angular deflections, of the order of 60 to 70 ′, both in elevation and circular allowing the firing of a machine under very wide conditions (shooting on very wide site (shooting on very target off-center of the longitudinal axis of the aircraft due to close combat conditions and strong incidence of the shooter, or close-range shooting with strong lateral deflections of the goal leading to trajectories at missile speed vector, and a fortiori at missile axis, very in front of the right missile-target) - minimal inertia of the moving parts which can have very rapid movements during the search - limitation of the size and in particular of the sections perpendicular to the longitudinal axis X - long storage life.

 These advantages also result from a choice of optomechanical elements constituting the different subsets of the opto-mechanical head. As regards the motorization, the torque motors controlling the direction of the line of sight are, according to a preferred embodiment of the motors with Samarium-Cobalt magnet, brushes with limited clearance capable of high peak torque, or motors with brushes according to the peak power available.

 The associated angular sensors can be linear inductive potentiometers, or resolvers. They give the viewing direction X1 of the optical axis.

 The infrared detector consists of a strip of sensitive elements in the spectral band envisaged and their exploitation is ensured by a charge transfer circuit integrated with the strip which allows the multiplexing of the elements. This strip is arranged according to a radius of the instantaneous field: in this way the rotary scanning system associated with the detector makes correspond to each element of the strip, a corresponding annular field. The strip of sensitivity of the strip can be the band 8-13 microns, or the band 3-5 microns. It is also possible to use two sensitive strips, one in the first of these strips and the other in the second, these strips being arranged in two different radii. This option allows a treatment by comparison between the detections in the two strips and target extraction on a decoy background.

The scanning technology described above makes it possible to obtain a very good homogeneity of the image and an improved sensitivity in the center. The multiplexer circuit associated with the bar allows preprocessing in the image plane, therefore a simplification of downstream electronics and wiring, and an improvement in reliability.

The cooling of the detector strip can be obtained in a known manner by a double expansion circuit Joule -Thomsonbigaz Argon and Nitrogen. Argon allows rapid cooling of the detectors and nitrogen maintains the temperature at approximately 80 Kelvin. It is also possible to envisage continuous cooling with compressed nitrogen.

FIG. 9 represents the assembly with the essential electronic processing circuits. Electronic processing is based on the exploitation of the imaging aspect of information from the opto-mechanical head. This operation is carried out digitally in order to allow greater flexibility of adaptability to the different operating conditions which may be envisaged, in the case in particular of a multifunction interception and combat missile, as well as to the different flight phases. . The general diagram block in FIG. 9 shows that the electronics include an analog part constituted by the servomechanism 61, control 62 and shaping 63 circuits, as well as unfigured supply and servitude operating circuits such as cryogenics. . The servomechanism circuits 61 allow the control of circular analysis of the instantaneous field at 7 and 70, the command and control of the line of sight rotation servos with the correcting networks necessary by commands from the motors 10 and 11. The circuit of command 62 allows the multiplexing of the detector to be controlled, the detected signals of which are then shaped in circuit 63 which has automatic gain control with continuous level correction according to known techniques to which reference may be made, in particular, to the aforementioned patents. After analogical digital coding, or corresponding decoding, in an interface part not shown, the circuits 64, 65, 66 representing the digital part comprise at 66 calculations for the search, stabilization and deviations, in 64 the preprocessing of the image comprising trimming operations, filtering signals by scanning circumference, spatial filtering by detector element, as well as changing coordinates and storing. In 65, the tracking processing includes the extraction of target at a long distance, in particular for acquisition with, for example, processing by decomposition into a certain number of gray levels, the extraction of mobile points on the ground, I contour and surface extraction, correlation with self-adjusting window for short distances. Thanks to imagery, it is possible to isolate a target element by a small tracking window and to prioritize tracking on the front sector of the target. The processing operations also make it possible to indicate the proximity of a target by comparing over time the evolution of a portion of the image, which makes it possible to obtain a possible final correction if necessary.

Claims (10)

 1. A video imaging device comprising, an optical assembly for producing an image of the field observed in the photodetection plane of a detector device (4,) this optical assembly grouping, an input objective (1), carried by a mounting orientable (2) which has at least two degrees of freedom and is provided with rotation drive means (10-11) around two perpendicular mechanical axes to orient the optical axis (Xl) of aiming around a center of rotation , and an image offset optic (3) to preserve the centering of the image in said plane, the detector being secured to a frame (5) which also supports the optical assembly by means of the orientable assembly , characterized in that the drive means comprise two motors arranged outside the assembly and mounted with the fixed outer cage, integral with the frame, so as to produce an orientable structure with low inertia and having angular deflections t very important along both axes.  2. Device according to claim 1, in which the orientable assembly is a universal joint with a first frame oriented in a circular manner and supporting a second frame oriented in elevation and which supports the input optics, characterized in that the motors comprise a motor circular to drive the first frame and a lifting motor to drive the second frame, the drive of the second frame being effected by a rod-crank assembly with mechanical couplings orientable at the ends of the crank, rod side and side respectively of the second frame.  3. Device according to claim 2, characterized in that said orientable couplings are constituted by universal joints with two axes of rotation, a first axis being integrally coupled to the drive member constituted by the connecting rod for one of the assemblies , and to the second frame constituting the member to be driven for the second mounting, the crank being integral with the second axis of said cardan joints and able to rotate around these axes.  4. Device according to claim 2 or 3, characterized in that the lifting motor drives the first frame according to an assembly constituted by a belt and pulleys of ends.  5. Device according to one of the sets of claims 2 and 4, or 3 and 4, characterized in that the drive by the circular motor is carried out in the ratio 1/1 by said belt and the drive by connecting rod-crank in elevation is carried out in a ratio which is a function of the angular deviation to be given in elevation as well as the deviation presented in circular.  6. Device according to any one of claims 1 to 5, characterized in that the image offset optic consists of plane reflecting mirrors and the device also comprises means for compensating for image rotation introduced by said plane mirrors, said compensation means comprising angular sensors for detecting the respective rotations in elevation and in circular, said angular sensors being also mounted outside, coupled to the corresponding motor and externally secured to the frame.  7. Device according to any one of claims 1 to 6, and in which are provided optical scanning means for scrolling the image in the detector plane where there is a detector strip, characterized in that said scanning means produce a rapid scan of an elementary field and the optical gimbal produces the relatively slow displacement of the center of this elementary field to cover a total field to be observed.  8. Device according to claim 7, characterized in that the slow displacement is determined to produce a spiral and that the rapid optical scanning is circular.  9. Device according to any one of claims 1 to 8, or according to all of claims 1 to 8, characterized in that the input objective is a Cassegrain assembly and that the optical gimbal is constituted by an assembly of several flat reflecting mirrors inclined at 450 on said axes of rotation.  10. Device according to claim 9, used to constitute a passive infrared seeker.