FR2891632A1 - Missile laser guidance system comprises two optical deflectors with vibrating mirrors with frictionless mountings controled by galvanometers such that the laser beam carries out a spatial scan in the form of the petals of a flower - Google Patents

Abstract

Missile laser guidance system has a laser (1) which emits a beam that impacts on two optical deflectors (2, 3) in the form of vibrating mirrors (9, 11) controlled by galvanometers (8, 10) and supported on frictionless supports which however permit torsion or flexing and which provide quasi-sinusoidal, synchronized, amplitude modulated, deflections of the laser beam. The resultant spatial scan pattern is analogous to the petals of a flower.

Description

- 1 -

  LASER BEAM GUIDING DEVICE PERFORMING A BALAYA-GE OF ROSACEOUS SPACE.

  The invention relates to a device for remote optical guidance of a missile on a target by means of a beam transmitter, said beam coming from a laser source whose image projected at the receiver (or disparometer) mounted on the missile is a circular spot which sweeps the guide field by means of optical deflectors, said transmitter and receiver being synchronized by clocks from the missile.

  The time of each passing of the beam on the receiver during scanning is known accurately since the corresponding clock is synchronized. The law of displacement of said spot as a function of time is thus known as well as the position of the missile with respect to the guide axis. It is then enough to act on its control surfaces to bring it along this axis.

  The precision of the sweep makes it necessary to obtain a spot as small as possible. It is quickly limited in this sense by the diffraction effect due to the optics of the transmitter. Furthermore, the complete scan of the field corresponding to the information renewal time on board the missile is even faster than the task is smaller in size. We are then limited by the inertia of the scanning system. If this scanning is done in one direction, e.g. line by line (television mode scan), this results in poor resolution in the direction perpendicular to the scan due to the relative size of the spot.

  This scanning mode is used in a determination apparatus known from French Patent No. 2,516,664. In this apparatus the scanning is carried out by means of an acousto-optical deflector passing the infra-red beam through a crystal traversed by high frequency acoustic waves. The deviation through the network created by said on-ones is proportional to their frequency. This deviator is no longer limited by inertia, but there are other technological problems that are not yet under control. In order to improve the resolution, this same scan is performed in two successive directions, horizontally and vertically. The scanning speed in this mode is very high. If it is necessary, for example, 100 lines to cover the entire guide field in one direction, the accuracy of the receiver clock must be 100 times greater than for a scanning system of said field by a beam in the form of a single rectangular bar. Obtaining such a degree of precision is not currently possible in the manufacture of clocks.

  Another scanning mode is described in European Patent No. 0 102 466. The linear radial scan is achieved by essentially mechanical means such as the rotation of a reflective polygon and a DoVe prism. The necessary mechanical precision must then be very great so that the errors due to the games and the vibrations are compatible with the precision required for the guidance of a modern missile.

  The object of the present invention is to create a remote optical guiding device of the type mentioned in the preamble and which, for a given rate of information renewal, makes it possible to use beam deflection systems that can operate at frequencies relatively low, of the order of a few hundreds of Hertz, in quasi-sinusoidal mode, so as to avoid the amplitude and phase control of high order harmonics, said device implementing simpler mechanical means without bearings balls or other pivots having a significant "friction noise". - 3

  This object is achieved because said transmitter comprises two optical deflectors which respectively move the beam in two mutually perpendicular directions to animate said image of the laser source of a translation movement so as to make it scan the field rosette for guiding, each of said deviators providing a wt, in-phase and amplitude-modulated wavelength deviation respectively as sinQt and cosQt, the deflection elements of said optical deflectors not being rotated but carried by galvanometric bending motors or tension of bar (s) or blade (s) recessed (s) and without mechanical play.

  According to an advantageous embodiment of the invention, said deflection elements are constituted by a first and a second mirror vibrating at w pulsations, in phase, and whose amplitude is modulated respectively as sinQt and cosQt, the ratio Q / w being a function of the ratio of the field covered by the diameter of said spot to the diameter of the guiding field.

  Said first and second mirrors being respectively actuated by a first and a second galvanometer whose axes are perpendicular to each other and respectively perpendicular to the incident beam on the first mirror and to the beam reflected on the second mirror, the latter beam being brought back in the direction of guide beam by any deflection device.

  FIG. 1 represents the block diagram of the transmitter used in the laser beam guidance device according to the invention.

  FIG. 2 represents a sequence of rosette scan patterns executed by a laser beam projected by the device of FIG. 1.

  Fig. 3 is a timing diagram showing the sweep variation along orthogonal axes oy and oz shown in Fig. 2. - 4 -

  Fig. 4 is a timing chart showing the scaling of signals received at various points in the field.

  Figure 5 shows the block diagram of the guide receiver.

  The guide transmitter shown in FIG. 1 comprises a laser emission 1 (CO2 laser) at 10.6 microns, for example), a first optical deflector 2 which moves the beam in the plane yoz, a second optical deflector 3, which de-place the beam on a cone axis oy very close to the yox plane and a pupil transfer 4 formed of a set of two or more lenses between the two deviators. At the output of the second deviator, this beam is concentrated at 6 at the focus of the lens 5 coinciding with that of the zoom lens 7 or zoom which projects the image of the laser at the missile, that is to say say practically to infinity.

  This lens with variable focus allows to adapt the illuminated field to the desired value at any time. If the desired missile width is constant, the focal length should vary in proportion to the distance traveled by the missile.

  The image projected at the missile level is a circular spot whose half-power diameter is determined by the laws of diffraction. The energy concentration is maximum if the emission optics has the maximum diameter allowed given the possible size of the beam projector and the necessary correction of optical aberrations.

  This image is animated by a translation movement following the rosette sweep figure.

  The frequency of the scans is chosen so that the full scan time of the guide tunnel is equal to the repetition period of the desired deviations at the receiver and the distance between the scanning curves is less than or equal to the diameter of the spot. laser in the guiding field. - 5 -

  It can thus be seen that the scanning frequency must be proportional to the resolution of the transmission optics, which can cause problems with regard to the deviators. In practice we will choose the best compromise between the energy concentration and the possible scanning speed.

  There is shown in Figure 2 a sequence of rosette scan patterns corresponding to a scanning period that is to say the time during which the entire field is explored. The projected image S of the laser spot centered at the point A indicated in the figure is referenced with respect to the rectangular coordinate system oyz. By setting wt = x and fixing w / Q = 4, the coordinates of the point A in this system are written: y = A sin 4x cosx z = A sin sin sinx Figure 3 represents the variation as a function of time of the scan signal along axes oy and oz.

  FIGS. 4a and 4c respectively show the timing as a function of time of the signals received for example at the points A and 0 of FIG. 2, as well as the signals of a clock (FIG. 4b) embarked on the missile and synchronous of the scanning of diverters.

  For the sake of clarity, FIGS. 2, 3 and 4 have been drawn with a ratio w = 40. In a real case, the results 2, 3 and 4 could correspond to the following typical case: - Laser CO2, at 10.6 microns; Diameter of the emission optics: 100 mm; - Spot diameter at 4000 m: 50 cm; - Width of the guiding field: 3.5 m; - Recurrence period: T = 20 ms; Q = 314 rd / s; - Frequency of scanning 700 Hz; w = 4396 rd / s.

  Each optical deflector of FIG. 1 is advantageously constituted by a vibrating mirror actuated by a gal-vanometer, the least expensive solution for low speeds. The deflector 2 comprises the galvanometer 8, the axis of which carries the planar mirror 9 of axis ox and making in its equilibrium position an angle of 45 with the axis oy.

  The deflector 3 comprises the galvanometer 10 whose axis parallel to oy carries the plane mirror 11 having in its equilibrium position an inclination of 45 on the axis ox.

  The beam from the laser source 1 strikes the plane mirror 9 vibrating around the axis of the galvanometer 8 parallel to ox. After a first reflection on the plane mirror 9, the laser beam passes through a pupil transfer 4 and undergoes a second reflection on the plane mirror 11 vibrating around the axis of the galvanometer 10 parallel to the axis oy. Thus, when the vibrating mirrors 9 and 11 pass through their equilibrium position, the beam reflected on the mirror 11 leaves parallel to ox. A not shown return system returns this beam in the direction of the guide beam.

  To reduce the pulsation w, taking into account the diameter of the spot we can use an amplitude A slightly greater than the guide field R (Figure 2). For example, R / A = 0.8, which gives a temporal yield of 60% and makes it possible to scan the entire field of R = 3.5 m with a spot of 50 cm in diameter and a ratio w / Q = 14 The laser emission can be cut off during idle time. The cutoff of the laser emission can be performed easily at medium speed but the reignition of the laser can not operate as easily. We will therefore limit ourselves to reducing the power of emission of the laser instead of cutting off this emission when the image of the spot sweeps the edges of the field.

  The guide receiver installed aboard the missile 30 and shown schematically in FIG. 5 comprises: an optical filter 12 centered on the wavelength of the laser; an optical concentrator 13 for recovering the maximum of energy; a detector 14 adapted to the working wavelength (10.6 p with a CO2 laser); a cryogenic nitrogen or argon cooling system which can go below 100 K if necessary; a preamplifier 16 followed by a logarithmic amplifier 17 necessary because of the enormous dynamics of variation of the signal and making it possible to judiciously place a comparison threshold; a comparator 18 and a logic for setting the threshold 19; The digital part which follows ensures the processing of the instants of reception of the pulses coming from the comparator; it comprises a time counter 20 driven by a crystal oscillator 21 and a digital processor 22 whose output provides the values of the y and z deviations. This processor includes a memory of the movement described by the spot as a function of time. Each passage of the laser beam on the detector makes it possible to determine a pair of coordinates by simply reading the memory. This assumes that the time scale on board the receiver is the same as that of the transmitter. It is necessary that there is synchronization of the clocks before the departure and maintenance of this synchronism during all the flight. In the case of FIG. 2, a precision of 1% of the field corresponds to a time error of T / 20000. A flight of 10 s with T = 20 ms therefore requires a relative accuracy of 10'7.

  This accuracy will be achieved easily if a resynchronization occurs in the receiver during the flight of the missile. This operation is possible because the scanning is symmetrical as shown in Figure 4. The signals are generally received in pairs whose medium is always fixed in time.

  The clock servo circuit will use the relation e = 2 to synchronization (FIG. 4a). - 8

  The accuracy is improved in the center of the guiding field because if the radial distance D is less than or equal to half the diameter S of the spot (D t S / 2), 4 k of information is received, resulting in an average decrease of the noise of difference in the ratio 1 / 2Ik or 0.19 in the example chosen.

  This improvement is of the order of 1 / 2lkArc (sin Sf2 D) when S / 2 <D <R. These noises can be due to instabilities of the laser or atmospheric turbulence.

  The device of the invention has among others the following advantages.

  - use of a single laser that may not be modulated; optical schema of the transmitter very simple; - use of beam detectors without friction or mechanical bearings, so as to reduce the "noise" of position not compatible with the desired accuracy; - excellent concentration of energy (limited by diffraction); - increased resolution in the center of the guiding field; - relatively simple signal processing on reception; - possibility of resynchronization of the receiver with the transmitter.

  The intended use is the guidance of a missile. Clocks can be synchronized just before the launch of this missile. It can be maintained with sufficient precision for the duration of the flight thanks to the redundancy and symmetry of the sweep without resorting to ultra-stable quartz oscillators. - 9

Claims (4)

CLAIMS:   1. Device for remote optical guidance of a missile on a target by means of a beam transmitter, said beam coming from a laser source which can be modulated or not, the image projected at the level of the receiver mounted on the missile is a circular spot which scans the field of the guide by means of optical deflectors, said transmitter and receiver being synchronized on clocks from the missile, characterized in that said transmitter comprises two optical deviators which respectively move the beam in two directions perpendicular to each other for animating said image of the laser source of a translation movement with a substantially constant linear velocity so as to cause it to scan the rosette-shaped guide field, each of said deviators respectively providing a deflection A sin wt cos Qt and A sin wt sin Qt, the deflection elements of said optical deflectors not being driven by a rotatio n but carried by galvanometric motors working by bending or twisting bar (s) or blade (s) recessed (s) and without mechanical play.   2. Device according to claim 1, the emitter of which furthermore comprises a pupil transfer between the first and second optical deflector and following said second deviator a lens which concentrates the beam at the focal point of a variable-focus lens. , characterized in that said deflection elements are constituted by a first and a second mirror vibrating at identical pulsations and in phase but whose amplitude is modulated in cosQt and sinQt, the ratio w / Q being defined by the ratio of the diameter said spot and the diameter of the guiding field, said first and second mirrors being respectively actuated by a first and a second galvanometer whose axes are perpendicular to each other and respectively perpendicular to the incident beam on the first mirror and to the beam reflected on the second mirror, the latter beam being brought in the direction of the guide beam by any deflection device.   - 10 - 3. Device according to claim 1, characterized in that said receiver comprises an optical filter centered on the wavelength of the laser followed by an energy concentrator, a detector (cooled or not by a cryogenic device according to the wavelength of the laser), a logarithmic amplifier connected to an input of a comparator whose other input is brought to a threshold and whose output is connected to a counter controlled by a quartz oscillator and synchronized at the departure of the missile on the scanning laws of the transmitter, a digital processor connected to said counter providing the value of said threshold and performing the decoding of the data to indicate the coordinates of the missile.   4. Device according to claims 1 to 3, characterized in that said laser source may be a CO2 laser operating at a wavelength of 10.6 p or any other laser providing a circular section beam.