GB2433306A

GB2433306A - Laser beam guidance device scanning space in a rosette pattern

Info

Publication number: GB2433306A
Application number: GB8707643A
Authority: GB
Inventors: Roland Albert Allezard
Original assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Current assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Priority date: 1986-06-17
Filing date: 1987-03-31
Publication date: 2007-06-20
Anticipated expiration: 2007-03-31
Also published as: DE3719135B3; FR2891632A1; GB8707643D0; GB2433306B; SE8702413D0

Abstract

Device for the optical guidance of a missile by means of a beam transmitter including for example a CO2 laser source (1) functioning at 10.6 and whose beam carries out a rosette pattern scan of the field of guidance by means of two optical deflectors (2 and 3) constituted by vibrating mirrors (9 and 11) activated respectively by galvanometers (8 and 10) and supported by systems without friction but working by torsion or by flexion, providing quasi-sinusoidal deflections modulated in amplitude. The beam emerging from the zoom (7) scans the said field about the missile on board which is mounted the guidance receiver synchronized at the launch with the transmitter and including the detector (14) cooled by cryogenics. The detected signal, amplified (at 17), enters the comparator (18) whose output is connected to the counter (20). The digital processor (22) connected to the counter provides the threshold value and decodes data to indicate the coordinates of the missile and to correct the synchronism errors. Application to the guidance of a missile.

Description

1 2433306

LASER BEAM GUIDANCE DEVICE

The invention relates to a device for the ren'o'te optical guidance of a missile onto a target by means of a beam transmitter, the said beam coming from a laser source whose projected image at the receiver (or deviation measurer) mounted on the missile is a circular spot which scans the field of guidance by means of optical deflectors, the said transmitter and receiver being synchronized by clocks when the missile is launched.

*The time of each passage of the beam over the rec-eiver during the scanning is known accurately as the corresponding clock is synchronized. The law of displace-ment of the said spot as a function of time is therefore known as is the position of the missile with respect to the guidance axis. It is therefore sufficient to operate its fins in order to bring it back to this axis.

The scanning accuracy requires as small a spot as possible to be obtained. Limitations are soon encountered in this respect because of the diffraction effect due to the transmitter optics. Also, the complete scanning of the field corresponding with the data refresh time on board the missile becomes faster as the spot becomes smaller. There is then a limitation due to the inertia of the scanning system. If this scanning is carried out in a singLe direction, for example line by line (television mode scanning), this results in poor resolution in the direction perpendicular to the scanning due to the rela-tive size of the spot.

This scanning mode is used in a determination device known from French Patent No 2 516 664. In this device the scanning is carried out by means of an acousto-optical deflector making the infra-red beam pass through a crystal throogh which high-frequency acoustic waves pass. The deflection through the system created by the said waves is proportional to their frequency. Tiiis deflector is no longer limited by inertia but it presents other technical problems that are not yet overcome. In order to improve resolution, this same scanning is carried out in two successive directions, horizontally and then vertically.

The scanning speed in this mode is very high. If, for example, it is necessary to have 100 lines to cover the totality of the field of guidance in one direction, the accuracy of the clock of the receiver must be 100 times higher than for a system of scanning the said field by a beam having the shape of a single rectangular bar. The obtaining of such a degree of accuracy is not currently possi.blein the manufacture of clocks.

Another scanning mode is described in European Patent No 0 102 466. The linear radial scanning is ob- tained by essentially mechanical means such as the rota- tion of a reflecting polygon and a Dove prism. The neces-sary mechanical precision must therefore be very high in order that the errors due to play and vibration are com-patible with the accuracy required for the guidance of a modern missile.

The object of the present invention is to create a remote optical guidance device of the type mentioned in the preamble and which, for a given data refresh rate, permits the use of beam deflection systems that can func-tion at relatively low frequencies in the order of a few hundred Hertz, in quasi-sinusoidaL mode, in order to avoid the amplitude and phase controL of high order har-monics, the said device implementing simpler mechanical means without ball bearings or other pivots having a large amount of "friction noise".

This object is achieved because the said transmitter includes two optical deflectors which respectively dis-pLace the beam in two directions perpendicular to each other in order to move the said image of the laser source with a translation movement that will make it carry out a rosette-shaped scan of the field of guidance, each of the said deflectors providing a deflection with a phase pulsation wand amplitude modulated as sin 2t and cos 2t respectively, the deflection elements of the said def-lectors not being driven in rotation but carried by galvanometric motors working by f$.exion or tension of built-in bar(s) or strip(s) having no mechanical play.

According to one advantageous embodiment of the invention, the said deflection elements are constituted by a first and a second mirror vibrating with phase pul-sations of w, and whose amplitude is modulated as sin t and cos t respectively, the ratio c2/w depending on the ration of the field covered by the diameter of the said

spot to the diameter of the field of guidance.

The said first and second mirrors being activated by a first and a second galvanometer, respectiveLy, whose axes are perpendicular to each other and respectively perpendicular to the incident beam on the first mirror and the reflected beam on the second mirror, this latter beam being brought into the direction of the guidance beam by any type of reflecting device.

Figure 1 is a diagrammatic representation of the transmitter used in the laser beam guidance device accord-ing to the invention.

Figure 2 represents a sequence of rosette-shaped scan-ning patterns carried out by a laser beam projected by the device in Figure 1.

Figure 3 is a time diagram showing the scanning variation in the orthogonal oy and oz axes indicated in Figure 2.

Figure 4 is a time diagram showing the time spacing of the signals received at various points in the field.

Figure 5 is the block diagram of the guidance rec-e i ye r.

The guidance transmitter shown in Figure 1 includes a laser 1, transmitting at 10.6 microns (CO2 laser) for example, a first optical deflector 2 which displaces the beam in the yoz plane, a second optical deflector 3 which displaces the beam over a cone of axis oy very close to the plane yox and a beam transfer device 4 formed from an assembly of two or more lenses between the two deftec-tors. At the output of the second deflector, this beam is concentrated at 6 at the focus of the lens 5 coincident with that of the variable focal length or zoom lens 7 which projects the image of the laser to the missile, i.e. practically to infinity.

This variable focal length lens enables the illum-inated field to be adapted to the desired value at any time. If the width desired at the missile level is con-stant, the focal length must vary in proportion to the distance traveled by the missile.

The image projected at missile level is a circular spot whose diameter at half-power is determined by the laws of diffraction. The concentration of energy is maxi-mum of the transmission optics have the maximum permitted diameter taking into account the possible size of the beam projector and of the necessary correction of optical aberrations.

This image is moved with a movement of translation following the rosette scanning pattern.

The frequency of the scans is chosen such that the complete scanning time of the guidance tunnel is equal to the repetition period of the deviation measurements re-quired at the receiver and such that the distance between the scanning curves is less than or equal to the diameter

of the laser spot in the field of guidance.

It can be seen therefore that the scanning frequency must be proportional to the resolution of the transmission optics, which raises problems in relation to the deflec- tors. In practice the best compromise will be chosen be- tween the concentration of energy and the possible scan-ning speed.

Figure 2 shows a sequence of scanning patterns in rosette format corresponding with one scan period, i.e. the time during which the totality of the field is scan-ned. The projected image S of the laser spot centered at point A shown in the figure is referenced with respect

S

to the system of rectangular coordinates oyz. By making wt = x and making w/cZ = 4, the coordinates of the point A in this system are written: y = A sin 4x cos x z = A sin 4x sin x Figure 3 represents the variation of the scanning signal as a function of time in the oy and oz axes.

Figures 4a and 4c respectively show the time spac-ing of the signals received for example at points A and 0 of Figure 2, and the signals from a clock (Figure 4b) instaLled on board the missile and synchronous with the scanning of the deflectors.

For purposes of drawing clarity Figures 2, 3 and 4 have been drawn with a ratio w = 42. In an actual case Figures 2, 3 and 4 could correspond with the following typical case: -CO2 laser, at 10.6 microns; -Diameter of transmission optics: 100 mm; -Diameter of spot: 50 cm;

-Width of guidance field: 3.5 m;

-Recurrence period: I = 20 ms, Q = 314 rad/s; -Scanning frequency 700 Hz; u = 4396 rad/s.

Each optical deflector in Figure 1 is advantageously constituted by a vibrating mirror activated by a galvan-ometer, the least onerous solution for tow speeds. The deflector 2 includes the galvanometer 8 whose spindle carries the plane mirror 9 of axis ox and, in its equi-librium position, making an angle of 450 with the axis oy.

The deflector 3 includes the galvanometer 10 whose axis which is parallel to oy carries the plane mirror 11 having, in its equilibrium position, an inclination of 450 with respect to the axis ox.

The.beam coming from the laser source 1 strikes the plane mirror 9 which is vibrating about the spindle of the galvanometer 8 which is parallel with ox. After a first reflection on the plane mirror 9, the Laser beam passes through a beam transfer device 4 and undergoes a second reflection on the plane mirror 11 vibrating about the spindle of the galvanometer 10 parallel with the axis oy. Thus, when the vibrating mirrors pass through their equilibrium positions, the beam reflected on the mirror 11 emerges parallel to ox. A second reflection not shown brings this beam into the direction of the guidance beam.

In order to reduce the pulsation w, taking account of the diameter of the spot, it will be possible to use an amplitude A slightly greater than the guidance field R (Figure 2). For example R/A = 0.8 will be chosen which gives.a time efficiency of 60% and enables the whole field of R = 3.5 m to be scanned with a spot of diameter 50 cm and a ratio /Q = 14. The laser transmission can be cut off during the dead times. The cutting off of the laser transmission can be carried out easily at medium speed but switching the laser back on is not so easily achieved.

The operation will therefore be limited to reducing the transmission power of the Laser instead of switching it off when the image of the spot scans the edges of the

field.

The guidance receiver installed on board the missile and diagrammatically represented in Figure 5 includes: -an optical filter 12 centered on the wavelength of the laser; -an optical concentrator 13 to retrieve the maximum of energy; -a detector 14 matched to the working wavelength (10.6 p with a CO2 laser); -a cryogenic cooling system 15 using nitrogen or argon able to drop below 100 K if necessary; -a preamplifier 16 followed by a logarithmic amplifier 17 which is necessary because of the enormous dynamic range of variation of the signal and which enables the judicious setting of a comparison threshold; -a comparator 18 and a threshold setting logic 19.

The digital section which follows processes the reception times of pulses coming from the comparator; it includes a time counter 20 controlLed by a quartz ascii-t.ator 21 and a digital processor 22 whose output provides the deviation measurement values y and z. This processor includes a memory of the movement described by the spot as a function of time. Each passage of the laser beam over the detector enables the determin-ation of a pair of coordinates simply by reading the memory. This assumes that the time scale on board the receiver is the same as that of the transmitter. It is therefore necessary for the clocks to be synchronized before. the launch and for this synchronism to be maintained throughout the flight.

In the case of Figure 2, an accuracy of 1% of the field corresponds with a time error of 1/20000. A flight of 10 sec with I = 20 ms therefore requires a relative accuracy of This accuracy will be obtained easily only if resyn-chronization occurs in the receiver during the flight of the missile. This operation is possible as the scanning is symmetrical as shown in Figure 4. The signals are gener-ally received in pairs whose center is always fixed in time.

The clock control circuit will use the relationship A1 = A2 at synchronization (Figure 4a).

The accuracy is improved in the center of the field of guidance because if the radial distance D is less than or equal to the spot diameter S CD S/2), 4k data are received resulting in a mean reduction in deviation measurement noise in the ratio 1/2 ir or 0.19 in the chosen example. This improvement is in the order of 1/2 1kArc(Sifl S/2 D) when S/2 < D < R. These noises can be due to instabilities of the laser or to atmos-pheric turbulence.

The device of the invention has, among others, the fol low ing advantages.

-the use of a single laser which it is possible not to modulate; -a very simple optical diagram of the transmitter; -the use of frictionless beam deflectors without mech-anical bearings, in order to reduce the positional "noise" that is not compatible with the desired accuracy; -excellent energy concentration (limited by diffraction); -increased resolution at the center of the field of guidance; -relatively simple processing of the received signal; -the possibility of resynchronizing the receiver with the transmitter.

The use foreseen is the guidance of a missile.

The synchronization of the clocks can take place just before the launch of this missile. It can be maintained with sufficient accuracy throughout the duration of the flight because of redundancy and the scanning symmetry without having to use ultra-stable quartz osciLlators.

Claims

CLAIMS

1. Device for the remote optical guidance of a missile onto a target by means of a beam transmitter, the said beam coming from a modulable or non-modulable Laser source whose image projected at the level of the receiver mounted on the missile is a circular spot which scans the field of guidance by means of optical deflectors, the said transmitter and receiver being synchronized on clocks at the missile launch, characterized in that the said trans- mittefl includes two optical deflectors respectively dis-placing the beam in two directions perpendicular to each other in order to move the said image of the laser source with a translation movement at almost constant linear velocity in such a way as to make it scan the field of guidance in the pattern of a rosette, each of the said deflectors respectively providing a deflection A sin wt cos t and A sin wt sin 2t, the deflection elements of the said optical deflectors not being driven in rotation but carried by galvanometric motors working by flexion or torsion of built-in bar(s) or strip(s) having no mechanical play.

2. Device according to claim 1 whose transmitter also includes a beam transfer device between the first and the second optical deflector and following the said second deflector a lens which concentrates the beam at the focus of a variable focal length lens, characterized in that the 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 cos t and sin 2t, the ratio w/2 being defined by the ratio of the diameter of the said spot and the diameter of the field of guidance, the said first and second mirrors being activated respectively by a first and a second galvano-meter whose spindles are perpendicular to each other and respectively perpendicular to the incident beam on the first mirror and to the reflected beam on the second mirror, this latter beam being brought into the dir-ection of the guidance beam by any type of reflecting device.

3. Device according to claim 1, characterized in that the said receiver includes an optical filter cen-tered on the wavelength of the laser followed by an energy concentrator, a detector (cooled or not cooled by a cryogenic device depending on the wavelength of the laser), a logarithmic amplifier connected to one input of a comparator whose other input is taken to a threshold and whose output is connected to a counter controlled by a quartz oscillator and synchronized at the missile launch to the scanning laws of the transmitter, a digital processor linked to the said counter providing the value of the said threshold and decoding data to indicate the coordinates of the missile.

4. Device according to claims 1 to 3, characterized in that the said laser source can be a CO2 laser func- tioning at a wavelength of 10. 6 p or any other laser pro-viding a beam of circular section.

5. A device for the remote optical guidance of a missile onto a target by means of a beam transmitter including a laser, substantially as described herein-before with reference to and as shown in the accompanying drawings.

Amendments to the claims have been filed as follows 1. In combination, a beam transmitter, receiver and missile for the remote optical guidance of the missile onto a target by means of the transmitter, the said beam coming from a modu]ab].e or non-modulab].e laser source whose image is detected by a detector included in the receiver which is mounted on the missile the image being a circular spot which scans the field of guidance by means of optical deflectors, the said transmitter and receiver being synchronized on clocks at the missile launch, characterized in that the said transmitter includes two optical deflectors respectively displacing the beam in two directions perpendicular to each other in order to move the said image of the laser source with a translation movement at almost constant linear velocity in such a way as to make it scan is the field of guidance in the pattern of a rosette, each of the said deflectors respectively providing a deflection A sin wt cos Qt and A sin wt sin Qt, where w Is a multiple of S2 the deflection elements of the said optical deflectors being carried by galvanometric motors working by flexion or torsion of built-in bar(s) or strip(s) having no mechanical play and which cause the deflectors to rotationally oscillate or vibrate at a frequency of
2. The combination claimed in claim I whose transmitter also includes a beam transfer device between the first and the second optical deflector and following the said second deflector a lens which concentrates the beam at the focus of a variable focal length lens, characterized in that the 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 cos 9t and sin2t, the ratio w/Q being defined by the ratio of the diameter of the said spot and the diameter of the field of guidance, the said first and second mirrors being activated respectively by a first and a second galvanometer whose spindles are perpendicular to each other and respectively perpendicular to the incident beam on the first 1'7.' mirror and to the reflected beam on the second mirror, this latter beam being brought into the direction of the guidance beam by any type of reflecting device.

3. The combination claimed in claim 1, wherein the said receiver includes an optical filter centered on the wavelength of the laser followed by an energy concentrator, a detector (cooled or not cooled by a cryogenic device depending on the wavelength of the laser), a logarithmic amplifier connected to one input of a comparator whose other input is taken to a threshold and whose output is connected to a counter controlled by a quartz oscillator and synchronized at the missile launch to the scanning laws of the transmitter, a digital processor linked to the said counter providing the value of the said threshold and decoding data to indicate the coordinates of the missile.

&. The combination claimed in claims I to 3, wherein the said laser source can be a CO2 laser functioning at a wavelength of 10.6w or any other laser providing a beam of circular section.

5. The combination of a beam transmitter, receiver and missile for the remote optical guidance of the missile onto a target by means of the beam transmitter including a laser, and substantially as described hereinbefore with reference to arid as shown in the accompanying drawings.