GB1605212A

GB1605212A - Systems for position determination with reference to light beams

Info

Publication number: GB1605212A
Application number: GB30725/70A
Authority: GB
Original assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Current assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Priority date: 1970-10-24
Filing date: 1970-10-24
Publication date: 1983-12-21

Description

(54) "IMPROVEMENTS IN AND RELATING TO SYSTEMS FOR POSITION DETERMINATION WITH REFERENCE TO LIGHT BEAMS" (71) We TELECOMMUNICATIONS RADIOELECTRIQUES ET TELEPHONIQUES T.R.T. 88, rue BrillatSavarin, Paris (l3 - France, a French body corporate do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to systems of position determination by light beam permitting the detection of the positiontof a moving object with reference to a position-determining axis defined by this beam. The light beam may be of visible wavelength, of infra-red wavelength or of ultraviolet wavelength.Such a system is applicable, for example, to the guiding of a machine in its start from a launcher following the trajectory or an axis of guidance - the machine altering its position towards the interior of the beam in correcting its position with reference to the latter axis. The system according to the invention can also be applied in a guided missile sys teni for the detection and then the pursuit of a target.

In known systems of this kind used for guiding a machine, and in particular for the launching of rockets at a target, a transmitter, usually situated at the launcher, transmits a light beam following the chosen axis of guidance through an aiming pattern for modulation so that it sweeps the field of guidance in a plane perpendicular to another plane, for example vertical, passing through the axis of guidance; the pattern provides two slotted regions and the periodicity of the slots is different for the two regions. In this way the beam is swept at regular intervals through two zones of different frequency modulation, the zones being calculated in order that, the receiver being placed in the vertical plane passing through the axis of guidance, it receives the radiation corresponding to the two zones for an equal time.If the receiver, or if the machine, is not in the said vertical plane, it is possible to calculate its position with reference to the latter as a function of the time of the reception of the two frequencies.

In order to provide that the machine is guided according to the axis of guidance, it is necessary to provide two completely separate systems, sweeping the beam in accordance with two orthogonal planes, the intersection of these two planes defining the axis of guidance; this will be seen to constitute an inconvenience.

Moreover, such systems have on the one hand: an angular sensitivity which is depefident upon the range (when the range increases the distance from the machine to the axis is defined with less precision) and on the other hand a signalfnoise ratio decreasing with range (according to a law Ild2). It is however, possible to provide that the guidance fileld (defined by the size of the beam) can be decreased by using variable focal length objectives, in such manner that an increasing angular precision is obtained; but this can lead to deviations of the optical axis and hence of the axis of guidance and in any case the signal/noise ratio is not increased.

Further, the modulation frequencies of the beam are dependent on the speed of sweeping of the pattern and on the minimal resolution limiting the frequency band of the transmitter.

Now it is useful to use a modulation frequency which is relatively high compared with that of the noise (resulting for example from the flare of the machine in the optical field of reception, this noise varying approximately inversely with the frequency.

The present invention relates to a system of guidance by light beam; which can be applied particularly to the guidance of machines, which makes it possible for the signal/noise ratio to be kept substantially constant independently of the range of the moving object and makes it possible to obtain an angular sensitivity which increases with the range (or follows any other desired law).

According to the invention, a system for determining the position of a mobile object with respect to a position-deterrnining axis using a light beam defining a field of position determination, comprises a transmitter of the light beam including an emitting source and an objective lens, and a receiver deternining the position of the said moving object with respect to the axis of position detbrmination, and the said transmitter comprising:: - means for providing that the light beam comprises elementary beams each having a distinctive characteristic, defining, in a section in a plane perpendicular to the axis of position determination, different sectors, - means to provide movement of the beam such that its axis describes a conical surface defining the field of position determination, the apex of the cone being in the proximity of the optical centre of the objective lens and the section of the light beam in a plane perpendicular to the axis of position determination carrying out a movement of translation applied to the corresponding base of the cone, while the receiver comprises means to measure the intervals of time respectively defined by the reception of the elementary beams.

Other features of the invention will appear more clearly from the following description; concerning more particularly the guiding of a machine and given by way of example, with reference to the accompanying drawings in which: Figure 1 a is a general schematic view illustrating the principle of the system according to the invention; Figure ib is a cross-section of the beam in the focal plane of the objective lens; Figure 2 is a perspective view of a simplified first transmitter according to the invention; Figure 3 is a schematic drawing of a variant of a deflection element included in part of the transmitter; Figure 4 is a perspective view of a second transmitter according to the invention; Figure 5 is a schematic cross-sectional view of a particular embodiment of the optical means for providing a transmitted guiding beam; and Figure 6 is a schematic cross-sectional view of a variation of the construction of the said optical means for providing the beam.

Explanation of the principle of position determination, in the example described of guiding, with the aid of a system according to the invention will now be given with reference to Figures la and Ib, then examples will be described with reference to Figures 2 to 6.

Figure la shows the axis of guidance XX' along which a light beam is transmitted by a transmitting arrangement, centred upon the axis XX' (situated on a launcher) in which are represented diagrammatically only those elements necessary for the understanding of the invention: a modulation pattern 1 situated at the focus of an objective lens 3, and a deflecting element 4 arranged between the pattern and the objective lens. The beam projects the image 2 of the pattern of modulation 1 into space. In the example of Figure la, the modulation pat tern is provided from a disc-shaped emitting surface having four sectors defined by two ner pendicular lines intersecting on the axis XX'.

Each sector emits a different elementary light beam, for example modulated at different fre quencies. The light beam emitted by the pat tern 1 is thus circular in cross-section in the plane perpendicular to the axis XX', and com prises four different elementary light beams.

The pattern may be self-luminous or may be illuminated from behind and many variants of pattern are possible.

The deflecting element 4, for example a prism, deflects the axis of the beam with reference to the axis XX' through an angle a variable with the distance D between the prism 4 and the pattern 1; the image 2 is eccentric with respect to the axis XX'. If, for example, the prism has a uniform rotational movement about the axis XX', the axis of the beam describes a cone of revolution having a semi-vertical angle of a.

The cross-section of this cone in a plane perpendicular to the axis XX' at the position of the image 2 is a circle 5. The diameter of the circle S is equal to the radius of the circle 2 and the two circles are tangential. It will be seen that in a plane perpendicular to the axis of guidance, the image 2 follows a circular path along the circle of the section 5 of the cone, the said cone defining the linear field of guidance, as will be explained hereafter. This type of sweeping according to the surface of a cone of revolution is given only as an example and is to be considered as a particular case of sweeping according to the surface of a cone or a pyramid of any desired section.

A point A situated in the plane of the image 2 and on the axis XX' is successively swept by the four sectors of the image 2 for a time which is equal for each of the sectors. If on the contrary the point A is not on the axis of guidance, it will be swept successively by the four sectors for unequal times. The comparison of these times permits the position of the point A to be determined with reference to the axis XX'.

For a better understanding of the operation of the arrangement, it is easier to consider the reverse case, in considering the image A' of the point Al which is formed in the plane of the pattern 1 at a distance r from the centre such that r = D tan 6 (D being the distance between the prism 4 and the pattern 1 and the angle of deflection which is constant for a given prism) or, as is equivalent, F tan a (F being the focal distance of the objective lens 3). The prism 4 turns about XX' and the path of A' which is shown in Figure 1 a is a circle 72 of radius r of which the centre is at the centre of the pattern.

In Figure lb, the circular pattern 1 of centre 0 is shown on a larger scale. It comprises four sectors I, II, III and IV. The circular path 71 of A has a radius r and is concentric at 0 with'the pattern 1.

On the other hand if a point A1 (Figure la) in the plane of the image 2 is off the axis XX' and on an axis at an angle P thereto, as can be seen from Figure lb, the image A'1 of Al in the plane of the pattern 1 describes a circle 72 of the same radius r as the circle yl but ecc entric (centre 02) with respect to the circle 1 delimiting the pattern. The eccentricity 002 is equal to F tan ss. From the value of this eccen tricity, the position of the point Al in the plane of the image 2 with respect to the axis XX', may be calculated.

With reference to the axes defined by the bisectors of the four sectors of the pattern (represented in chain dot lines in Figure lib), the centre 02 of the circular path 72 of A', may be defined by its coordinatesx andy calculated from the time that A'1 is situated in the different sectors I, II, III, IV of Figure 1 b. These times are Tl, Till, Tlll and TIV respectively.It can be shown that: x is substantially proportional to the difference between the times TIV - TIt, and y is substantially proportional to the difference between the times Till - Ti. This is possible only if the eccentricity 002 is less than the radius r of the circular path 72 of A'1 that is: Ftan/3 < DtanorFtan/3 < Ftana,thatis to say P < a. The law of the deviation measure is very generally linear for a pattern having four equal sectors, this does not exclude the possibility of utilising a pattem with a different number of sectors, for which the law of deviation measure will be different.

If at the point Al (Figure 1 a) in the plane of the image 2, a receiver is provided on the machine, this receiver is swept successively through each of the four different elementary beams of the pattern 1 for periods which are the same as the times TI, TII, Till and TIV during which the mobile image A1 of A1 is on the circle 72 in the four sectors I, II, 111 and IV of the pattern 1. From these times, the receiver can determine the position with respect to the axis of guidance and supply to the machine corresponding information to provide that the correction control is effected and hence the guidance of the machine.

In the same manner in the case of position determination of a target, a receiver placed on the ground in the proximity of the transmitter receives a part of the beam reflected by the target being swept successively by each of the sectors, the receiver can determine the position of the target with reference to the axis of guidance as a function of the duration in each of the sectors. The transmitter can be controlled by a servo mechanism which receives information from the receiver in such manner as to direct continuously the axis of the beam towards the target in order to permit the pursuit of this target and to centre it in the field of fire.

It is possible to focus the guidance field either by movement of the prism 4 parallel to the axis XX' in the direction of objective lens 3 - pattern 1 , or by using a variable focal length objective lens 3. If the movement of the prism 4 is effected according to a predetermined law, the accuracy of guidance follows a corresponding law. It is thus possible to provide that after a period of low accuracy when the engine is first taken under control, the accuracy is increased with increasing range.

It is advantageous to arrange that each of the four elementary beams emitted by the pattern illuminates only the guidance field so that light is not lost. This is the case illustrated in Figure 1 a in which the diameter of the circular image 2 of the pattern is equal to the radius of the circle 5 corresponding to the field of guidance and these two circles are tangential. In other words, the aperture angle 2 7 of the beam emitted by the pattern is advantageously about twice the angle 2 a at the apex of the cone defining the field of guidance.

If the guidance field defined by the angle a is made variable, it is advantageous also to make the aperture angle 2 z of the beam emitted by the pattern 1. This angle 2 wy can be made variable for instance by variation of the useful radiating surface of the pattern 1. If the pattern 1 is illuminated from behind, the luminous flux which illuminates it can be focused so that it illuminates only the useful part of the pattern.

The variation in the focusing of the luminous flux is of interest in particular if the sources of illumination used are electroluminescent sources or are gas, solid or liquid lasers; it permits at the time when the machine is first controlled, that is to say at short distances, to widen the flux field, and thereafter to narrow it in proportion to the range in a manner such that the signal/noise ratio is retained substantially constant whatever is the range (or following another desired law).

Referring now to Figure 2 an example of a transmitter of a position determining system according to the invention will be described in the case of its application to the guidance of a machine.

The elements of the transmitter are mounted on a base 6 fixed to the launcher of the machine' to be guided and parallel to the axis of guidance XX'.

A support 7, perpendicular to the base 6, supports four electroluminescent diodes 8, 9, 10, 11 (gallium arsenide diodes cooled to the temperature of liquid nitrogen, for example) each associated with four condenser lenses respectively 12-13-14-15, the distance diode condenser lens remaining constant (the arrangement may be of known type and is not shown in the Figure except schematically). The four diodes are modulated differently as will be explained hereafter. The support 7 is displaceable in the direction of the axis XX' in order to permit variation of the focusing of the luminous flux. A fixed optical unit 16, formed by the combination of four identical glass prisms is centred upon the axis XX' and provided behind the support 7 in a manner such that each of the prisms corresponds with one condenser lens, the exit surface of the unit 16 is positioned in the focal plane of the objective lens 3. Each prism has a form such that a ray parallel to the axis XX is twice totally internally reflected and emerges parallel to the latter. The surfaces of each of the prisms, with the exception of the entry and exit surfaces, are metallised in order to avoid interference between the radiation emerging from each of the condenser lenses.This optical unit may be grouped together according to the axis XX', the four axes being parallel, spaced apart from the other elements in order to leave sufficient space for the securing of the condenser lenses and the diodes with their cooling systems. The surfaces of the prisms are metallised so that the four sectors are partitioned and separated (see above with reference to the pattern) at the exit surfaces of the four prisms. After the position of the support 7, on the exit surface of the unit 16 or in the neighbourhood thereof, a single image of the four sources is obtained on the axis XX', and consequently at the exit surface of the unit, a spot centred upon the axis of dimensions which are variable with the position of the support 7, in such manner as to provide a radiating pattern situated at the focus of the objective lens.Each sector corresponds to an elementary beam and the sectors are luminous. The delimitation of the four sectors is thus clear cut, without overlapping and without a dark zone.

On a support 17 perpendicular to the axis XX', a deflecting prism 4, of glass and of small angle, is mounted in a rotary mounting; it is compensated dynamically by a compensating ring 18. A motor 19 mounted upon the support 17 provides the rotational movement of the prism. The support 17 can be displaced in the direction of the axis XX' between the optical unit 16 and the objective lens 3 mounted at the rear end of the base 6 in such manner as to permit variation of the focusing of the guidance field.

The axis of guidance XX' is thus defined by the optical unit 16 and the objective lens 3, and the support 17 is suspended with reference to the base 6 in order to avoid effects on the position of the optical axis due to vibration which can be occasioned by small errors in the compensation of the rotary parts. Such an elastic suspension is made possible by the fact that the precision required in the positioning of the rotary prism 4 is not high, faulty positioning due to mechanical trouble not giving rise to any substantial deviation of the axis.The simultaneous movement of the components carried by the supports 7 and 17 is provided by an appropriate mechanical system which can be controlled by a servo mechanism or set of cams, for example, - not shown - each according to an appropriate law, predetermined as a function of the characteristics of the machine, in such manner as to obtain a respective variation of the field of guidance and of the light flux, the variations being linked as has been indicated in the explanation of the principle.

The receiver assembly fixed at the back of the machine is of known type and is not shown.

For reasons of symmetry and of the position available on the machine, the reception optical means is divided into four elementary optical systems arranged in parallel in such manner as to provide the sum of the received flux. Each comprises, for example, a silicon detector placed at the focus of a parabolic mirror translating the flux received into an electric signal, the sum of the four signals constituting the received signal.

It will be obvious that the use of four optical systems is not limitative and it can be envisaged, if the engine has a suitable position available, that a single optical system can be used.

An interference filter set in an aperture limits the spectral bandwidth ot the detector to a band centred upon the wavelength of the transmitter and provides, at the same time, for tightness. The transmitter and receiver circuits are of known type and are not shown.

As an example, the four diodes 8, 9, 10 and 11 can receive four signals of different frequencies obtained with the aid of a quartz oscillator (at 1.25 Mc/sec., for example) and of an RC oscillator (resistance-capacity at 1, 2, 3 and 4 kc/ sec.). The signals are amplified by a transistor circuit. The receiver comprises a quartz oscillator of the same frequency as that at the transmitter; after amplification of the received signal the frequency is changed with the aid of a local oscillator, so that the four frequencies of the RC oscillators are regained. These frequencies are separated by filters; the signals at the outputs of these filters are corrected in amplitude and then the energies which they carry are compared two by two so as to deduce the proportional quadrant error voltages in each of the separate sectors.

In the description given above, the deflecting prism 4 has a fixed angle and is situated in the path of convergent light between the objective lens 3 and the pattern here provided by the four diodes 8, 9, 10 alid 11 and corresponding to the pattem 1 of Figure la. This simple arrangement is of interest only if the diameter of the prism and the field of guidance are not too great.

In the contrary case, it could be of interest to use the system shown in Figure 3; the deflecting prism has a variable angle and comprises two prisms 41 and 42 of equal angles (diasporameter). If the direction of tapering of one prism is altered with reference to the other, the angle of deflection of the combination is varied. Such a system is preferably placed in parallel light, that is to say in front of the position of the objective lens 3 shown in Figure la.

In order to reduce the dimensions of the diasporameter and its means of control if the objective lens 3 has a large diameter, the combination can be used of an objective lens 31 of focal distance f focused on the pattern and then an afocal system 32 of magnification g such that the combination has a resultant focal distance fg equal to the focal distance F chosen in the previous example for the objective lens 3. The diasporameter 41,42 is placed between the objective lens 31 and the afocal system 32.

Referring to the Figure 4, which is very schematic, a second type of transmitter will now be described.

The axis of guidance XX' is shown. The var ious optical elements which will be described are centred upon the axis of guidance XX', or, as may be the case, arranged in a symmetrical manner with respect to this axis. The stability of the axis of guidance depends then on the position of these different elements and a parti cular arrangement providing simple regulation will be described below after the description of the said system of guidance.

The emitting source is constituted by a continuous laser 51 for example a gas or solid laser, emitting a pencil of light of very small divergence. A prismatic separator 52 divides the incident beam, not shown, into four elementary beams of the same geometrical characteristics and of equal intensities. These four beams, of which the axes are represented by 53, 54, 55 and 56, are parallel to the principal axis of the system and are equidistant from it. Four objective lenses 57, 58, 59 and 60 focus the four beams to four image points 61,62, 63 and 64, positioned for example in the same plane perpendicular to the axis of guidance, at the corners of a square centred upon it. A mechanical modulator 65 is provided in this plane.It is constituted for example by a disc having engravings, centred upon an axis yy' parallel to the axis XX', carrying four concentric tracks 651,652,653,654 with radial markings alternately transparent and opaque, the number of markings being different for each of the tracks.

In this transmitter, each of the images 61, 62, 63 and 64 are formed on a different track and when rotated with a constant speed around the axis'YY', the disc modulates each of the four beams with four different frequencies. At the output of the modulator there are provided four different elementary beams.

Four anamorphic optical systems 66, 67, 68 and 69 provide from the combination of the four point images 61,62,63 and 64, an image 70 in the focal plane of the objective lens 3 having the form of a cross. Each elementary beam modulated by a different frequency has in a section in a plane perpendicular to the axis of the beam one of the branches of the cross.

These branches can be luminous up to the centre or in certain cases over a part of their lengths but symmetrically with respect to the centre.

In the following the case considered is limited to that where the branches are luminous up to the centre. This cross provides the pattern referred to with reference to Figure 1. Here, the pattern is self-luminous, in the form of a cross of which the arms constitute two orthogonal axes delimiting the four sectors I, II, III, IV. In the preceding example described with reference to Figure 2, the sectors are luminous; in the present case on the contrary the axes are luminous, in the form of the arms of the cross and the sectors are dark. Such a system has the advantage that it leads to a better concentration of the light flux; the optical system 71 forms the image 72 of the cross in the plane of the stop 73, the stop limits the emitted beam to the guidance field.As a result, in the guidance field the receiver receives four impulses corresponding to the four branches of the cross, these four impulses being necessary at the receiver in order to evaluate its position with reference to the axis as will be explained hereafter. Outside the guidance field, the receiver receives a number tif impulses less than four so that an error of interpretation is introduced. A rotary prism 74 is placed between the system 71 and the stop 73, in such manner as to deflect the beam through an angle a corresponding to the half length of a branch of the cross, (if the branches are luminous up to the centre).

The stop aperture is chosen in a manner such that its diameter is less than or preferably equal to the length of one branch of the cross. When the prism turns, the axis of the beam describes a cone of half vertical angle a. In the plane of the stop, the centre of the cross follows the contour of the aperture. The objective lens 3 projects into space the image of the eccentric part of the pattern passing through the stop (this is not shown in the Figure). The vertical angle of the field of guidance is a function of the aperture of the stop and of the focal length of the objective lens. The objective lens 3 has a variable focal length; the focal length follows a predetermined law in such manner as to provide for the sensitivity of guidance a corresponding law.In contradistinction to the preceding example, in the present case, a variable focal length objective lens is very simple to realise if for example a laser is used which makes it possible to use small apertures and provides a monochromatic radiation. With such objective lens the field can easily be varied by a factor of ten, which is more than enough for practical purposes and at the same time, the objective lens concentrates the light flux.

The receiver, not shown, receives the radiation corresponding to each of the elementary beams. It can evaluate, as a function of the four separate periods of reception of impulses corresponding to different successive radiations, its position with respect to the axis XX' of position determination. In the case of guidance of a machine, it transmits to the machine the corresponding information in order to provide the correction control and hence the guidance of the machine.

In the same Figure 4, is shown a particular arrangement permitting the obtaining of a good stability of the axis of guidance XX'. It will be evident that the application of this arrangement is not limited to the system shown in this Figure.

First, a small part of the beam emitted by the objective lens 3 is separated with the aid of a semi-transparent plate 75. A receiver comprising an objective lens 76 and a point detector 77 receives the emitted beam. An arrangement 78 amplifies the signals and provides a deviation voltage proportional to the angle of error. This error voltage is transmitted to a servo mechan ism 79 which controls a variable angle deflector 80, provided on the axis of position determination at the outlet of the laser 51. Such an arrangement tends to reduce the angle of error to a negligible value: the axis of position determination is rigidly determined with respect to the axis constituted by the three elements 75, 76 and 77.It is then sufficient to position the three elements once for all with a high degree of precision and it is very simple to position the different elements of the system with the same precision, and in such manner as to permit thereafter the correction at any time of errors due to deformation or mechanical play.

Figure 5 relates to a particular embodiment of the invention. The transmitting source for the ray is constituted by a continuous gas or crystal laser 101, for example transmitting a pencil of parallel monochromatic light of very small divergence. The beam first passes through.

an afocal system, comprising two lenses L1 and L2, which permits the adaptation of the laser to the optical system of the guiding transmitter to give a beam of convenient diameter. At the exit of the afocal system, the cross-section of the beam in a plane A perpendicular to the axis of guidance XX' has the form A';that is a homogeneous beam. The beam then falls on a pyramid L3, comprising four thick plates with parallel surfaces, inclined with respect to the axis, which divides the beam into four equal sectors and displaces the said sectors by a predetermined extent. The cross-section of the beam in a plane B normal to the axis XX' has the form indicated by B'.

The four lenses L4 focus the partial beams at four points situated at the four comers of a square on a modulating disc d which turns with a constant speed abut an axis YY' parallel to the axis XX'. This disc carries at its periphery four concentric tracks with radial markings altternately transparent and opaque, the number of the markings being different for each of the tracks. The disc d thus modulates each of the four partial beams with a different frequency.

There are then provided beyond the modulating disc, four differentiated elementary beams.

The set of four lenses L5 provides four parallel beams in the form of four sectors which in the cross-section taken on the plane C perpendicular to the axis XX' has the form indicated by C'.

The set of lenses L6 and L7 and of plates Ls inclined with respect to the axis are symmetrical with the sets of lenses Ls, L4 and plates Ls with respect to the plane C, so that in the plane E a parallel light beam is reformed of which the circular cross-section is indicated by E'. This beam is obtained by recombination of the four modulated sectors of different frequencies.

The single lens Lg focuses this beam on the lens Llo which forms the image of the plane E in the plane F. In the plane F therefore a parallel beam is obtained by recombination which has a cross-section identical with that of the beam at the plane E.

The oscillatory movement of the beam is provided by the rotation, about the axis XX', of a prism L11 of small vertical angle. This oscillatory movement permits the definition of a guidance field. It is to be noted that the prism L1 is not subjected to a translational movement as is the case with the prism 4 in the arrangement described above (Figure 2).

As a matter of fact, the variation of the vertical angle of the cone described by the guiding beam is provided by the variation of the focal length of a variable focal length objective lens, comprising a set of divergent lenses L13 and a set of convergent lenses Ill 4. The focal plane of the said objective lens coincides with the plane F. It projects to infinity the cross-section of the beam in the plane F, that is to say the image of the four sectors of the circle.

A field diaphragm D positioned in the plane F limits the useful field of guidance. The field lens L12 focuses the beam in the plane of the aperture of the variable focal length objective lens.

The variable focal length objective lens (L13 and L14) is such that its optical axis coincides with the axis of guidance XX' very accurately throughout the variation of the focal distance which follows a predetermined programme.

This programme can be such, for example, that the cross-section of the beam in a plane containing the moving object to be guided, has a constant diameter whatever the range of the said moving object. In these conditions, the il- lumination due to the guiding beam and the accuracy of guiding of displacements with respect to the axis, are constant as a function of the range.

A cube L15 having a semi-transparent plate reflects a very small part of the beam and provides in the plane F' symmetrical with F with respect to the semi-transparent plate, a crosssection of the beam identical with that obtained in the plane F. It is thus possible to provide, from the plane F', monitoring of the satisfactory operation of the optical system and of the power of the guiding beam: a diaphragm D' having an aperture of very small diameter is positioned in the plane F'. The image of the reflected beam is received at a detector Det.

Figure 6 shows diagrammatically a variation of the arrangement of Figure 5, permitting a better output of the emitted power to be obtained. It can be seen that in the arrangement of Figure 5. the field diaphragm D positioned in the plane F, permits only a part of the beam provided by the optical system to pass.

In the variation shown in Figure 6, the optical elements L3 to L1 3 are identical with those shown in the arrangement according to Figure 5. There is provided in the afocal system comprising the lenses L1 and L2 and the set of plates L3, a thick plate L1 6 having parallel surfaces inclined with respect to the axis and which turns about the axis XX' in synchronism with the prism L1 l of small vertical angle. This plate imposes on the parallel beam emitted by the source (not shown in Figure 6) a lateral deflection in such manner that the said beam always passes through the image of the field diaphragm D which is formed in the plane F.

With this arrangement it is possible to obtain at the output of the afocal system a beam of smaller diameter and thus to obtain a higher flux density. The gain in power obtainable is of the order of 2 to 3 with respect to the power emitted by the guiding emitter.

The invention is not limited to the embodiments described and modifications can be made without departing from the limits of the invention in particular in the number and the form of the sectors, of the distinctive characters of the elementary beams and of the means employed to provide that the beam describes a cone.

For example, the distinctive characters of the elementary beams may be: different wavelength, different modulation frequency in which case the modulator may be electron-optical, different phase with a common frequency of the sectors, impulse coding or state of polarisation of the emitted radiation. Further, the deflecting prism may be replaced, for example, by a liquid prism of variable angle or by any kind of rotary system which is adapted to de flect the beam by an angle 6: dioptric or catadioptric system which is eccentric by a variable amount, thick plate with parallel faces and a variable inclination, plane mirrors associated in a particular manner etc . . .. The arrangement defining the sectors (pattern) may also itself be moved in a circular translatory manner with a variable amplitude.Within the same limits of the idea, the focusing of the light flux may be obtained with the aid of variable focal length condenser lenses.

WHAT WE CLAIM IS: 1. A system for determining the position of a mobile object with respect to a position deter- mining axis using a light beam defining a field of position determination, and comprising a transmitter of the light beam including an emitting source and an objective lens, and a receiver determining the position of the said moving object with respect to the axis of position determination, and the said transmitter comprising:: - means for providing that the light beam comprises elementary beams each having a distinctive characteristic, defining, in a section in a plane perpendicular to the axis of position determination, different sectors, - means to provide movement of the beam such that its axis describes a conical surface defining the field of position determination, the apex of the cone being in the proximity of the optical centre of the objective lens and the section of the light beam in a plane perpendicular to the axis of position determination carrying out a movement of translation applied to the corresponding base of the cone, while the receiver comprises means to measure tlle intervals of time respectively defined by the reception of the elementary beams.

2. A system for position determination as claimed in Claim 1, in which the cone is a cone of revolution of which the axis coincides with the axis of position determination, and of' which the angle at the apex is variable.

3. A system for position determination as claimed in Claim 2, comprising fixed means for providing that the light beam comprises elementary beams and in which the means for moving the beam comprises a deflecting element posi- tioned between the source and the objective lens, centred upon the axis of guidance and capable of rotation about the latter.

4. A system for position determination as claimed in Claim 3, in which the deflecting element is translatable parallel to the axis of position determination in a manner such as to permit the variation of the vertical angle of the cone.

5. A system for position determination as claimed in Claim 3 or 4, in which means are provided to focus the light flux of the beam according to a law such that the relationship between the vertical angle of the light beam and the vertical angle of the cone is about 2.

6. A system for position determination as claimed in Claim 5, in which the objective lens has a variable focal length.

7. A system for position determination as claimed in any of the preceding claims, in which the means for providing that the light beam comprises elementary beams is such that four elementary beams are provided defining four sectors offset relative to one another by a rotation of 90" about the axis of the beam and the receiver comprises means to measure the intervals of time respectively defined by the reception of the said four elementary beams and means for determining the differences between the intervals of time corresponding to each pair of opposite sectors.

8. A system for position determination as claimed in Claim 7, in which the sectors are luminous quarter circles and the receiver comprises means to determine the differences between the durations of reception of radiation corresponding to each pair of opposite sectors.

9. A system for position determination as claimed in Claim 7, in which the four sectors together provide four dark quarter circles separated by two orthogonal axes visible, at least in part, as luminous arms defining a luminous cross centred upon the axis of the beam and the receiver comprises means to determine intervals of time respectively defined by the reception of radiation corresponding to the arms and for determining the differences between the durations corresponding to each pair of opposite sectors.

10. A system for position determination as claimed in any of the preceding claims in which the means for providing that the light beam comprises elementary beams comprises four ele

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. with the prism L1 l of small vertical angle. This plate imposes on the parallel beam emitted by the source (not shown in Figure 6) a lateral deflection in such manner that the said beam always passes through the image of the field diaphragm D which is formed in the plane F. With this arrangement it is possible to obtain at the output of the afocal system a beam of smaller diameter and thus to obtain a higher flux density. The gain in power obtainable is of the order of 2 to 3 with respect to the power emitted by the guiding emitter. The invention is not limited to the embodiments described and modifications can be made without departing from the limits of the invention in particular in the number and the form of the sectors, of the distinctive characters of the elementary beams and of the means employed to provide that the beam describes a cone. For example, the distinctive characters of the elementary beams may be: different wavelength, different modulation frequency in which case the modulator may be electron-optical, different phase with a common frequency of the sectors, impulse coding or state of polarisation of the emitted radiation. Further, the deflecting prism may be replaced, for example, by a liquid prism of variable angle or by any kind of rotary system which is adapted to de flect the beam by an angle 6: dioptric or catadioptric system which is eccentric by a variable amount, thick plate with parallel faces and a variable inclination, plane mirrors associated in a particular manner etc . . .. The arrangement defining the sectors (pattern) may also itself be moved in a circular translatory manner with a variable amplitude.Within the same limits of the idea, the focusing of the light flux may be obtained with the aid of variable focal length condenser lenses. WHAT WE CLAIM IS:

1. A system for determining the position of a mobile object with respect to a position deter- mining axis using a light beam defining a field of position determination, and comprising a transmitter of the light beam including an emitting source and an objective lens, and a receiver determining the position of the said moving object with respect to the axis of position determination, and the said transmitter comprising:: - means for providing that the light beam comprises elementary beams each having a distinctive characteristic, defining, in a section in a plane perpendicular to the axis of position determination, different sectors, - means to provide movement of the beam such that its axis describes a conical surface defining the field of position determination, the apex of the cone being in the proximity of the optical centre of the objective lens and the section of the light beam in a plane perpendicular to the axis of position determination carrying out a movement of translation applied to the corresponding base of the cone, while the receiver comprises means to measure tlle intervals of time respectively defined by the reception of the elementary beams.

mentary sources each having a distinctive characteristic and associated with four elementary condenser lenses directing the four elementary beams emitted by the elementary sources each to the corresponding one of four prisms joined together but separated optically from one another, the means being such as to provide a single image of the elementary sources on the axis of position determination and substantially in the plane of the exit surfaces of the said prisms, the exit surfaces being substantially in the focal plane of the objective lens.

11. A system for position determination as claimed in Claims 3 and 10, comprising position determining means for translation displacement of the deflecting element and the condenser lenses with rates of displacement such that the relationship between the variations of the vertical angle of the light beam and the vertical angle of the cone is about two.

12. A system for position determination as claimed in Claims 9 and 10 comprising four identical anamorphic optical systems arranged in the same plane perpendicular to the axis of position determination at the corners of a square centred upon the latter and for transforming the four elementary beams to provide the four dark quarter circles separated by the two visible orthogonal axes.

13. A system for position determination as claimed in any of Claims 10 to 12, in which the said transmitting source is a continuous source of light, the said means for dividing the beam into elementary beams comprising inter alia a separator having four prisms providing four elementary beams and associated with four optical systems forming four image points of the said source respectively at four different positions of a mechanical modulator which is rotatable with a constant speed about an axis substantially parallel to the axis of position determination in such manner as to constitute the four said elementary sources.

14. A system for position determination as claimed in any of Claims 1 to 13, comprising an arrangement for guiding to the axis of position determination having on the axis of position determination, after the said objective lens, a semi-transparent plate for separating a part of the said light beam in such manner as to constitute a secondary beam of different axis, received by a detector providing information concerning the deviation of the angle of the said secondary beam from an axis of reference defined by the position of the said plate and the said detector and transmitting the said information, after amplification and conversion, to a deviation angle controlling element.

15. A beam transmitting arrangement when used in a system for position determination as claimed in any of the preceding claims, in which an initial parallel monochromatic light beam, is emitted from a single source, is divided into a plurality of differentiated elementary means means are provided to cause the axis of the beam to describe a cone so that a field of guidance of variable vertical angle is defined ac cording to a predetermined programme as a function of the characteristics of mobile object to be guided, in which optical elements com prising an arrangement of thick plates having their parallel surfaces inclined with respect to the axis, first divide the initial beam into a plur ality of sectors constituting a corresponding number of elementary beams and displace them by a predetermined extent from the axis, opti cal means which then focus the image of the el ementary beams on a modulating means which differentiates them, a second arrangement of thick plates having their parallel surfaces in clined with respect to the axis to recombine the differentiated partial beams into a single beam having the same cross-section and the same diameter as the initial beam, deflecting means, op erated by a single movement of rotation to impress on the recombined beam an oscillatory movement so that a guidance field is defined, the variation of the angle at the apex of the cone described by the guiding beam being obtained with the aid of a variable focal length objective lens.

16. A beam transmitting arrangement as claimed in Claim 15, in which the predetermined programme of variation of the focal distance of the variable focal length objective lens is such that the cross-section of the beam in a plane containing the mobile object to be guided has a constant diameter whatever the range of the mobile object.

17. A beam transmitting arrangement as claimed in Claim 15, comprising, situated before the first set of thick plates with parallel surfaces, a thick plate having parallel surfaces inclined with respect to the axis and rotated in synchronism with the deflecting element to subject the parallel beam emitted from the source to a transverse displacement such that the beam always passes through the image of the field diaphragm formed in a plane after the inclined thick plate and before the initial beam is divided.

18. A system of position determination as claimed in Claim 1, substantially as herein described with reference to any of Figures 2 to 6.