GB2093213A

GB2093213A - Optical Radiation Receiving Apparatus

Info

Publication number: GB2093213A
Application number: GB8204108A
Authority: GB
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1981-02-16
Filing date: 1982-02-12
Publication date: 1982-08-25
Also published as: GB2093213B

Abstract

Radiation receiving and image stabilising apparatus particularly for use as a missile seeker head includes relatively movable optical wedge forming elements 11, 12 of which one 12 is gyro-stabilised and a focussing lens 2 upstream of the wedge and operable to focus a radiation image onto a detector 3 downstream of the wedge. The gyro 5 is impelled by air jets 8 and floats as a gas bearing (air inlets 7). <IMAGE>

Description

SPECIFICATION Optical Radiation Receiving Apparatus This invention relates to optical radiation receiving apparatus. More particularly, but not exclusively, it concerns a gyroscopical ly stabilised electro-optical seeker head for guiding a missile or projectile towards a source of optical radiation or target. The term "optical" is used here and in the rest of the specification in its wider sense to include wavelengths, such as IR wavelengths, outside the visible range.

In a known gyroscopic seeker, a gyroscopic rotor carries a reflective element which receives radiation from a target and directs it to a fixed photo-sensitive element such that the element forms an electric signal indicative of the target position in relation to the rotor. Using this signal, the rotor can be precessed until its axis of rotation lines up on the target and then the missile is guided along that axis. It is desirable that the target image be stabilised with respect to the photo-sensitive element to maintain the target in view despite any pitch or yaw oscillations of the missle.To provide a certain amount of such stabilisation, it has been proposed to provide a second reflective element to reflect radiation received from the source via a first such element to a photosensitive element mounted close to the point about which relative oscillation of the missile and gyro rotor takes place.

There has also been proposed an image stabiliser, for a camera, telescope or the like, in which a frictionably driven, generally spherical gyroscopic rotor having a hole therethrough has a lens mounted at one end of the hole so that the lens is gyroscopically stabilised. A fixed lens is mounted next to the one mounted to the rotor and the two lenses of which one has a convex surface facing a concave surface of the other, form a variable optical wedge such that a beam of light, received at an optical focussing system of the camera or the like via the fixed and stabilised lenses and the hole in the rotor, is maintained steady in the face of movement of the image stabiliser and camera relative to the scene from which the light is received.In this proposed image stabiliser, the light from the scene viewed is received first at the fixed lens which, with the stabilised lens, forms the variable optical wedge while the focussing lenses for the camera or like device are all downstream of the rotor.

According to one aspect of the invention, there is provided optical radiation receiving apparatus comprising a receiving element for receiving a beam of optical radiation, and an optical system including converging lens means for focussing said beam and, between the lens means and the receiving element, two relatively movable transmissive elements arranged for said beam to pass through the transmissive elements from one to another side of each and together operable to deflect the beam by an amount dependent upon the relative positions of the two transmissive elements, the apparatus further including stabilising means for stabilising the position in space of one of said transmissive elements and thereby to control the deflection of said beam and stabilise it with respect to said receiving element.

Advantageously, said stabilising means comprises a gyroscope rotor coupled to said one transmissive element. The receiving element may comprise a radiation sensitive element for forming electrical signals representative of received radiation. Said two transmissive elements can comprise two members made of material transparent to said optical radiation and operable to refract the radiation beam at respective surfaces thereof by an overall amount dependent upon the relative orientation of the surfaces.

According to a second aspect of the invention there is provided optical radiation source seeker apparatus comprising support means, a gyroscope rotor supported by the support means for rotation with respect thereto and for movement relative to the support means to maintain its spatial position a radiation sensitive element for receiving a beam of optical radiation from said source, and an optical system including fixed converging lens means for focussing said beam and, between the lens means and the sensitive element, two relatively movable radiation transmissive elements arranged for said beam to pass through the two transmissive elements from one to another side of each and together operable to deflect said beam by an amount dependent upon the relative positions of the two transmissive elements, one of said transmissive elements being fixed with respect to said support means and the other being coupled to said gyroscope rotor for movement relative to said one element to stabilise said beam with respect to the radiation sensitive element.

The seeker apparatus may comprise an aperture for receiving radiation from the source, the gyroscope rotor may have an aperture therein, through which aperture said beam passes to reach the radiation sensitive element, and said other transmissive element may be positioned in said aperture while the said one tranmissive element is arranged closely adjacent the upstream side of the other.

According to a third aspect of the invention, there is provided an electro-optical seeker head consisting of a body mounted lens and a body mounted detector with its sensitive element in the image plane of the lens with, in an intermediate position, two optical elements, one being mounted to the body and the other being mounted to a free gyroscope, the two elements having respective adjacent curved surfaces of which the radius of curvature is substantially equal to the distance of the surfaces from the centre of suspension of the gyroscope.

The two optical elements may be plano-convex and plano-concave respectively. Then, advantageously the plano-convex element is attached to the gyroscope rotor and revolves about its axis of symmetry.

The gyroscope may consist of a gas bearing supported part spherical rotor.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, the single figure of which is a sectional view of part of an electrooptical seeker head for guiding a missile or projectile.

The illustrated seeker head comprises a housing 1 which is mounted in a projectile (not shown) and carries in an opening at its forward end a lens 2 which forms an image of the forward view on the sensitive surface 3 of a detector system 4. The detector system is rigidly mounted to the body 1 by fixing means (not shown).

Mounted in a part spherical space within the body 1 is a spherical gyro rotor 5, separated from the body 1 by a radial gap 6 which in operation is part of a spherical gas bearing, fed from numerous orifices 7 connected to a high pressure gas supply (not shown). The gas bearing may be a conventional aerostatic bearing or one of the type known as 'compliant' i.e. in which one surface of the bearing is made from a compliant material.

The rotor 5 is accelerated to a high angular velocity about an axis parallel to the optical axis of lens 2 by an air jet system 8 acting on depressions 9 in the surface of the rotor 5. Rotor 5 has a cylindrical hole 10 through its centre parallel to its axis of rotation. Mounted between the detector 4 and the lens 2 are radiation transmissive optical elements 11 and 12 attached to the body 1 and rotor 5 respectively. The refractive index of each of the elements 11 and 12 is greater than 2. Elements 11 and 12 are plano-concave and plano-convex respectively with approximately equal radii of curvature such that element 12 may rotate with rotor 5 about any axis relative to element 11 and still maintain a constant and small gap between them.This necessitates the radius of curvature being equal to the distance between the gap and the centre of the spherical surface of rotor 5. The plane face of element 11 is normal to the optical axis of lens 2 and the plane face of element 12 is normal to the axis of rotation of rotor 5.

Note that though in the figure the detector sensitive surface 3 is shown at the centre of the rotor 5 this is not an essential feature.

It is a function of the gyroscopicaily stabilised seeker head to maintain the image of the forward scene produced by lens 2 and optical elements 11 and 12 in a constant position on detector sensitive surface 3 when the body is rotated by some angle in pitch or yaw. This is ensured, in the system described and shown, for small angles of rotation if relative positioning of the optical components is described by the following eauation: b=f+(,u-1 )t (equation 1) 1 (-1 )a=(-2)b-(± -2)t At (equation 2) where fis the focal length of lens 2 ,u is the refractive index of elements 11 and 12.

t is the combined thickness of elements 11 and 12.

b is the distance between the lens 2 and the detector sensitive surface 3 a is the distance between the lens 2 and plane face of element 11.

It is usually desired to fit a seeker head with angular position pick-offs and/or precession means by which the direction of look may be steered. These can be any of the means well known in the art i.e. the pick-off may consist of a photo-electric device 13 which views a pattern marked on the surface of the rotor 5. Alternatively a magnetically soft armature 14 attached to rotor 5 may form part of the inductive circuit 1 5 attached to the body 1. The difference in inductance between circuit 1 5 and a similar circuit (not shown) on the opposite side of armature 14 is a measure of the relative angle between rotor 5 and body 1. An inductance device as described can also be used to apply precessional torques to rotor 5.

In use gas such as air is supplied to the gas bearing from the supply thereof (this may be before launch of the projectile if the launch shocks are low or after launch if the launch shocks are high) and then to the jets 9 for a short period until the rotor has reached its desired high angular velocity about an axis parallel to the optical axis of lens 2 and for the remainder of the flight the rotor is allowed to coast. If the projectile is deviated from the original flight direction, optical element 11 and 12 form an optical wedge of angle equal to this deviation which, if the conditions of equations 1 and 2 are satisfied will maintain the original line-of-sight on the detection surface 3.

Signals from the gyroscope angular pick-offs and/or signals to the gyroscope torque motors are used in the guidance of the projectile as is well known in the art.

The plane face of element 12 may make a small angle to the axis of rotation, so scanning the image of the viewed scene in a circular path over the face of the detector 1 6. This causes the detector signals to be modulated at the frequency of rotation of the rotor and enables improved signal processing techniques, known in the art, to be used. The elements 11 and 12 need not necessarily each have one plane face. Instead, the relevant faces could be convex and/or concave to give a desired optical effect.

It will be seen that the seeker head described herein with reference to the drawing does not use any reflective elements for directing the source radiation to the surface 3 and for that and other reasons constitutes a head which, suitably designed, might be made at less cost and with a ruggedness of particular benefit for example for a high acceleration subject device such as a gun launched projectile.

Because the lens 2 is mounted upstream of the elements 11 and 12, the latter elements can be substantially smaller than if they received light direct from the source.

Also, the lens 2 can perform the function of a front-window which would be needed if the lens 2 were downstream of elements 11 and 12 in order to protect the elements and gyroscopic rotor from the airstream, i.e. because the lens 2 is upstream of elements 11 and 12, this lens protects the elements and rotor and no front window is needed.

It is not essential that the refractive index of element 11 be the same as that of element 12, in which case the average of the two indices can be used for the term y in equations 1 and 2 herein.

The term t means the sum of the individual thicknesses, along the optical axes, of elements 11 and 1 2 and not the distance from the front face of one to the back face of the other.

Generally, however, the length of the air-gap between the two elements is so small that these two values will be very little different from one another.

Claims

1. Optical radiation receiving apparatus comprising a receiving element for receiving a beam of optical radiation, and an optical system including converging lens means for focussing said beam and, between the lens means and the receiving element, two relatively movable transmissive elements arranged for said beam to pass through the transmissive elements from one to another side of each and together operable to deflect the beam by an amount dependent upon the relative positions of the two transmissive elements, the apparatus further including stabilising means for stabilising the position in space of one of said transmissive elements and thereby to control the deflection of said beam and stabilise it with respect to said receiving element.

2. Optical radiation source seeker apparatus comprising support means, a gyroscope rotor supported by the support means for rotation with respect thereto and for movement relative to the support means to maintain its spatial position, a radiation sensitive element for receiving a beam of optical radiation from said source, and an optical system including fixed converging lens means for focussing said beam and, between the lens means and the sensitive element, two relatively movable radiation transmissive elements arranged for said beam to pass through the two transmissive elements from one to another side of each and together operable to deflect said beam by an amount dependent upon the relative positions of the two transmissive elements, one of said transmissive elements being fixed with respect to said support means and the other being coupled to said gyroscope rotor for movement relative to said one element to stabilise said beam with respect to the radiation sensitive element.

3. An electro-optical seeker head characterized by a fixed focussing lens and a fixed radiation sensitive element at the image plane of said lens and, in an intermediate position, two optical elements of which one is fixed and the other is supported by gyroscope means and which have respective adjacent curved surfaces of which the radius of curvature is substantially equal to the distance of the surfaces from the centre of suspension of the gyroscope, the seeker head being such that, at least substantially, the following relationships are satisfied, that is to say: b=f+(-1 )t 1.

(At-1 )a=(,u-2)b-(-2+1,'t 2.

where t is the focal length of said focussing lens, y is the refractive index of said optical elements, t is the combined thickness of said optical elements, b is the distance between said focussing lens and said radiation sensitive element, and a is the distance between said focussing lens and said optical elements.

4. A seeker head according to claim 3, wherein said optical elements are respectively plano convex and plano-concave.

5; A seeker head according to claim 4, wherein the plano-convex element is attached to said gyroscope means.

6. A seeker head according to claim 3, 4 or 5, wherein one of said optical elements is so inclined as to cause scanning of the viewed scene over the face of said sensitive element in a circular path.