Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF, AND APPARATUS FOR, FURNISHING INFORMATION TO
DETERMINE THE POSITION OF A BODY
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to the determination of the
7L0 position of a body and more particularly, but not exclusively, is
concerned with the determination of the position within a guidance
beam of a moving body, e.g. a missile, in order that the body may
pursue a predetermined path along the beam.
Prior Art
British Patent Specification No. 1395246 describes a
method of, and apparatus for, sensing the position of a body, the
apparatus described specifically and illustrated in the
20 specification having a reticle which shutters the beam
progressively. This shuttering restricts the radiance of the
transmitted beam and it is an object of the invention to
ameliorate this disadvantage of the prior art system.
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SUMMARY OF THE INVENTION
According to a broad aspect of the present invention
there is provided apparatus for furnishing information sufficient
to determine two polar co-ordinates (r, 0) of a point position
within a two-dimensional bounded space, said polar coordinates
measured with respect to a center of said space and a reference
angle within said space, comprising:
means for generating a beam of radiation, and for
shaping said beam to illuminate a part only of the space, said
part extending from the center to a circumference of the space,
said part including an edge defined by the following equation:
d = r cos [27f Kr]
where d is a distance between said edge and said reference angle
when said part is positioned at a reference position corresponding
to said reference angle, K is a constant falling within the range
of 0 L K Lsubstantially 0.25, and r is normalized to unit
radius,
means for revolving the beam around the center of the
space whereby the beam is incident upon the said point position
during discrete spaced periods of time a duration of said periods
of time defining said r coordinate, and
means for modulating the beam in a phase with the
revolution of the beam thereby to define said 0 co-ordinate.
In one embodiment the apparatus comprises means for
generating a shaped, pulsed laser beam and means (e.g. a Pechan
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prism) for rotating it around an axis. The pulse frequency is
high enough to provide a multitude of pulses in each revolution of
the beam. The shape may be chosen such that the duration of
illumination of any point in a bounded space illuminated by the
beam is indicative of the radical distance r of the point from the
axis, while the frequency of repetition pulses varies cyclically,
in phase with the rotation of the beam, so that the repetition
frequency of pulses received during each period of illumination is
indicative of the angular displacement 8 of the point from a
reference axis.
It will be appreciated that, in this last-mentioned
arrangement, sufficient information is provided in each
illumination period to identify both co-ordinates of the point.
In prior Specification No. 1395246, a set of two consecutive
periods is needed, to convey this amount of information.
Nevertheless, the present invention is not restricted to
arrangements in which a single period provides, by itself, fully
comprehensive positional data.
The method of invention preferably includes the steps of
(i) providing a beam of radiation comprising a first beam
component and a second beam component, each said
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component being so shaped as to illuminate a part only of
said space, (ii) repeatedly scanning the first beam
component across the space in a first scanning direction
at a first scanning frequency, (iii) repeatedly scanning
the second beam component across the space in a second
scanning direction inclined to the first scanning
direction and at a second scanning frequency, and (iv) so
modulating the first beam component in phase with the
first scanning frequency and so modulating the second
beam component in phase with the second scanning
frequency that the beam radiation incident on the point
position from the first and second beam components
provides at the point sufficient information to identify
both of the two co-ordinates.
Conveniently, the first and second scanning directions
are mutually perpendicular and the first and second
scanning frequencies are equal, with the first and second
beam components scanning alternately in time across the
bounded space.
The apparatus of the invention preferably comprises means
to generate a beam of radiation comprising a first beam
component and a second beam component, each said
component being so shaped as to illuminate a part only of
the space, means for repeatedly scanning the first beam
component across the space in a first scanning direction
at a first scanning frequency, means for repeatedly
scanning the second beam component across the space in a
second scanning direction inclined to the first scanning
direction and at a second scanning frequency, and means
for so modulating the first beam component in phase with
the first scanning frequency and so modulating the second
if
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beam component in phase with the second scanning
frequency that the beam radiation incident on the point
position from the first and second beam components
provides at the point sufficient information to identify
both of the co-ordinates.
Conveniently the apparatus comprises means for generating
first and second shaped, pulsed laser beam components and
means for sweeping them across the space in first and
second scanning directions respectively. The pulse
frequency of each component is high enough to provide a
multitude of pulses in each scan of that beam component
across the space. In one embodiment, the frequency of
pulses of each component varies in phase with the sweep
of the beam component, so that the frequency of pulses
received during each period of illumination is indicative
of the displacement in predetermined X and Y directions.
Apparatus in accordance with the invention may be used in
conjunction with a radiation sensor to be secured to a
body, and position computing means, either on the body or
remote from it, which in use receives as input
information as to the radiation incident upon the sensor.
The invention has particular application to remote
guidance of a body (e.g. a guided missile) moving along a
laser guidance beam. According to the invention,
therefore, there is also provided, in combination, a body
to be remotely guided along a pulsed modulated guidance
beam and comprising a radiation sensor and position
computing means programmable with information about the
modulation of the beam sufficient for computation of the
position of the body within the cross-section of the beam
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relative to a longitudinal axis of the beam by inspection
of an output from the radiator sensor, and means for
generating said modulated beam, including means for
moving the beam cyclically over a bounded space and means
for modulating the beam in phase with the said cyclical
movement.
Although the invention is applicable to any moving body
whether it be on land, in/or on the sea, in the air or
outer space, it is particularly suited to moving bodies
such as missiles which incorporate means for modifying
the trajectory of the body. It can also be applied to
moving bodies which do not possess such means but which
are required to receive, at any moment, data concerning
the position of the moving body with respect to a given
trajectory or sight line. The invention may be applied
to moving bodies whose orientation in space is required
to be determined at a fixed or moving point away from the
moving body.
The invention has particular application to a missile
guidance system in which a laser beam control pattern is
directed at a target and a missile fired into the beam
pattern. The missile carries on its rear end a sensor
which is operative to sense a spatially modulated laser
beam and feed signals in response thereto to a control
device within the missile which produces signals
dependent on the position of the missile in the
cross-section of the control pattern. Once this position
has been calculated the missile is controlled to move
towards the centre or other designated position within
the control pattern. By tracking the target with the
control pattern, the missile is guided to the target.
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BRIEF DESCRIPTION OF THE DRAWINGS.
Reference will now be made, by way of example, to the
accompanying drawings, in which:
Fig. 1 is a diagram of a first control beam shape in
cross-section showing the meaning of parameters r, R and
9 and identifying four points D within the beam
cross-section;
Fig. 2 is a graph of received beam intensity I against
time, expressed as one cycle of revolution of the beam
around the axis 11, for the four points shown in Fig. 1
the graph indicating the form of the beam as it is
incident upon each of the four points D;
Figs. 3 and 4 are graphs which further characterise the
form of the beam section shown in Figs. 1 and 2;
Fig. 5 is a graph similar to Fig. 3 showing a second
shape of a beam section;
Fig. 6 is a graph showing a third shape and Fig. 7 is a
graph which characterises the form of that beam section;
Fig. 8 is a view from one side of apparatus according to
the invention for generating a beam having a section as
shown in Figs. 1, 2, 3 and 4;
Fig. 9 is a diagram of a control beam in cross-section
showing the meanings of parameters X and Y and
identifying four points within the beam cross-section;
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Fig. 10 is a graph of received beam intensity I against
time expressed as one scan cycle of the two beam
components, for the four points shown in Fig. 9, the
graph indicating the form of the beam as it is incident
upon each of the four points P;
Fig. 11 is a diagram of an optical arrangement of double
source/double scanner/double aperture beamriding
transmitter, being a first embodiment of apparatus
according to the invention for generating the beam of
Fig. 9; and
Fig. 12 is a diagram of an optical arrangement of a
double source/single scanner/double aperture beamriding
transmitter, being a second embodiment of apparatus
according to the invention for generating the beam of
Fig. 9.
DETAILED DESCRIPTION.
With reference to Figs. 1 to 7 of the drawings, and in
particular Fig. 1, spatial modulation of a control beam
cross-section 10 is achieved by (1) rotating a laser beam
in a direction f about the central axis 11 of a circular
control pattern area 12 having radius R, (2) pulsing the
laser beam at a frequency which varies with its angular
displacement (e.g. 9 after time t) from a vertical or
other reference direction 13 in the plane of the control
pattern. Each revolution of the laser beam takes time T.
During it, the pulse repetition frequency (or pulse
interval) is varied between two limits, and the cycle is
continually repeated at a pre-determined rate.
Measurement by a sensor D of the time interval Tg between
it
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received laser pulses provides a measure of the position
of the sensor in terms of an angular displacement 9 from
the reference direction 13, and (3) so shaping the
control beam that the total time Tr during which a sensor
receives electromagnetic waves per revolution of the beam
pattern provides a measure of the position of the sensor
in terms of a radial displacement r from the axis of
rotation 11 of the beam.
Figs. 1 to 4 illustrate the simple case of a beam so
shaped that during rotation thereof at a steady angular
velocity w, the proportion Tr of the total time T for one
revolution of the beam during which laser radiation is
incident upon a point varies strictly linearly with the
radial distance r of that point from the rotational axis
11 of the beam 10, and varies from 25% for a point just
off the axis of the beam to 0% for a point on the very
edge of the beam. This has been termed a "rotating leaf"
beam modulation system.
Figs. 1 and 2 illustrate schematically the variation in a
series of electromagnetic (laser) pulses 14 received for
various sensor positions (Dl, D2, D3 and D4) within the
area 12 controlled by the beam 10.
Referring to Figs. 3, 4 and 5, a range of beam shapes
governed by the expression
d = r cos[2TVK r]
possess the characteristic that the time Tr during which
electromagnetic waves are received by a sensor D
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decreases linearly with increasing radial co-ordinate r.
These shapes are identified herein as Shape I.
With K = 0.125 the second curve illustrated in Fig. 5 is
generated. However, other values of K within the range
0-0.25 could be chosen, these curves all providing a
linear change (with radial position) of the time during
which electromagnetic waves are received by the sensor.
An alternative range of shapes (Shape II) may be
generated, and these are characterised in that the time
during which electromagnetic waves are received by the
sensor D increases with increasing radial co-ordinate r.
These shapes correspond to those generated when the first
set Shape I are subtracted from a quadrant of a circle to
yield, for example, the unshaded area in Fig. 5.
A third set of shapes (Shape III) may be generated by the
rotation of a beam of rectangular cross-section about one
corner of the rectangle. Fig. 6 illustrates the beam
shape and the variation of time of receipt of
electromagnetic radiation with radial co-ordinate r. It
will be seen from Fig. 7 that in a circular region radius
d around the centre 11 of the pattern no precise radial
co-ordinate information is available though angular
information may be derived from the pulse interval Te in
the same way as the first and second sets of-shapes.
Outside this central region the detector will receive
both radial and angular co-ordinate information. The
relative size of the central region is determined by the
width/length ratio d/L of the rectangular shape. Various
d/L ratios may be considered for example ratios from 1 to
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0.067 and below may be easily employed in practical
beamriding transmitters.
The use of a rectangular beam has the advantage of simple
beam shaping (with particular laser sources). This
system would be useful in special beamriding systems
wherein the body being guided is required to either
remain in the general region (x < d) around the centre of
the control pattern and/or be guided to precise points
outside this central region x > d.
Although a multitude of rotating beam shapes are suitable
for position definition these three particular shapes are
of special significance in missile beamriding systems
because of factors such as the ease of generation of
particular shapes of beams of electromagnetic waves or
the linearity of the sensor response to the modulated
radiation received.
Of the three types already identified, Shape I has the
most general application to missile beamriding systems,
while Shapes II and III may find application in
particular situations.
Referring again to Fig. 3, it will be seen that the beam
shape comprises a straight radial edge 16 which, when 9=
0 is co-incident with the reference direction 13, a
curved edge 17 and a short circumferential edge 18
extending in a circumferential direction to complete the
shape but which is, in the Fig. 3 limiting case, of zero
length. The beam generated by an aperture or other means
may be arranged to rotate in either direction by the beam
rotation system.
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The nature of the curved edge 17 and the relative
length of the short circumferential edge 18 may be varied to mould
the linearity of the radial information provided to the missile
guidance system. In this first example, Shape Ia, Tr decreases
linearly with increasing radial co-ordinate value r, in accordance
with the expression:
d = r cos f?r.r7
2
where d and r are defined in Fig. 3.
The shape may be chosen such that the length of the
circumferential edge 18 is finite thereby ensuring that the
receipt time of electromagnetic radiation by the sensor does not
approach zero at the edge R of the controlled space 12. The
following are examples of ways which may be used to achieve this:
(1) A simple removal of the aperture tip illustrated by the
dashed circumferential edge 19 in Fig. 3.
(2) Design of a curved edge which satisfies a general
expression of which equation (1) is a particular example.
The resolution of the detecting apparatus for beam
shapes I and II in both the angular and radial directions varies
with the angular coordinate.
With beam shape III, the resolution of the detecting
apparatus in both angular and radial directions again vary with
the angular position from the reference angle.
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Referring to Fig. 8 the illustrated embodiment of optical
beam transmitting apparatus comprises a divergent laser
source S providing a laser beam 20 which is collimated by
a fibre optic integrating system 21.
Alternatively the diverging beam can be collimated by a
collimating lens, the collimated beam being partially
diffused by a diffusing plate or optical scrambler rod.
The resultant substantially parallel beam 22 is then
passed through a fixed shaped aperture 23 which is
provided in a housing (not shown) which has an interior
matt black finish. The shape of the aperture 23
corresponds to the shape of the transmitted laser beam
20.
Arranged in front of the fixed aperture 23 is a beam
rotation optical system 24 (in this case a pechan prism)
which is driven by a motor (not shown) so as to rotate
the shaped beam normally at a constant angular velocity
on an axis 25 passing through the centre of the beam
rotation optical system and one end of the aperture. The
beam then passes to a main optical system indicated by a
zoom lens 26 which focuses the laser beam as required.
The lens is of the "flick on" type, in that it features
step changes in optical gain to match what would be
expected for the maximum and minimum optical gain of a
normal zoom lens.
A laser trigger mechanism linked to the beam rotation
system pulses the laser as the beam is rotated. The
laser trigger consists of a light source 27 and light
sensor 28. The light transmitted by the source 27 is
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modulated by a pulsing pattern comprising alternate
opaque and transparent regions provided around the
periphery of a reticle 29 mounted to the beam rotation
system. The sensor produces, in response to the pulsed
light signals, a pulsed output signal which triggers a
solid state switch 30 to intermittently connect a source
31 of high power to the laser source to produce
corresponding pulses of the laser beam. A progressive
increase or decrease in the width of the opaque areas
around the circumference of the reticle will produce
during each rotation an increase or decrease in the time
interval between pulses of the laser beam within an upper
and lower limit.
With reference to Figs. 9 to 12 of the drawings and in
particular Fig. 9, spatial modulation of the control
pattern cross-section is achieved by (1) sweeping a first
beam component 40 across a rectangular control pattern
area 41 in a first scanning direction X and at a first
scanning frequency (No. of scans in unit time), (2)
sweeping a second beam component 42 across the same
control pattern area 41 in a second scanning direction Y
which is perpendicular to direction X and at the same
scanning frequency, the scans of the second beam
component 42 being arranged to illuminate points within
the control pattern area 41 at times which alternate with
the times of illumination of the point by the first beam
component 40, and (3) pulsing each beam component at a
frequency which varies with its displacement (x or y)
from the origin 43 of the X, Y scanning axes in the plane
of the control pattern. During each sweep of a beam
component, the pulse repetition frequency (or pulse
interval) is varied between two limits, with alternate
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sweeps preferably possessing different limits and non-overlapping
ranges, and the two sweep cycles being continually repeated at a
predetermined rate.
A sensor is actuated by laser radiation of the pulse
train of each sweep of a beam component. Measurement by the
sensor of the time intervals (Tx, Ty) between received laser
pulses in two consecutive pulse trains provides a measure of the
position of the sensor relative to mutually perpendicular
reference axes.
Fig. 10 illustrates schematically the variation in a
series of electromagnetic (laser) pulses received for various
sensor positions (P1, P2, P3, P4) within the control beam.
Generation of the control pattern by alternate sweeps of
a rectangular shaped beam in orthogonal directions may be achieved
by a variety of optical systems, for example, a single or double
scanner, a single or double source and a single or double lens
(i.e. aperture). Figs. 11 and 12 respectively illustrate two
optical arrangements selected from the range of possible
scanner/source/lens combinations.
The choice of arrangement for a particular application
will be governed by overall system design considerations.
Referring to Fig. 11, the illustrated embodiment of
optical beam transmitting apparatus comprises a first 50 and a
second 51 divergent laser source (for example laser
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diodes) providing first 52 and second 53 laser beams
which are coupled into first 54 and second 55 fibre optic
elements possessing rectangular cross-section output
faces. The divergent beams from the shaped rectangular
sources then pass to a main optical system indicated by
first 56 and second 57 lenses which focus the beams as
required.
Alternatively (1) the diverging beams from the laser
sources can be collimated by collimating lens systems,
the collimated beams being partially diffused by
diffusing plates or optical scrambler rods. The
resultant substantially parallel beams are then passed
through fixed rectangular shaped apertures to the main
optical system or (2) the diverging beams can be both
collimated and shaped by double cylindrical lens systems
within the main optical system.
Arranged between the rectangular shaped sources 54 and 55
and the main optical system 56 and 57 or alternatively
after the main focussing lenses 56 and 57 are first 58
and second 59 scanning mirrors which are driven by first
60 and second 61 torque or other motor so as to sweep the
beam usually at a constant angular velocity alternatively
in orthogonal directions about the control region.
Fig. 12 shows a modification in which a single, double
sided, scanning mirror 80 replaces the two scanning
mirrors 58 and 59, being driven by a single motor 81.
After reflection at the mirror 80, the beam 52 is
directed to the lens system 56 by a pair of mirrors 82,
and the beam 53 by mirrors 83.
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Laser trigger mechanisms linked to the beam scanning
systems pulse the appropriate laser as the beam is
scanned. Each laser trigger consists of a 'pick-off' or
sensor 90,91 which senses the position of the scanning
mirror and feeds information thereon to a control device
92 which produces a pulsed output signal to trigger a
solid state switch 30, in phase with the scanning mirror
positions, intermittently connecting a source 31 of high
power to the laser source to produce corresponding pulses
of the laser beam. During each scan of the mirrors each
control device 92 will produce a progressive increase or
decrease in the time interval between pulses of the laser
beam within upper and lower limits.
Alternative Apparatus is possible, for example:
A continuous wave laser (or a non-lasing pulsed or
continuous source) may be used instead of a laser pulsed
at source. The output from a continuous wave laser could
be pulsed i.e. its intensity modulated before or after
beam shaping using for example:
(a) Mechanical shutters for example a rotating reticle
comprising alternate transparent and opaque sectors, in
which the width of the opaque sectors increases or
decreases around the reticle. In Figs. 11 and 12 the
scanning mirrors are driven appropriately in phase with
the beam shuttering.
(b) Electro-optic, acousto-optic or other shutter
triggered from the beam rotation system or main control
device.
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(c) Direct laser cavity (Length) modulation inducing
wavelength modulation (and received intensity modulation)
by end mirror displacement using piezoelectric elements.
Alternative trigger mechanisms may be considered. These
may be triggered directly from the beam scan by
electromechanical or electro-optic means or from the beam
by electro-optic means for example:-
(a) Electronic pulse interval variation using an
electronic ramp triggered pulse per beam scan from a
light source/reticle/light sensor system.
(b) Monitoring the generated beam position (i.e.
direction by means of a quadrant detector or other
optical position sensitive device.
Furthermore r and 8, or x and y positional information
could be provided by changing a different characteristic
of the beam other than the time interval between pulses
of electromagnetic radiation for example:
(i) The wavelength (colour) of the beam may be varied as
the beam is scanned across the control region, by means
of a tunable laser source or optical filter.
(ii) The intensity may be varied using a variable
density optical filter or variable input laser drive
power.
(iii) The axis of polarisation of a linearly polarised
laser beam may be varied as the beam is scanned across
the control region (e.g. the axis may be varied by 180
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as the beam is rotated by 360 ) and a polarisation
sensitive detector(s) mounted on the moving body.
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