Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
13415342
TITLE: MISSILES AND CONTROL SYSTEMS THEREFOR.
This invention relates to homing missiles and
control systems therefor.
A homing missile will generally comprise a
target tracker mounted in the missile tip, which tip
is formed as a window giving the tracker a field-of-
view extending forwardly from the missile tip and,
evenly and symmetrically, all round the missile roll
axis. Normally the tracker is spatially stabilised,
for example by gyroscopic means, so that it maintains
its view of the target despite manoeuvring of the
missile, and that spatially stabilised position is
controlled, for example by gyro precessing means, so
that it maintains its view of the target despite
relative movement of missile and target which produces
a change in the direction of the line-of-sight between
missile and target. Meanwhile, in response to signals
indicative of the relative position of the target
tracker and the missile, a control system guides the
missile according to some chosen navigation law to
intercept the target.
It may be advantageous to move the target
tracker and its w.indow back away from the missile tip
'the shape and material of the tip can then be
optimised, say for the high degree of kinetic heating
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at this point, without having to consider transparency
and lack of distortion for the tracker-sensitive
radiation, while the window may become less subject to and/
or more easily protectable against such kinetic heating.
Then, however, it may be difficult or impossible to
provide the tracker with an all round field-of-view.
In fact, it is not inherently essential that the tracker
should have an all round field-of-view since,
theoretically at least, it is possible, say by
manoeuvring the missile and target tracker, to move
the restricted field-of-view. For example, if the
field-of-view is of rectangular cross-section and
extends between the roll axis and one side only of the
missile, then the missile and tracker can be rolled
around to maintain the target within the restricted
field. Now, however, due mainly to limitations' in
the capabilities of available target trackers, the
space stabilising means for the tracker, and coritrol
systems, particularly the autopilot guidance system
which generally forms part of such a control system,
problems of instability may arise and/or it may
simply not be possible to move the tracker fast-
enough to keep the target in view.
An object of the present invention is to provide
a control system for a missile of which the target
tracker has a restricted field-of-view, the system
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nevertheless permitting adequate performance for
navigating the missile to target interception. A
preferred further object is to provide a control
system which in some circumstances, is able to tolerate
some loss of sight of the target by the target tracker.
According to the present invention, there is
provided a homing missile comprising a target tracker
and drive motor means for producing relative movement
of inembers comprised in the missile including control
members for steering the missile in a lateral direction,
the missile being such that said target tracker has
a field-of-view which is restricted at any instant to
less than an all-round field-of-view and, to compenstLte
for said restriction, said drive motor means is
operable for rotating the said restricted field-of-view
with.respect to the missile roll axis, for example by
controlling roll of the missile itself, the missile
further comprising a control system connected between
said target tracker and said drive motor means and
including look-angle demand signal deriving means for
deriving signals indicative of desired positions of a
target relative to the missile, said signals including
lateral steering signals and also including angle
signals indicative of the angular arguments of the
polar coordinates of said desired position, first
limiting means connected to said signal deriving means
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to receive said angle signals and for limiting these signals
to between two limit values, signal processing means for
receiving said limited angle signals and for forming
therefrom signals for controlling the rotation of said
5 field-of-view by said drive motor means, second limiting
means for receiving said lateral steering signals and
limiting them to less than a limit value, and combining
means connected to said first and second limiting means and
said drive motor means and operable for combining the
limited angle signals and the limited lateral steering
signals to form steering control signals for controlling
lateral steering of the missile.
For a better understanding of the invention,
reference will now be made, by way of example, to the
accompanying drawings in which:-
figure 1 is a diagram illustrating the restricted
field-of-view of the target tracker of a homing missile,
figures 2 and 3 are simplified circuit diagrams of
respective parts of the control system of the missile.
The control system to be described is for use in a
missile (not shown) having a gyro stabilised target tracker
(not shown) with a narrow rectangular or slit-shaped field-
of-view 1 as shown in figure 1. The field 1 includes but is
asymmetrically disposed with respect
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to the missile roll axis X. Axes Y and Z are
Cartesian axes by reference to which there can be
defined respective coordinates LH and LJ indicative of
an actual position of a target relative to the missile
and coordinates LHri and LJn indicative of a demanded
or desired position of the target relative to the
missile, i.e. these latter coordinates are those at
which the missile control system wants the target to
be having regard to the chosen navigation law for the
missile flight. In effect, LJ and LH and LJn and LHn
define actual and demanded look angles between roll
axis X and the sight line to the target. The demands
LJn and LHn may be derived by a part of the control
system constructed according to known techniques and
in accordance with any chosen navigation law which may
be a standard known law. By way of example, the law
may provide 'Acceleration Vectored Navigation' as
described in Guided Weapon Control Systems by P. Garnell
and D.J. East (Pergamon Press 1977) Section 9.9. This
reference also provides information about examples of
the construction and design of parts of the missile and
its control system, such as the tracker and the control
system part which derives LJn and LHn, which may be
standard and are not shown herein. For the example
being described, it is assumed that the responses by
the missile to the navigation demands for desired
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lateral look-angle values (LJn and LHn) are approx-
imately linear in axes not spinning about the missil-e
but corresponding to it.
The part of the control system shown in figure 2
receives the demands LJn and LHn and modifies them in accord-
ance with constraints imposed by the limited field-of-
view l. In order to move the field-of-view 1 to
maintain its view of the target, it is assumed that
the missile and the target tracker are to be rolled
about axis X. This invention is still of course
relevant if such rolling-does not take place. Other
compensating manoeuvres of the missile can achieve
rotational movement of the field or some mechanism
for relatively moving different parts of the missile
can be used to achieve movement of the filed in
addition or alternatively to manoeuvring the missile.
Within the control system part of figure 2,
the Cartesian components LJn and LHn are fed to a
converter device 1 which may be of known construction
and which converts the components to polar coordinates
0 where : -
0 (arg (LJn, LHn) -rc <~ 5 ~t
0 otherwise
and amplitude
~ _ ~(LJn2 + LHn2)
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Block 2 computes the absolute value of 0 and
block 3 is a threshold detector of which the output
signal controls a logic changeover switch 4. If the
threshold value of detector 3 is not exceeded, switch 4
passes the current value of 0 from the converter 1 to
a limiter 6. This current value is also fed via a
delay unit 5 to the second input of switch 4, which
input thereby has available a value of g1 previous to
the current value. If, due to noise or sudden
perturbations, the threshold value of detector 3 is
exceeded, switch 4 passes this previous value of gf
onto the limiter 6 rather than the current value there-
by reducing the tendency towards violent missile
manoeuvres that might otherwise resdl.t. The function
of the limiter 6 is to provide a value 0 which is
equal to the received value of 0 if that value lies
between positive and negative limits or which, if the
magnitude of 0 equals or exceeds that of each limit,
is of the same sign as the received value of 0 but has
a magnitude equal to the limit value. The magnitude
of each limit imposed by limiter 6 is controlled by a
logic changeover switch 7. It is either the polar
coordinate magnitude A multiplied, at gain element 9,
by a gain factor K or it comprises a value Omax derived
as described later. The control of switch 7, and hence
the choice of which of the two mentioned limit values
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is imposed by limiter 6 is also described later.
The output value Od from the limiter 6 is multiplied
by a gain Kp 8 to give the demand Pd to a roll rate
control system. Filtering of Pd using standard
techniques may be required.
The magnitude output X.from converter 1 is also
fed as an input to threshold detector 10 from which a
O or 1 logic output goes to the OR gate 12. The other
input to gate 12 is derived from a threshold detector 11.
The input to detector 11 comprises a signal in3icative
of the target sight line separation from the missile axis.
This signal is calculated by a modulus forming device
from the actual target position signals LH and LJ
supplied by the target tracker. For example the signal
could be calculated as rLH 2 + LJ2. The output Bd
from gate 12 is used to operate the aforementioned logic
switch 7 and also two further logic switches 14 and 15.
As well as to the converter 1, the look angle demand
signal LJn is fed via a series arrangement of two
limiters 16 and 17 to both a combining device 18 and to
an output line 30 of the'illustrated part of the control
system. The limiter 16 sets one of two lower limits LIJ and
LOJ for LJn, the particular limit chosen being set. by
switch 15. Meanwhile, a fixed upper limit for LJn is
provided by limiter 17. The output Ljd from 17 is the
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demand in one axis. A computation by device 18 using the
outputs Od from 6 and LJd from 17 gives L'Hd where
L' Hd = LJd tan Od
The error between L'Hd and LHn is the input to a
limiter 19 controlled by switch 14. The output of limiter
19 is added to L'Hd and is the demand for the other axis
LHd.
The output from converter 1 gives a measure of the
angle between the raw demanded look angle direction and the
current direction of the field-of-view slit. This is used
after some modification to provide an error signal to drive
the missile roll demands. Using the raw look angle demands
instead of the measured look angle to drive the rolling
motion of the missile has the advantage of providing the
missile with advanced warning of the intended rolling motion
so the missile tends to roll in the right direction long
before the measured look angle makes this necessary. In the
presence of noise and roll limiting by limiter 6 the demands
Pd can change sign rapidly causing time to be lost before
the missile is demanded to roll in the correct direction for
instance if the demands are near n . rad. The function
performed by blocks 2, 3, 4, and 5 in the drawing tend to
reduce this by keeping the demand nearly fixed when
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the demands are near +)zradians and the limiter 6 is
included so that the missile does not roll fast
enough to cause an out of plane or underdamped response
from a standard autopilot that is responding to Lhd
and LJd.
The arrangement of 1, 10, 11, 13 and 12 is used
to indicate when the demands and look angle are small
enough to make the missile fly momentarily with the
targetwithin the axis proximate portion A of the field-
of-view, shown in figure 1. The output from 12 Bd is
false (zero) in this case.
.When Bd equals zero i.e. when the target is close
to axis X, the rolling of the missile has little
consistent effect on the.pitching of the missile while
the limited H direction look angle capability of the
target tracker is sufficient to retain the target. Thus,
it is appropriate then to limit the roll control demands
to a magnitude equal to KA instead of the normal fixed
limit Omax. A similar effect could be achievedby
changing the value of the gain Kp given by gain element
8 instead of the limit given by limiter 6. 18 is used
to give a demand for LH that matches the roll demand Od
and thus keeps the target near the LJ axis in figure :2.
Normally the limits in 19 are set to zero when
Bd = 1 and opened up when Bd = 0 so that the lateral
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look angle capability of the missile can be used in
this case. Similarly for the control of the limits
of 16 by 15. The limit 17 is not essential but may
be included to slightly extend the missile tracking
capability in cases when the field-of-view is needed.
In other words, when the magnitude part X of
the polar coordinates of the demanded look angle and/
or the actual separation- between the target sight line
and missile axis, exceeds the respective limits imposed
by limiters 10 and 11, the signal Bd from gate 12 is
logical 1 whereby the limits imposed by limiter 19 are
each zero, i.e. the limiter does not pass on the signal
received thereby and the output demand signal LHd is
equal to the output signal provided by'combiner 18.
Meanwhile the negative limit of LJd is reduced in
magnitude while the limit set by limiter 6 is at its
higher value Omax. In this state then, a certain
proportion of the control of the missile to achieve
apparent movement of the target to the demanded position
relative to the missile is achieved by rolling the
missile, the maximum roll rate set by Omax being chosen
to give adequate stability. When both X and the
actual target/axis separation are less than the
respective limits, the proportion of the controlling
effect performed by roll control is curtailed while
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the proportion performed by lateral steering about the
yaw and pitch axes of the missile, 'i.e. cartesian control,
is increased. In this state, the Z axis demand signal
LHd equals L'Hd plus (Lhn-L'Hd) if (Lhn - L'Hd) is
between the limits L1H imposed by limiter 19 at this
time, i.e. Lhd equals LHn. If the magnitude of
(Lhn - L'Hd) exceeds the L1H, then Lhd equals L'Hd',
plus the limit value.
The signal Pd is fed to a roll rate autopilot
(not shown) of any suitable known construction and
operation. Alternatively, provided that relatively
minor changes are introduced into the function by.
which Pd is produced, for example at the limiter 6, a
roll position autopilot could be used. The afare-,'.:
mentioned reference by Garnell and East illustrates
examples of autopilots and the theory of missile
control needed to adapt the illustrated arrangement
to any particular situation. For example, a suitable
roll position autopilot is described in section 6.10
of the reference.
Figure 2 is illustrated in function block form
since it could be implemented by a variety of electro-
mechanical or electronic devices of nature that will
be clear to those skilled in the art. Advantageously,
however, the apparatus is implemented by one or more
computer processors, particularly microprocessor
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devices, and any necessary associmlle# dnCftobi;Os.
For this, the blocks in the figure should be regarded
as function representative blocks of an overall
algorithm rather than discrete items of hardware.
The signals LHd and LJd may be fed to two identical
autopilots one for pitch and one for yaw control-
suitable examples being shown in Garnell and East
section 6.3. However, the roll capability of the
missile may be improved by advancing the demands in
the direction of rotation as a function of roll rate
to compensate for the lag produced by the actuator.
Fig. 3 shows a possible implementation. Those
skilled in the art will recognise that the priorities
of roll signals and conditioned steering signals may
be varied to advantage for other applications.
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