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Description  |
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BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates generally to a stabilized sighting device
suitable for use on all types of vehicles, while of particular interest on
low speed aircraft such as helicopters.
Most stabilized sighting devices utilized in the past have included a core
unit provided with a gyro and including the payload, which may include
sensors and/or illuminators, mounted on a second unit for rotation about
an axis (typically an elevation axis). The second unit is mounted on the
vehicle for rotation about another axis (typically a bearing or azimuth),
transversal to the first axis, by a motor or torquer controlled by a
servoloop.
Often times, the device should have the ability to operate within a large
range of positive and negative angular positions about the elevation axis.
For instance, aiming sights for land vehicles, nose mast or fuselage
mounted sights for helicopters should accept movements of considerable
amplitude while maintaining a satisfactory accuracy and resolution in
spite of movements and vibrations of the support. On the other hand,
stabilization about the line of sight is generally unnecessary.
In such prior art devices where the motor for angularly locating the second
(bearing) axis is controlled by a servo control loop, parasitic or
perturbating inertial torques C.sub.e are applied about that axis, which
constitutes an external axis:
C.sub.e =(I.sub.g +I.sub.v).omega. tan s (1)
where
I.sub.g is the moment of inertia of the first unit (bearing gimbal in
general) alone, about the external axis (bearing axis);
I.sub.v is the moment of inertia of the core about the sighting line;
.omega. is the rotational acceleration about an axis perpendicular to the
other two axes (elevational axis and bearing axis), i.e. an axis which is
the roll axis at zero bearing;
s is the elevation angle.
Torque C becomes high when tan s becomes substantial, i.e. when the system
is far away from the canonical position, in which the sighting line is
orthogonal to the two rotation axes (bearing and elevational axes).
In view of the large values of torque C at high elevation angles, proper
design of the system and determination of the stabilizing residuum imply
that the actions to which the device will be subjected (particularly
angular accelerations and energizing frequencies) are perfectly known,
which is far from being always the case.
This problem is very different from those which arise in the construction
of platforms stabilized along three axes and comprising gimbals mounted
about pitch, roll and azimuth axes; the latter problems include flip of
the roll gimbal when the amount of pitch exceeds 90.degree.. The use of
two roll gimbals in cascade has been provided in this case, i.e. an inner
gimbal having a small range of freedom and an outer gimbal equipped with a
motor controlled by a synchrotransmitter carried by the internal gimbal
(U.S. Pat. No. 3,188,870): the purpose of this arrangement is to
facilitate flip. Applicants have found that a fundamentally different
problem exists in respect of sighting devices in which the core has a much
higher mass and inertia and consists in decreasing the incidence of the
characteristics of the carrier on those of the device, by minimizing the
multiplication coefficient of .omega. in the expression of the
preponderant parasite torque in formula (1).
It is an object of the invention to provide a sighting device in which the
tangent term of formula (1) is rendered small by maintaining the
stabilizing system in a position close to the canonical position, whatever
the elevation of the sighting line.
It is a more general object to provide an improved sighting device which
has a high degree of accuracy when mounted on a vehicle.
According to the invention, a stabilized sighting device comprises a first
gimbal unit mounted for rotation about an external axis, a drive motor for
rotating said first gimbal unit about said external axis, a second gimbal
unit mounted for rotation on said outer unit about an intermediate axis
perpendicular to said external axis, and a sight unit provided with a
gyroscope and mounted on aid second gimbal unit for rotation within a
predetermined limited angular range about an inner axis located in a plane
perpendicular to said intermediate axis. First and second motor means are
provided for driving said second gimbal unit and sight unit about said
intermediate and inner axes respectively. First and second servo loop
circuits are connected to first and second outputs of said gyroscope for
controlling said first and second motor means, respectively. Detector
means are arranged to deliver a signal representative of the deviation of
said first gimbal unit from the canonical position. A position reproducing
loop controlled by said detector means actuates said drive motor.
In such a device, stabilization is obtained by controlling the motors of
the angular movement servo loops about the intermediate and inner axes
(elevational and "lateral" movement axes in most cases) directly from
signals supplied by the gyro, whose spin axis is located parallel to the
line of sight. A correction network will be provided in each loop for
ensuring stability of the servocontrol.
On the other hand, the motor for moving the first gimbal unit angularly is
controlled by a simple position copy system. The latter should fulfil one
important condition: under all operating conditions, it must have a
sufficiently low time constant for the angular deflection assumed by the
sight unit about the inner axis to be small, typically within a range
which does not exceed one to a few degrees. Thus, the sensitive axes of
the gyroscope and the rotational axes about which rotation is controlled
by the servo-control motors always remain practically co-linear and no
network for compensating variations of the mechanical gain depending on
the angular extent of movement about the intermediate axis (elevation
angle) is necessary.
The position copying loop will comprise a compensation network for
compensating variations of the mechanical gain responsive to angular
amplitude of movement about the intermediate axis. But the implementation
of such a compensation network raises much less problems than in a
stabilization loop. One reason is that a stabilization loop must provide
maximum values of stiffness and passband, which requires high gains and
phase advance networks. To the contrary, the gain of the position copying
system can be relatively low.
In the device of the invention, the critical or relevant parasitic torque
is torque C.sub.s about the axis of the second unit (elevation axis in
general) rather than torque C.sub.e. It is given by the formula:
C.sub.s =(I.sub.v +I.sub.s).omega.tg1 (2)
where
I.sub.s is the inertia of the second unit (elevation gimbal in general)
about the intermediate axis, such assembly being considered alone, to the
exclusion of the sight unit,
.omega. is the acceleration about the axis perpendicular to the elevation
axis and "lateral" movement axis;
1 is the "lateral" movement angle (angle between the line of sight and the
axis perpendicular to the elevation axis and lateral movement axis).
The arrangement which has just been described substantially decreases the
effects of the torques which appear about the intermediary axis. To
further improve the performance, it is desirable to reduce the amplitude
of the torque. In an attempt to reduce I.sub.s, the assembly consisting of
the sight and second unit will be advantageously given an inverted
structure. Instead of forming the second unit as a fork straddling the
sight unit, bulky parts of the latter will be located in parts placed on
both sides of the second assembly and may thus have a low rotational
inertia. Reduction of I.sub.s may be accompanied by an increase in
I.sub.v, but the latter will generally be lower and the overall effect
will be of advantage.
It will be appreciated that the favorable effect of the latter arrangement
only exists because of implementation of the first one, which causes
I.sub.s to appear in the parasite torque to be taken into consideration
(formula 2).
The invention will be better understood from the following description of
particular embodiments thereof, given by way of examples only.
SHORT DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified diagram showing the components of a stabilized
sighting device in accordance with the invention;
FIG. 2 is a phantom isometric view showing a typical distribution of the
components of a sighting device in accordance with the invention, for use
as a roof sight on a helicopter;
FIG. 3 is a sketch illustrating how the device of FIG. 2 may be mounted on
a helicopter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, there is shown a stabilized sighting device which
comprises a first unit 10 formed by a gimbal mounted on a fixed support
for rotation about an axis 12, which will be assumed in the following to
be the bearing axis (typically parallel to the yaw axis of the carrying
vehicle). The first unit 10, forming a bearing gimbal, supports a second
unit 13 by bearing means disposed so that the second unit may rotate about
an intermediate axis 14 perpendicular to the first axis and forming an
elevation axis. The second unit 13 supports the sight unit 15 of the
sighting device. The core 15 is not fixed to the second unit 13, but
supported by bearing means for rotation about an internal axis 16, which
will be referred to as a lateral movement axis, located in a plane
perpendicular to elevation axis 14, like the bearing axis 12.
A stabilization control two-axis gyro 17 is mounted directly on core 15.
The spin axis of the gyroscope 17 is directed along line of sight 18. One
of the input axes of the gyroscope is directed along the lateral movement
axis 16 and the other input axis is then at an angle with elevation axis
14, which is equal to the angle of deviation of the core about the lateral
movement axis, from the canonical position as defined above.
The core 15 and second assembly 13, in the form of an elevation gimbal, are
each provided with a stabilizing closed loop servo-control circuit whose
detector is gyroscope 17. The elevation stabilizing loop comprises
electronic circuit means 19 for controlling an elevation motor 20.
Similarly the lateral movement servo-control loop comprises circuit means
21 which receives input signals from gyroscope 17 and which controls a
motor 22 for angularly moving the core about the bearing axis.
As already indicated above, the device must be designed so that the
gyroscope 17 deviates little from the canonical position, i.e. from the
position in which the sensitive axes of gyroscope 17 are parallel to the
axes about which the stabilizing motors 20 and 22 rotate the respective
units. For that purpose, the first unit 10 (bearing unit 10) is caused to
rotate so that the angle of deviation of the core about the lateral
movement axis 16 remains quite small. The first unit 10 is provided with a
copying or position reproducing system comprising a detector 23 detecting
the rotations of core 15 about the lateral movement axis, an electronic
processing circuit 24 and a motor 25 for rotating unit 10.
Under these conditions, the angular movements of core 15 about the lateral
movement axis 16 are limited to a very low amplitude, typically limited to
a range not exceeding a maximum which may be of from one to a few degrees
depending upon the required accuracy. In practice, the amplitude will be
frequently less than a degree since it corresponds solely to the copy
errors. The copying system comprises a network for compensating variations
of the mechanical gain of the linkage as a function of the elevation. The
compensating network may be quite simple, since the gain of the system is
not critical, which would not be the case in a stabilization loop.
The system may be conventional in design. It will generally comprise a
potentiometric or inductive sensor 23, a low-gain copy system with a
compensation network and a drive motor. The whole system may be analog and
may control motor 25 by pulse width modulation of DC square waves in
response to the measured deviation. The compensation network may consist
of an amplifier whose gain is modified by steps responsive to the value of
the elevation angle to approximate the theoretical compensation law. The
elevation stabilization loop will comprise an electronic circuit having a
higher amplification gain, but it will not comprise a network for
compensating variations of the mechanical gain, due to the small extent of
rotation of the core about the lateral movement axis 16.
Last, the loop for stabilization about the lateral movement axis may be
quite simple in design, considering the low value of the angular movements
of core 15. In particular, the drive motor may be of a type having a very
small angular range of movement (brushless motor for example).
It can be seen that the device of the invention removes the limitations in
the stabilization caused by large elevation angular fields. This angular
field will now be limited by optical or mechanical problems only. There is
no need for a full analysis of the forces to which the device will be
subjected in operation, and particularly the perturbating frequencies and
accelerations of the carrier.
There is shown schematically in FIGS. 2 and 3 a device 30 for use on a
helicopter schematically shown at 31. It comprises an external casing 32
having a flange 33 for roof mounting. In FIG. 3, the case is shown
provided with a shroud 34 for protecting the active parts. In FIG. 2, the
shroud is replaced by cylindrical bulges 35 provided with windows 35a.
The geometry of the device shown in FIG. 2 is reversed with respect to that
shown in FIG. 1. The first unit 10a is formed by a gimbal which projects
axially upwardly from its support, instead of downwardly in FIG. 1; the
more bulky parts of the core are placed on each side of the first unit
(instead of being placed between the branches of the gimbal which forms
it), which reduces considerably the moment of inertia of the first unit
10a about the bearing axis 12a and that of the second unit about the
elevation axis 14a.
In the device shown in FIG. 2, where the elements corresponding to those of
FIG. 1 are shown by the same reference number to which the index a has
been added, the second or intermediate unit comprises a tubular sleeve 36.
Two lateral plates 38 located each on one side of the first unit 10a are
connected by a shaft 37. The plates carry the sensors: for example, one of
plates 38 may carry an assembly 39 formed by a laser range finder and a TV
camera; the other plate 38 may then carry a thermal IR camera 40. Shaft 37
may project through elongated passages in the two legs of the first unit
10a and constitutes a section of a cross shaped piece shose other branches
are directed along the lateral movement axis 16a and support the core. For
the sake of clarity, FIG. 2 only shows the lateral movement mirror 41 of a
system for optical sighting from the cabin of the helicopter. The optical
system may comprise successively, along the path of the incident light
from the line of sight 18a, the lateral movement mirror 41, an elevation
mirror 42 carried by the second assembly, two deflecting mirrors 43, the
bearing mirrors 44 and a sight fitted with an eyepiece 46.
A device of the kind illustrated in FIG. 2 has been designed which achieves
stabilization with an accuracy of 30 .mu.rad for angular accelerations as
high as 6 r/s.sup.2 and frequencies exceeding 70 Hz.
Furthermore, it can be seen that the arrangement shown in FIG. 2 allows
easy access to the different sensors for maintainance repair or
replacement.
It is clear that the invention is not limited to the particular embodiments
shown and described by way of examples and it should be understood that
the scope of the present patent extends to variations which will appear to
those familiar with the art to which the invention relates.
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