Azimuth damper

Fluid damper for damping aiming movements, for instance when tracking a target. The damper comprises a substantially disc-shaped cylindrical housing (9) in which a drum-wheel (11) is supported for rotation so that an inner gap spacing (24) is provided between the drum-wheel and a slipping ring (14) integrated in the outer cylindrical wall (13) of the damper housing. Said gap spacing (24) is filled with a high-viscosity damping fluid so that a damping torque is obtained which is linearly increasing to the angular velocity for velocities up to a certain level but which are limited for velocities exceeding said level due to the fact that the slipping ring (14) starts rotating with respect to the other parts of the damper housing. The slipping ring (14) is made of a material which is temperature stable and which can be made with a high accuracy with respect to its dimensions for instance carbon-fibre reinforced epoxy resin.

The present invention relates to a fluid damper for damping aiming 
movements, for instance when tracking a target. The damper comprises a 
substantially disc-shaped cylindrical damper housing in which a drum wheel 
is supported for rotation so that an inner gap spacing is provided between 
the drum wheel and the damper housing which gap is filled with a highly 
viscous damping fluid. 
For following a target or other object with high aiming accuracy it is 
necessary to introduce some sort of damping means for damping the aiming 
movements. Specifically in military applications some weapon systems 
require an accurate optical tracking of a target. For example in a 
portable missile launcher optical tracking of a target is provided for 
throughout the entire flight of the missile by the operator keeping the 
line of sight on the target. An accurate tracking is not required, 
however, only in military applications; also in civilian applications, 
such as optical sighting devices, television and cinematographic cameras, 
an accurate tracking is often required. 
Waver, overshoot and other undesirable effects of a human operator should 
be eliminated by the damper. Even a very small deviation of the guidance 
control signals in response to waver is capable of causing a deviation in 
the missile trajectory so that the missile passes at the side of the 
target. Prior dampers which have been used for limiting such undesired 
deviations have usually been of the fluid damper type. In this case the 
necessary damping is provided by means of a highly viscous fluid which 
fills one or more gaps between a rotatable disc and some part of the 
damper housing. Such a damper generally provides for a linearly increasing 
damping torque to the angular aiming velocity. This means an increasing 
resistance to rapid aiming velocities which is usually no problem during 
the target tracking phase at which only low angular velocities are 
required. During target acquisition, however, when rapid slewing about the 
azimuth or elevation axes often is required, the torque of a conventional 
damper is often too high. It is therefore previously known to provide the 
damper with disengaging or overriding means to limit the increasing torque 
required at higher angular velocities specifically slipping couplings such 
as a maximum torque coupling. 
In Swedish Pat. No. 74.13625-0 a fluid damper with a nonlinear damping is 
described which provides for an optimal and critical damping for low 
angular velocities but in which the damping torque has been limited for 
higher annular velocities so that rapid slewing for instance during target 
acquisition is facilitated. A highly viscous damping fluid as used in said 
patent gives a substantial damping of high-frequency vibrations such as 
trembling, jitter or the like caused by the human operator. The viscosity 
of the damping fluid has been decreased at higher shear rates by means of 
a specific design of the fluid gap of the damper. 
It is evident that a nonlinear damping torque is required in such a way 
that the damping torque is linearly increasing up to a certain angular 
velocity, for instance 30 to 40 mrad/s, which corresponds to normal target 
tracking. For higher velocities, however, the increase of damping torque 
is limited and a low, constant damping torque is desired for angular 
velocities above said level. It has been difficult, however, to provide 
for such an ideal damping characteristic and it is also evident from FIG. 
3 of said patent that no constant damping torque is obtained for higher 
angular velocities corresponding to the target acquisition phase. 
Especially at low temperatures the damping torque will be too high. This 
is unsatisfactory especially in portable weapon systems of the 
light-weight type in which a too high torque very easily brings the weapon 
out of position during target acquisition. 
Accordingly, it is an object of the present invention to provide a fluid 
damper with high aiming accuracy for low aiming velocities i.e. velocities 
up to approximately 30 mrad/s, but in which the linear increase of the 
damping torque is minimized for velocities exceeding said value to permit 
a rapid target acquisition. 
It is also evident that the damping torque of a fluid damper clearly 
depends on the temperature. As there is a general requirement for a weapon 
system to have an acceptable function over a temperature range of at least 
-30.degree. C. to +60.degree. C. some sort of temperature compensation is 
required. 
Another object of the invention is therefore to provide a fluid damper with 
a damping torque and a temperature dependence as small as possible within 
a large temperature range, but which is still designed to withstand 
military field use, has a light weight and inexpensive design. 
A main characterizing feature of the invention is that said fluid gap 
spacing is formed between the peripheral, cylindrical surface of the drum 
wheel and a slipping ring integrated in the outer cylindrical wall of the 
damper housing to the desired maximum torque couping. 
The maximum damping torque can be easily controlled by adjusting the 
clamping force of the slipping ring within the damper housing. By making 
the slipping ring of a material which is substantially in dependent of the 
temperature, preferably a carbon fibre reinforced epoxy resin material, a 
good temperature compensation is obtained. 
In the following the invention will be described more in detail in 
connection with the accompanying drawings which illustrate a preferred 
embodiment of the invention.

FIG. 1 schematically illustrates a target tracking device to be 
incorporated for instance in a weapon system in which the operator should 
keep the line of sight on a target with high aiming accuracy. The target 
tracking device further comprises an optical sight 1 through which the 
target and its background can be observed by the operator. 
In order to make it possible to follow a moving target the sight is 
supported for rotation in both elevation and traverse, i.e. about a 
horizontal elevation axis 2 and a vertical azimuthal axis 3. The sight is 
aimed by the operator by means of a handle 4. 
The sight 1 is mounted on a support 5 provided with a tripode 6 supported 
on the ground. The support is provided with telescopic legs 7 to permit 
the support to be raised and lowered. 
In order to improve the target tracking operation an azimuth damper 8 is 
arranged between the support 5 and the sight 1. The damper comprises two 
main parts which are supported for rotation relative to each other about 
an axis of rotation which coincides with the azimuthal axis. The damper is 
described more in detail in connection with FIG. 2 below. 
As already mentioned in the introductory portion of our specification a 
damper is required for eliminating undesired vibrations such as human 
trembling or waver and overshoot. In this case a damper is used only for 
the azimuthal axis but of course a damper could be used also for the 
elevation axis. For a weapon system, however, a damper for the azimuthal 
axis is more important as both the angular velocity and acceleration about 
the azimuthal axis exceeds the same for elevation axis. 
FIG. 2 shows one example of a damper according to the invention. The damper 
comprises a substantially cylindrical, thin outer housing 9 in which a 
drum wheel 10 is supported for rotation. The housing 9 comprises a 
circular bottom part 11 and an upper circular cover 12 so that the damper 
housing forms a substantially I-shaped section. The damper housing 9 
further comprises a slipping ring 14 integrated in the cylindric wall 13 
of the housing which ring will be described more in detail below. The 
bottom part 11 and the cover 12 are held together by means of a number of 
screws 15, preferably three, which are distributed about the peripheral, 
cylindrical flange of the housing. In order to be able to adjust the 
joining force between the two parts of the housing and thereby also the 
clamping of the slip ring 14 the screws 15 are provided with a package of 
cup springs 16. 
The cover 12 of the damper housing is provided with means 17 for connecting 
the damper to the rotatable optical sight 1. The center of the cover is 
provided with a hollow shaft 18 on which the drum wheel 10 is supported 
for rotation by means of an angular contact ball bearing 19 as well as a 
nail bearing 20. 
The drum wheel 10 is provided with a corresponding inner hub 21 supported 
on the damper housing, three spokes 22 extending in the small annular gap 
of the housing and an outer ring 23 conforming to the slip ring 14 so that 
a small gap spacing 24 is formed between the outer cylindrical surface of 
the drum wheel and the slip ring. The spokes of the drum wheel are formed 
so that the drum wheel can be turned an angle of .+-.45.degree. about the 
axis 25 before the movement is prevented by the joining screws 15. The 
inner hub 21 of the drum wheel is provided with a fastening plate 26 for 
mounting on the support 5 of the target tracking device. 
The desired damping of the aiming movements are provided by means of the 
annular gap 27 formed between the drum wheel and the damper housing which 
gap also includes said gap spacing 24 and which is filled with a high 
velocity damping fluid. The fluid gap 27 is sealed by means of two 
annular, fluid-tight dynamic seals 28, 29 arranged between the hub 21 of 
the drum wheel and the cover and bottom part of the damper housing. By 
locating the fluid-tight seals comparatively close to the rotational axis 
25, the friction loss in the seals is minimized, but on the other hand the 
fluid volume is rather large, which means that specific means are required 
to even out the pressure differences of the fluid. Therefore the cover 12 
of the damper housing is provided with an additional spacing 30 connected 
to the fluid gap but divided from it by means of a membrane 31. The 
additional spacing is also provided with an airing screw 32. The 
peripheral flange of the damper housing is provided with screws 33, 34 for 
filling and draining the damping fluid. The outer part of the fluid gap is 
also sealed by means of two O-rings 35, 36 disposed between the slipping 
ring and the cover and bottom part of the damper housing, respectively. 
The damper fluid, e.g. silicone oil, should have a viscosity of 
approximately 10.sup.-6 centistokes at room temperature. The dimension of 
the gap spacing 24 is approximately 0.2 millimeters at room temperature 
for producing a suitable damping torque. 
If the target tracking device is incorporated in a portable weapon system 
it is important that the damper has a light weight. In order to reduce the 
weight of the damper the housing and the drum wheel is preferably made of 
magnesium. The slipping ring is made of a carbon fibre reinforced epoxy 
resin, which material is light and strong and which can be made with a 
high precision with respect to its dimensions which is very important 
considering the very small dimension of the gap spacing. This material is 
also very stable at different temperatures, its thermal expansivity is 
practically zero for the temperature range in question. This will 
automatically give a temperature compensation as the gap is formed between 
the slipping ring made of epoxy resin) and the drum wheel (made of 
magnesium). 
The described fluid damper has a linearly increasing damping torque to 
angular velocity for velocities up to approximately 40 mrad/s. For angular 
velocities exceeding this level the damping torque is limited and 
substantially constant due to the fact tht the start friction of the 
slipping ring has been reached and the slipping ring starts to be rotated 
with respect to the other parts of the damper housing. 
The invention is not limited to said embodiment but can be varied within 
the scope of the accompanying claims. Even if the invention is described 
in connection with a light-weight portable weapon system it should be 
understood that the damper can be used also in other applications in which 
a damping of aiming movements is desired. The outer slipping ring 14 which 
is an important part of the damper is not necessarily made of a 
carbon-fibre reinforced epoxy resin only; in applications in which the 
requirement of temperature stability is not soo high a steel slipping ring 
can be used.