Optical sights

The invention provides a stabilizable optical gun sight including an optical system having an eye piece and an objective lens and between the eyepiece and objective lens, two optical wedges which are arranged to be driven differentially and together in order to deflect the line of sight through the optical system in response to servo control signals applied to the driving means.

This invention relates to optical sights and in particular to optical 
sights which are required to be stabilised. 
Whilst the invention is applicable generally to optical sights which are 
required to be stabilised, such as may be used for cameras in some cases, 
the principal concern of the present invention is optical gun sights for 
use with the gun control system of fighting tanks. 
The present invention seeks to provide improved optical sights and in 
particular improved stabilisable gun sights for fighting tanks. 
According to this invention an optical sight comprises an optical system 
including an eye piece and an objective lens and between said eye piece 
and said objective lens two serially arranged optical wedges mounted so as 
to be rotatable both relatively one with respect to the other and together 
about said optical axis whereby the line of sight through said optical 
system may be moved. 
Preferably said optical sight is a stabilisable optical sight and means are 
provided for applying servo control signals to driving means for said 
optical wedges whereby in operation said sight tends to be stabilised 
against movements of a body on which said sight is carried. 
Preferably said two wedges are mounted on parts which are arranged freely 
to be rotatable within a casing each part carrying a bevel gear with the 
two bevel gears arranged to be driven differentially by a bevel pinion 
driven by a first servo motor assembly. 
Preferably said first servo motor assembly is itself mounted upon a further 
part which is arranged freely to be rotatable within said housing which 
further part is driven by a further servo motor assembly mounted on said 
casing whereby operation of said further servo motor assembly causes said 
further part and said first servo motor assembly and said differential 
pinion gear to be rotated together thus causing said two optical wedges to 
be rotated together. 
Preferably each servo motor assembly comprises a servo motor and a gear box 
drive and a positional transducer and for each servo motor assembly a 
servo amplifier control loop is provided comprising an input terminal for 
error control signals, a comparator connected to derive input from said 
input terminal and from the output of said positional transducer (usually 
via scaling means e.g. a potentiometer), and a servo amplifier connected 
to the output of said comparator via a signal processing circuit as 
required, the output of said amplifier being connected to drive said servo 
motor. 
According to a feature of this invention a gun control system for a 
fighting tank, including a gun barrel stabilisation system comprising 
means for developing stabilisation error signals resulting from movement 
of said tank and means for utilising said stabilisation error signals to 
drive a gun barrel in elevation and a turret carrying said gun barrel in 
azimuth to provide stabilisation, includes a gun sight as described above 
arranged to provide compensation for stabilisation error remaining in said 
gun barrel stabilisation system.

Referring to FIG. 1 the sight consists of an eye piece and an objective 
lens 2 forming an optical system through which the tank gunner looks along 
a line of sight 3. 
Positioned between the eye piece 1 and the objective lens 2 is a series 
combination of two optical glass wedges 8 and 9. It will be seen that one 
is inverted relative to the other. The wedges 8 and 9 are rotatable about 
the optical axis 3 both one relative to the other and together. 
Controlling the relative rotation of one wedge with respect to the other 
and the absolute rotation of the combination of the two wedges will enable 
the line of sight 3 to be moved in any co-ordinate direction as best seen 
from FIGS. 2 and 3. 
In FIG. 1 the wedges 8 and 9 are shown orientated with their vertical axis 
in alignment. FIG. 2 shows the two wedges 8 and 9 after rotation through 
angles of +.crclbar. and -.crclbar. respectively. The movement of the 
scene to the gunner's eye depends upon the geometry of the wedges 8 and 9 
(referring to FIG. 3 the angle .alpha. of the wedges and the spacing D) 
and also upon the angles +.crclbar. and -.crclbar. through which the 
wedges 8 and 9 have been turned. 
The movements of the wedges 8 and 9 are controlled by a servo control 
system 6 which derives error signals in X and Y co-ordinates which 
represent the deviation required of the line of sight 3 in order to 
stabilise the system and take into account gun pointing error arising from 
movement of the tank. Control signals are applied to driving servos (not 
shown in FIG. 1) of the wedges 8 and 9 in order to provide the required 
compensating rotation. 
It is, of course, well known per se to derive such error signals and the 
majority of present day fighting tanks are equipped with stabilizing 
systems, based upon gyroscopes for example, and commonly such error 
signals are utilised to adjust the direction in which the gun barrel is 
pointing by means of servos adjusting the elevation of the gun barrel and 
the rotation of a turret carrying the said gun barrel. For the purposes of 
the present explanation it may be assumed that the gun sight schematically 
illustrated in FIG. 1 is utilised in such a tank in which a basic 
stabilisation system is employed utilising servos to control elevation of 
the gun barrel and the rotation of the turret. Thus in this case the 
stabilisation provided within the gun sight itself is not required to 
achieve total stabilisation but merely to take into account any remaining 
stabilisation error in the system. 
The assembly comprising the wedges 8 and 9 and any driving servos therefor 
is mounted as a unit such that under control of the gunner the unit may be 
swung out of the optical path of the system so as to afford the gunner 
with a "genuine bore sight" view. 
Referring to FIG. 4 this shows in section an instrument assembly containing 
the two wedges 8 and 9 which is interposed between the eye piece 1 and the 
objective lens 2 of the gun sight represented in FIG. 1 in the position 
occupied in that figure. The optical axis of the assembly is represented 
at 10. Wedge 9 is mounted upon a part 11 which is freely rotatable about 
the optical axis 10 on bearings 12. The part 11 housing wedge 9 is formed 
with a flange on which is cut a bevel gear 14 which engages with a bevel 
pinion 13. 
Wedge 8 is mounted upon another part 14 which is also arranged to be freely 
rotatable about the axis 10 by means of bearings 12. Part 14 is also 
formed with a flange upon which a bevel gear 15 is cut. 
Bevel gear 15 is also in mesh with bevel pinion 13 so that when bevel 
pinion 13 is rotated bevel gears 14 and 15 are driven differentially. 
Bevel pinion 13 is driven by a worm and wheel gear assembly 16 by means of 
a motor/gear box assembly 17. 
Whilst not separately represented, the motor/gear box assembly 17 also 
includes a positional transducer in the form of a digital encoder or an AC 
resolver or a conducting plastics potentiometer. 
As will now be appreciated, as bevel gear 13 is turned via the gear 
assembly 16 by motor/gear box 17 a differential movement is imparted to 
the two optical wedges 8 and 9 with respect to a third, freely rotatable, 
part 18 which houses the motor/gearbox 17. Third part 18 carries a slip 
ring assembly 19 consisting of polished coin silver slip rings spaced by 
paxolin shims and insulated from part 18 by a paxolin tube 20. A brush 
holder 21 carries precious metal wire brushes which contact the slip rings 
of slip ring assembly 20. Flying leads to the brushes of brush gear holder 
21 are taken via a suitable grommet in one end cover 22 of the assembly. 
The slip rings and brush gear assembly 19 and 21 permits supplies and 
positional feedback signals to be routed to and from the differential 
wedge-driver motor/gearbox assembly 17. The whole assembly so far 
described is encased by the aforementioned end cover 22, an opposite end 
cover 23 and an outer cylindrical main frame 24. 
In order to provide for absolute rotation of the optical glass wedges 8 and 
9 together, the freely rotatable part 18 is itself provided with a gear 25 
which is meshed with a driving bevel gear 26 extending through a suitable 
aperture in the main frame 24. Bevel gear 26 is itself driven by a second 
motor/gearbox/positional transducer assembly 27 fixedly mounted to the 
outside of main frame 24 and housed under a protective cover 28. 
Thus by applying suitable servo control signals to motor assembly 27 the 
wedges 8 and 9 can be rotated together about the axis 10 whilst the 
relative rotation of the wedges 8 and 9 may be determined by servo control 
signals applied to the motor assembly 17. As will be appreciated, 
referring again to FIG. 3, defining the required shift of the line of 
sight in R.crclbar. terms the relative rotation of the wedges 8 and 9 will 
determine R and the absolute rotation of the wedges 8 and 9 together will 
determine .crclbar..