GB1559218A - Optical sight-line stabilisation apparatus - Google Patents

Description

(54) OPTICAL SIGHT-LINE STABILISATION APPARATUS (71) We, FERRANTI LIMITED, a Company registered under the Laws of Great Britain, of Hollinwood, in the County of Lancaster, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to stabilisation apparatus, and in particular to apparatus for the stabilisation of optical sight-line.

Stabilised sighting devices are used, in general, for allowing observations to be made of a distant object from a platform subject to rapid and often unnpredictable movement, such as a helicopter. Many types of such devices are known, and almost all make use of a gyroscope coupled to one or more mirrors which form part of the optical sight-line. Frequently such known devices use mechanical linkages connecting the mirror to the gyroscope to give the 2:1 ratio of movement necessary for correct pitch stabilisation. Other known devices use two mirrors to achieve the same result without the need for these linkages. Both types of device have obvious disadvantages.

Our British application No. 1,491,117 related to a stabilised sighting device in which the mirror is mounted in a frame capable of movement about two perpendicular axes.

The frame also carries a twcdegree of freedom gyroscope having inner and outer gimbal axes parallel to the two axes of the frame, and control means are provided responsive to movements of the platform in which the device is mounted to ensure that the frame moves so as to correct for displacement of the sight line.

In devices of this type movement of the mirror requires a torque to be developed either by the gyro itself, if the control means are mechanical, or by a torque motor if a servo system is used. The size of the torque motor depends upon the torque to be developed, and the use of large mirrors may therefore require large torque motors.

Tt is an object of the invention to provide a stabilised sighting device in which the torque which has to be developed to ensure stabilisation may be reduced.

According to the present invention there is provided apparatus for stabilising an optical sight-line against movements of a platform, which includes a mirror arranged to deflect the axis of the sight-line and mounted in a frame capable of movement about two axes respectively parallel to the pitch and azimuth axes of the platform, a gyro having two degrees of freedom pivotally mounted in the same frame about its outer gimbal axis and having its inner and outer gimbal axes respectively parallel to said pitch and azimuth axes, control means responsive to movements of the platform about its pitch and azimuth axes to move the frame in such a manner as to correct for displacement of the sight-line resulting from such movements, a balanced inertial mass rotatably supported by the frame about an axis parallel to said pitch axis, and a mechanical drive interconnecting the inertial mass and the frame such the inertial mass rotates in the opposite direction to the frame, the drive ratio between the inertial mass and the frame and the ratio of their masses being such that the torque necessary to rotate the frame is effectively reduced.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of stabilisation aparatus according to a first embodiment of the invention: Figure 2 is a rear view of part of the embodiment of Figure 1; and Figure 3 is a similar diagram of a second embodiment of the invention Referring now to Figure 1, a structure 10, forming part of the movable platform, supports a frame 11 which in turn carries a mirror 12. Frame 11 is pivoted about a horizontal axis, parallel to the pitch axis of the platform, and is itself supported about a vertical axis by a supporting member 13 pivotally mounted on the structure 10. The vertical axis is arranged to be parallel to the azimuth axis of the platform. The platform itself may be a vehicle such as a helicopter. A torque motor 14, comprising rotor 15 and stator 16, is provided to control movement of the supporting member 13.

The frame 11, besides carrying the mirror 12, also carries a two-degrees-offreedom gyro 17, which is pivoted in the frame about the supporting axis of its outer gimbal 18. Nominally this outer gimbal axis is parallel to the azimuth axis of the platform, and hence to the axis of rotation of the supporting member 13. The axis of the inner gimbal 19 is arranged to be nominally parallel to the pitch axis of the platform, and hence to the axis of rotation of the frame 11. The spin of the gyro rotor is arranged to be parallel with the incident sight-line 22 of light striking the mirror 12.

The reflected sight-line 23 passes through an optical system, shown schematically by a lens 24 carried by the structure 10.

Attached to the support member 13 is photoelectric autocollimator 25. This comprises a light source and a quadrant detector the latter comprising four separate detectors each occupying a 90" segment of a circle. Light is projected from the autocollimator in a direction parallel to the axis 23 of the reflected sight-line, on to a small mirror 26, fixed to the frame 11 on its axis of rotation. From mirror 26 the light is directed onto a second small mirror 27 carried on the inner gimbal axis of the gyro 27. The two mirrors are aligned so that when the gyro spin axis 20 is parallel to the incident sight-line axis 22, and the projected beam from the autocollimator 25 is parallel to the reflected sight-line axis 23, the mirrors 26 and 27 reflected the projected beam back along its own path.

The quadrant detector is connected so that output signals from the segments sensitive to movement about the azimuth axis are applied through an amplifier 28 to the azimuth torque motor 14. Similarly signals from the segments sensitive to movement about the pitch axis are applied through an amplifier 29 to a similar torque motor 30 on the axis of frame 11. A torque motor 31 is provided on the outer gimbal 18 of the gyro for a purpose to be described later.

As is shown more clearly in Figure 2, a balanced inertial mass in the form of a flywheel 40 is rotatably mounted on the member 13 about an axis parallel to the hori zonal axis one the frame. A pulley 41 on the mirror support spindle is connected by a drive belt 42 to a smaller pulley 43 on the flywheel spindle. The belt 42 has a single twist so that the mirror and flywheel rotate in opposite directions.

When the sight-line is stabilised, the autocollimator is providing a balanced output, and known error signals are applied to amplifiers 28 and 29.

If the platform carrying the apparatus is subjected to a pitching motion, then the inner gimbal 19 of the gyro 17 will move relative to the outer gimbal. In fact, since the gyro motor is space-stabilised it is actually the structure 10, supporting member 13 and frame 11 which moves. The relative movement results in a rotation of the small mirror 27 about the inner gimbal axis. As a result light falling on mirror 27 from mirror 26 is no longer reflected back along its path, but is deflected, striking the photo-sensitive detector of the autocollimator 25 offcentre.

The resulting out-of-balance output from the autocollimator 25 is applied via amplifier 29 to the torque motor 30 on the pitch axis of the apparatus. This causes the frame 11 to rotate about its mounting in the supporting member 13 until the mirror 26 is moved to such a position that the autocollimator again produces a balanced output. The movement of frame 11 to produce this balance is such as to restore the direction of the axis 23 of the reflected sight-line to its original direction.

The inertial compensator 40 is effective during movements of the platform, when its inertia will react with that of the mirror and frame to reduce the torque which must be applied by the torque motor 30. The drive ratio between the inertial mass and the frame, and the ratio of their masses determines the degree of compensation provided by the flywheel. With suitable values of these two ratios it is possible to counteract all except the frictional torques, so that the workload of the torque motor 30 becomes very small.

If the platform is subject to a displacement in azimuth, then the gyro pivots in the frame about its outer gimbal axis. This again moves mirror 27, this time in a direction perpendicular to that referred to above.

Again the light from the autocollimator 25 reflected back along a different path, and produces an azimuth error signal. This is applied via amplifier 28 to the stator 16 of the torque motor 14 on the axis of the supporting member 13. The resultant rotation of the supporting member 13 rotates frame 11, though the gyro 17 remains space-stabilised, and the mirror 26 is returned to its original orientation with respect to mirror 27, thus nullifying the error signal from the autocollimator 25.

Any movement of the platform involving a combination of pitch and azimuth movements results in a combination of the two corrective actions referred to above.

If it is desired to move the stabilised siht- line, then steering signals may be applied to torque motors on the gyro inner and outer gimbal axes. Only one such torque motor, motor 31 on the outer gimbal axis, is shown.

The application of signals to these torque motors will cause the gyro to precess, resulting in corrective action as already described.

The embodiment described above is simple, but may suffer from the disadvantages usually associated with optical elements. Figure 3 shows an alternative embodiment which uses servo control loops to avoid these problems.

It will be seen that many of the elements in Figure 3 are the same as those in Figure 1, and the same reference numerals have been used. The two small mirrors 26 and 27 of the first embodiment have been replaced by synchros 35 and 36 respectively. In addition a third synchro or pick-off 37 is placed on the gyro outer gimbal axis. The stator output of synchro 36 is fed to the stator of synchro 35 and the signal from the rotor of 35 is supplied to amplifier 38 and applied to the pitch axis torque motor 30. A null output is obtained when the angle of the rotor of synchro 35 is equal to the angle of the rotor of synchro 36. Similarly the output of the pick-off 37 is applied via amplifier 39 to the azimuth axis torque motor 14. The autocollimator is not required in this embodiment.

As before, the inertial compensator is a flywheel 40 mounted on member 13 about an axis parallel to that of the inner gimbal of the gyro, and connected to the mirror axis by a reversing belt drive.

If the platform is subjected to a pitching movement, the rotor 21 of the gyro remains space stabilised. The resulting relative movement between the inner and outer gimbals of the gyro produces an output from synchro 36. This applied via synchro 35 to amplifier 38. The amplifier output drives torque motor 30 to rotate frame 11 and mirror 12, resulting in a change in the output from synchro 35. When the input of amplifier 38 is nulled by the action of synchro 35, the output to torque motor 30 ceases. This occurs when the mirror has been rotated through half the angle of the original pitch disturbance, as is necessary to restore the direction of the sight-line.

The effect of the inertial compensator is exactly as described in the first embodiment.

If the platform is subjected to a disturbance in azimuth, then, this is detected by synchro 37 on the outer gimbal axis of the gyro. The output signal from this syncho is applied through amplifier 39 to the azimuth axis torque motor 14, which rotates supporting member 13 and hence frame 11 until the disturbance has been corrected.

This is indicated by the rotor and stator of synchro 37 being restored to their original relative position.

As before, a combination of pitch and azimuth disturbances results in a combination of the two corrections described above.

Similarly the apparataus may be "steered" to move the sight-line in the manner already described with respect to the first embodiment.

Stabilisation apparatus according to either of the above embodiments will require the usual means for caging the gyro and for preventing toppling of the gyro.

The inertial compensator need not be belt driven as described above. Any other suitable reversing drive may be used, such as gears or linages. However, the belt drive probably offers the best compromise between accuracy of coupling and fricational effect.

WHAT WE CLAIM IS:- 1. Apparatus for stabilising an optical sight-line against movements of a platform, which includes a mirror, arranged to deflect the axis of the sight-line and mounted in a frame capable of movement about two axes respectively parallel to the pitch and azimuth axes of the platform, a gyro having two degrees of freedom pivotally mounted in the same frame about its outer gimbal axis and having its inner and outer gimbal axes respectively parallel to said pitch and azimuth axes, control means responsive to movements of the platform about its pitch and azimuth axes to move the frame in such a manner as to correct for displacement of the sight-line resulting from such movements, a balanced inertial mass rotatably supported by the frame about an axis parallel to said pitch axis, and a mechanical drive interconnecting the inertial mass and the frame such that the inertial mass rotates in the opposite direction to the frame, the drive ratio between the inertial mass and the frame, and the ratio of their masses, being such that the torque necessary to rotate the frame is effectively reduced.

2. Apparatus as claimed in Claim 1 in which the mechanical drive is provided by a belt interconnecting pulleys on the axes of the inertial mass and the frame.

3. Apparatus for stabilising an optical sight-line against movements of a platform substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. The application of signals to these torque motors will cause the gyro to precess, resulting in corrective action as already described. The embodiment described above is simple, but may suffer from the disadvantages usually associated with optical elements. Figure 3 shows an alternative embodiment which uses servo control loops to avoid these problems. It will be seen that many of the elements in Figure 3 are the same as those in Figure 1, and the same reference numerals have been used. The two small mirrors 26 and 27 of the first embodiment have been replaced by synchros 35 and 36 respectively. In addition a third synchro or pick-off 37 is placed on the gyro outer gimbal axis. The stator output of synchro 36 is fed to the stator of synchro 35 and the signal from the rotor of 35 is supplied to amplifier 38 and applied to the pitch axis torque motor 30. A null output is obtained when the angle of the rotor of synchro 35 is equal to the angle of the rotor of synchro 36. Similarly the output of the pick-off 37 is applied via amplifier 39 to the azimuth axis torque motor 14. The autocollimator is not required in this embodiment. As before, the inertial compensator is a flywheel 40 mounted on member 13 about an axis parallel to that of the inner gimbal of the gyro, and connected to the mirror axis by a reversing belt drive. If the platform is subjected to a pitching movement, the rotor 21 of the gyro remains space stabilised. The resulting relative movement between the inner and outer gimbals of the gyro produces an output from synchro 36. This applied via synchro 35 to amplifier 38. The amplifier output drives torque motor 30 to rotate frame 11 and mirror 12, resulting in a change in the output from synchro 35. When the input of amplifier 38 is nulled by the action of synchro 35, the output to torque motor 30 ceases. This occurs when the mirror has been rotated through half the angle of the original pitch disturbance, as is necessary to restore the direction of the sight-line. The effect of the inertial compensator is exactly as described in the first embodiment. If the platform is subjected to a disturbance in azimuth, then, this is detected by synchro 37 on the outer gimbal axis of the gyro. The output signal from this syncho is applied through amplifier 39 to the azimuth axis torque motor 14, which rotates supporting member 13 and hence frame 11 until the disturbance has been corrected. This is indicated by the rotor and stator of synchro 37 being restored to their original relative position. As before, a combination of pitch and azimuth disturbances results in a combination of the two corrections described above. Similarly the apparataus may be "steered" to move the sight-line in the manner already described with respect to the first embodiment. Stabilisation apparatus according to either of the above embodiments will require the usual means for caging the gyro and for preventing toppling of the gyro. The inertial compensator need not be belt driven as described above. Any other suitable reversing drive may be used, such as gears or linages. However, the belt drive probably offers the best compromise between accuracy of coupling and fricational effect. WHAT WE CLAIM IS:-

1. Apparatus for stabilising an optical sight-line against movements of a platform, which includes a mirror, arranged to deflect the axis of the sight-line and mounted in a frame capable of movement about two axes respectively parallel to the pitch and azimuth axes of the platform, a gyro having two degrees of freedom pivotally mounted in the same frame about its outer gimbal axis and having its inner and outer gimbal axes respectively parallel to said pitch and azimuth axes, control means responsive to movements of the platform about its pitch and azimuth axes to move the frame in such a manner as to correct for displacement of the sight-line resulting from such movements, a balanced inertial mass rotatably supported by the frame about an axis parallel to said pitch axis, and a mechanical drive interconnecting the inertial mass and the frame such that the inertial mass rotates in the opposite direction to the frame, the drive ratio between the inertial mass and the frame, and the ratio of their masses, being such that the torque necessary to rotate the frame is effectively reduced.

2. Apparatus as claimed in Claim 1 in which the mechanical drive is provided by a belt interconnecting pulleys on the axes of the inertial mass and the frame.

3. Apparatus for stabilising an optical sight-line against movements of a platform substantially as herein described with reference to the accompanying drawings.