GB2345155A

GB2345155A - Sightline stabilisation system using inertial reference sensors

Info

Publication number: GB2345155A
Application number: GB9828274A
Authority: GB
Inventors: Norman Frederick Watson
Original assignee: Marconi Avionics Ltd
Current assignee: Allard Way Holdings Ltd
Priority date: 1998-12-23
Filing date: 1998-12-23
Publication date: 2000-06-28
Anticipated expiration: 2018-12-23
Also published as: GB9828274D0; GB2345155B

Abstract

A sightline stabilisation system comprises a base unit 1, on which is mounted inertial reference sensors 13 for determining the rates of rotation of the base unit 1 about its free axes. Attached to the base unit 1 is an optical device 2 having a sightline S incident on a gimbal mounted mirror 6, the position of which controls the sightline direction. Inertial reference sensors 19 and 20, mounted on the mirror 6, determine angular displacement of the mirror. Data from these inertial reference sensors 19 and 20 together with data obtained from the inertial reference sensors 13 enable a processor 21 to determine the position of the mirror 6 relative to the base 1. The processor then controls the position of the mirror accordingly, using gimbal drive motors 4 and 10. The inertial reference sensors may be angular rate sensing gyroscopes or angular accelerometers. Use of inertial reference sensors on the mirror 6 improves the high frequency performance of the sightline stabilisation system.

Description

SIGHTLINE STABILISATION The present invention relates to a sighdine stabilisation system employing a mirror which is controlled to stabilise a sightline incident on the mirror.

In certain applications there is a requirement to stabilise a sightline, that is the optical path of an optical device which may either be a transmitter or a receiver, mounted on a moving platform.

One example of a requirement for sightline stabilisation is where an imaging device, for example a camera, is mounted aboard an aircraft. It is often desirable to stabilise the sightline of the imaging device in order to compensate for changes in attitude of the aircraft or to reduce the effects of vibration of the aircraft on the received image.

Another example would be the requirement to stabilise the optical output of a device such as a laser in a laser designator.

Although sightline stabilisation systems of the type with which the present invention is concerned are equally applicable to transmitting or receiving radiation, very accurate sightline stabilisation is particularly important in imaging systems where an imaging device such as a CCD will have a"stare"period. Any movement of the image across the imager in this period will significantly degrade the information content of the image.

Various sightline stabilisation systems exist and many comprise a gimballed platform where each axis of the gimbal is driven to compensate for movement detected in the structure on which the platform is mounted. The platform may have a camera mounted directly on it, referred to as a stabilised camera, with a pair of gyroscopes also mounted on the platform. The gimbals are then driven in dependance on signals received from the gyroscopes to maintain platform stability.

A disadvantage of a stabilised camera is that the mass of the camera is on the platform and therefore the device for driving the platform, normally motorised gimbals, have to be relatively powerful and therefore may themselves be relatively large and heavy.

The overall device thus has a relatively large mass which makes it difficult to move the sightline quickly to interrogate targets, especially in the case of multiple targets.

Also volume and mass are both undesirable in most applications.

To overcome the above problems a second technique commonly employed uses a stabilised mirror. In such devices a camera is fixed relative to a vehicle or some other such structure and is arranged to view the target object by way of the object's reflection in a mirror. The mirror is mounted on a gimbal and the orientation of the mirror is controlled to maintain desired sightline direction.

One possible stabilised mirror arrangement is illustrated in figure 1. A vehicle structure 1 has mounted to it a camera 2 and sightline stabilisation apparatus indicated generally as 3. The stabilisation apparatus 3 comprises a gimbal drive motor 4 supporting a frame 5 on which is mounted mirror 6. Also on the frame 5 is mounted a stabilised platform 7 supporting gyroscopes 8 and 9, mounted with mutually perpendicular axis in the plane of the mirror, or equivalent.

A gimbal drive motor 10 directly drives the platform 7 and also the mirror 6 via a"two to one link 11". That is for every unit of rotation the support 5 experiences with respect to platform 7 the mirror 6 rotates half a unit relative to support 5. The output of the gyroscopes 8 and 9 is used in a feedback loop to control gimbal drive motors 4 and 10 such as to maintain platform 7, and thus mirror 6, in the desired position.

Note that the two to one mechanism is required because, for the sightline direction to remain stable during a vehicle manoeuver, the mirror must move through only half the angle which the vehicle rotates.

A disadvantage of the mirror stabilisation system described above is that it is difficult to provide a sufficiently stiff linkage between the platform 7 and mirror 6 to cope with relatively high frequency motion that that can be experienced, for example due to vibration of the vehicle. To overcome this drawback it has previously been proposed to adopt the system illustrated in figure 2.

Referring to figure 2, three gyroscopes 13 are mounted directly on the vehicle structure in what is termed a"strap down"arrangement, one gyroscope associated with each rotation axis of the vehicle. The gyroscopes 13 provide data from which the instantaneous attitude of the vehicle structure I can be calculated, while angular displacement sensors 14 and 15 provide the angular position of the mirror 6 relative to the vehicle structure 1. This enables the direction of the sightline to be computed and controlled by the gimbal drive motors 4 and 10, alleviating the need for a linkage between the stabilised platform and the mirror in the figure 1 arrangement. However the figure 2 arrangement tends to suffer limited high frequency performance due to angular displacement sensors generally having low accuracy at high read out rates.

Also errors arise due to compliance in the gimbal bearings and this limits overall stabilisation performance.

With either of the two systems described above the optical aperture of the system and magnitude of vehicle manoeuvres which are to be accommodated by the system often require the mirror 6 to be relatively large. Because it is also necessary to ensure that the mirror surface does not distort significantly the mirror structure itself must be relatively stiff and this results in a mirror structure having a significant mass. The mass of the mirror in tum effectively limits the control bandwidth which can be achieved by feedback control signals applied to the gimbal drive motors. To address this problem it has also previously been proposed to employ a second small controllable mirror 6 (see figure 3) with a limited angular displacement, in the optical path between a focal plane 17 of the camera 2 and an objective lens 18. The smaller "fine"mirror, can be far smaller than the first,"coarse"mirror, and operate at higher frequencies. The angular displacement required by the fine mirror is small because it only needs to compensate for small residual errors resulting from poor high frequency performance of the coarse mirror.

Although employing the second mirror helps to improve high frequency operation of the figure 1 and 2 embodiments, the present inventor has realised that there is an inherent problem with these systems, namely that the residual errors of the coarse mirror mechanism are not accurately known and these are necessary in order to compensate for these residual errors.

According to a first aspect of the present invention there is provided a sightline stabilisation system comprising: a base unit; an optical device fixed to the base unit, the optical device having a sightline which is to be stabilised; a mirror; a support structure for supporting the mirror relative to the base such that the sightline is incident on the mirror, the support structure permitting the mirror to be rotated relative to the base unit about at least one axis and comprising a drive mechanism for controlling rotation of the mirror about the axis, a first inertial reference sensor fixed relative to the base unit; a second inertial reference sensor fixed relative to the mirror; and a processor for receiving inputs from the first and second inertial reference sensors and determining the rotational position of the mirror relative to the base unit.

By employing the present invention the inertial reference sensor (IRS) which would typically be an angular rate sensing gyroscope or angular accelerometer, mounted on the base unit determines the angular position of that base unit and operates at a relatively high frequency. However by having an IRS also mounted directly on the mirror the angular displacement of the mirror can also be determined at higher frequencies than would be possible with conventional angular displacement sensors.

Furthermore compliance within the gimbal bearings does not significantly reduce the accuracy to which the sightline direction is known since an IRS on the mirror measures the mirror motion with respect to inertial space.

The output of the first IRS relates to the angular position of the base unit. The drive mechanism support structure can thus advantageously be controlled in dependance on the output of the first IRS.

It is preferred that the support structure comprises a two axis gimbal arrangement and two drive mechanisms, one associated with each axis of rotation of the gimbal, the system further comprising at least two IRS's fixed relative to the mirror and three IRS's fixed relative to the base. The three IRS's mounted to the base are sufficient to provide all inertial rotation rates of the base whilst the two IRS's on the mirror provide the inertial rates of the mirror about its two free rotation axes. Thus if the initial directions of the base and the mirror are known the directions of the sightline at any subsequent time can be computed by processing the data from the gyroscopes.

Preferably the or each drive mechanism is controlled by the processor in dependance of the output on the first and second IRS's, or sets of IRS's.

Advantageously the system comprises an angular displacement sensor associated with each axis of rotation of the support structure, the processor controlling the or each drive mechanism in dependance upon signals received from first and second IRS's and at least one angular displacement sensor.

The use of angular displacement sensors enables the relative angles of the mirror to the base to be determined. This overcomes problems which may otherwise be encountered due to poor long term performance of the mirror mounted IRS's especially if gyroscopes where gyroscope biases (rate measurement offsets) and noise may introduce an angle measurement error which over time may become significant.

Also the angular displacement sensors prevent gyroscope scale factor errors which may otherwise introduce unacceptable errors in the computed sightline direction as the mirror position changes with time. In summary the incorporation of the angular displacement sensors overcomes low frequencies errors caused by long term drift within the gyroscopes.

Advantageously the processing means incorporates a Kalman filter, to permit gimbal angle sensor data sampled at low frequency to be used to calibrate the mirror mounted gyroscopes.

Advantageously the system additionally comprises a second mirror, positioned in the optical path such as to allow the orientation of the sightline to be controlled, and a drive mechanism for controlling the positions of the second mirror relative to the base unit in dependance on the output of the processor. The output of the first and second gyroscopes can be used to determine any residual error between the true position of the first mirror and its desired position, arising due to the poor high frequency performance of the first mirror. This residual error can then be compensated for by the second mirror, the second mirror and associated drive mechanism being more accurate at a higher frequency operation than the first mirror and associated drive mechanism.

The optical device may be a laser, but the system is particularly advantageously employed where the optical device is an imager. With an imager if the sightline is not stabilised to a high degree of accuracy a considerable amount of information is lost from the resultant image, particularly where the imager moves during the stare period of that device.

Where the optical device is an imager, image processing means may be employed and the image processed dependant on the output of the processor of the system. Residual errors in the position of the stabilisation mirror, as detected by the IRS's mounted on the mirror, can be used to"shift"subsequent frames of the image to bring those frames into alignment.

According to a second embodiment of the invention there is provided a method of stabilising a sightline, the method comprising positioning a mirror in the sightline such as to reflect the sightline, determining the position of a base on which an optical device associated with the sightline is mounted, determining the position of the mirror from the output of an inertial reference sensor mounted on the mirror and controlling the position of the sightline in dependance thereon.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying figures of which: Figures 1 and 2 illustrate prior art sightline stabilisation mechanisms described above; Figure 3 schematically illustrates a dual mirror sightline stabilisation system; Figure 4 is a schematic illustration of a sightline stabilisation system in accordance with the present invention; and Figures 5 is a flow diagram indicated the steps performed by the processor illustrated in figure 4.

Referring now to figure 4, and in accordance with the numbering system used in respect of the earlier figures, there is illustrated a sightline stabilisation system comprising a base unit 1. The base unit 1 may either be part of the structure of a vehicle to which the system is to be fitted, or fixed to that structure, possibly by means of an anti-vibration mounting. Secured to the base unit 1 is an optical device 2, which in this embodiment is referred to generally as a camera but could equally be any type of optical receiving device or a laser for projecting radiation along the sightline.

The base unit I additionally has securely mounted to it three gyroscopes 13 arranged to sense rotation about three mutually perpendicular axes. Also mounted on the base unit 1, via outer gimbal drive motor 4, is a frame 5 supporting mirror 6 via inner gimbal drive motor 10. To the rear surface of the mirror 6 is mounted gyroscopes 19 and 20 for detecting angular displacement of the mirror 6 about its free axes A and B respectively. In addition the frame 5 carries angle sensors, shown schematically as arrows 14 and 15, to determine the position of the mirror relative to axes A and B respectively.

A processor 21 is provided which receives inputs from each of the gyroscopes 13, gyroscopes 19 and 20, and angle sensors 14 and 15. The processor 21 also receives additional information relating to the desired sightline direction, of the camera 2, which need not be constant. The processor 21 processes this information as described with reference to figure 5 and controls outer gimbal drive motor 4 and inner gimbal drive motor 10 accordingly. In addition the processor 21 may control the position of fine mirror 16 of figure 3, which is mounted within the housing of camera 2. In a third alternative arrangement the processor may provide an output to image processing means associated with camera 2 such that any displacement of the sightline S from its intended position, as detected by gyroscopes 19 and 20, can be corrected within the image processor.

Referring now to figure 5, the function performed by processor 21 of figure 4 is illustrated. The processor receives inputs 22 and 23 from the base unit gyroscopes 13 and mirror mounted gyroscopes 19 and 20 respectively. The processor also receives at input 24 the measured gimbal angles from sensors 14 and 15, and at input 25, the desired sightline orientation with respect to the axes of the base unit 1.

The processing block 26 integrates the inputs from the three gyroscopes 13 to provide base unit attitude angles from the measurements of inertial rotation within three degrees of freedom of the base unit 1, in accordance with standard techniques used on inertial navigation systems and elsewhere.

The mirror rotation rates, or accelerations as deduced from gyroscopes 19 and 20 are received at input 23 and corrected in processor block 27 to compensate for inertial sensor errors estimated in processing block 29, such as drift or scale factor errors, so that the subsequent computation of mirror angles are more accurate. The compensated mirror rates, or accelerations, are received in processing block 28 together with data from the base unit gyroscopes 13. This data is integrated in accordance with differential equations relating relative mirror angles (gimbal angles) to the measured inertial rates. The processing technique is digital and a discrete approximation to the exact continuous equations is used to provide inertially derived gimbal angles. These are combined with measured gimbal angles received at input 24 in the processing block 29 indicated by a broken line. Processing block 29 uses the measured gimbal angles, which are accurate at low frequency, with the inertially derived gimbal angles to derive the errors in the inertially derived gimbal angles. The processor block 29 comprises a Kalman filter 30 which provides an estimate of the sensor errors which is fed back to processing block 27 to provide sensor error compensation to the mirror rates or accelerations received from input 23.

The desired sightline orientation received at input 25 is processed by block 31 to provide the mirror (gimbal) angles necessary to achieve the required sightline orientation defined externally to the system. The required inner axis angle and outer axis angle is then transmitted respectively to the inner axis and outer axis digital servos 32 and 33, which also receive the corrected inertially derived gimbal angle estimates from processing block 29. The inner and outer axis digital servos 32 and 33 provide control outputs to the gimbal drive motors 4,10 via digital to analogue converters 35 and 36.

In addition to providing the drive demands to the gimbal motors, servos 32 and 33 also provide a measure of the residual error between the desired and achieved gimbal angle (and hence sightline angle). This information is received by processing block 34 which derives the correction demands required for any subsequent stage of sightline error compensation, such as a second mirror having a higher operating frequency than the first mirror or image processing means, where the image may be shifted correct for any residual error. The processing block 34 transforms the gimbal angle errors to the necessary demand signals which is a function of the geometry of the subsequent stabilisation means.

The present invention has been described by way of example only with reference to one embodiment illustrated in the accompanying figures 4 and 5. It is realised that many variations and modifications within the scope of the appended claims will occur to the skilled practitioner.

Claims

CLAIMS 1. A sightline stabilisation system comprising:

i) a base unit;

ii) an optical device fixed to the base unit, the optical device having a sightline which is to be stabilised;

iii) a mirror;

iv) a support structure for supporting the mirror relative to the base such that the sightline is incident on the mirror, the support structure permitting the mirror to be rotated relative to the base unit about at least one axis and comprising a drive mechanism for controlling rotation of the mirror about the axis v) a first inertial reference sensor fixed relative to the base unit;

vi) a second inertial reference sensor fixed relative to the mirror; and

vii) a processor for receiving inputs from the first and second inertial reference sensors and determining the rotational position of the mirror relative to the base unit.
2. A system as claimed in Claim 1 wherein the drive mechanism is controlled in dependance on the output of the first inertial reference sensor.
3. A system as claimed in Claim 1 or 2 wherein the support structure comprises a two axis gimbal arrangement and two drive mechanisms, one associated with each axis of rotation of the gimbal, the system further comprising at least two inertial reference sensors fixed relative to the mirror and three inertial reference sensors fixed relative to the base.
4. A system as claimed in Claim 1,2 or 3 wherein the drive mechanism is controlled by the processor in dependance on the output of the first and second inertial reference sensors
5. A system as claimed in Claim 4 further comprising an angular displacement sensor associated with each axis of rotation of the support structure, the processor controlling the, or each, drive mechanism in dependance on input signals received from first and second inertial reference sensors and at least one angular displacement sensor.
6. A system as claimed in Claim 5 wherein the processing means incorporates a Kalman filter.
7. A system as claimed in any preceding claim comprising a second mirror, positioned in the optical path such as to allow the orientation of the sighdine to be controlled, and a drive mechanism for controlling the positions of the second mirror relative to the base unit in dependance on the output of the processor.
8. A system as claimed in Claim 7 wherein the second mirror and associated drive mechanism is capable of operating at a higher frequency than the first mirror and associated drive mechanism.
9. A system as claimed in any preceding claim wherein the optical device is a laser.
10. A system as claimed in any one of Claims 1 to 8 wherein the optical device is an imager.
11. A system as claimed in Claim 10 further comprising image processing means in which an image obtained from the imager is processed in dependance on the output of the processor.
12. A system is claimed in preceding claim wherein one or more inertial reference sensors comprises a gyroscope.
13. A sightline stabilisation system substantially as hereinbefore described with reference to, and/or as illustrated in figures 4 and 5 of the accompanying drawings
14. A method of stabilising a sightline, the method comprising positioning a mirror in a sightline to be stabilised such as to reflect the sightline, determining the position of a base on which an optical device associated with the sightline is mounted, determining the position of the mirror from the output of inertial reference sensors mounted on the mirror and controlling the position of the sightline in dependance thereon.
15. A method as claimed in Claim 14 wherein gyroscopes affixed to the base and the mirror are used to determine the relative rotational position of the mirror relative to the base.
16. A method as claimed in Claim 13 or 14 wherein the position of the sightline is controlled in dependance in the position of the mirror relative to the base by controlling the position of a second mirror.