GB2123935A - Relative attitude determining system - Google Patents

Abstract

Apparatus for determining the relative attitude of two objects on board a movable support such as a vehicle or ship comprises two inertial measurement units arranged to indicate the state vectors of the two objects and a signal processing system, which may incorporate a Kalman filter, for obtaining an indication of the relative attitude. By way of example, the objects may be a weapon (a gun or a missile or torpedo launch tube) and a weapon aiming device (a radar installation or an optical or electro-optical sighting device) in a weapon system on board a ship. The apparatus may assist the weapon system to remain more accurately operable despite wave-induced flexure of the ship hull or even in the face of a reasonable degree of static misalignment due to damage sustained by the ship.

Description

SPECIFICATION Relative attitude determining system This invention relates to apparatus for determining the relative attitude of two or more objects, parts of an object or components of a system which are comprised in or are on board a movable support, such as a vehicle, ship orthe like. By way of example, the apparatus might be operable for determining the relative attitude of two components of a weapon system on board a ship or boat.

A sea vessel borne weapon system may comprise the weapon itself (a gun or a missile or torpedo launch tube say) and separate target detection aiming apparatus (a radar installation or an optical or electro-optical sighting device perhaps). When the ship is fitted-out, these components obviously have to be quite carefully aligned one with another.

However, particularly if the components are widely spaced and/or if the ship is of light-construction (one having a fibre-reinforced plastics hull for example), the components may move relative to one another in a dynamic, i.e. time-varying, manner due to flexing of the hull. Also, "static" misalignment may come about due perhaps to the effect of an opposing weapon. The actual damage produced by the opposing weapon may be only slight but, nevertheless, if it has in anyway produced substantial static misalignment of the weapon system components, it may have rendered the ship ineffective for as long as it takes to repair and realign the system.

According to one aspect of the present invention, there is provided apparatus for determining the relative attitude of two members comprised in or on-board a movable support, the apparatus comprising for each said member an inertial measurement unit which is located at least near or is coupled to the associated member so as to respond to movement of said support with the production of signals indicative of the instant attitude of the associated member, and comparing means arranged to receive said signals and to formulate therefrom an indication of the relative attitude of the members.

According to a second aspect of the present invention, there is provided a weapon system intended for use on board a vehicle or craft such as a ship and comprising at least two system components and, for each said component, an inertial measurement unit arranged to produce, as a result of movements of the vehicle or craft, signals indicative of the attitude of the associated component, the system further including comparing means for receiving said signals and for formulating therefrom an indication of the relative attitude of the components.

According to a third aspect of the invention there is provided a weapon system intended for use on board a vehicle or craft such as a ship and comprising a weapon delivery part and a separate target detection and/orweapon aiming part, the target detection and/or weapon aiming part being operable for delivering first signals for manually or automatically controlling the aiming of the weapon, and the system further comprising, for each said part, an inertial measurement unit arranged to respond to movements of said vehicle or craft to produce second signals which are indicative of the attitude of the associated part, comparing means for receiving said second signals and for formulating therefrom an indication of the relative attitude of the compo nents, and signal processing means which is operable, in response to said comparing means, for processing said first signals to take account of any misalignment of said parts.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing the single figure of which is a block diagram of relative attitude determining apparatus.

The apparatus to be described is operable for determining the relative attitude and/or alignment of two orthogonal sets of axes associated with component parts of a system, which parts are positioned relatively remote from one another. The two parts comprise a weapon and a target detector respectively on a warship. The weapon could be a missile, a gun or a torpedo while the detector could be a surveillance radar system or an electro-optical device. The ship, weapon and target detector themselves are not shown. Inertial measurement units 1 and 2, each capable of providing measurements of specific force and angular rate, are mounted one on or near to the detector and one on or near to the weapon. Provided that the ship is not stationary, the difference in the measured quantities may be used to compute the attitude of the weapon with respect to the detector.In order that good estimates of this attitude may be obtained from measurements which are likely to be corrupted by noise, and in the presence of dynamic flexural deformations of the vessel, the measurements are processed using a stochastic filter.

Alignment (i.e. measurement of relative attitude) of the sets of axes is accomplished by comparing inertial measurements, i.e. specific force and angular rate, obtained at each location and processing these measurements using a stochastic filtering technique.

In the drawing the reference IMU (inertial measurement unit) 1 provides measurements of angular rate and specific force (non-gravitational acceleration) in reference axes. The slave IMU 2 provides similar measurements in slave axes. Both IMU's or strapdown units provide three axes of angular rate and specific force information with respect to their respective co-ordinate frames. The outputs are taken to be in digital form which may be input directly to a digital processor unit 3.

Within the processor 3, a comparison unit 4 uses the measured angular rates to compute the relative attitude X of the slave and reference units. This attitude may be described in terms of quaternion parameters and computed by solving the standard quaternion differential equations. The X information is passed to an attitude correction unit 5 where it is corrected using estimates of the attitude errors (AX) derived via a Kalman filter 6 at discrete intervals of time. Thus improved estimates of relative attitude (X) are obtained.

The Kalman filter is based primarily on an error model of the strapdown attitude system, and uses the difference (AZ) between the measured inertial quantities output by the slave and reference IMU's to compute AX. The reference measurements (denoted by Z in the figure) are compared with estimates of the same quantities (Z) derived from the slave system measurements after resolution into reference axes. In addition to computing attitude errors, the filter may also provide estimates of the dominant components of acceleration and angular rate arising due to flexural motion between the locations at which the slave and reference systems are mounted.

These estimates are combined with the resolved slave system measurements to determine Z.

Finally, it is noted that the filter may also be used to obtain estimates of errors in the inertial instrument outputs, i.e. to calibrate the instruments as part of the alignment procedure. It is therefore possible to use relatively low quality gyroscopes and accelerometers in the IMU; whilst still achieving an accurate alignment.

In order to estimate the relative attitude, i.e. to align the slave and reference systems, it is essential that some motion of the craft containing the equipment should occur. For instance, in a ship, the normal wave induced rolling and pitching motion is believed to be sufficient. In the case of an aircraft alignment, the craft may be required to perform an alignment manoeuvre such as a weave, or simply a longitudinal acceleration over a short period oftime.

In the absence of sufficient vehicle motion, the various quantities which the Kalman filter is required to estimate are not separately identifiable and full alignment cannot be accomplished.

In the system described above, both acceleration and angular rate measurements are used to achieve an alignment. In the presence of suitable vehicle motions however, acceleration matching or angular rate matching alone may be used to enable an alignment to take place. Such systems have the advantage of reduced algorithm complexity (although the period of time needed for the align ment to take place may generally be increased).

As will be appreciated, when setting up the illustrated system for use, it would be checked for alignment and calibrated. Periodic re-alignment is also generally desirable after the system has been put into service. Such checks can be done in various ways generally making use of some relatively more sophisticated equipment defining a set of reference axes and, in turn, re-aligning each inertial measure ment unit of the illustrated system against those axes.A warship will often comprise a very accurate inertial measurement unit as part of its on-board navigation equipment. if so, then it can be used to test the illustrated system simply by coupling it into the system in the reference IMU position and then placing each system IMU in the slave position in turn - not necessarily by physically moving the system IMU's of course but instead, for preference at least, by constructing the system so that it can be easily set into a calibration mode with the interconnections re-arranged appropriately. The system is then oper ated to align each system MU against the main ship IMU.

When in use, it is not essential for the illustrated system to relate to local geographical axes as long as the components within the system are aligned with each other. In this case, a 'static' misalignment may be compensated to some extent. It will be realised, of course, that damage to either IMU or relatively large misalignments, may disable the system.

Claims (4)

1. Apparatus for determining the relative attitude of two members comprised in or on-board a movable support, the apparatus comprising for each said member an inertial measurement unit which is located at least near or is coupied to the associated member so as to respond to movement of said support with the production of signals indicative of the instant attitude of the associated member, and comparing means arranged to receive said signals and to formulate therefrom an indication of the relative attitude of the members.

2. A weapon system intended for use on board a vehicle or craft such as a ship and comprising at least two system components and, for each said component, an inertial measurement unit arranged to produce, as a result of movements of the vehicle or craft, signals indicative of the attitude of the associated component, the system further including comparing means for receiving said signals and for formulating therefrom an indication of the relative attitude of the components.

3. A weapon system intended for use on board a vehicle or craft such as a ship and comprising a weapon delivery part and a separate target detection and/or weapon aiming part, the target detection and/orweapon aiming part being operable for delivering first signals for manually or automatically controlling the aiming of the weapon, and the system further comprising, for each said part, an inertial measurement unit arranged to respond to movements of said vehicle or craft to produce second signals which are indicative of the attitude of the associated part, comparing means for receiving said second signals and for formulating therefrom an indication of the relative attitude of the components, and signal processing means which is operable, in response to said comparing means, for processing said first signals to take account of any misalignment of said parts.

4. Aweapon system substantially as hereinbefore described with reference to the accompanying drawing.