GB2081910A - Azimuth measurement from an unstable platform - Google Patents

Abstract

A system for measuring bearing of an object from an unstable platform includes a sighting device 10, trained on the object. Movement of the sighting device 10 in azimuth is digitised by a shaft encoder 15 and the resulting digital signal is combined with the output of a gyro compass 20 by an arithmetic unit 26 to give as output a true bearing regardless of platform movement. The sighting device 10 is preferably a laser rangefinder, whereby bearing and range can be obtained simultaneously. <IMAGE>

Description

SPECIFICATION Bearing (azimuth) measurement The present invention relates to measuring the bearing or azimuth of an object with respect to a datum from a stable or unstable platform, and preferably also measuring the range of the object from the observer.

As one example, in underwater oil exploration it is necessary to position anchors very accurately in range and bearing with respect to a drilling rig or barge. This is done by observing range and bearing from the rig or barge and giving a signal to drop the anchor from a tender when the tender is in the correction position. However, a simesubmersible rig or a drilling barge provides a platform which is far from stable; in particular the rig or barge is likely to experience rotational movement which makes the measurement of bearing angle difficult and inaccurate.

An object of the present invention is to overcome or mitigate this problem, and to provide a bearing measurement system which may be used on an unstable platform.

The invention accordingly provides a system for measuring the azimuth of an object from a viewing station, comprising: sighting means rotatably mounted to be trained on the object by an observer, a transducer mechanically coupled to the sighting means for producing a signal representing rotation of the sighting means, direction datum means for providing a signal representing a reference which is fixed in space irrespective of movement of the viewing station, and circuit means connected to receive said signals and adapted to generate therefrom an output signal representing the azimuth of the sighting means with respect to the datum.

Suitably the transducer is a rotary shaft encoder, and the direction datum means is a gryo compass providing a digital output as known peruse. The circuit means preferably comprises an up/down counter connected to receive pulses from the encoder and an arithmetic unit adapted to combine the count held by the counter with the gryo signal.

Means may be provided for setting an initial count in the counter.

The sighting means is preferably a laser rangefin der of known type.

The output signal may drive a visual display and/or supply a local or remote computer, plotter or other peripheral.

Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure lisa block diagram of a range and bearing system according to the invention; and Figures 2 and 3 are block diagrams of alternative embodiments.

Referring to Figure 1, a laser rangefinder 10 (known per se) is mounted on a shaft 12 via a trunnion 13. The shaft 12 is rotatablyjournalled in suitable bearings within a housing 14 which also encloses a known optical shaft encoder 15 which provides a pulse for each 0.1 of rotation with discrimination of direction of rotation.

The rangefinder 10 may for example be a rangefinder LP7 manufactured by Lasergage Limited, Masons Avenue, Croydon. The shaft encoder 15 may suitably be a Ferranti optical increment encoder type 23L47/5116.

The output of the shaft encoder 15 is decoded by a decoder circuit 16 to provide up or down pulses which increment or decrement a register R2. The register R2 is a 14-bit up/down BCD counter. A register R1, also a BCD counter, stores a reference bearing entered manually by means of switches in a control unit 18. The reference bearing may be loaded from R1 into R2 when starting to take a bearing to give a result in terms of degrees from the reference.

A gyro compass 20 of known type (for example a gyrocompass type SKR 80 manufactured by Robertson Radio of Norway and distributed by Oilfield Hydrographic Systems Limited) provides on line 22 a serial digital signal defining the azimuthal orientation of the gyro platform with respect to a datum direction. The signal on line 22 is converted by circuit 24 into BCD form and passed to an arithmetic unit 26, which also receives the contents of the register R2.

The arithmetic unit 26 adds the contents of R2 and the gyro signal, and if these total more than 260 subtracts 360" from the total to produce an output signal which is fed to a digital display 28.

In use, the laser rangefinder is aligned with a known axis, e.g. a reference object or true north. The bearing of the reference object (zero for true north) is entered in register R1. Means are provided to load the contents of R1 into R2. Subsequent rotation of the rangefinder causes the bearing of any object aimed at relative to true north or other reference to be displayed, irrespective of movement of a ship or the like on which the apparatus is mounted. The rangefinder is operated in the normal way to provide range data.

In Figure 2, the items R1, R2, 16, 18 and 24-28 of Figure 1 are indicated as a single unit 30. Azimuth data is derived as before and combined with range data from the rangefinder 10 in a mixer 32. The combined data is processed by a computer 34 to drive a plotter 36 to indicate graphically the position of an object with respect to the user.

Figure 3 is similar to Figure 2, but the computer 34 and plotter 36 are at a remote location and are supplied via a telemetry transmitter 38 and receiver 40. For example, the range and bearing apparatus may be on a drilling barge and the plotter on a tender. With an operator keeping the rangefinder trained on the tender, the plotter will show con tinuouslythe relative position of the tender to a high degree of accuracy in both range and azimuth.

It is also possible to use the computer to combine the range and bearing data with data defining the position coordinates of the platform, to produce the position coordinates of the object being observed, or to produce a heading and distance required to bring the object to a desired location.

Claims (8)

1. A system for measuring the azimuth of an object from a viewing station, comprising: sighting means rotatably mounted to be trained on the object of an observer, a transducer mechanically coupied to the sighting means for producing a signal representing rotation of the sighting means, direction datum means for providing a signal representing a reference which is fixed in space irrespective of movement of the viewing station, and circuit means connected to receive said signals and adapted to generate therefrom an output signal representing the azimuth of the sighting means with respect to the datum.

2. The system of claim 1, in which the sighting means is a laser rangefinder.

3. The system of claim 1 or claim 2, in which the transducer is a rotary shaft encoder.

4. The system of any preceding claim, in which the direction datum means is a gyro compass providing a digital output.

5. The system of claim 3 and claim 4, in which the circuit means comprises an up/down counter connected to receive pulses from the encoder and an arithmetic unit arranged to combine the count held by the counter with the gyro signal.

6. The system of claim 5, in which the arithmetic unit adds the count held by the counter and the gyro signal and, if the result is greater than 360, subtracts 360 to form the output signal.

7. The system of claim 2 or any claim dependent thereon, in combination with a digital computer arranged to combine range and bearing information from the system with positional data relating to the sighting station.

8. An azimuth measuring system, substantially as herein described with reference to the drawings.