GB2218517A

GB2218517A - Navigational Aid

Info

Publication number: GB2218517A
Application number: GB8906111A
Authority: GB
Inventors: Martin John Cahill
Original assignee: SEAFARER INTERNATIONAL Ltd
Current assignee: SEAFARER INTERNATIONAL Ltd
Priority date: 1988-03-16
Filing date: 1989-03-16
Publication date: 1989-11-15
Also published as: GB8906111D0; GB8806275D0

Abstract

A navigational aid to enhance the performance of an autopilot (10), especially for boats travelling on a northerly heading at high speed, comprises a rate-of-turn sensor (14), e.g. based upon a gyroscope, arranged to supplement a compass (12). A microprocessor (18) uses the output from the rate-of-turn sensor (14) to control the autopilot when a rate of turn greater than a preset rate is detected, and only uses the output from the compass when the sensor indicates that the rate of turn is less than said preset rate. <IMAGE>

Description

NAVIGATIONAL AID SPECIFICATION This invention relates to navigational aids, and is also concerned with a navigational system which overcomes one of the problems of conventional navigational systems.

The present invention is particularly concerned with navigational systems for boats, although it could also be applied to aircraft or land-based vehicles.

Autopilots using a magnetic or fluxgate compass as the main direction Sensor all suffer from a condition called "north seeking" or "turning" error. The cause of this problem is due to the earth's magnetic field not being horizontal to the surface of the earth in latitudes away from the equator. At latitude 50 N the "angle of dipl' is approximately 680. When a boat is travelling fast on a northerly heading and makes a course change or is knocked off course by a wave or some other effect, the centrifugal force which acts on the gimbals of the compass during the time the boat is turning forces the compass card or fluxgate sensor to leave the horizontal position in which it would normally lie.In this position the compass card is then pointing down the lines of magnetic flux and is no longer pointing in the direction of "magnetic north". In other words, the compass is "looking" at the lines of the earth's magnetic field from a different angle and therefore thinks the boat is on a different course from the one which it is truly on. In the northern hemisphere, the error will be seen as an under-reading. In the southern hemisphere, under similar conditions, the error will be seen as an overreading. The phenomenon becomes more marked the faster the boat is travelling. As a result, the signal from the compass to the autopilot signifies an error when there is no error.This incorrect information causes the autopilot to make a course change which takes the boat off course and causes instability, resulting in the boat wandering about the required northerly course and eventually becoming unstable and possibly going out of control.

Generally, the smaller the craft, the quicker it can turn. The point at which effective control is lost depends upon many factors, for example hull shape, sea conditions and rudder size. As a rough guideline, an autopilot on a boat of under: 20 feet in length will lose effective control at 10 knots.

25 feet in length will lose effective control at 12 knots.

30 feet in length will lose effective control at 15 knots.

40 feet in length will lose effective control at 18 knots.

50 feet in length will lose effective control at 20 knots.

It is therefore an object o the present invention to provide a navigational aid, and a system incorporating a navigational aid, which overcomes this problem.

It is a further object of the present invention to provide a navigational aid, and a navigational system incorporating a navigational aid, which enables one to achieve more accurate steering under normal conditions as well, i.e. when not travelling north at relatively high speed.

In accordance with the present invention there is provided a navigational aid in which a compass used as a main direction sensor is supplemented by a rate-ofturn sensor.

Preferably, the compass is used as a main heading device and the rate-of-turn sensor is used on a shortterm basis to hold the course heading for the short term.

In a preferred arrangement, the compass and the rate-of-turn sensor are connected to microprocessor means for controlling a steering system, the microprocessor using the output from the rate-of-turn sensor when a rate of turn greater than a predetermined rate is detected, and using the output from the compass when the rate-of-turn sensor indicates that the rate of turn is less than said predetermined rate.

The navigational aid of the present invention is particularly adapted for use with an autopilot. The rate-of-turn sensor can be provided as an optional extra for a basic autopilot and can be fitted as an accessory to existing autopilot systems. Although the navigational aid of the present invention will be highly desirable on certain fast boats, it will still positively enhance the performance of an autopilot on all boats.

In order that the invention may be fullv understood, one presently preferred embodiment of navigational aid in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a block schematic diagram illustrating the use of the navigational aid in conjunction with an autopilot; Fig. 2 is a side view of the rate-of-turn sensor; and Fig. 3 is a top plan view of the gyroscope of the sensor shown in Fig.2.

Referring first to Fig. 1, this shows the general arrangement of an autopilot 10 which is controlled both by a compass 12 and by a rate-of-turn sensor 14. The compass 12 may be a magnetic or fluxgate compass for example and serves as the main direction sensor for the autopilot 10. The compass 12 incorporates a sense unit 16 which reacts to movements of the compass and which provides an output signal to a microprocessor 18. The microprocessor 18 provides control signals for the autopilot 10. The rate-of-turn sensor 14, further details of which will be described hereinafter, provides an analogue output to an interface unit 20, the output being information as to the rate of change of direction of the boat or other craft. The interface unit 20 provides an output signal the frequency of which varies with changes in the output from the rateof-turn sensor.The output frequency signal from the interface unit 20 is fed to the microprocessor 18.

The microprocessor 18, which in practice is built into the autopilot 10, continuously monitors both the output from the compass sense unit 16 and the output from the interface unit 20. When the frequency signal from the interface unit 20 exceeds a predetermined value, representing a rate of turn greater than a preset figure, the microprocessor 18 uses the signal from the rate-of-turn sensor 14 to control the boat and ignores the compass output signal until the boat is again on a steady course and the rate of turn, as detected by the sensor 14, has returned to an acceptable level, i.e. has fallen below a predetermined figure.When this happens, the microprocessor 18 will then switch back to accepting information from the compass sense unit 16 for the control of the autopilot In other words, the information from the rate-of-turn sensor 14 is used to steer the boat over relatively short periods of time, whilst the main compass is used to maintain the boat on the correct course, the information from the compass only being used when the boat is sensed not to be turning at a rate greater than a selected threshold rate.

The rate-of-turn sensor 14 is used automatically bv the autopilot 10 as a back-up to the compass sense unit 16 when the rate of change of direction of the boat reaches a level which could cause short-term errors from the main compass sense unit 16. When the autopilot 10 is first switched on, i.e. when power is applied to it, the output frequency from the interface unit 20 at zero rate of directional change is sampled and is used as a reference signal. It is therefore important that the autopilot should be switched on in fairly calm conditions in order to enable this output frequency from interface unit 20 at zero rate of directional change to be sampled. This will be achieved either in the standby or in the automatic mode. If this reference level is not established for the microprocessor then the rate-of-turn sensor will be ignored by the autopilot and the autopilot will respond only to signals from the compass sense unit 16.

The rate-of-turn sensor 14 and the associated electronics, including the interface unit 20, are preferably encased together in a housing of plastics material. As the rate-of-turn sensor 14 preferably comprises a gyroscope, associated with which is a magnetic field, the sensor 14 should be positioned away from the compass 12 and away from the compass sense unit 16. A separation of at least 500mm is desirable.

The rate-of-turn sensor 14 is preferably mounted securely on a rigid horizontal surface, preferably amidships on the centre line of the boat, away from excessive vibration.

Referring now to Figs. 2 and 3, these show one particular embodiment of rate-of-turn sensor. The rate-of-turn sensor preferably is based upon a gyroscope. As shown in the drawings, the sensor comprises a housing 24, preferably of plastics material, in the form of a box having a detachable lid 26. The housing 24 is provided on its underside with a mounting plate 28 by means of which it can be secured to a rigid horizontal surface. Within the housing 24 is mounted a printed circuit board 30 which constitutes the interface unit. A gyroscope, indicated generally at 32, is mounted within the housing above the printed circuit board on a mounting plate 34. In this embodiment the gyroscope 32 comprises a small electric motor 36 with a pair of rotatable wheels 37, 38 mounted one on each side of the motor on a shaft 40 which extends centrally through the motor.The mounting plate 34 includes an upstanding backplate 42 which is provided with a pair of lugs 44 at the upper corners thereof. Between these lugs 44 and a central support shaft 46, which is rigid with the motor 36, are a pair of springs 48,49. The motor/wheel assembly 36,37,38 is mounted to be pivotable about a horizontal axis extending fore and aft through the centre of the motor 36, as indicated at 50. Below the body of the motor 36 is mounted a Hall-effect sensor and magnet, indicated at 52. An output lead 54 from the Hall-effect sensor is connected to the printed circuit board 30.

As the boat turns, the gyroscopic effect will cause the gyroscope to tilt about the pivot axis 50 against the constraint of the springs 48,49. This movement is measured by the Hall-effect sensor and magnet 52 situated below the motor, and the analogue output from the Hall-effect sensor 52 is passed to the interface circuit where it is converted into a frequency which is transferred to the microprocessor of the autopilot, via output lead 56. When a fast rate of turn is detected by the gyroscope, i.e. in excess of a preset threshold level, the microprocessor of the autopilot will use the signal from the gyroscope to control the boat and will ignore the compass output until the boat is again on a steady course and the rate of turn is at an acceptable level, at which time information on the heading of the boat is again taken from the compass.

Although the gyroscope shown in Figs. 2 and 3 is in the form of a motor and a pair of wheels, other forms of gyroscope could alternatively be used, for example a gyroscope in the form of a solid state device, rather like a tuning fork.

Claims

CLAIMS:

1. A navigational aid comprising a compass arranged to be used as a main direction sensor and a rate-of-turn sensor arranged to supplement the compass.

2. A navigational aid as claimed in claim 1, in which the rate-of-turn sensor comprises a gyroscope.

3. A navigational aid as claimed in claim 2, in which the gyroscope comprises an electric motor and a pair of rotatable discs mounted one at each end of the motor, the motor and discs being pivotable about a tilt axis against the constraint of spring means, and the motion of the motor and discs being detected by sensor means.

4. A navigational aid as claimed in claim 3, in which the sensor means comprises a Hall-effect device.

5. A navigational aid as claimed in claim 3 or 4, in which the output from the rate-of-turn sensor is a signal the frequency of which varies with changes in the rate of turn.

6. A navigational aid substantially as heroinbefore described with reference to the accompanying drawings.

7. A navigational system comprising a naigationl aId as claimed in any preceding claim, microprocessor means arrange to receive the output from both the compass and the rate-of-turn sensor, and an autopilot controlled by the microprocessor means.

8. A navigational system as claimed in claim 7, in which the microprocessor means uses the output from the rate-of-turn sensor to control the autopilot when > rate of turn greater than a predetermined rate is detected, and only uses the output from the compass when the rate-of-turn sensor indicates that the rate of turn is less than said predetermined rate.

9. A navigational system substantially as hereiabefor described with reference to the accompanying drawings.