GB2042724A

GB2042724A - Magnetic Compasses

Info

Publication number: GB2042724A
Application number: GB7906071A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-02-21
Filing date: 1979-02-21
Publication date: 1980-09-24

Abstract

A non-pendulously pivoted compass mounted on a platform 54 stabilised by gyroscopic means to keep its plane horizontal, the mounting of the compass being such that it responds only to the component of terrestrial magnetism in the plane of the platform. The compass can be a "flux valve" or a magnetic needle 50 to give a direct reading of heading and/or drive an encoder to feed a remote display, or a coil of wire 11, 12 driven to rotate about an axis 13 perpendicular to the plane of the platform 16 and to the axis of the coil thus producing a voltage induced by terrestrial magnetism which goes through zero when the coil axis points North-South, means being provided to measure the orientation of the coil with respect to the vehicle. <IMAGE>

Description

SPECIFICATION A Magnetic Compass System This invention relates to compasses for boats, aircraft etc (which will be referred to generically as "vehicles"), and embodies features of my earlier invention described in U.K. patent application 39370/78, which it supersedes. Its object is to provide a vehicle compass system which overcomes certain inaccuracies inherent in known magnetic compass systems, while avoiding the high cost of known gyro-magnetic compass systems as used particularly in aircraft.

One major problem of magnetic compass devices stems from "magnetic dip", i.e. the inclination of the earth's magnetic field to the horizontal. Because of this, gross errors in compass indication can result if the magnetic needle is not accurately horizontal. For instance, at the latitude of Britain, the compass reading can be nearly 3 degrees in error for every degree of departure of the needle from the horizontal. The same problem applies to other devices used for detecting the direction of magnetic North, such as the "flux valves" (or "flux gates") used in aircraft gyro-magnetic compass systems, which are based on magnetic saturation phenomena.

To overcome this problem, it is normal practice to make the detecting device, whether magnetic needle or "flux valve", pendulous, so that it remains horizontal at least when the vehicle is in steady motion. When the vehicle is accelerating however, and more particularly when it is turning, any pendulously suspended device aligns itself with the apparent vertical, which is the direction of the resultant of gravity and acceleration forces, and which can deviate from the true vertical by a large fraction of 900, so that gross errors can result in such manoeuvres.

With plain magnetic compasses, these errors are inevitable. With gyro-magnetic compasses, the magnetic detecting device (a flux valve) is combined with a horizontal gyro-scope in a manner which minimises the errors, but at a relatively high cost, since both the gyroscope and the flux gate are quite expensive devices.

In a compass system according to this invention, a magnetic sensing device is nonpendulously mounted on a gyroscopically stabilised platform ("stable platform"), i.e. a platform which is kept substantially horizontal by gyroscopic means, in such a manner as to make it respond only to the component of terrestrial magnetism in the plane of the platform.

The elimination of pendulous mounting would normally represent a simplification, but stable platforms have hitherto been complicated and expensive devices. Furthermore, known stable platforms are dependent on electric motors for their operation, and therefore generate stray magnetic fields, which make them unsuitable as mountings for a device which is to sense terrestrial magnetism. In U.K. patent application 39371/78 Serial No. 2031149 however I have described a novel type of stable platform of relatively simple and inexpensive construction, and which moreover is inherently independent of electric motors or other electro-magnetic devices.

A platform of that type is envisaged as the mounting for a magnetic sensing device in accordance with this invention.

Various types of magnetic sensing device may be used. For instance, one may use a flux valve as used in existing gyro-magnetic compass systems, but fixed rigidly to the platform instead of being pendulously suspended. Apart from this detail such a system would be identicai to any known system based directly (i.e. without horizontal gyroscope) on a flux valve, and no further description is therefore necessary. Alternatively, and as described in more detail hereunder, the sensing device could be a magnetised needle or it could be a rotating coil of wire.

In the accompanying drawings, Figure 1 is a schematic illustration of one embodiment of the invention, Figure 2 is a schematic illustration of an alternative embodiment Figure 3 is a schematic illustration of a modified form of the embodiment shown in Figure 2.

Referring to Figure 1, which describes a system based on a magnetised needle, the needle 50 is fixed to a shaft 51 running in bearings 52 and 53 fixed to a stable platform 54, so that the axis of shaft 51 is maintained vertical to the degree of accuracy that the plane of the platform is maintained horizontal, and therefore, the needle responds to the component of the earth's magnetic field in the plane of the platform. Also fixed to shaft 51 is a compass card 55 and/or an optical encoder disc 56, the latter cooperating with light sources and photo-sensitive devices to give a signal which may be used in known manner to drive a remote heading display if a remote display is required, while the compass card gives a local indication of heading if that is required.

In the alternative embodiment shown in Figure 2, the magnetic sensing device consists of a coil of conducting wire wound on an axis 10, and conveniently made in two halves 11 and 12 (for the sake of dynamic balance) connected in series, and fixed to a shaft 1 3 rotating about its axis in bearings 14 and 1 5 located in a stable platform 16. The shaft and coil are spun by a pneumatic motor of any convenient type (or any other form of rotary motor not depending on electromagnetic effects).

Rotation of the coil induces in it a voltage caused by the earth's magnetic field, and this voltage goes through zero when the axis of the coil is aligned North-South. This voltage is fed to the necessary electric circuitry by brushes T8, 19 bearing on slip-rings 20,21 fixed to the shaft, and connected to the ends of the coil winding.

The shaft also carries a disc 22 which acts as an angle encoder, i.e. a means of giving signals which measure the orientation of the coil with respect to some fixed direction (say the fore-andaft axis) in the vehicle. The encoder is preferably of the optical type, and is so illustrated for purposes of description. The encoder could be of the direct type-well known in the art-which generates a digital (usually binary)) signal which expresses its orientation relatively to a fixed direction.

Alternatively the encoder could, as illustrated in Figure 1, consist of a single light source 23 and a photo-sensitive device 24, the encoder disc having a short opaque or transparent "shutter" segment, thus generating a pulse when the disc passes through some reference orientation. This system would work in conjunction with a clock pulse generator and an electronic counter, set to count the time interval from the last passage through the reference direction and also the time interval between the last two passages, plus an electronic computing circuit to divide the second interval into the first, the result expressing the angle of the coil from the reference direction expressed as a fraction of one revolution.

Or again, the encoder could consist of a light source 23, photo-sensitive device 24 and short "shutter" segment as just described, giving a signal denoting the passage of the coil through a reference orientation, plus a second photosensitive device and a light shining on it through a pattern of alternate opaque and transparent segments on the disc, giving a series of pulses fed to an electronic counter which is zeroed by the signal from device 24, so that the count represents the rotation of the coil from the reference direction.

Figure 3 shows an alternative construction of the sensing device, which avoids the need for siiprings. In this version the coil halves 1 1,12 are in series with a coil 30 coaxial with and rotating with the shaft, coil 30 forming the primary winding of a transformer, the secondary winding of which is a coil 31 fixed to the platform, in close proximity to coil 30 and coaxial with it. The shaft again carries a disc 22 acting as an angular encoder, as before.

Information provided by the encoder is processed in known manner to derive a signal expressing the heading of the vehicle, which is subtracted from a feedback signal derived from a compass display instrument, the difference being amplified and fed to a servo-motor driving the display, thus causing the latter to display the heading as sensed by the sensing device.

Claims

1. A compass system for boats, aircraft, or other vehicles, consisting of a device capable of sensing the direction of the earth's magnetic field, said device being mounted on a gyroscopically stabilised platform of a type which does not generate significant stray magnetic fields, the mounting being such that the magnetic sensing device responds only to the component of the local magnetic field in the plane of the platform, which is maintained close to the horizontal plane.

2. A compass system as claimed in claim 1, in which the sensing device is a "flux valve" of known type rigidly mounted on the platform, to provide a signal to a remote heading display.

3. A compass system as claimed in claim 1, in which the sensing device consists of a magnetised needle fixed to a shaft running in bearings fixed in the platform, the axis of rotation of the shaft being perpendicular to the plane of the platform.

4. A compass system as claimed in claim 3, in which the shaft carries a compass card or a pointer to give a direct indication of heading.

5. A compass system as claimed in claim 3 or claim 4, in which the shaft carries an optical encoder cooperating with light sources and photosensitive devices to give a signal of shaft orientation which is capable of being fed to a remote heading display.

6. A compass system as claimed in claim 1, in which the sensing device is a coil of conducting wire mounted on a shaft rotating in bearings on an axis perpendicular to the plane of the platform, the axis of the coil being perpendicular to the axis of the shaft, both being rotated by a motor of a type which does not generate stray magnetic fields, means being provided for determining the orientation of the coil at the instant when the voltage induced in it by terrestrial magnetism passes through zero.

7. A compass system as claimed in claim 6, in which the voltage induced in the rotating coil is fed to measuring circuits by slip-rings.

8. A compass system as claimed in claim 6, in which the revolving coil is in series with a second coil coaxial with the spin axis of the revolving coil and rotating with it, and a third stationary coil coaxial with and close to the second coil is provided, the second and third coil forming a transformer, whereby the voltage generated in the sensing coil induces a voltage in the third coil, as a means of feeding the voltage in the sensing coil to a stationary circuit without resort to slip-rings.

9. A compass system as claimed in claim 6 and either of claims 7 or 8, where a signal expressing the orientation of the spinning coil is derived by an angle encoder rotating with that coil.

10. A compass system as claimed in claim 6 and either of claims 7 or 8, in which a datum point from which to measure the orientation of the coil is signalled by an optical shutter rotating with the coil and cooperating with a light source and a photo-sensitive device.

1 A compass system as claimed in claim 10, where the orientation of the coil is computed by counting clock pulses from the passage of the coil through the reference orientation and clock pulses between two successive such passages, and dividing the second count into the first.

12. A compass system as claimed in claim 10, where there is provided a disc rotating with the sensing coil and having alternate opaque and transparent segments acting in cooperation with a light source and photo-sensitive device to produce pulses in proportion to the rotation of the disc, and there is further provided a counting device to count the said pulses from the passage of the coil through the reference orientation thereby giving a measure of the orientation of the coil.