GB1565188A - Compasses - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO COMPASSES (71) We, ELLIOTT BROTHERS (LONDON) LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex CM1 1PL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to compasses and is concerned with compasses of the kind which utilise electro-magnetic coils to detect the earth's magnetic field. Currents are induced in the coils by the magnetic field in which they are situated, and known devices of this kind suffer from certain disadvantages, the most significant of which arises from the need to measure the horizontal component of the terrestial magnetic field without influence from the vertical component.Existing designs are mounted pendulously so as to align themselves with local vertical, but if the vehicle in which the compass is mounted is subject to horizontal acceleration as is the case when changing direction whilst in motion, the compass will tilt, thus erroneously including a part of the vertical component of the field in the measurement. A common method of reducing erroneous output is to connect a gyro such that it is subjected to a torque that tends to align it with the magnetic field measured by the compass.

In such a case the devices are often referred to as gyro magnetic compasses and compasses of this kind often introduce the further disadvantage that, unless constructed in a very expensive way, they do not allow the carrying vehicle, such as an aircraft, complete freedom of attitude; many aircraft manoeuvres causing the gyro mounting to become locked. The present invention seeks to provide an improved device in which the field sensor can be rigidly fixed within the carrying vehicle and permits the total magnetic field to be measured in a way that allows the true horizontal component to be extracted.

According to this invention, a compass includes four electromagnetic pick-up coils, each having its coil axes inclined at the same angle to each of the other three coil axes with the four axes of the four pick-up coils being arranged to extend outwardly from a common point, and means for deriving from the four pick-up coils electrical signals representative of the component of a magnetic field in which the device is situated and which component aligns with a respective axis, from which the direction of the magnetic field can be determined.

Preferably, each of the four pick-up coils surrounds a magnetic material and means are provided for cyclically varying the total magnetic field experienced by each pick-up coil so that the Inagnetic material is operated over a non-linear portion of its flux density magnetising force curve.

Preferably again. said means comprises four further electromagnetic coils, each being located within, and aligned with, a respective one of the four pick-up coils.

Preferably again. the electric currents through the four further coils are varied cyclically in synchronism with each other.

The four axes of the four pick-up coils are arranged to intersect and extend outwardly from a common point, and preferably the outer ends of each pick-up coil being coupled magnetically to a respective one of four equally sized magnetic pick-up plates, the four pick-up plates being shaped and arranged so that, together, they enclose a substantially closed volume in which the four pick-up coils are located.

The four pick-up plates must be magnetically isolated from each other, and a small air gap can be provided for this purpose between adjacent edges thereof.

Preferably, the outer surfaces of the four pick-up plates together define a sphere.

The invention is further described, by way of example, with reference to the accompanying drawings, in which Figure I illustrates, diagrammatically, the relative orientations of the axes of the four pick-up coils, Figure 2 illustrates part of a compass in accordance with the present invention, Figure 3 is an explanatory diagram, and Figure 4 illustrates one way in which the signals provided by the four pick-up coils can be combined together.

Referring to Figure 1, the upper drawing a illustrates an elevation view of the four axes W, X, Y, Z. These correspond to the axes of four electro-magnetic pick-up coils which form part of a compass in accordance with the present invention and the four axes are so arranged that each is spaced at the same angle from the other three. Drawing b of Figure 1 illustrates a plan view of the same four axes and, thus, from Figure 1 it can be seen that axes X and Y both lie in the same horizontal plane and are angularly inclined at 1200 to each other and that axes W and Z both lie in a vertical plane and are also angularly inclined at 1200 to each other. The four axes W, X, Y, Z intersect and extend from a common point 1.

Figure 2 shows the part of a compass associated with one of the four axes. A magnetic pick-up plate 2 is mounted symmetrically about one axis represented by the line 3, 4. The plate 2 is curved and represents one quarter of the surface of a sphere. A pick-up coil 5 and a further coil 6 are arranged symmetrically about the axis 3, 4 with the further coil 6 lying wholly within the pick-up coil 5. Both coils 5 and 6 are wound on a limb 7 formed of a soft magnetic material of a kind which exhibits very little magnetic hysteresis. The actual shape of the pick-up plate 2 can best be seen from drawing b of Figure 2 and it represents one quarter of the surface area of a sphere centred on the point 1 which lies on the axis 3, 4.

Drawing a is a section view taken on the line AA'.

A current flows through an electric coil when the magnetic field in which it is situated changes, but the current drops to zero if the magnetic field becomes constant. The further coil 6 is provided to enable the pick-up coil 5 to detect the existence of a steady unchanging magnetic field and the way in which this is achieved is exPlained with reference to Figure 3 which illustrates at 8 the variation of the flux density of the magnetic material 7 with magnetising force (H). As it is assumed that the material 7 exhibits very little hysteresis the curve 8 is drawn as a single line. It is assumed, for the purpose of explanation, that the magnetic field in which the device is situated has a component of value H1 in the direction of the axis 3, 4.Superimposed on this magnetic field is a further cyclically varying magnetic field which is generated by the further coil 6. This varying magnetic field is represented by the sinusoidal curve H2. It will be seen that the effect of both magnetic fields H1 and H2 is to shift the flux density of the magnetic material 7 onto a non-linear portion of the curve 8 so that the current flowing in the pick-up coil 5 has an asymmetrical characteristic 9. The degree of asymmetry is determined by the extent of the non-linearity of the curve 8, and the value of H1 can be determined from it, even when H1 is unchanging.The limb 7 is so arranged that the coil does not generate an external magnetic field, and to ensure that the B-H relationship is the same for H1 and H2 the effective area of the material of the inner cylinder 10 should be the same as the effective area of the outer cylinder 11, thereby giving identical waveforms 9a and 9b which have a relative phase shift of 1800. The resultant flux cutting the pick-up coil is given by 9a-9b. The coil 6 iss located within the annular space between these two cylinders 10 and 11.

Figure 2 illustrates only one of the four limbs that would be provided and a complete device would include four such limbs equally angularly spaced from one another so that, together, the four magnetic pick-up plates 2 form a substantially closed sphere. The four limbs are mounted on a common boss 12, also formed of a soft magnetic material. It is important that the four pick-up plates are mutually magnetically isolated from each other and this can be achieved by leaving a small air gap between each. The four further coils 6 are driven together at a common cyclically varying frequency. From the four signals produced from the four pick-up coils, the direction of the magnetic field in which the device is situated can be calculated.

To ease the calculation, a combining circuit, as shown in Figure 4, can be used. This circuit has four input terminals 41, 42, 43, 44, to which are applied the respective a.c.

voltages representative of the magnetic field detected by the four pick-up coils. These voltages are represented at Vx, Vv, Vz and V, and are fed directly to respective controllable attentuators 45 to 48, designated Ax, Av, Az and Aw. The attentuators have a range of gain from 1 decreasing through zero to - 1 . The voltages Vx and Vv are also fed to a point 49 where they are combined and passed to a further attentuator 50; designated A Similarly, voltages Vz and Vw are combined at point 51 and fed to variable attentuator 52. The outputs from attentuators 45 to 48 are combined at point 53, and fed to point 54 where they are combined with the outputs of attenuators 50 and 52. The resistors 55 represent the summing resistors.It may be necessary to insert impedance buffers at terminals 41 to 44, so that the current in the pick-up coils is kept low.

The attentuators Axy and A", are provided to ease the task of determining the actual direction of the earth's magnetic field. By adding Vw and Vz prior to being fed to the attentuator Awz, and by adding Vx and V prior to being fed to the attentuator Axy, the three-dimensional system can be reduced to two orthogonal two-dimensional systems.

The two planes so defined can be arranged to be in the pitch and roll axes of an aircraft, and by so doing the calculation of the direction of the earth's magnetic field is eased. The input signals can be represented as, Vx = cosO V = cos(6 + 1200) Vzw = cos(0 - 1200) Vw = cosX Vz = cos( + 1200) Vxy = cos( - 1200) where 0 and are the directions of the magnetic field to the two two-dimensional planes defined above.When obtaining the value of 0, attenuators Vw, Vz and Vx are set to the maximum attentuation, and when obtaining the value of 4), attentuators Vx, 'V,, and Vzw are set to the maximum attenuation. For each value of attentuation a positive or negative signal is obtained at the output terminal 54, and the value can be set iteratively until the presence of a stable null of minimum error signal is obtained.

Although several combinations of attentuation can result in a null, if they are related tirgonometrically with 6 and Q there exists only a single stable null. This process is performed for both 6 and zp in turn.

It would, of course, instead be possible to perform the calculation digitally in a fairly straight-forward manner, but the presence of noise at the input terminals could cause serious difficulties.

WHAT WE CLAIM IS: 1. A compass including four electro-magnetic pick-up coils, each having its coil axes inclined at the same angle to each of the other three coil axes with the four axes of the four pick-up coils being arranged to extend outwardly from a common point, and means for deriving from the four pick-up coils electrical signals representative of the component of a magnetic field in which the device is situated and which component aligns with a respective axis, from which the direction of the magnetic field can be determined.

2. A compass as claimed in claim 1 and wherein each of the four pick-up coils surrounds a magnetic material and means are provided for cyclically varying the total magnetic field experienced by each pick-up coil so that the magnetic material is operated over a non-linear portion of its flux density magnetising force curve.

3. A compass as claimed in claim 2 and wherein said means comprises four further electro-magnetic coils, each being located within, and aligned with, a respective one of the four pick-up coils.

4. A compass as claimed in claim 3 and wherein the electric currents through the four further coils are varied cyclically in synchronism with each other.

5. A compass as claimed in any of the preceding claims and wherein the outer ends of each pick-up coil are coupled magnetically to a respective one of four equally sized magnetic pick-up plates, the four pick-up plates being shaped and arranged so that, together, they enclose a substantially closed volume in which the four pick-up coils are located.

6. A compass as claimed in claim 5 and wherein the outer surface of the four pick-up plates together define a sphere.

7. A compass substantialy as illustrated in and described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. combined with the outputs of attenuators 50 and 52. The resistors 55 represent the summing resistors. It may be necessary to insert impedance buffers at terminals 41 to 44, so that the current in the pick-up coils is kept low. The attentuators Axy and A", are provided to ease the task of determining the actual direction of the earth's magnetic field. By adding Vw and Vz prior to being fed to the attentuator Awz, and by adding Vx and V prior to being fed to the attentuator Axy, the three-dimensional system can be reduced to two orthogonal two-dimensional systems. The two planes so defined can be arranged to be in the pitch and roll axes of an aircraft, and by so doing the calculation of the direction of the earth's magnetic field is eased. The input signals can be represented as, Vx = cosO V = cos(6 + 1200) Vzw = cos(0 - 1200) Vw = cosX Vz = cos( + 1200) Vxy = cos( - 1200) where 0 and are the directions of the magnetic field to the two two-dimensional planes defined above.When obtaining the value of 0, attenuators Vw, Vz and Vx are set to the maximum attentuation, and when obtaining the value of 4), attentuators Vx, 'V,, and Vzw are set to the maximum attenuation. For each value of attentuation a positive or negative signal is obtained at the output terminal 54, and the value can be set iteratively until the presence of a stable null of minimum error signal is obtained. Although several combinations of attentuation can result in a null, if they are related tirgonometrically with 6 and Q there exists only a single stable null. This process is performed for both 6 and zp in turn. It would, of course, instead be possible to perform the calculation digitally in a fairly straight-forward manner, but the presence of noise at the input terminals could cause serious difficulties. WHAT WE CLAIM IS:

1. A compass including four electro-magnetic pick-up coils, each having its coil axes inclined at the same angle to each of the other three coil axes with the four axes of the four pick-up coils being arranged to extend outwardly from a common point, and means for deriving from the four pick-up coils electrical signals representative of the component of a magnetic field in which the device is situated and which component aligns with a respective axis, from which the direction of the magnetic field can be determined.

2. A compass as claimed in claim 1 and wherein each of the four pick-up coils surrounds a magnetic material and means are provided for cyclically varying the total magnetic field experienced by each pick-up coil so that the magnetic material is operated over a non-linear portion of its flux density magnetising force curve.

3. A compass as claimed in claim 2 and wherein said means comprises four further electro-magnetic coils, each being located within, and aligned with, a respective one of the four pick-up coils.

4. A compass as claimed in claim 3 and wherein the electric currents through the four further coils are varied cyclically in synchronism with each other.

5. A compass as claimed in any of the preceding claims and wherein the outer ends of each pick-up coil are coupled magnetically to a respective one of four equally sized magnetic pick-up plates, the four pick-up plates being shaped and arranged so that, together, they enclose a substantially closed volume in which the four pick-up coils are located.

6. A compass as claimed in claim 5 and wherein the outer surface of the four pick-up plates together define a sphere.

7. A compass substantialy as illustrated in and described with reference to the accompanying drawings.