A SENSOR FOR A MAGNETIC GRADIOMETER

Abstract: Magnetic field sensor which can be used in a magnetic gradiometer. Gradiometer which includes two substantially coaxial windings (1, 2) with cores (3, 4) and an inner tube (5) arranged within an outer tube (9) having a length somewhat greater than the length of the inner tube, at least a central part of the inner tube being supported against radial movement relative to the outer tube, and at least the core being located within the inner tube, and further including an adjusting device (15) for adjusting the concentricity of the windings with cores, is so constructed that each core comprises a thin magnetic strip (6) located externally on a separate core carrier (7) which is individual to each core and which is short in comparison with the inner tube, and that the adjusting device is arranged to move a part of, but not the whole of the inner tube radially in relation to the outer tube, one core carrier, but not all core carriers, being located in the inner tube.

Description: A sensor for a magnetic gradiometer The present invention relates to a magnetic field sensor, and more particularly, although not exclusively, to a sen¬ sor intended for use in a magnetic gradiometer. Magnetic gradiometers are used to detect local variations in magnetic fields, and optionally also to measure the orientation and magnitudes of detected variations. Varia- - tions in magnetic fields may be due to various causes, e.g. the presence of metallic objects, such as mines, in the ground or in water, magnetic bodies, or electrical circuits, etc. A gradiometer normally comprises two substantially coaxial windings or coils with cores arranged within an outer tube, and electrical circuits which receive and process electrical signals emanated by the windings. The purpose of the two substantially coaxial windings is to ensure that magnetic fields which are mutually identical in the respective regions of the two windings or coils will give rise solely to such signals from the windings that will balance out each other in the electrical circuits. On the other hand, magnetic fields which vary in space, and which therewith differ in the region of the one winding from the region of the other winding, shall give rise to signals which do not totally balance-out one another in the electrical circuits, but instead cause the electrical circuits to produce a differential signal. In order to achieve this it is essential that the windings and cores do not deviate from a perfect coaxial position to any appreciable extent. The accuracy of the gradiometer is highly dependent on the degree of concentricity of the windings, i.e. the mutual coaxial state of the windings. It will be understood from this that one important problem resides in achieving the requisite degree of coaxiality between the respective windings and cores of a gradio¬ meter. This problem is aggravated by the fact that there is no guarantee that the original degree of concentricity of the windings and cores found to be sufficiently precise subsequent to manufacturing and delivering the gradiometer will be sustained throughout the effective working life thereof. The coaxiality of the windings and cores can be impaired as a result of ageing of the material from which the various parts of the gradiometer are constructed, and also by variations in the ambient environment, such as changes in temperature, pressure, air humidity, gravitational forces, etc. Gradiometer sensors comprising two or three essentially coaxial windings with cores, and an inner tube which is arranged in an outer tube that is somewhat longer than the inner tube, and in which at least part of the inner tube is supported against radial movement in relation to the outer tube, and in which at least the cores are located within the inner tube and an adjusting device is provided for adjusting the coaxiality or mutually axial state of the windings with cores, are known from, for example, US 2,620,381; US 2,996,663 and US 3,286,169. The sensors taught by these publications differ from one another with regard to the structural design of the windings, cores, and adjustment devices employed. The present invention is based on a combination of wind¬ ing, core, tube, and adjustment device configurations or features which are mutually adapted one to the other and which in co-action with one another solve the problem of attaining the requisite mutual concentricity of the wind- ings and cores. When taken individually, however, these structural features according to the invention afford no significant advantage, since in order to achieve the result desired a given construction or configuration of one part of the sensor is contingent on a given construc¬ tion or configuration of other parts in accordance with 5 the invention. According to the invention each core comprises a thin mag¬ netic strip arranged on a separate core carrier which is individual to each core and which is short in comparison 10 with the inner tube. Furthermore, the adjusting device shall be capable of moving a part of the inner tube, but not the whole of said tube, radially in relation to the outer tube, one, but not all, of the core carriers being located in this moveable part of the inner tube. 15 It is known per se, e.g. from US 2,981,885; US 3,319,161 and US 3,961,245, to manufacture the core of a magnetic sensor that includes windings and cores from a thin strip of material that possesses desirable magnetic properties. 2.0 It is also known per se to provide magnetic sensors with adjusting devices that are operative for moving an inner part in relation to an outer tube, thereby to obtain the desired concentricity. Examples of such arrangements are disclosed in US 2,620,381; US 2,996,663; US 3,050,679 and 2-- US 3,961,245. These known structural features, however, obtain a particular significance when applied together with other structural features according to the invention. For example, manufacture of a winding with core and its coaxial adjustment is made particularly simple when each 0 core within an inner tube and windings are placed exter¬ nally on the inner tube. A core which is constructed from a thin strip and is mounted externally on a separate core carrier can be inserted into the inner tube with far grea¬ ter ease, particularly when each core carrier is short in 5 comparison with the inner tube. At the same time, synchro¬ nous coaxial adjustment of winding and core can be readily ___-. _-_.._. PCT/SE87/00004 effected, through radial displacement of a part, but not all, of the inner tube, particularly when each core asso¬ ciated with a winding is located externally on a separate core carrier which is short in comparison with the inner 5 tube. Due to the fact that the wall of the outer tube is preferably thicker than the wall of the inner tube and is made of the same material as the inner tube, i.e. a mate¬ rial which exhibits high shape-stability within a tempera¬ ture range of from about -20°C to about +50°C, and TG because the outer tube is somewhat longer than the inner tube, the influence on coaxiality of rapid changes in temperature and pressure, etc., will be equalized, while the outer tube serves, at the same time, as an anvil sur¬ face when bending the inner tube in order to adjust the ^ concentricity or co-axial state of the windings with cores. Furthermore, because the cores are constructed from strip material arranged on core carriers located within the inner tube, it is possible to alter the length of the cores without needing to modify any other part of the sen- 2.0 sor. The combination of individual structural configura¬ tions given to the various parts of the sensor in accor¬ dance with the invention also affords other advantages. These advantages are too numerous to mention in their totality. It can be said, however, that the structural 5- configuration given to each part of the sensor in accor¬ dance with the invention is such that each structural con¬ figuration is not only positive and will not solely afford advantages in one respect, but is also active in contribu¬ ting towards and affording advantages in other respects, 0- due to the mutual co-action of each specific structural configuration in accordance with the invention with the structural configuration of other parts of the sensor according to the invention. There is obtained hereby a multiple synergistic effect which solves the technical 5 problems concerned in an advantageous manner. The features common to a sensor constructed in accordance with the invention are set forth in the following Claim 1, whereas the depending claims set forth embodiments which are of particular advantage in one respect or another. Embodiments of the invention are described below with reference to the accompanying drawing, in which Fig. 1 is a simplified longitudinal sectional view of a sensor according to the invention; Fig. 2 is a front view of a bottom support incorporated in the sensor; Fig. 3 is a front view of a support ring incorporated in the sensor; Fig. 4 is a longitudinal sectional view of a core incorpo¬ rated in the sensor, and Fig. 5 is a front view of an adjusting device forming part of the sensor. For the sake of simplicity, certain components of the sen¬ sor have been excluded from certain figures, and the sca- les used are not the same -for all figures. The illustrated proportions between the various components and the illu¬ strated length/breadth/thickness ratios between said com¬ ponents have been chosen for reasons of clarity with regard to the various illustrations shown and do not fully reflect the true proportions or ratios applied in reality. The embodiment of a sensor constructed in accordance with the invention and illustrated in Figs. 1-5 comprises two substantially coaxial windings or coils 1 and 2 with respective cores 3 and 4. The windings are wound around an inner tube 5 which is made of a shape-stable material, preferably a material which is highly shape-stable within a temperature range of about -20°C to about +50°C, for example a ceramic material of the kind retailed by KEBO- Grave AB, Stockholm under the name Pythagorasrδr (Pythago- 5 ras tubes), and which has an external diameter of about 6 mm. Each core comprises a thin magnetic strip 6 arranged on a respectivce core carrier 7, there being one such car¬ rier for each individual core. The core carriers prefer¬ ably have the form of quartz tubes presenting substanti- 10 ally cylindrical peripheral surfaces, and the strips are folded or wound around the peripheral surfaces of respec¬ tive carriers in a manner such that the length direction of the strips coincides generally with the axial direction of the peripheral surfaces of respective carriers. The 15 quartz tubes have a length in the region of 4 cm and are therefore short in comparison with the inner tube. The windings with cores and the inner tube are located in an outer tube, which has an outer diameter of about 1.5 2.0 cm. The length of the outer tube is in the order of 50 cm and is somewhat greater than the length of the inner tube, such that when the cores are located in their respective ends of the inner tube said cores are spaced about 40 cm apart. 25 An inner and central part of the inner tube is supported against radial movement in relation to the outer tube, by means of a silicone-rubber cylindrical element or layer 10. The outer and inner diameters of the silicone rubber 3 : 0 element 10 are accordingly adapted to the inner diameter of the outer tube and to the outer diameter of the inner tube respectively, and are about 11 mm and 6 mm respec¬ tively. The silicone rubber element extends " over the winding 1 and along the inner tube, through a distance 35 equal to at least approximately half the distance to the other, or outer, winding 2, and may be provided with small holes for accommodating electrical wire conductors. Alternatively, electrical wire connections for the inner winding 1 may be incorporated in the silicone rubber element when embodying the element between the inner and the outer tubes. The inner end of the inner tube may also be supported against radial movement in relation to the outer tube, by means of a bottom support 13 positioned in the end of the outer tube. The bottom support has a length of about 11 mm and is firmly seated in the outer tube, and presents a centrally located aperture 14 for accommodating the adjacent end of the inner tube. If suitably construc¬ ted, the silicone rubber element will also, of course, prevent the inner tube from moving axially in the outer tube, to varying extents. Arranged externally on the end of the outer tube remote from the bottom support is an adjusting device 15. At this end of the sensor assembly, the inner tube extends beyond the end of the outer tube and passes into a central aperture 16 in the adjusting device. The adjusting device incorporates four screws 17, 18, 19 and 20 by means of which forces can be applied to the inner tube, such as to move a part of the inner tube, but not the whole of said tube, radially in relation to the outer tube. These forces are operative in moving the end of the inner tube located in the adjusting device and accommodating one of the core holders 8, radially in relation to the end of the outer tube located in said adjusting device. The outer tube is manufactured from a material which is highly shape-stable, and has a greater wall thickness than the inner tube, so as to enable the outer tube to absorb the forces which occur when bending the inner tube, and to ensure stability in the coaxial setting, despite variations in the ambient environment. The tubes 5 and 9 are preferably of the kind sold by KEBO-Grave AB, Sweden, under the name Pythagoras- rδr. In accordance with one suitable, alternative embodiment, the adjustment made may take the form of a single, perma¬ nent adjustment, effected with the aid of wedges instead of screws. In this case, the assembled but non-adjusted instrument is placed in a holder which is orientated in an east-west direction, and is thus nominally perpendicular to the earth's magnetic field, such that the field sensed should be zero. When the instrument is subsequently rota¬ ted about its axis and coaxiality is lacking, it is possi- ble to establish a maximum "gradient signal" in a given position of the instrument. This position is obtained when the lack of coaxility falls in a plane coinciding with the vector of the total magnetic field. Zero gradient signal should be obtained when the cores are precisely parallel, and the sensed differential signal is proportional to any deviation in parallelism. In order to eliminate this deviation, a wedge is placed between the inner and the outer tubes and driven-in between said tubes until the deflection disappears. Additional wedges, normally two in number, are then inserted into the free end of the annular cavity thus formed, whereafter the wedges are secured with varnish, therewith completing the adjustment to the instrument. It will be understood that -the invention is not restricted to the aforedescribed embodiments illustrated in the accompanying drawing, and that other embodiments are con¬ ceivable within the scope of the following claims. For example, the length of the silicone rubber element arran- ' ged between the tubes need not be precisely the length illustrated in Fig. 1. Furthermore, the adjusting device for adjusting the coaxial state, or concentricity, of the tubes need not have precisely the form of an adjusting device provided with four screws in accordance with the embodiment of Fig. 5. It is preferred, however, that the inner tube protrudes at one end thereof beyond the outer tube, even when using more sophisticated adjusting means than simple screws, e.g. in order to achieve greater resolution and more precision when moving the inner tube radially. Neither is it absolutely essential for the core carriers to have the form of quartz tubes, since solid cylinders can be used instead. Furthermore, possible deviations from the illustrated embodiments will be readi¬ ly realized by those skilled in this art.

Claims: CLAIMS 1. A sensor for a magnetic gradiometer, comprising two substantially coaxial windings (1,2) with cores (3,4) and an inner tube (5) which is arranged within an outer tube (9) having a length which is somewhat greater than that of the inner tube, at least a central part of the inner tube being supported against radial movement in relation to the outer tube, and at least the cores being located within the inner tube, and further comprising an adjusting device (15) for adjusting the concentricity of the windings, cha- racterized in that each core includes a thin magnetic strip (6) located externally on a respective core carrier (7) which is individual to each core and which is short in comparison with the inner tube and is placed therein; and in that the adjusting device is intended to move a part, but not the whole, of the inner tube radially in relation to the outer tube, said part having one, but not all, of said core carriers located therein, thereby to enable the core located in said part to be brought into parallelism with another core located in an u-nmoved part of the inner - tube. 2. A sensor according to Claim 1, characterized in that the core carriers have a substantially cylindrical peri¬ pheral surface; and in that the strips are folded or wound around the peripheral surfaces of respective core carriers such that the length direction of the strips coincides substantially with the axial direction of the peripheral surfaces of respective core carriers. 3. A sensor according to Claim 1, characterized in that the adjusting device includes an adjusting means which is arranged externally on one end of the outer tube, and in that the adjusting means presents devices (17,18,19,20) by means of which forces can be applied to the inner tube for moving the end of the inner tube located in the adjusting device radially in relation to the one end of the outer tube located in said adjusting device. 4. A sensor according to Claim 3, characterized in that the other end of the inner tube is supported against radial movement in the outer tube by means of a bottom support (13) arranged in the other end of the outer tube. 5. A sensor according to any of Claims 1-4, characterized in that the windings are wound around the inner tube. 6. A sensor according to any of Claims 1-5, characterized in that the outer tube has a length in the order of 50 cm and a diameter in the order of 1.5 cm; in that the cores each have a length in the order of 4 cm; and in that the spacing between the cores is in the order of 40 cm. 7. A sensor according to Claim 3, characterized in that an inner and central part of the inner tube is supported against radial and axial movement with the aid of a sili¬ cone rubber element located between the inner and the outer tube, said silicone rubber element having a length extension which equals at least approximately half the distance between the windings. 8. A sensor according to Claim 1, characterized in that the adjusting device comprises wedges of non-magnetic material which, for the purpose of moving the moveable end of the inner tube, are intended to be inserted between said inner tube and the outer tube in a manner to effect said adjustment.

Inventors: OESTERLUND HANS OLOF, OESTERLUND, HANS, OLOF
Applicants: MARINEAGENT AB
Application year: 1987
Main classification: G01R33
IPC codes: G01R33