APPARATUS FOR MAGNETIC POSITION DETERMINATION

Abstract: Apparatus for magnetic position determination, with a magnet device which establishes a magnetic field, and a sensor for sensing the magnetic field. The magnet device (25) is provided in a body being symmetric about an axis and having a central opening for an elongate element (22) passing therethrough. The magnet device (25) is annular and encircles the central opening with the magnetic field extending a distance generally axially through the opening. The sensor (24a, b) is moved in or carried by the elongate element and comprises two ferromagnetic parts being arranged behind each other as seen in the longitudinal direction of the elongate element. Each of the two parts (24a, 24b) has a widened portion which narrows to a more restricted portion, these restricted portions of the two parts forming an airgap (20) between them. A sensing element (29) for magnetic flux is located in the airgap (20). A primary use of the apparatus is for level measurement in tanks, wherein a float constitutes the body in which the magnet device is mounted.

Description: Apparatus for Magnetic Position Determination. This invention relates to an apparatus for magnetic position determination, and is more specifically directed to the accurate position determination or level measure¬ ment in tanks and the like. It will be understood, however, that even though the invention has been developed primare- ly for the purpose of level measurement in tanks with a float on the liquid surface in the tank, it is obvious that the invention may also have other uses. It is previously known to employ magnetic induction as a basis for position detectors, inter alia in arrange¬ ments wherein a magnet device establishes a surrounding magnetic field and a sensor is adapted to sense the mag¬ netic field in various relative positions of the magnet device and the sensor. On the other hand there is known for example from Norwegian Patent 134 572 a level measurement apparatus based upon an annular float carrying a magnet adapted to influence a sensor which can be moved in a path through the float. Other examples of known techniques may be found in U.S. Patents 3,982,087, 3,437,771, 4,056,979, 4,361,835 and 4,466,284 as well as German Patent Applica¬ tions 2421552 and 2806773. This invention has for an object to provide an improved apparatus for magnetic position determination, in particu¬ lar for obtaining an increased accuracy at the same time as the apparatus is very robust and to a very high degree eliminates the risk of explosion. These are properties of much importance in position determination or measure¬ ment in many tank installations and for example in connec¬ tion with various measurement methods and operations within the field of oil and gas production. Thus, on the background of the prior art the invention is based upon an apparatus comprising at one hand a body provided with a magnet device for establishing a sur¬ rounding magnetic field, and on the other hand a sensor for sensing the magnetic field in various relative posi¬ tions of the body and the sensor, the body being sub¬ stantially symmetric about an axis and having a central opening for an elongate element passing therethrough, whereas the magnet device is annular and encircles the central opening, the magnetic field being adapted to extend a distance generally axially through the opening, and the sensor being guided in or carried by the elongate element. What is novel and specific to the apparatus according to the invention in the first place consists therein that the sensor comprises two ferromagnetic parts being located behind each other as seen in the longitudinal direction of the elongate element, that each of the two parts has a relatively widened portion which narrows to a more re¬ stricted portion, that the restricted portions of the parts form therebetween an airgap having a significantly reduced magnetic flux cross-sectional area compared to the widened portions, and that a sensing element for mag¬ netic flux is located in the airgap. The invention shall be explained more closely below with reference to the drawings, in which: Fig. 1 shows a simplified section through a part of a level measurement system with a float, Fig. 2 shows an embodiment of a magnet device and an associated sensor for position determination, Fig. 3 shows a diagram for explaining the operation of the apparatus in Fig. 2, Fig. 4 shows another embodiment of the magnet device and the associated sensor, in which the sensor is adapted to be moved within a narrow tube, Fig. 5 shows the sensor of Fig. 4 in more detail and at an enlarged scale, and Fig. 6 shows a diagram for further explanation of the operation of the apparatus in Fig. 4. Fig. 1 illustrates the primary use to which the in¬ vention is aimed. There is shown a float body 1 which floats on a liquid surface 3. The float 1 has a central opening la for the passing therethrough of an elongate element 2 which may be a tube, a measuring rod or the like. In Fig. 1 this is considered to be a tube 2 in the in¬ terior of which there may be moved a sensor 4, for example suspended by a measuring tape 4a. As seen in Fig. 1 the tube 2 extends with a good clearance through the opening la in the float 1, so that there will be only a small probability that the float will get stuck because of dirt or depositions. This is of significance in order that the float 1 shall be able to follow the liquid surface 3 exactly when the level varies. This means that the float 1 can also perform rotational movements about a vertical axis which is more or less coincident with the direction of the elongate element 2 through the opening la. In the float 1 there is schematically shown a magnet device 5 which is annular and thus encircles the opening la and the element 2 passing therethrough. The magnet device 5 is adapted to provide a magnetic field which can influence the sensor 4 when this is near the float, in particular when it is located at the same level as the magnet device 5. The cooperation between the magnet device and the sensor shall be explained more closely with reference to Fig. 2. The situation which is illustrated schematically and simplified in Fig. 1, may be found in analog forms in other fields of application, for example in connection with instrumentation or measurement during drilling for oil or gas. The elongate element 2 can be for example a cable which is extended into a borehole, whereas the body 1 may be a tool, instrument or the like for which the relative position in the longitudinal direction of the cable 2 is to be determined. In Fig. 2 there is indicated a length of a tube 22 corresponding to the elongate element 2 in Fig. 1, in the interior of which there is provided a sensor having two parts 24a and 24b of ferromagnetic material and being arranged behind or above each other in the longitudinal direction of the tube 22. Outside the tube 22 there is shown schematically an annular magnet 25 having a north pole at the top and a south pole at the bottom, so that there is established a magnetic field having field lines as indicated. The annular magnet 25 may quite simply be a permanent magnet or a permanent magnet element having associated pole pieces or the like. With such a magnet device the magnetic field as shown in the figure, will pass through the tube 22 along a length generally axially through the opening being encircled by the annular magnet 25, so that the sensor 24a, 24b may come into this magnetic field. Each of the parts 24a and 24b of the sensor has mutual¬ ly opposite portions or end parts being adapted to pick up the magnetic field and pass the same through the sensor. From these widened end portions each of the parts 24a, 24b is so shaped that they narrow to more restricted portions forming therebetween an airgap 20. Accordingly the magnetic field will be strongly concentrated in this airgap 20 between the parts 24a and 24b. In the airgap there is mounted a sensing element 29 for magnetic flux, and from the element there extend electrical wires as indicated at 29a, which may for example lead to electronic measuring equipment at the top of the tank. When the relative position of the magnet device 25 and the sensor 24 varies in the vertical direction, the con¬ centrated magnetic field in the airgap 20 and thereby through the sensing element 29, will vary correspondingly. A preferred form of sensing element is a Hall element which has proved to be very suitable for this purpose. With a configuration and relative dimensions as shown schematically in Fig. 2 the magnetic flux through the Hall element 29 will depend upon the relative position between the magnet device 25 and the sensor 24. The variation of magnetic flux H as a function of the relative position is in the principle as shown in Fig. 3. When the sensor is moved far upwards or far downwards in relation to the magnet device 25, the magnetic flux will be stabilized at a more or less constant value, whereas a rather strong change in the magnetic flux will occur in an operating region which in Fig. 3 is indicated between letters A and B, corres¬ ponding to relative positions where the sensor is located more or less directly in the zone opposite and within the magnet device 25. Since the magnetic flux varies much in this operating region it is possible to obtain a high de¬ gree of accuracy in the position determination, based upon a certain reference value of the magnetic flux. In the embodiment of the apparatus shown schematically in Fig. 4, it is contemplated that the sensor shown with parts 44a and 44b shall be able to be moved within a com¬ paratively narrow tube, which has led to another orien¬ tation of the airgap 40 between the parts 44a and 44b. As will appear from the field lines drawn, the magnetic field in the airgap 40 is directed transversally to the axial direction of the arrangement, being in contrast to the broadly axial field direction in the airgap 20 in Fig. 2. Moreover, in Fig. 4 the widened portions of the parts 44a and 44b remote from the airgap 40, are more elongate, inter alia in order to pick up the field lines with their side surfaces which face the inside tube wall (not shown) . This results in a total larger length of the sensor and this has led to the construction of the magnet device with two annular magnets 45a and 45b respectively, having a radial direction of magnetization. The radial direction of magnetization has opposite polarities in the two magnets 45a and 45b. This arrangement gives a compara¬ tively long and generally axial magnetic field in the cen¬ tral opening which makes it possible to move the sensor 44 vertically in relation to the magnet device. Fig. 5 shows in more detail a longitudinal section through the sensor of Fig. 4. The upper part 44a and the lower part 44b have widened end portions which may for example have a cylindrical shape adapted to the interior diameter of a tube in which the sensor shall be mounted or moved. The parts 44a and 44b may advantageously be made of a ferrite material. From the widened end portions of the parts 44a and 44b there extend more narrow portions towards the airgap 40 in which the magnetic field lines through the sensor are concentrated. In the airgap 40 there is located a Hall element 49 which is mounted on a semiconductor carrier 48. This carrier may possibly com¬ prise certain electronic circuits or components which are involved in a manner known per se, when using a Hall element. At the lower end of the semiconductor carrier 48 there are mounted terminals 47 for the connection of conductors 48a which lead to a measuring system or the like at ' a dis¬ tance from the measuring point concerned. In order that the sensor shall constitute a unitary and robust assembly, there may be moulded a suitable moulding mass into the cavities indicated at 41a and 41b. Fig. 6 shows an example of a characteristic for an arrangement as shown in Figs. 4 and 5. As in the diagram of Fig. 3 the abscissa gives the relative position (in centimeters) whereas the ordinate is the output voltage in volts from the Hall element. The curve or characteris¬ tic in Fig. 6 shows the signal from the Hall element as a function of the relative position of the sensor in the magnetic field, similar to Fig. 3, but with a somewhat different shape because of the dual magnet device as shown in Fig. 4. Advantageously there is selected an ope¬ rating point or region at one of the steep portions of the curve in Fig. 6, i.e. as indicated with letters C to D. The curve in this operating region will have small changes when there is drift in various parameters, for example temperature, so that it is possible to obtain a very high degree of accuracy in the position determination. The reference point may for example be a voltage of 5,0 volts. Accuracies of about 1/10 mm are obtainable in a practical arrangement as illustrated in Fig. 1. Another advantage with the operating region stated, is that the effect of magnetic hysteresis will not be significant. It appears from Fig. 3 and in particular from Fig. 6 that the stated operating region lies adjacent an inverting tangent in the characteristics shown. Above and with reference to the figures of drawings there are described two examples of embodiments according to the invention, but it is obvious that these embodiments may be modified in various directions and still have the same fundamental function as stated in the invention. As it has appeared from the description, in particular in relation to the characteristics shown, the method is directed to determining a point or a position at which the magnetic field gives a defined magnetic flux or out¬ put voltage from the Hall element, i.e. a predetermined reference value. It has not been closely described how the output sig¬ nal from the Hall element is processed in a suitable electronic circuit, but this is conventional, included a suitable temperature compensation. The permanent magnet described may possibly be replaced by electromagnets, energized either by direct current or by alternate current. It is clear, however, that in locations with an explosion risk permanent magnets are preferred. The permanent magnets may with advantage consist of several segments, for example 4 segments distributed symmetrically about the circumference. In the specific case wherein the relative radial movement of the sensor and the magnet device is negligible, possibly a single per¬ manent magnet segment may be sufficient for establishing the necessary magnetic field, while retaining the desired accuracy in the position determination. From Fig. 1 it appears that the relative movement between the magnet device and the sensor is substantially axial and rectilinear. It will be realized that there may be uses in which this path of movement follows a line, but this line does not necessarily have to be rectilinear.

Claims: CLAIMS : 1. Apparatus for magnetic position determination com¬ prising at one hand a body (1) provided with a magnet device (5, 25, 45a, b) for establishing a surrounding mag¬ netic field, and at the other hand a sensor (4, 24a, b, 44a, b) for sensing the magnetic field in various relative positions of the body and the sensor, the body (1) being substantially symmetric about an axis and having a central opening (la) for an elongate element (2) passing there¬ through, whereas the magnet device (5, 25, 25a, b) is annular and encircles the central opening, the magnet field being adapted to extend a distance generally axially through the opening, and the sensor being moved in or carried by the elongate element, characterized in that the sensor comprises two ferromagnetic parts (24a, 24b, 44a, 44b) lying behind each other as seen in the longitudinal direction of the elongate element, that each of these two parts has a relatively widened portion which narrows to a more restricted portion, that the restricted portions of the parts (24a, 24b, 44a, 44b) form an airgap (20, 40) therebetween with a significantly reduced magnetic flux cross-sectional area in relation to the widened portions, and that a sensing element (29, 49) for magnetic flux is located in the airgap (20, 40). 2. Apparatus according to claim 1, characterized in that the sensing element is a Hall element (29, 49). 3. Apparatus according to claim 1 or 2, characterized in that the airgap (20) extends generally transversally to the longitudinal direction of the elongate element. 4. Apparatus according to claim 1 or 2, characterized in that the airgap (40) extends generally parallel to the longitudinal direction of the elongate element. 5. Apparatus according to any one of the preceding claims, characterized in that the magnet device comprises at least an annular permanent magnet. 6. Apparatus according to any one of claims 1 - 4, characterized in that the magnet device comprises a number of symmetrically distributed permanent magnet segments. 7. Apparatus according to any one of the preceding claims, characterized in that the magnet device comprises two sets of magnets (45a, 45b) being mutually displaced in the longitudinal direction of the elongate element. 8. Apparatus according to any one of the preceding claims, characterized in that the determination of position is based upon a reference value of magnetic flux in, resp. at a steep portion of the characteristic thereof.

Inventors: HOLMEN HANS KRISTIAN, HOLMEN, HANS, KRISTIAN
Applicants: J L TIEDEMANNS TOBAKSFABRIK JO, J.L. TIEDEMANNS TOBAKSFABRIK JOH H ANDRESEN
Application year: 1985
Main classification: G01F23
IPC codes: G01F23