Magnetic field detector

Abstract: A pair of Hall sensors are symmetrically positioned between the poles of a horseshoe magnet. The Hall sensors are mounted in a plane flush with the ends of the magnet faces, and are located equidistant from the centerline of the magnet. A small magnetic object in the field of the magnet, preferably located on the magnet axis, has an induced dipole moment which is detected by the Hall sensors. The small magnetic object dipole field is such that the flux at each Hall sensor is in opposite directions, and by differentially sensing the outputs of the Hall sensors, a signal proportional to the magnetic object is derived. Leakage flux from the magnet as well as the flux of the earth's magnetic field, are in the same direction at both Hall sensors, and their effects are zeroed out by the differential sensing of the Hall sensors' outputs. When the poles of the magnet are drawn across the surface of a non magnetic sheet of conductive material, eddy currents are induced in the sheet by the moving magnetic field. Due to the symmetry of the magnet/sensor arrangement, in a uniform sheet these currents generate identical magnetic fields which result in sensor outputs that cancel. However, a crack or other anomaly in the sheet results in unequal signals at the two sensors, and an output signal proportional to the anomaly occurs.

Description: 1. Field of the Invention This invention relates to magnetic field measurement, and in particular to detection of a magnetic field due the presence of a magnetic object or due to the presence of an anomaly or discontinuity in a moving conductive sheet. 2. Description Relative to the Prior Art A wide variety of devices for detection of a magnetic object exist in the art. Applications widely range from detecting the position of teeth in a rotating gear, (U.S. Patent 5,304,926) to measuring the separation of a railroad car axle from the railroad track (U.S. Patent 4,943,772). The technique employed and the configuration of the implemented hardware strongly depend upon the size of the object to be detected, and its separation from the measurement device. U.S. Patent 5,304,926 discloses a position sensor having two magnetically sensitive devices associated with a U-shaped magnet. The sensor is disposed proximate a rotatable member having at least one discontinuity in its surface, such as a rotatable gear. The two magnetically sensitive devices, such as Hall effect transducers, each provide output signals that represent the direction and magnitude of the magnetic field in which its respective transducer is disposed. An algebraic sum of the first and second output signals from the magnetically sensitive devices is provided as an indication of the location of the rotatable member that is disposed proximate the sensor. U.K. Published Patent Application GB 2 159 954A discloses apparatus for locating a discontinuity in a metal pipe or conduit. The apparatus includes a magnet for inducing a magnetic field in the pipe, two Hall effect devices with matched outputs but inverted to cancel the effect of one magnetically induced pole of the pipe. In both cases, the Hall sensors are placed so that they are responsive to flux from the U-shaped magnet. The present invention as defined by claim 1 can be applied to the detection of a small magnetic object under conditions where the detector may be brought into close proximity to the region thought to include the magnetic particle. It may be applied in the medical field where doctors need a means to sense the presence of foreign magnetic material within a patient, particularly within the eyes, before placing the patient in the large magnetic field of a diagnostic scanner. The large field gradient of the scanner can cause the magnetic object to move and inflict injury. The invention can also be used before and during surgery to detect and locate a magnetic object within a patient. The structure of the present invention may also be used to detect an anomaly in the eddy current magnetic field due to a crack or other discontinuity in a moving planar sheet of electrically conductive material. SUMMARY OF THE INVENTION A pair of Hall sensors are symmetrically positioned between the poles of a horseshoe magnet. The Hall sensors are mounted in a plane flush with the ends of the magnet faces, and are located equidistant from the centerline of the magnet. A small magnetic object in the field of the magnet, preferably located on the magnet axis, has an induced dipole moment which is detected by the Hall sensors. The small magnetic object dipole field is such that the flux at each Hall sensor is in opposite directions, and by differentially sensing the outputs of the Hall sensors, a signal proportional to the magnetic object is derived. Leakage flux from the magnet as well as the flux of the earth's magnetic field, are in the same direction at both Hall sensors, and their effects are zeroed out by the differential sensing of the Hall sensors' outputs. When the poles of the magnet are drawn across the surface of a non magnetic sheet of conductive material, eddy currents are induced in the sheet by the moving magnetic field. Due to the symmetry of the magnet/sensor arrangement, in a uniform sheet these currents generate identical magnetic fields which result in sensor outputs that cancel. However, a crack or other anomaly in the sheet results in unequal signals at the two sensors, and an output signal proportional to the anomaly occurs. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described with respect to the drawings of which: Fig. 1 is the drawing of the apparatus of the invention used in detecting the presence of a magnetic object, Fig. 2 is a drawing of a second embodiment of apparatus of the invention, and Fig. 3 is a drawing illustrating the use of the invention in detection of a crack in a planar sheet of non magnetic material. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Fig. 1, a permanent horseshoe magnet 10 has coplanar pole faces 12, 14. The magnet 10 is preferably made of high coercivity material, so that its leakage flux is minimum, i.e., substantially all flux enters and leaves the magnet at the faces 12,14. A pair of Hall sensors 16,18 are located in a plane that is coplanar with the pole faces 12,14; the sensors 16,18 being symmetrically positioned with respect to the magnet pole faces 12,14. The sensitive axes of the Hall sensors 12,14 are parallel to the "z" direction, and along orthogonal directions to the "z" axis there is no sensor response. (The sensitive direction is perpendicular to the top surface of the Hall sensors 16,18, as seen in Fig. 1.) As stated above, the magnet 10 material is selected to have minimum leakage flux, and it will be noted that the largest component of any residual leakage flux is along an insensitive direction for the Hall sensors 16,18. A small magnetic object 20 to be detected is shown on the "z" axis, and is influenced by the field, e.g. 22,24 of the magnet 10, which induces a responsive dipole field in the magnetic object 20. Typical field lines of the induced dipole field are 26,28,30. Notice the field line 30 is in the minus "z" direction as it enters the Hall sensor 18, and in the plus "z" direction as it enters the Hall sensor 16. The output signals from the Hall sensors are differentially detected so that the preceding field signals add, indicating the presence of the magnetic object 20, while signals from external magnet fields, such as the earth's magnetic field, which enter the sensors in the same directions, and thermal effects in the sensors are zeroed out. Referring to Fig. 2, a magnet 10', characterized by a larger fringing field than the magnet 10 of Fig. 1, is provided with Hall sensors 16, 18' in a second embodiment of the invention. (In the drawings, different but related elements have the same reference numbers, albeit they are distinguished by primes.) Curving field lines 32,34,36 bridge across the U of the magnet, and rather than placement of the Hall sensors flush with the end faces of the magnet 10, the elements 16',18' are cocked so that the fringing field line, e.g. 34 runs parallel to the top surfaces of the Hall sensors instead of the top surfaces being flush with the magnet end faces. In this way, the fringing flux has no component along the sensitive axes of the Hall sensors 16',18', and the invention may be implemented in terms of a less expensive magnet which may have an appreciable fringing field. Referring to Fig. 3, a magnet 10'', and sensors 16'',18'' mounted flush with the pole faces of the magnet 10'' are shown in contact with a planar conductive non magnetic sheet 40. By moving the magnet 10'', for example in the direction of the arrow 42 across the surface of the sheet 40, eddy currents having resultant magnet fields are generated by the changing flux in the sheet 40. If the sheet is homogeneous, the resultant fields appear identical at the Hall sensors 16'',18''and there is no detectable output from the differentially sensed Hall effect sensors 16'',18''. But a crack 44 results in distortion of the eddy current generated fields, and the field at one Hall sensor 16'' will differ from that at the other Hall sensor 18''. In this way the presence of the crack 44 or other anomaly may be detected. The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as claimed.

Claims: A magnetic detector for detecting the presence of a magnetic object (20), the detector comprising: a) a horseshoe magnet (10) for generating a magnetic field; and b) first and second detection elements (16, 18), the first detection element (16) being a first Hall sensor having a first sensitive axis, and the second detection element (18) being a second Hall sensor having a second sensitive axis, characterized in that the first Hall sensor (16) is mounted proximate to the horseshoe magnet (10) with the first sensitive axis orthogonal to a flux line (34) of said magnet, and the second Hall sensor (18) is mounted proximate to the horseshoe magnet (10) with the second sensitive axis orthogonal to the flux line (34) of said magnet, whereby said first Hall sensor and said second Hall sensor are not responsive to flux from the horseshoe magnet.

Inventors: CHAMBERLAIN IV FREDERICK ROCKW, CHAMBERLAIN, IV, FREDERICK ROCKWELL
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1996
Main classification: G01R33
IPC codes: G01N27, G01R33, H01L43, G01V3