Determining properties of magnetic elements

• US 6,769,612 B1
• Filed: 05/21/2002
• Issued: 08/03/2004
• Est. Priority Date: 08/12/1999
• Status: Expired due to Term

• Current Assignee: Hsinyi Investment LLC
• Original Assignee: BTG International Limited (Boston Scientific Corporation)
• Inventors: Dames, Andrew Nicholas, England, James Mark Carson
• Primary Examiner(s): Lee, Michael G.
• Assistant Examiner(s): PAIK, STEVE S
• Application Number:
  US10/049,420
• Time in Patent Office: 805 Days
• Field of Search: 235/385, 235/449, 235/493, 235/487, 360/1, 360/2, 340/551, 340/552
• US Class Current: 235/385
• CPC Class Codes: G01N 27/72 by investigating magnetic variables
• CPC Class Codes: G06K 19/06196 Constructional details
• CPC Class Codes: G06K 7/084 sensing magnetic material by relative movement detecting flux changes without altering its magnetised state
• CPC Class Codes: G06K 7/087 flux-sensitive, e.g. magnetic, detectors G06K7/10336 takes precedence

Abstract

A method of determining differing characteristics of magnetic dipole elements such as orientation, coercivity, bias and response amplitude and a tag reader for reading magnetic tags containing such elements. The elements are scanned by a rotating magnetic field and two sets of transition data are determined. The transition data sets are associated with respective elements and analyzed to determine mean field values resolved along the element vectors. These field values are used to determine properties of the elements, such as coercivity.

Claims

1. A method of reading a magnetic tag having at least one magnetic element, comprising;

interrogating the tag with a scanning magnetic field;

determining transition data associated with changes in the magnetisation state of the at least one magnetic element;

associating the transition data with one or more respective magnetic elements; and

for each magnetic element, determining the element direction which corresponds to the transition data for that magnetic element.

2. A method according to claim 1, wherein the step of determining the magnetic element direction comprises selecting the direction which minimises the scatter of transition field vectors resolved along the direction of the magnetic element.

3. A method according to claim 1, including grouping the transition data by the type of magnetic element transition.

4. A method according to claim 3, comprising grouping first and second types of magnetic element transition.

5. A method according to claim 4, wherein the first type of magnetic element transition comprises a forward transition and the second type of magnetic element transition comprises a reverse transition.

6. A method according to claim 4, wherein a signal defining a transition is received by one or more receiver coils, including determining the type of transition in accordance with the polarity of the rate of change of the field vector in the direction of the magnetic element.

7. A method according to claim 4, including determining information relating to the switching fields for each of the first and second types of transition.

8. A method according to claim 7, comprising determining magnetic element characteristics from said switching field information relating to transition data associated with a magnetic element.

9. A method according to claim 8, further comprising calculating the coercivity of the magnetic element as substantially half the difference between first and second switching fields.

10. A method according to claim 8, further comprising calculating the bias field on the magnetic element as substantially the sum of first and second switching fields.

11. A method according to claim 9, wherein the first switching field comprises the mean value of the switching fields for the first type of transition and the second switching field comprises the mean value of the switching fields for the second type of transition.

12. A method according to claim 1, including associating the transition data with one or more respective magnetic elements using a receiver vector whose components represent the amplitudes of the signals in one or more receive coils.

13. A method according to claim 1, comprising scanning the tag using a rotating magnetic field.

14. A method according to claim 13, in which the tag comprises a plurality of magnetic elements, further comprising associating transition data with respective magnetic elements in accordance with the order in which the magnetic elements transition in response to the rotating field.

15. A method according to claim 1, comprising determining the coercivity, the local magnetic field bias resolved in the direction of each magnetic element and the orientation of each magnetic element relative to a known interrogation field reference frame.

16. A method according to claim 1, further comprising determining the amplitude response of each magnetic element to the applied magnetic field.

17. A method of distinguishing between a plurality of magnetic elements, comprising the steps of:

applying a scanning magnetic field to the magnetic elements;

determining the direction of each of the magnetic elements;

for each of the magnetic elements, determining the components of the field in the direction of the magnetic element at which the magnetic element switches magnetisation states; and

from said components, determining, for each of the magnetic elements, respective characteristics of the magnetic element.

18. A method according to claim 17, comprising determining first and second switching components as the components when the rate of change of the field along the direction of the magnetic element is positive and negative respectively.

19. A method according to claim 17 wherein the respective characteristics comprise the coercivities of the magnetic elements.

20. A method according to claim 17, comprising storing data by reference to the respective characteristics of the magnetic elements.

21. A method according to claim 20, wherein data is storable by reference to any one or more of orientation of the magnetic elements, coercivity, bias field along the magnetic element and amplitude response.

22. A method according to claim 20, wherein data is storable by reference to parameters relating to any one or more of rate of change of applied field, perpendicular field, response time, characteristic response shape and the statistical distribution of the parameters.

23. A method of determining, for a magnetic element, any one or more of a plurality of characteristics comprising the coercivity of the magnetic element, the local magnetic field bias resolved in the direction of the magnetic element and the orientation of the magnetic element, comprising:

applying a varying magnetic field to the magnetic element;

determining the direction of the magnetic element;

determining the components of the field in the direction of the magnetic element at which the magnetic element switches magnetisation states; and

from said components, determining the one or more characteristics of the magnetic element.

24. A computer program, which when run on a computer, is configured to carry out a method of reading a magnetic tag having at least one magnetic element comprising:

interrogating the tag with a scanning magnetic field;

determining transition data associated with changes in the magnetisation state of the at least one magnetic element;

associating the transition data with one or more respective elements; and

for each element, determining the element direction which corresponds to the transition data for that element.

25. A magnetic tag reader for reading a magnetic tag having at least one magnetic element, comprising:

means for interrogating the tag with a scanning magnetic field;

means for determining transition data associated with changes in the magnetisation state of the at least one magnetic element;

means for associating the transition data with one or more respective magnetic elements; and

means for determining, for each magnetic element, the magnetic element direction which corresponds to the transition data for that magnetic element.

26. A tag reader according to claim 25, wherein the scanning field comprises a rotating magnetic field.

27. A tag reader according to claim 25, further comprising means for selecting the magnetic element direction which minimises the scatter of transition point field vectors resolved along the direction of the magnetic element.

28. A tag reader according to claim 25, wherein the transition data includes data defining first and second switching fields at which a magnetic element undergoes first and second transitions.