Methods and systems for reactively compensating magnetic current loops

• US 6,960,984 B1
• Filed: 11/27/2000
• Issued: 11/01/2005
• Est. Priority Date: 12/08/1999
• Status: Expired due to Fees

• Current Assignee: DBTag, Inc., University of South Carolina
• Original Assignee: DBTag, Inc., University of South Carolina
• Inventors: Dettloff, Wayne D., Vicci, Leandra
• Primary Examiner(s): Zimmerman, Brian
• Assistant Examiner(s): YANG, CLARA I
• Application Number:
  US09/723,356
• Time in Patent Office: 1,800 Days
• Field of Search: 343/742, 343/842, 343/866, 343/867, 343/744, 340/10.1
• US Class Current: 340/10.1
• CPC Class Codes: G06K 19/0701 at least one of the integrated circuit chips comprising an arrangement for power management
• CPC Class Codes: G06K 19/0715 the arrangement including means to regulate power transfer to the integrated circuit
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06K 7/10336 the antenna being of the near field type, inductive coil
• CPC Class Codes: H01Q 1/22 by structural association with other equipment or articles
• CPC Class Codes: H01Q 1/2208 associated with components used in interrogation type services, i.e. in systems for information exchange between an interrogator/reader and a tag/transponder, e.g. in Radio Frequency Identification [RFID] systems methods or arrangements for sensing record carriers, e.g. for reading patterns G06K7/00; record carrier for use with machines and with at least a part designed to carry digital markings G06K19/00
• CPC Class Codes: H01Q 7/00 Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop
• CPC Class Codes: H01Q 7/04 Screened antennas H01Q7/02, H01Q7/06 take precedence

Abstract

Methods and systems for compensating magnetic current loops provide current magnitude and phase uniformity within the magnetic current loops. A magnetic current loop is divided into k sections. Each of the k sections has a series reactance. Series reactive compensation is added to each of the k sections such that the reactive compensation substantially cancels the series reactance of each section. Adding reactive compensation to the loop that cancels the series reactance of each section of the loop provides current magnitude and phase uniformity along the loop at any given instant in time. As a result, the magnitude and phase of the magnetic field at a point in space can be controlled with precision to achieve a desired result, such as precise field cancellation or precise field generation.

Claims

1. A magnetic current loop system adapted to produce strong near fields and weak far fields, the magnetic current loop system comprising:

(a) first and second magnetic current loops for generating a local magnetic field, the magnetic current loops being divided into k sections, k being an integer, each of the k sections having a series reactance at a frequency;

(b) k reactive compensation elements for controlling generation of the local magnetic field, each reactive compensation element being coupled to one of the k sections and having a reactance that substantially cancels the series reactance of each section at the frequency; and

(c) a current source coupled to the first and second magnetic current loops such that current flows in a first direction in the first magnetic current loop and in a second direction, opposite the first direction, in the second magnetic current loop, the current producing a strong magnetic field near the first and second magnetic current loops and substantially canceling dipole magnetic fields produced by the magnetic current first and second loops at a distance far from the first and second magnetic current loops, the reactive compensation elements producing substantial uniformity in phase and magnitude of the current flowing through each loop and thereby enabling the cancellation of the dipole magnetic fields.

2. The magnetic current loop system of claim 1 wherein the series reactance of each of the k sections comprises an inductive reactance and each of the k reactive compensation elements comprises a capacitor.

3. The magnetic current loop system of claim 1 wherein the k sections are substantially equal in length and the series reactances of the k sections are substantially equal.

4. The magnetic current loop system of claim 1 wherein at least some of the k sections are unequal in length and some of the series reactances of the k sections are not equal.

5. The magnetic current loop system of claim 1 wherein the current source is adapted to produce a current having a frequency of about 13.56 MHz.

6. The magnetic current loop system of claim 1 wherein the first magnetic current loop is located in a first plane and the second magnetic current loop is located in a second plane spaced from and parallel to the first plane.

7. The magnetic current loop system of claim 1 wherein each of the first and second magnetic current loops includes n turns, n being an integer.

8. The magnetic current loop system of claim 7 wherein n is equal to one.

9. The magnetic current loop system of claim 7 wherein n is greater than one.

10. The magnetic current loop system of claim 1 wherein the first and second magnetic current loops are coaxial with each other.

11. The magnetic current loop system of claim 10 wherein sections of the first magnetic current loop are substantially equal in length to adjacent sections of the second magnetic current loop.

12. The magnetic current loop system of claim 11 wherein the reactive compensation elements associated with sections of the first magnetic current loop are substantially equal in reactance to reactive compensation elements of the adjacent sections of the second magnetic current loop.

13. The magnetic current loop system of claim 1 wherein the first and second magnetic current loops are located in different planes.

14. A magnetic current loop system adapted to produce strong near fields and weak far fields, the magnetic current loop system comprising:

(a) first and second magnetic current loops for generating a local magnetic field, the magnetic current loops being divided into k sections, k being an integer, each of the k sections having a series reactance at a frequency;

(b) k reactive compensation elements for controlling generation of the local magnetic field, each reactive compensation element being coupled to one of the k sections and having a reactance that substantially cancels the series reactance of each section at the frequency; and

(c) a current source coupled to the first and second magnetic current loops such that current flows in a first direction in the first magnetic current loop and in a second direction, opposite the first direction, in the second magnetic current loop thereby substantially canceling a dipole field at a distance spaced from the first and second magnetic current loops, wherein the series reactance of each of the k sections comprises an inductive reactance and each of the k reactive compensation elements comprises a capacitor, and wherein each capacitor has a capacitance value C_k such that $\frac{1}{\mathrm{\omega <br><br>}<br><br><br><br>C_k}=\mathrm{\omega <br><br>}<br><br>L_k,$

wherein ω

the angular frequency of the current source and L_k is the series inductance of the k^th section of the magnetic current loops.

15. A reader for a magnetic-current-loop-based communication system, the reader comprising:

(a) first and second magnetic current loops for generating a local magnetic field, each being divided into n sections, n being an integer, each section having a series reactance, the first and second magnetic current loops having a total of 2n sections;

(b) 2n reactive compensation elements for controlling generation of the local magnetic field, one element being associated with each of the 2n sections, such that the reactive compensation elements substantially cancel the series reactance of each of the sections; and

(c) circuitry operatively associated with the first and second magnetic current loops for communicating with a device when the device is within a predetermined distance of the first and second magnetic current loops, wherein the circuitry includes a current source for driving the first and second magnetic current loops to produce a strong magnetic field near the first and second magnetic current loops and to substantially cancel dipole magnetic fields produced by the first and second magnetic current loops at distances far from the first and second magnetic current loops, the reactive compensation elements producing substantial uniformity in phase and magnitude of the current flowing through each loop and thereby enabling the cancellation of the dipole magnetic fields.

16. The reader of claim 15 wherein the first and second magnetic current loops are coaxial with each other.

17. The reader of claim 16 wherein the first and second magnetic current loops are connected to each other so that current flows in a first direction through the first magnetic current loop and in a second direction, opposite the first direction, through the second magnetic current loop.

18. A reader for a magnetic-current-loop-based communication system, the reader comprising:

(a) first and second magnetic current loops for generating a local magnetic field, each being divided into n sections, n being an integer, each section having a series reactance;

(b) 2n reactive compensation elements for controlling generation of the local magnetic field, one element being associated with each of the 2n sections, such that the reactive compensation elements substantially cancel the series reactance of each of the sections;

(c) circuitry operatively associated with the first and second magnetic current loops for communicating with a device when the device is within a predetermined distance of the first and second magnetic current loops, wherein the first and second magnetic current loops are coaxial with each other;

(d) a third magnetic current loop positioned between and equidistant from the first and second magnetic current loops for coupling to a magnetic field from the device; and

(e) circuitry operatively associated with the third magnetic current loop for processing a signal modulated on the magnetic field from the device.

19. The reader of claim 18 comprising a microprocessor operatively associated with the circuitry for performing a predetermined function in response to the signal from the device.

20. The reader of claim 19 wherein the microprocessor is adapted to perform an authentication function in response to the signal from the device.

21. The reader of claim 19 wherein the microprocessor is adapted to store at least some of the information contained in the signal from the device in a memory device.

22. A magnetic current loop system comprising:

(a) a plurality of magnetic current loops for generating a local magnetic field, each magnetic current loop being divided into n sections, n being an integer, each of the n sections having a series reactance at a frequency;

(b) n reactive compensation elements for controlling generation of the local magnetic field, the reactive compensation elements being respectively coupled to each of the n sections of each loop, each of the n reactive compensation elements having a reactance that substantially cancels the series reactance of the corresponding section at the frequency, thereby producing substantial current magnitude and phase uniformity along the magnetic current loop; and

(c) a current source coupled to the magnetic current loops for producing a current in the magnetic current loops, wherein the magnetic current loops are coupled to each other such that a strong magnetic field is produced near the magnetic current loops and such that dipole magnetic fields resulting from the current flowing through each loop cancel at distances far from the current loops, the reactive compensation elements producing substantial uniformity in phase and magnitude of the current flowing through each loop and thereby enabling the cancellation of the dipole magnetic fields.

23. The system of claim 22 wherein the series reactance of each of the n sections comprises an inductive reactance and the reactance of each of the respective compensation elements comprises a capacitive reactance.

24. The system of claim 22 wherein each of the n sections includes a series resistance, a series inductance, a shunt capacitance, and a shunt resistance, the shunt capacitance and the shunt resistance of each section having a first time constant, and wherein each of the reactive compensation elements has a reactance value such that the series resistance and an effective capacitive series reactance of each of the sections has a second time constant that is substantially equal to the first time constant.

25. A magnetic current loop system comprising:

(a) n magnetic current loops for generating a local magnetic field, n being an integer, each of the n magnetic current loops being divided into sections, each section having a series reactance, the magnetic current loops being coupled to each other and magnetized in opposite directions to produce a strong magnetic field near the magnetic current loops and to substantially cancel a dipole magnetic field at distances far from the magnetic current loops, the reactive compensation elements producing substantial uniformity in phase and magnitude of the current flowing through each loop and enabling the cancellation of the dipole magnetic fields; and

(b) reactive compensation elements respectively coupled to the sections for controlling generation of the local magnetic field, each of the reactive compensation elements having a reactance that substantially cancels the series reactance of the respective section.

26. The system of claim 25 wherein n is equal to one.

27. The system of claim 25 wherein n is greater than one.

28. The system of claim 27 comprising a current source coupled to each of the magnetic current loops such that current flows in the same direction in all of the magnetic current loops.

29. The system of claim 25 wherein the n magnetic current loops comprise first, second, and third magnetic current loops being coaxial with each other, the third magnetic current loop being located between the first and second magnetic current loops.

30. The system of claim 25 wherein the n magnetic current loops comprise first and second pairs of magnetic current loops being coaxial with each other.

31. The system of claim 30 wherein the first and second pairs of magnetic current loops each include an inner magnetic current loop and an outer magnetic current loop, and the inner magnetic current loops of each pair are adjacent to each other.

32. The system of claim 31 comprising a current source coupled to the outer magnetic current loop of each pair such that current flows in a first direction in the outer magnetic current loop of each pair and to the inner magnetic current loop of each pair such that the current flows in a second direction opposite the first direction in the inner magnetic current loop of each pair.

33. A magnetic current loop system comprising:

(a) n magnetic current loops for generating a local magnetic field, n being an integer, each of the n magnetic current loops being divided into sections, each section having a series reactance, wherein the n magnetic current loops comprise first, second, and third magnetic current loops being coaxial with each other, the third magnetic current loop being located between the first and second magnetic current loops;

(b) reactive compensation elements respectively coupled to the sections for controlling generation of the local magnetic field, each of the reactive compensation elements having a reactance that substantially cancels the series reactance of the respective section; and

(c) a first current source coupled to the first and second magnetic current loops adapted to produce a first current having a first magnitude and a first direction in the first and second magnetic current loops and a second current source coupled to the third magnetic current loop adapted to produce a second current having a second magnitude and a second direction in the third magnetic current loop, the second direction being opposite the first direction and the second magnitude being twice the first magnitude.

34. A method for reactively compensating magnetic current loops, the method comprising:

(a) dividing first and second magnetic current loops into k sections, k being an integer, each of the k sections having a series reactance at a frequency;

(b) adding reactive compensation to each of the k sections such that the reactive compensation substantially cancels the series reactance of each of the k sections and controls generation of a local magnetic field by the magnetic current loops;

(c) driving the magnetic current loops with a current source having a frequency such that current flows in a first direction in the first magnetic current loop and in a second direction in the second magnetic current loop and producing a strong magnetic field near the magnetic current loops; and

(d) placing the first and second magnetic current loops in close proximity to each other to substantially cancel dipole magnetic fields produced by the magnetic current loops at distances far from the magnetic current loops, the reactive compensation elements producing substantial uniformity in phase and magnitude of the current flowing through each loop and thereby enabling the cancellation of the dipole magnetic fields.

35. The method of claim 34 wherein the series reactance of each of the k sections is a series inductive reactance and adding reactive compensation to each of the k sections includes adding a capacitor to each of the k sections.

36. The method of claim 34 wherein dividing the first and second magnetic current loops into k sections includes dividing the first and second magnetic current loops into k sections having substantially equal lengths such that the series reactances of the k sections are substantially equal.

37. The method of claim 34 wherein dividing the first and second magnetic current loops into k sections includes dividing the first and second magnetic current loops into k sections, at least some of which are unequal in length, such that the series reactances of at least some of the k sections are not equal.

38. The method of claim 34 wherein driving the magnetic current loops with the current source comprises driving the magnetic current loops with the current source having the frequency substantially centered about 13.56 MHz.

39. A method for reactively compensating a magnetic current loop, the method comprising:

(a) dividing each of first and second magnetic current loops into k sections, k being an integer, each of the k sections having a series reactance at a frequency;

(b) adding reactive compensation to each of the k sections such that the reactive compensation substantially cancels the series reactance of each of the k sections at the frequency, thereby making the amplitude and phase of a current in the loop at the frequency substantially uniform throughout the loop and thereby providing more precise control over generation of a magnetic field at the frequency; and

(c) coupling the magnetic current loops to each other and driving the magnetic current loops to produce a strong magnetic field near the magnetic current loops and to substantially cancel dipole magnetic fields produced by the magnetic current loops at distances far from the magnetic current loops, the reactive compensation elements producing substantial uniformity in phase and magnitude of the current flowing through each loop and thereby enabling the cancellation of the dipole magnetic fields.