Magnetic field emission and differential receiver coil configuration for discriminating response magnetic field from transponder tag

• US 6,268,723 B1
• Filed: 09/24/1998
• Issued: 07/31/2001
• Est. Priority Date: 09/26/1997
• Status: Expired due to Term

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: Wherenet Corporation (Zebra Technologies Corporation)
• Inventors: Belcher, Donald K., Boyd, Robert W., Hash, Ronald J.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US09/159,555
• Time in Patent Office: 1,041 Days
• Field of Search: 324/239, 324/243, 324/241, 340/551, 340/572.1, 340/572.4
• US Class Current: 324/243
• CPC Class Codes: G06K 7/083 inductive G06K7/10336 takes precedence

Abstract

A magnetic field excitation and detection coil arrangement is configured to produce an electrical output signal representative of a differential combination of responses from a plurality of magnetic fields, including the field emitted from the excitation coil, a response magnetic field emitted from an object-tagged transponder interrogated by the excitation coil, and other magnetic fields from extraneous sources, such as industrial machinery. This differential combination of the responses from the detection coils will contain only the desired signal (e.g., response from an object-tagged transponder), as the other far away fields cancel, and therefore produces a net output signal level for a downstream signal (transponder output) processor.

Claims

1. An apparatus comprising:

an exciter coil having a coil axis;

a plurality of magnetic field detection coils spaced apart from one another along said coil axis on opposite sides and in proximity of said exciter coil;

said exciter coil being operative to emit an excitation magnetic field that stimulates a transponder located in the direction of said coil axis, but beyond said plurality of detection coils and said exciter coil therebetween, to emit a response magnetic field; and

said plurality of magnetic field detection coils being operative to detect said response magnetic field emitted by said transponder, and being arranged and electrically differentially coupled with one another to produce a composite, transponder-discriminating electrical output, in which responses of said magnetic field detection coils to said excitation magnetic field are canceled, but being representative of the difference in said response magnetic field emitted by said transponder as detected by said plurality of magnetic field detection coils.

2. An apparatus according to claim 1, wherein said plurality of magnetic field detection coils comprise first and second magnetic field detection coils equally spaced apart from and on opposite sides of said exciter coil.

3. An apparatus according to claim 1, further including a core of high magnetic permeability material around which said exciter coil is wound.

4. An apparatus according to claim 1, wherein said plurality of magnetic field detection coils comprise a first plurality of first magnetic field detection coils spatially arranged in a first generally planar distribution on a first side of and parallel to said exciter coil, and a second plurality of second magnetic field detection coils spatially arranged in a second generally planar distribution on a second side of and parallel to said exciter coil.

5. An apparatus according to claim 4, wherein said first and second pluralities of magnetic field detection coils are smaller than said exciter coil.

6. A method of detecting an object comprising the steps of:

(a) providing an exciter coil having a coil axis;

(b) providing a plurality of magnetic field detection coils spaced apart from one another along said coil axis on opposite sides and in proximity of said exciter coil, and being arranged and electrically differentially coupled with one another to produce a composite electrical output;

(c) providing said object with a transponder that is operative to emit a response magnetic field in reply to an excitation magnetic field coupled thereto;

(d) emitting from said exciter coil an excitation magnetic field that stimulates said transponder located in the direction of said coil axis, but beyond said plurality of detection coils and said exciter coil therebetween, to emit said response magnetic field; and

(e) detecting at said plurality of magnetic field detection coils said response magnetic field emitted by said transponder, and thereby producing said electrical output representative of the difference in said response magnetic field emitted by said transponder as detected and discriminated by said plurality of magnetic field detection coils, in which responses of said magnetic field detection coils to said excitation magnetic field are canceled.

7. A method according to claim 6, wherein said plurality of magnetic field detection coils include first and second magnetic field detection coils equally spaced apart from said exciter coil.

8. A method according to claim 7, wherein said exciter coil is wound around a core of high magnetic permeability material, and wherein said first and second magnetic field detection coils are spaced apart from opposite ends of said core.

9. A method according to claim 6, wherein said plurality of magnetic field detection coils comprise a first plurality of first magnetic field detection coils spatially arranged in a first generally planar distribution on a first side of and parallel to said exciter coil, and a second plurality of second magnetic field detection coils spatially arranged in a second generally planar distribution on a second side of and parallel to said exciter coil.

10. A method according to claim 9, wherein said first and second pluralities of magnetic field detection coils are smaller than said exciter coil.