Apparatus and method for detecting a magnetic marker

• US 4,710,752 A
• Filed: 08/08/1986
• Issued: 12/01/1987
• Est. Priority Date: 08/08/1986
• Status: Expired due to Term

• Current Assignee: Pitney Bowes Incorporated (Böwe Systec AG)
• Original Assignee: Pitney Bowes Incorporated (Böwe Systec AG)
• Inventors: Cordery, Robert A.
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US06/894,429
• Time in Patent Office: 480 Days
• Field of Search: 340/551, 340/572
• US Class Current: 340/551
• CPC Class Codes: G08B 13/2408 using ferromagnetic tags
• CPC Class Codes: G08B 13/2477 Antenna or antenna activator circuit
• CPC Class Codes: G08B 13/2488 Timing issues, e.g. synchronising measures to avoid signal collision, with multiple emitters or a single emitter and receiver

Abstract

This invention relates to an apparatus and method for detecting the presence of a magnetic marker in an interrogation zone. A dual frequency magnetic field is generated causing a magnetic marker to produce amplitude modulated side band signals at harmonics of the higher frequency field. These side band signals are readily distinguishable from amplitude noise.

Claims

1. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising:

first generating means for generating a first magnetic field in the interrogation zone at a first frequency,

second generating means for generating a second magnetic field in the interrogation zone at a second frequency,

said second frequency having a value less than approximately one-fifth of said first frequency and

means for detecting amplitude modulated signals produced by a magnetic marker present in the interrogation zone when said first and second magnetic field generating means are enabled.

2. The system of claim 1 wherein said first frequency is greater than 5 kHz and said second frequency is less than 2 kHz.

3. The system of claim 1 wherein the ratio of the frequency of said first magnetic field to said second magnetic field varies from 5:

1 to 100;

1.

4. The system of claim 1 wherein the ratio of the frequency of the first magnetic field to the second magnetic field is approximately 20:

1.

5. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising:

first generating means for generating a magnetic field in the interrogation zone at a first frequency and first amplitude,

second generating means for generating a second magnetic field in the interrogation zone at a second frequency and a second amplitude,

said second frequency having a value of less than one fifth of said first frequency, and

means for detecting amplitude modulated signals produced by a magnetic marker present in the interrogation zone.

6. The system of claim 5 wherein the ratio of the amplitude of said first magnetic field to the amplitude of the second magnetic field varies from 2:

1 to 30;

1.

7. The system of claim 5 where the ratio of the amplitude of said first magnetic field to the amplitude of the second magnetic field is approximately 5:

1.

8. The system of claim 5 wherein the ratio of the frequency of the first magnetic field to the frequency of the second magnetic field varies from 5:

1 to 100;

1.

9. The system of claim 5 wherein the ratio of the frequency of the first magnetic field to the frequency of the second magnetic field is 20:

1.

10. The system of claim 6 wherein said first frequency is greater than 5 kHz, said first amplitude is greater than 3 Oe, said second frequency is less than 2 kHz and said second amplitude is less than 0.5 Oe.

11. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising:

first generating means for generating a first magnetic field in the interrogation zone at a first frequency,

second generating means for generating a second magnetic field in the interrogation zone at a second frequency,

said second frequency having a value less than approximately one-fifth of said first frequency and

means for measuring amplitude variations of a predetermined harmonic frequency of said first magnetic field produced by a magnetic marker present in the interrogation zone when said first and second magnetic field generating means are enabled.

12. The system of claim 11 wherein said first frequency is greater than 5 kHz and said second frequency is less than 2 kHz.

13. The system of claim 11 wherein the ratio of the frequency of the first magnetic field to the frequency of trhe second magnetic field varies from 2:

1 to 100;

1.

14. The system of claim 11 wherein the ratio of the frequency of the first magnetic field to the frequency of the second magnetic field is 20:

1.

15. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising:

first generating means for generating a magnetic field in the interrogation zone at a first frequency and first amplitude,

second generating means for generating a second magnetic field in the interrogation zone at a second frequency and a second amplitude,

said second frequency having a value less than one-fifth of said first frequency and said second amplitude having a value of less than one-fourth of said first amplitude, and

means for detecting amplitude variations about a predetermined multiple of said first magnetic field produced by a magnetic marker present in the interrogation zone when said first and second magnetic field generating means are enabled.

16. The system of claim 15 wherein said first frequency is greater than 5 kHz, said first amplitude is greater than 3 Oe, said second frequency is less than 2 kHz and said second amplitude is less than 0.5 Oe.

17. The system of claim 15 wherein the ratios of the amplitude of said first magnetic field to the amplitude of the second magnetic field varies from 2:

1 to 30;

1.

18. The system of claim 15 wherein the ratio of the amplitude of said first magnetic field to the amplitude of the second magnetic field is approximately 5:

1.

19. The system of claim 15 wherein the ratio of the frequency of the first magnetic field to the frequency of the second magnetic field varies from 5:

1 to 100;

1.

20. The system of claim 15 wherein the ratio of the frequency of the first magnetic field to the frequency of second magnetic field is 20:

1.

21. A method of detecting a magnetic marker in an interrogation zone, the steps comprising:

(a) generating a first magnetic field in the interrogation zone at a first frequency,

(b) generating a second magnetic field in the interrogation zone at a second frequency having a lower value than said first frequency, and

(c) detecting the presence of a magnetic marker in the interrogation zone by detecting side band signals generated by the magnetic marker as a result of being exposed to the first and second magnetic fields simultaneously.

22. The method of claim 21 including the step of generating a magnetic field with a first frequency of greater than 5 kHz and generating a second magnetic field with a frequency of less than 2 kHz.

23. The method of claim 21 wherein the step of generating the first and second magnetic fields includes generating said first and second fields in a manner such that the ratio of the frequency of the first magnetic field to the frequency of the second magnetic field varies from 2:

1 to 100;

1.

24. The method of claim 21 wherein the step of generating the first and second magnetic fields includes generating said first and second fields in a manner such that the ratio of the frequency of the first magnetic field to the frequency of the second magnetic field is 20:

1.

25. A method of detecting a magnetic marker in an interrogation zone, the steps comprising:

(a) generating a first magnetic field in the interrogation zone at a first frequency and first amplitude,

(b) generating a second magnetic field in the interrogation zone at a second frequency having a lower value than the first frequency and a second amplitude having a lower value than the first amplitude, and

(c) detecting the presence of a magnetic marker in the interrogation zone by detecting side band signals about a predetermined multiple of the frequency of the first magnetic field generated by the magnetic marker as a result of being exposed to the first and second magnetic field simultaneously.

26. The method of claim 25 wherein the step of generating the first and second magnetic fields includes generating the first field with a frequency greater than 5 kHz and an amplitude greater than 3 Oe, and generating the second magnetic field with a frequency of less than 2 kHz and an amplitude of less than 0.5 Oe.

27. The method of claim 25 wherein generating the first and second magnetic fields yields a ratio of the amplitude of said first magnetic field to the amplitude of the second magnetic field that varies from 2:

1 to 30;

1.

28. The method of claim 27 wherein generating the first and second magnetic fields yields a ratio of the amplitude of the first magnetic field to the amplitude of the second magnetic field of approximately 5:

1.

29. The method of claim 25 wherein generating the first and second magnetic fields yields a ratio of the frequency of the first magnetic field to the frequency of the second magnetic field which varies from 5:

1 to 100;

1.

30. The method of claim 25 wherein generating the first and second magnetic fields yields a ratio of the frequency of the first magnetic field to the frequency of the second magnetic field of approximately 20:

1.

31. A method of detecting the presence of a magnetic marker in an interrogation zone, the steps comprising:

(a) generating a dual frequency magnetic field of frequencies f₁ and f₂ in the interrogation zone, wherein f₁ is at least twice the value of f₂,

(b) passing a magnetic marker through the interrogation zone to generate sideband signals around the harmonics of the frequency f₁ and

(c) determining the presence of the magnetic marker in the interrogation zone by detecting the sideband signals.

32. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising:

(a) means for generating a magnetic field of frequencies f₁ and f₂ in the interrogation zone, wherein f₁ is at least twice the value of f₂,

whereby sideband signals around the harmonics of the f₁ frequency are generated upon a magnetic marker'"'"'s being passed through the interrogation zone and

(b) means for determining the presence of a marker in the interrogation zone by detecting said sideband signals.