Single winding magnetometer with automatic distortion compensation

• US 5,039,945 A
• Filed: 03/27/1990
• Issued: 08/13/1991
• Est. Priority Date: 03/27/1990
• Status: Expired due to Term

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Webb, Spencer L.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US07/499,957
• Time in Patent Office: 504 Days
• Field of Search: 324/253, 324/254, 324/255, 324/117 R, 340/870.33, 33/361
• US Class Current: 324/253
• CPC Class Codes: G01R 33/04 using the flux-gate principle

Abstract

The effects of non-equal saturation spikes in a magnetic oscillator magnetometer utilizing a magnetic core sensing inductor are compensated by adding the average inductor current to both the high and low current magnitude limits. In particular, each of the current limits is selected to be a fixed current (which is greater than that required to saturate the core) offset by the average current flowing in the core.

The effects of magnetic core hysteresis and series resistance between the inductor and its drive sources are compensated by controlling the drive voltage levels so that the time during which the inductor is driven by current in one direction is equal to the time during which it is driven by current in the other direction.

Claims

1. In a magnetometer, having a first voltage source with a potential greater than a reference potential, a second voltage source with a potential less than said reference potential, an inductor, switch means for connecting said first voltage source and said second voltage source to said inductor to cause a current with a magnitude to run therethrough to said reference potential, means responsive to said current magnitude for controlling said switch means to connect said inductor to said first voltage source when said current magnitude reaches a first limit magnitude and to connect said inductor to said second voltage source when said current magnitude reaches a second limit magnitude, the improvement comprising:

means responsive to said current magnitude for generating a time averaged value thereof; and

means responsive to said time averaged value for adjusting said first limit magnitude and said second limit magnitude so that the average of said first limit magnitude and said second limit magnitude equals said time averaged value.

2. In a magnetometer, the improvement according to claim 1 wherein said adjusting means is responsive to said time averaged value for adding said time averaged value to said first limit magnitude and to said second limit magnitude.

3. In a magnetometer, the improvement according to claim 1 wherein said generating means comprises a low pass filter.

4. In a magnetometer according to claim 1 wherein said controlling means comprises a first comparator responsive to said current magnitude and to said first limit magnitude for generating a set signal, a second comparator responsive to said current magnitude and to said second limit magnitude for generating a reset signal and a flip-flop responsive to said set signal and to said reset signal for controlling said switch means, the improvement according to claim 1 wherein said adjusting means is responsive to said time averaged value for adding said time averaged value to said first limit magnitude and to said second limit magnitude so that said first and said second comparators generate said set and reset signals at magnitudes of said current dependent on said time averaged value.

24. In a magnetometer according to claim 21 wherein said controlling means comprises a first comparator responsive to said current magnitude and to said first limit magnitude for generating a set signal, a second comparator responsive to said current magnitude and to said second limit magnitude for generating a reset signal and a flip-flop responsive to said set signal and to said reset signal for controlling said switch means, the improvement according to claim 1 wherein said first adjusting means is responsive to said time averaged current magnitude value for adding said time averaged current magnitude value to said first limit magnitude and to said second limit magnitude so that said first and said second comparators generate said set and reset signals at magnitudes of said current dependent on said time averaged current magnitude value.

5. An improved magnetometer comprising:

a first voltage source with a potential greater than a reference potential;

a second voltage source with a potential less than said reference potential;

an inductor;

switch means for connecting said first voltage source and said second voltage source to said inductor to cause a current with a magnitude to run therethrough to said reference potential;

means responsive to said current magnitude for controlling said switch means to connect said inductor to said first voltage source when said current magnitude reaches a first limit magnitude and to connect said inductor to said second voltage source when said current magnitude reaches a second limit magnitude;

means responsive to said current magnitude for generating a time averaged value thereof; and

means responsive to said time averaged value for adjusting said first limit magnitude and said second limit magnitude so that the average of said first limit magnitude and said second limit magnitude equals said time averaged value.

6. An improved magnetometer according to claim 5 wherein said adjusting means is responsive to said time averaged value for adding said time averaged value to said first limit magnitude and to said second limit magnitude.

7. An improved magnetometer according to claim 5 wherein said generating means comprises a low pass filter.

8. An improved magnetometer according to claim 5 wherein said controlling means comprises a first comparator responsive to said current magnitude and to said first limit magnitude for generating a set signal, a second comparator responsive to said current magnitude and to said second limit magnitude for generating a reset signal and a flip-flop responsive to said set signal and to said reset signal for controlling said switch means and wherein said adjusting means is responsive to said time averaged value for adding said time averaged value to said first limit magnitude and to said second limit magnitude so that said first and said second comparators generate said set and reset signals at magnitudes of said current dependent on said time averaged value.

9. An improved magnetometer according to claim 5 wherein said inductor has a saturation current magnitude and said first limit magnitude and said second limit magnitude are greater than said saturation current magnitude so that said inductor saturates before said first limit magnitude and said second limit magnitude are reached.

10. An improved magnetometer comprising:

a voltage source with a potential positive with respect to ground;

a voltage source with a potential negative with respect to ground;

an inductor having a winding around a core fabricated of magnetically-saturable material which saturates at a saturation current;

switch means for connecting said positive voltage source and said negative voltage source to said inductor winding to cause a current with a magnitude to run therethrough to ground;

means for establishing a first predetermined current limit magnitude greater than said saturation current;

means for establishing a second predetermined current limit magnitude greater than said saturation current;

a first comparator responsive to said current magnitude for generating a set signal when said current magnitude reaches said first predetermined current limit magnitude;

a second comparator responsive to said current magnitude for generating a reset signal when said current magnitude reaches a second predetermined current limit magnitude;

a flip-flop responsive to said set signal for controlling said switch means to connect said inductor to said positive voltage source and responsive to said reset signal for controlling said switch means to connect said inductor to said negative voltage source;

a low pass filter means responsive to said current magnitude for generating a time averaged value thereof; and

means responsive to said time averaged value for adding said time averaged value to said first predetermined limit magnitude and to said second predetermined limit magnitude.

11. An improved magnetometer according to claim 10 wherein said low pass filter means comprises sensing means for converting said current magnitude into an analog voltage and a low pass filter for filtering said voltage.

12. An improved magnetometer according to claim 10 wherein said means for establishing said first predetermined current limit magnitude comprises a resistor and a current source for drawing a predetermined current through said resistor.

13. An improved magnetometer according to claim 10 wherein said means for establishing said second predetermined current limit magnitude comprises a resistor and a current source for drawing a predetermined current through said resistor.

14. In a magnetometer, having a first voltage source with a first potential greater than a reference potential, a second voltage source with a second potential less than said reference potential, an inductor, switch means for connecting said first voltage source and said second voltage source to said inductor to cause a current with a magnitude to run therethrough to said reference potential, means responsive to said current magnitude for controlling said switch means to connect said inductor to said first voltage source when said current magnitude reaches a first limit magnitude and to connect said inductor to said second voltage source when said current magnitude reaches a second limit magnitude so that said magnetometer oscillates with a duty cycle, the improvement comprising:

means responsive to said duty cycle for genrating a time averaged value thereof; and

means responsive to said time averaged value for adjusting said first potential and said second potential until said duty cycle becomes fifty percent.

15. In a magnetometer, the improvement according to claim 14 wherein said adjusting means is responsive to said time averaged value for adjusting said reference potential thereby changing said first potential and said second potential.

16. In a magnetometer, the improvement according to claim 14 wherein said generating means comprises an integrator.

17. An improved magnetometer comprising:

a first voltage source with a first potential greater than a reference potential;

a second voltage source with a second potential less than said reference potential;

an inductor;

switch means for connecting said first voltage source and said second voltage source to said inductor to cause a current with a magnitude to run therethrough to said reference potential;

means responsive to said current magnitude for controlling said switch means to connect said inductor to said first voltage source when said current magnitude reaches a first limit magnitude and to connect said inductor to said second voltage source when said current magnitude reaches a second limit magnitude so that said magnetometer oscillates with a duty cycle;

means responsive to said duty cycle for generating a time averaged value thereof; and

means responsive to said time averaged value for adjusting said first potential and said second potential until said duty cycle becomes fifty percent.

18. In a magnetometer, the improvement according to claim 17 wherein said adjusting means is responsive to said time averaged value for adjusting said reference potential thereby changing said first potential and said second potential.

19. In a magnetometer, the improvement according to claim 17 wherein said generating means comprises an integrator.

20. An improved magnetometer comprising:

a first voltage source with a potential positive with respect to a reference potential;

a second voltage source with a potential negative with respect to said reference potential;

an inductor having a winding around a core fabricated of magnetically-saturable material which saturates at a saturation current;

switch means for connecting said first voltage source and said second voltage source to said inductor winding to cause a current with a magnitude to run therethrough to said reference potential;

means connected to said switch means for generating a duty cycle signal having a first voltage magnitude when said first voltage source is connected to said inductor winding and having a second voltage magnitude when said second voltage source is connected to said inductor winding;

means for establishing a first predetermined current limit magnitude greater than said saturation current;

means for establishing a second predetermined current limit magnitude greater than said saturation current;

a first comparator responsive to said current magnitude for generating a set signal when said current magnitude reaches said first predetermined current limit magnitude;

a second comparator responsive to said current magnitude for generating a reset signal when said current magnitude reaches a second predetermined current limit magnitude;

a flip-flop responsive to said set signal for controlling said switch means to connect said inductor to said first voltage source and responsive to said reset signal for controlling said switch means to connect said inductor to said second voltage source;

an integrator responsive to said duty cycle signal for generating a time averaged value thereof; and

means responsive to said time averaged value for adjusting said reference potential until said time averaged value is zero.

21. In a magnetometer, having a first voltage source with a potential greater than a reference potential, a second voltage source with a potential less than said reference potential, an inductor, switch means for connecting said first voltage source and said second voltage source to said inductor to cause a current with a magnitude to run therethrough to said reference potential, means responsive to said current magnitude for controlling said switch means to connect said inductor to said first voltage source when said current magnitude reaches a first limit magnitude and to connect said inductor to said second voltage source when said current magnitude reaches a second limit magnitude so that said magnetometer oscillates with a duty cycle, the improvement comprising:

first means responsive to said current magnitude for generating a time averaged value thereof;

first means responsive to said time averaged value for adjusting said first limit magnitude and said second limit magnitude so that the average of said first limit magnitude and said second limit magnitude equals said time averaged value

second means responsive to said duty cycle for generating a time averaged value thereof; and

second means responsive to said time averaged value for adjusting said first potential and said second potential until said duty cycle becomes fifty percent.

22. In a magnetometer, the improvement according to claim 21 wherein said first adjusting means is responsive to said time averaged current magnitude value for adding said time averaged current magnitude value to said first limit magnitude and to said second limit magnitude.

23. In a magnetometer, the improvement according to claim 21 wherein said first generating means comprises a low pass filter.

25. In a magnetometer, the improvement according to claim 21 wherein said second adjusting means is responsive to said time averaged duty cycle value for adjusting said reference potential thereby changing said first potential and said second potential.

26. In a magnetometer, the improvement according to claim 21 wherein said second generating means comprises an integrator.

27. An improved magnetometer comprising:

a first voltage source with a potential greater than a reference potential;

a second voltage source with a potential less than said reference potential;

an inductor;

switch means for connecting said first voltage source and said second voltage source to said inductor to cause a current with a magnitude to run therethrough to said reference potential;

means responsive to said current magnitude for controlling said switch means to connect said inductor to said first voltage source when said current magnitude reaches a first limit magnitude and to connect said inductor to said second voltage source when said current magnitude reaches a second limit magnitude so that said magnetometer oscillates with a duty cycle;

first means responsive to said current magnitude for generating a time averaged value thereof;

first means responsive to said time averaged value for adjusting said first limit magnitude and said second limit magnitude so that the average of said first limit magnitude and said second limit magnitude equals said time averaged value

second means responsive to said duty cycle for generating a time averaged value thereof; and

second means responsive to said time averaged value for adjusting said potential and said second potential until said duty cycle becomes fifty percent.

28. An improved magnetometer according to claim 27 wherein said first adjusting means is responsive to said time averaged current magnitude value for adding said time averaged current magnitude value to said first limit magnitude and to said second limit magnitude.

29. An improved magnetometer according to claim 27 wherein said first generating means comprises a low pass filter.

30. An improved magnetometer according to claim 27 wherein said controlling means comprises a first comparator responsive to said current magnitude and to said first limit magnitude for generating a set signal, a second comparator responsive to said current magnitude and to said second limit magnitude for generating a reset signal and a flip-flop responsive to said set signal and to said reset signal for controlling said switch means and wherein said first adjusting means is responsive to said time averaged current magnitude value for adding said time averaged current magnitude value to said first limit magnitude and to said second limit magnitude so that said first and said second comparators generate said set and reset signals at magnitudes of said current dependent on said time averaged current magnitude value.

31. An improved magnetometer according to claim 27 wherein said inductor has a saturation current magnitude and said first limit magnitude and said second limit magnitude are greater than said saturation current magnitude so that said inductor saturates before said first limit magnitude and said second limit magnitude are reached.

32. In a magnetometer, the improvement according to claim 27 wherein said second adjusting means is responsive to said time averaged duty cycle value for adjusting said reference potential thereby changing said first potential and said second potential.

33. In a magnetometer, the improvement according to claim 27 wherein said second generating means comprises an integrator.