AC magnetic tracker for operation close to metallic objects

• US 6,528,989 B1
• Filed: 03/21/2000
• Issued: 03/04/2003
• Est. Priority Date: 03/21/2000
• Status: Expired due to Fees

• Current Assignee: Skysense, Ltd.
• Original Assignee: Skysense, Ltd.
• Inventors: Hansen, Per Krogh
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Aurora, Reena
• Application Number:
  US09/532,825
• Time in Patent Office: 1,078 Days
• Field of Search: 324/207.12, 324/207.15, 324/207.16, 324/207.17, 324/225, 324/226, 324/234, 324/239, 324/240, 702/150-153, 342/450, 342/451, 342/453
• US Class Current: 324/207.12
• CPC Class Codes: G01B 7/003 for measuring position, not involving coordinate determination coordinate measuring G01B7/004
• CPC Class Codes: G01B 7/004 for measuring coordinates of points
• CPC Class Codes: G01B 7/30 for measuring angles or tapers; for testing the alignment of axes
• CPC Class Codes: G01D 5/2046 by a movable ferromagnetic element, e.g. a core
• CPC Class Codes: G01D 5/2053 by a movable non-ferromagnetic conductive element

Abstract

Embodiments of an AC magnetic tracker include frequency-based compensation for metal distortion. The tracker measures the position and orientation of the receiver relative to the transmitter in up to six dimensions, position (x, y, z) and orientation (azimuth, elevation, roll). In some embodiments, magnetic fields are transmitted at selected frequencies and the received magnetic fields are measured in-phase with the transmitted magnetic field. In other embodiments, magnetic fields are transmitted at selected frequencies and the received magnetic fields are measured both in-phase and in-quadrature with the transmitted magnetic field. In yet other embodiments, a transition signal is transmitted and the received decaying signal is measured. Other embodiments disclose some combination of these features. The metal'"'"'s inductive limit, decay time constant and polarity sign are calculated from the received signals. The present invention determines if the metals are conductive or ferrous and compensates for the metal distortion accordingly. The present invention contemplates a scheme allowing compensation for distortions commonly occurring when conductive and ferrous metals are adjacent a measuring space and, thus, overcomes the negative effects of metal distortion caused by such adjacency of such metals.

Claims

1. A magnetic position and orientation measurement system for operation close to metallic objects comprising:

a) at least one transmitter means for transmitting a non-DC-based magnetic field in a space;

b) at least one receiver means in said space for receiving said magnetic field;

c) computer means including compensation means for receiving said magnetic field and compensating for metal distortion directly from said received magnetic field without resort to mapping, said computer means being connected to said transmitter means and receiver means;

d) said computer means further including means for computing position and orientation of said receiver means relative to said transmitter means from said received magnetic field free of metal distortion.

2. The system of claim 1, said compensation means further including:

a) control means for transmitting said magnetic field at a frequency f, and for storing received signal strength; and

b) said frequency f being chosen to be low enough to preclude creation of eddy current distortion of said magnetic field due to presence of conductive metal in or adjacent said space.

3. The system of claim 1, said compensation means further including:

a) first control means for transmitting said magnetic field at a first frequency f1 and at a second frequency f2; and

b) second control means for measuring signal strength received by said at least one receiver means in-phase and in-quadrature to said transmitted magnetic field at said frequency f1 and at said frequency f2; and

c) processing means for calculating net signal strength of said received signal, free of ferrous as well as conductive metal distortion, from said in-phase and in-quadrature measurements.

4. The system of claim 3, wherein second processor means further calculates metals'"'"' inductive limit, decay time constant, and polarity sign;

and

a) said second processor means further calculates an expected difference between said in-phase signals; and

b) said second processor means further finds the net signal strength free of metal distortion, if said calculated difference is equal to the actual measured difference, from said measured signals and said calculations.

5. The system of claim 1, said compensation means further including:

a) processor means for selecting a frequency f0 at which said transmitter means is driven, said frequency f0 being chosen based upon knowledge of frequency ranges in which ferrous metals are likely to induce distortion; and

b) control means for transmitting said magnetic field at said frequency f0, and for storing said received signals of ferrous metal distortion.

6. The system of claim 1, said compensation means further including:

a) first processor means for selecting transmission frequencies fh and fl, where fh is a relatively high frequency at which conductive and ferrous metals have reached their inductive limit, and fl is a relatively low frequency at which distortion of a magnetic field is quantifiable;

b) first control means for transmitting said magnetic field at said first frequency fh, and for storing a received field strength, Ih, in-phase with said transmitted magnetic field at frequency fh;

c) second control means for transmitting said magnetic field at said second frequency fl, and for storing a received field strength, I1, in-phase with said transmitted magnetic field at frequency fl;

d) second processor means for determining a ferrous metal distortion free received signal from said first and second received signals corresponding to said first and second magnetic fields, respectively.

7. The system of claim 1, said compensation means further including:

a) first processing means for selecting transmission frequencies fh and fl where fh is a relatively high frequency at which conductive and ferrous metals have reached their inductive limit, and fl is a relatively low frequency at which distortion of a magnetic field is approximately the same as found for DC; and

b) second control means for (1) controlling a last portion of a transmitted said magnetic field at the transmitter means at its maximum level for a desired time or (2) changing said magnetic field from any initial level to its maximum level during said desired time and then transitioning it from said maximum level to a zero level in a desired fall time; and

c) third control means for measuring received signal strength after said transitioning has occurred at least twice; and

d) second processing means for adding or subtracting a signal resulting from said transitioning to or from said received signal strength resulting in a net received signal strength free of distortion from conductive and/or ferrous metals.

8. The system of claim 7, said computer means further including third processor means to select frequency f such that an inductive limit for both the ferrous and the conductive metals has been reached.

9. The system of claim 7, further including:

a) fourth control means for transmitting an additional magnetic field at a frequency closely approximating 1/T; and

b) fifth control means for measuring received signal strength in-quadrature to said transmitted field at a frequency closely approximating 1/T, Q_T; and

c) third processing means for finding the new inductive limit from said received signal and said inductive limit and thus eliminating a need for generating a transition signal from time to time.

10. The system of claim 9, wherein said computer means includes sixth control means for allowing said receiver means to be held stationary, said transition signal only being used when said receiver means is stationary with relation to said transmitter means.

11. The system of claim 7, further including:

a) fourth control means for transmitting an AC magnetic field at a frequency close to 1/T and an AC magnetic field at a frequency defined by k1/T;

b) fifth control means for measuring received signal strength in-quadrature to said transmitted field at a frequency close to 1/T, Q1, and at a frequency defined by k1/T, Q2; and

c) third processing means for finding the new inductive limit from said received signals and said inductive limit and thus eliminating a need for generating a transition signal from time to time.

12. The system of claim 7, wherein said computer means includes means for refraining from using said transition signal when Q1(t)/Q2(t) at a current measurement is equal to Q1(t0)/Q2(t0) at a time said transition signal was used.

13. The system of claim 7, wherein said second processing means determines said transition compensation value from the formula Q1(t)/Q1(t0)*IT(t0).

14. The system of claim 7, further including:

a) fourth control means for receiving in-phase and in-quadrature signals relative to a transmitted magnetic field; and

b) means for storing in-phase and in-quadrature received signals.

15. The system of claim 1, said compensation means further including:

a) first control means for transmitting said magnetic field at a frequency f, and for storing received signal strength;

b) second control means for transmitting said magnetic field at a frequency f1 and at a frequency f2, both frequencies f1 and f2 being lower than said frequency f;

c) third control means for measuring received signal strength in-phase and in-quadrature with respect to said transmitted fields at frequency f1, and at frequency f2;

d) first processing means for finding inductive limit, decay time constant and polarity sign from said received signals collectively referred to as found parameters; and

e) second processing means for calculating net received signal strength, free of distortion from conductive and/or ferrous metals, from said received signals and said found parameters.

16. The system of claim 15, further including processor means for selecting the frequency f such that an inductive limit for both ferrous and conductive metals has been reached.

17. The system of any one of claims 2, 3, 5, 6, 7 or 15, wherein said computer means includes means for selecting appropriate transmission frequencies based upon knowledge of frequency response of various metals.

18. The system of any one of claims 2, 3, 5, 6, 7 or 15, wherein said computer means includes means for selecting appropriate transmission frequencies based on knowledge of properties of metals located in said space.

19. The system of any one of claims 2, 3, 5, 6, 7 or 15, wherein said at least one transmitter means comprises at least three transmitters, and said at least one receiver means comprises at least two receivers, said computer means including means for calculating position (x, y, z) and orientation (az, el, rl) in six degrees of freedom.

20. The system of any one of claims 2, 3, 5, 6, 7 or 15, wherein said at least one transmitter means comprises at least two transmitters, and wherein said receiver means comprises at least three receivers, and wherein said computer means calculates position (x, y, z) and orientation (az, el, rl) in six degrees of freedom.

21. The system of any one of claims 2, 3, 5, 6, 7 or 15, wherein said transmitter means comprises at least five transmitters, and wherein said computer means calculates position (x, y, z) and orientation (az, el) in five dimensions.

22. The system of any one of claims 2, 3, 5, 6, 7 or 15, wherein said at least one receiver means includes means for measuring dB/dt and wherein said computer means calculates magnetic field strength.

23. The system of any one of claims 2, 3, 5, 6, 7 or 15, further including further control means for rotating measurements at the different frequencies or transition measurements between the transmitter axis and further processor means in order to calculate the position and orientation using a present measurement and two most recent previous measurements.

24. The system of claim 23, said computer means further including further processor means for interpolating between measurements of a same frequency or transition measurement taken at different times but generated from a particular transmitter axis for determining all relevant measurements for a position and orientation measurement as if they were all taken simultaneously.

25. The system of any one of claims 15 or 14, further including:

a) said magnetic field being transmitted at a high frequency f_hp at which metals have not yet reached their inductive limit;

b) said computer means including;

i) third processing means for calculating expected quadrature signal Q_c at frequency f_hp; and

ii) fourth processing means for determining a signal free of metal distortion when measured quadrature signal is equal to a calculated said quadrature signal.

26. The system of claim 25, including further transmitting means for transmitting at a low frequency f₁ at which metal distortion is approximately the same as at DC.

27. The system of claim 25, including:

a) further transmitting means for transmitting AC signals at a frequency f_n different from f_hp;

b) said computer means including;

i) fifth processing means for utilizing curve fitting using quadrature signals to find a frequency f_h where an in-quadrature signal is equal to zero;

ii) sixth processing means using in-phase signals for finding in-phase signal I_h at the frequency f_h; and

iii) seventh processing means for finding a signal free of metal distortion from an in-phase signal at frequency f_h.

28. The system of any one of claims 15 or 14, wherein said transmission frequency f is chosen from knowledge of properties of metals, the computer means further including:

a) second processing means for calculating an expected in-quadrature signal at a desired frequency f;

b) third processing means for determining a signal that is free of metal distortion, when said measured in-quadrature signal is equal to said calculated in-quadrature signal, from said measured signals and said calculations; and

c) fourth processing means for determining a signal that is free of metal distortion, when said measured in-quadrature signal is not equal to said calculated in-quadrature signal, from the measured in-phase signal.

29. The system of claim 28, wherein third processor means further includes means for utilizing curve fitting using said in-phase signals in order to find the expected in-phase signal at a sufficiently low frequency, and said third processor means further finding the net signal free of metal distortion from said calculated in-phase signal and said calculated parameter.

30. A method of measuring the position and orientation of an object for operation close to metallic objects including the steps of:

a) providing a magnetic position and orientation system including;

i) at least one single axis transmitter;

ii) at least one single axis receiver attached to said object;

iii) magnetic field transmission means;

iv) magnetic field receiving means;

v) computer means for controlling said transmitter and receiver means;

b) programming said computer means to transmit a non-DC-based magnetic field at a selected frequency;

c) further programming said computer means to store received signals;

d) further programming said computer means to calculate metal distortion from said received signals without resort to mapping;

e) further programming said computer means to find said received signals free of metal distortion; and

f) further programming said computer means to calculate position and orientation of said object from said metal distortion free signals.

31. The method of claim 30, wherein said programming step includes the step of selecting the selected frequency low enough for conductive metals to be approximately equal to the magnetic field at DC.

32. The method of claim 31, wherein said selecting step includes the step of choosing two frequencies, both low enough so that an inductive limit for said objects has not been reached.

33. The method of claim 32, further including additional means for measuring both in-phase and in-quadrature signals, and further including the steps of:

a) further programming the computer means to calculate inductive limit, decay time constant and polarity sign from said in-quadrature signals;

b) further programming the computer means to calculate expected difference between two in-phase signals;

c) further programming the computer means to find a signal free of metal distortion from said signals and said calculations if the expected difference is equal to the measured difference;

d) further programming the computer means to use curve fitting of said two in-phase signals in order to find the in-phase signal at a sufficiently low frequency if the expected difference is not equal to the measured difference; and

e) further programming the computer means to find a signal free of metal distortion from said signals and said calculations.

34. The method of claim 30, wherein said selecting step includes the step of choosing a frequency at which in-phase signal of ferrous metal distortion is approximately equal to zero.

35. The method of claim 30, further including the step of holding the magnetic field at its maximum level for a desired time period and then shifting to zero very fast.

36. The method of any one of claims 32 or 35, with additional means for measuring both in-phase and in-quadrature signals, and:

a) further programming the computer means to calculate inductive limit, decay time constant and polarity sign from two in-quadrature signals;

b) further programming the computer means to transmit at an additional frequency;

c) further programming the computer means to calculate an expected in-quadrature signal at the additional frequency;

d) further programming the computer means to calculate the signal free of metal distortion from said signals and calculations if said expected in-quadrature signal is equal to a measured in-quadrature signal;

e) further programming the computer means to transmit at yet one more additional frequency and using curve fitting on said in-quadrature signals in order to find a frequency where the in-quadrature signal is equal to zero if said expected in-quadrature signal is not equal to said measured in-quadrature signal;

f) further programming the computer means to utilize the found frequency from sub-paragraph e) above, and curve fitting on in-phase signals in order to find an in-phase signal at said frequency; and

g) further programming the computer means to calculate a signal free of metal distortion from said signals and said calculations.

37. The method of any one of claims 32 or 35, with additional means for measuring both in-phase and in-quadrature signals, and:

a) further programming the computer means to calculate inductive limit, decay time constant and polarity sign from two in-quadrature signals;

b) further programming the computer means to transmit at an additional AC frequency where ferrous metal distortion in-phase signal is approximately equal to zero;

c) further programming the computer means to calculate expected in-quadrature signal at the additional frequency;

d) further programming the computer means to calculate a signal free of metal distortion from said signals and calculations if said expected in-quadrature signal is equal to said measured in-quadrature signal; and

e) further programming the computer means to calculate a signal free of metal distortion from said signals and said calculations if said expected in-quadrature signal is not equal to said measured in-quadrature signal.