Electromagnetic position and orientation sensing system

• US 8,723,509 B2
• Filed: 05/21/2013
• Issued: 05/13/2014
• Est. Priority Date: 04/28/2009
• Status: Active Grant

• Current Assignee: Brown University
• Original Assignee: Brown University
• Inventors: Patterson, William R. III, Silverman, Harvey F., Silverman, Kenneth J.
• Primary Examiner(s): Assouad, Patrick
• Assistant Examiner(s): BROWN, LAMARR A
• Application Number:
  US13/899,044
• Publication Number:
  US 20140043017A1
• Time in Patent Office: 357 Days
• Field of Search: 324/202, 324/207.15, 324/207.16, 324/207.17, 324/207.26, 340/539.13, 702150-153
• US Class Current: 324/207.15
• 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 7/14 for measuring distance or clearance between spaced objects or spaced apertures G01B7/30 takes precedence
• CPC Class Codes: G01D 5/204 by influencing the mutual induction between two or more coils G01D5/22 takes precedence
• CPC Class Codes: G01D 5/2086 by movement of two or more coils with respect to two or more other coils
• CPC Class Codes: G01R 35/005 Calibrating; Standards or reference devices, e.g. voltage or resistance standards, "golden" references G01R33/0035, G01R35/002 take precedence
• CPC Class Codes: G01V 13/00 Manufacturing, calibrating, cleaning, or repairing instruments or devices covered by groups G01V1/00 – G01V11/00
• CPC Class Codes: G01V 3/081 the magnetic field is produced by the objects or geological structures characterised by the method of magnetic field measurement G01R33/00

Abstract

Magnetic tracking systems and methods for determining the position and orientation of a remote object. A magnetic tracking system includes a stationary transmitter for establishing a reference coordinate system, and at least one receiver. The remote object is attached to, mounted on, or otherwise coupled to the receiver. The transmitter can include a set of three mutually perpendicular coils having a common center point, or a set of three coplanar coils with separate centers. The receiver can include a set of three orthogonal coils. The position and orientation of the receiver and the remote object coupled thereto is determined by measuring the nine mutual inductances between the three transmitter coils and the three receiver coils. The magnetic tracking system provides reduced power consumption, increased efficiency, digital compensation for component variation, automatic self-calibration, automatic synchronization with no connections between transmitter and receiver, and rapid low-cost implementation.

Claims

1. A transmitter for use in a magnetic tracking system, the transmitter being powered by a power source, the transmitter comprising:

a plurality of resonant circuits, each of the plurality of resonant circuits having;

a transmitter coil; and

a resonating capacitor electrically coupled in parallel with the transmitter coil;

a plurality of digitally controllable switches, each of the plurality of digitally controllable switches being electrically coupled in series between one end of a respective one of the plurality of resonant circuits and one terminal of the power source, each of the other ends of the plurality of resonant circuits being electrically coupled to the other terminal of the power source; and

a plurality of pulse generators, each of the plurality of pulse generators being operative to generate periodic digital pulses for controlling a respective one of the plurality of digitally controllable switches, thereby sustaining continuous, periodic, and substantially sinusoidal currents in the plurality of resonant circuits,

wherein, responsive to the respective continuous, periodic, and substantially sinusoidal currents, the transmitter coils are operative to generate a corresponding plurality of electromagnetic fields defining a reference coordinate system for the magnetic tracking system.

2. The transmitter of claim 1 wherein the continuous, periodic, and substantially sinusoidal currents sustained in the plurality of resonant circuits have different frequencies, and wherein each of the plurality of resonant circuits is operative to resonate at or slightly above the frequency of the sinusoidal current sustained in the respective resonant circuit.

3. The transmitter of claim 1 wherein the plurality of pulse generators are further operative to generate the respective periodic digital pulses at different frequencies, the different frequencies corresponding to different integer sub-multiples of a master clock frequency.

4. The transmitter of claim 3 wherein the plurality of pulse generators are further operative to generate the respective periodic digital pulses at the different frequencies, the different frequencies being in integer ratios to each other.

5. The transmitter of claim 1 further including a plurality of current limiting resistors, each of the plurality of current limiting resistors being coupled in series between a respective one of the plurality of resonant circuits and the digitally controllable switch electrically coupled to the respective resonant circuit.

6. The transmitter of claim 1 wherein the plurality of resonant circuits comprises three resonant circuits.

7. The transmitter of claim 6 wherein the respective transmitter coils have a common center point and are mutually perpendicular to each other.

8. The transmitter of claim 1 wherein each of the transmitter coils has coil turns disposed generally in a common plane, and wherein each of the transmitter coils has a center displaced from the centers of the other transmitter coils.

9. The transmitter of claim 8 wherein the centers of the respective transmitter coils are arranged to form an equilateral triangle.

10. The transmitter of claim 8 wherein the centers of the respective transmitter coils are arranged to form an isosceles triangle.

11. A transmitter for use in a magnetic tracking system, the transmitter being powered by a power source, the transmitter comprising:

a plurality of resonant circuits, each of the plurality of resonant circuits having;

a transmitter coil; and

a resonating capacitor electrically coupled in parallel with the transmitter coil;

a plurality of digitally controllable switches, each of the plurality of digitally controllable switches being electrically coupled in series between one end of a respective one of the plurality of resonant circuits and one terminal of the power source, each of the other ends of the plurality of resonant circuits being electrically coupled to the other terminal of the power source; and

a plurality of pulse generators, each of the plurality of pulse generators being operative to generate periodic digital pulses for controlling a respective one of the plurality of digitally controllable switches, thereby energizing the respective resonant circuits with a conduction angle substantially within a range of 5 to 15 degrees,

wherein, responsive to being energized via the plurality of digitally controllable switches, the transmitter coils are operative to generate a corresponding plurality of electromagnetic fields defining a reference coordinate system for the magnetic tracking system.

12. The transmitter of claim 11 wherein each of the plurality of pulse generators is further operative to generate the periodic digital pulses for closing the respective one of the plurality of digitally controllable switches for a time of approximately 1 microsecond in a cycle of approximately 50 microseconds.

13. The transmitter of claim 11 wherein each of the plurality of pulse generators is further operative to generate the periodic digital pulses for closing the respective one of the plurality of digitally controllable switches for a time substantially within a range of 0.5 to 1.5 microseconds in a cycle of approximately 50 microseconds.

14. The transmitter of claim 11 wherein the plurality of resonant circuits comprises three resonant circuits.

15. The transmitter of claim 14 wherein the respective transmitter coils have a common center point and are mutually perpendicular to each other.

16. The transmitter of claim 11 wherein each of the transmitter coils has coil turns disposed generally in a common plane, and wherein each of the transmitter coils has a center displaced from the centers of the other transmitter coils.

17. The transmitter of claim 16 wherein the centers of the respective transmitter coils are arranged to form an equilateral triangle.

18. The transmitter of claim 16 wherein the centers of the respective transmitter coils are arranged to form an isosceles triangle.

19. A transmitter for use in a magnetic tracking system, the transmitter being powered by a power source, the transmitter comprising:

a plurality of resonant circuits, each of the plurality of resonant circuits having;

a transmitter coil;

a resonating capacitor electrically coupled in parallel with the transmitter coil; and

a tuning capacitor switchingly coupled in parallel with the resonating capacitor;

a first set of digitally controllable switches, each of the first set of digitally controllable switches being electrically coupled in series between one end of a respective one of the plurality of resonant circuits and one terminal of the power source, each of the other ends of the plurality of resonant circuits being electrically coupled to the other terminal of the power source;

a first set of pulse generators, each of the first set of pulse generators being operative to generate periodic digital pulses for controlling a respective one of the first set of digitally controllable switches, thereby sustaining continuous, periodic, and substantially sinusoidal currents in the plurality of resonant circuits

wherein, responsive to the respective continuous, periodic, and substantially sinusoidal currents, the transmitter coils are operative to generate a corresponding plurality of electromagnetic fields defining a reference coordinate system for the magnetic tracking system;

a second set of digitally controllable switches, each of the second set of digitally controllable switches being electrically coupled in series between one end of a respective one of the tuning capacitors and the other terminal of the power source, the other end of the respective tuning capacitor being electrically coupled to the same end of the respective resonant circuit as the respective digitally controllable switch included in the first set of digitally controllable switches;

a second set of pulse generators, each of the second set of pulse generators being operative to generate digital pulses for controlling a respective one of the second set of digitally controllable switches, thereby switchably coupling the respective tuning capacitors in parallel with the respective resonating capacitors;

sense circuitry operative to measure the currents sustained in powering the plurality of resonant circuits; and

a controller operative to control timing of the digital pulses generated by the second set of pulse generators based on the current measurements of the sense circuitry, thereby controlling operation of the second set of digitally controllable switches for reducing power consumption of the plurality of resonant circuits.

20. The transmitter of claim 19 further including a plurality of current limiting resistors, each of the plurality of current limiting resistors being coupled in series between a respective one of the plurality of resonant circuits and the digitally controllable switch included in the first set of digitally controllable switches electrically coupled to the respective resonant circuit.

21. The transmitter of claim 19 wherein each of the plurality of resonant circuits is operative to resonate at a frequency substantially in a range of 1 percent to 5 percent above a predetermined resonating frequency while the tuning capacitor within the respective resonant circuit is de-coupled from the resonating capacitor within the respective resonant circuit.

22. The transmitter of claim 21 wherein each of the plurality of resonant circuits is operative to resonate at a frequency substantially in a range of 1 percent to 5 percent below the predetermined resonating frequency while the tuning capacitor within the respective resonant circuit is coupled in parallel with the resonating capacitor within the respective resonant circuit.

23. The transmitter of claim 19 wherein the plurality of resonant circuits comprises three resonant circuits.

24. The transmitter of claim 23 wherein the respective transmitter coils have a common center point and are mutually perpendicular to each other.

25. The transmitter of claim 19 wherein each of the transmitter coils has coil turns disposed generally in a common plane, and wherein each of the transmitter coils has a center displaced from the centers of the other transmitter coils.

26. The transmitter of claim 25 wherein the centers of the respective transmitter coils are arranged to form an equilateral triangle.

27. The transmitter of claim 25 wherein the centers of the respective transmitter coils are arranged to form an isosceles triangle.

28. A transmitter for use in a magnetic tracking system, the transmitter being powered by a power source, the transmitter comprising:

a plurality of resonant circuits, each of the plurality of resonant circuits having;

an autotransformer coil having a winding with a tap; and

a resonating capacitor electrically coupled in parallel with the autotransformer coil;

a plurality of digitally controllable switches, each of the plurality of digitally controllable switches being electrically coupled in series between the tap of a respective one of the autotransformer coil windings and one terminal of the power source, each of the plurality of resonant circuits having one end electrically coupled to the other terminal of the power source; and

a plurality of pulse generators, each of the plurality of pulse generators being operative to generate periodic digital pulses for controlling a respective one of the plurality of digitally controllable switches, thereby sustaining continuous, periodic, and substantially sinusoidal currents in the plurality of resonant circuits,

wherein, responsive to the respective continuous, periodic, and substantially sinusoidal currents, the transmitter coils are operative to generate a corresponding plurality of electromagnetic fields defining a reference coordinate system for the magnetic tracking system.

29. The transmitter of claim 28 wherein the winding of each autotransformer coil comprises bifilar wire.

30. The transmitter of claim 28 wherein the winding of each autotransformer coil comprises trifilar wire.

31. The transmitter of claim 28 further including a plurality of current limiting resistors, each of the plurality of current limiting resistors being coupled in series between the tap of a respective one of the autotransformer coil windings of the plurality of resonant circuits and the digitally controllable switch electrically coupled to the respective resonant circuit.

32. The transmitter of claim 28 further including a plurality of snubber networks, each of the plurality of snubber networks being electrically coupled in parallel with a respective one of the plurality of digitally controllable switches.

33. The transmitter of claim 32 wherein each of the plurality of snubber networks includes a resistor, and a capacitor coupled in series with the resistor.

34. The transmitter of claim 28 wherein the plurality of resonant circuits comprises three resonant circuits.

35. The transmitter of claim 34 wherein the respective autotransformer coils have a common center point and are mutually perpendicular to each other.

36. The transmitter of claim 28 wherein each of the autotransformer coils has coil turns disposed generally in a common plane, and wherein each of the autotransformer coils has a center displaced from the centers of the other autotransformer coils.

37. The transmitter of claim 36 wherein the centers of the respective autotransformer coils are arranged to form an equilateral triangle.

38. The transmitter of claim 36 wherein the centers of the respective autotransformer coils are arranged to form an isosceles triangle.

39. A coil assembly, comprising:

first and second substrates, each of the first and second substrates having an outer face, the first and second substrates being spaced apart and generally parallel to each other;

a plurality of spacing members mounted between the first and second substrates and defining a spacing between the spaced apart and generally parallel substrates, the first and second substrates and the plurality of spacers forming a frame; and

first, second, and third coils disposed around the frame such that each of the first, second, and third coils is generally orthogonal with respect to the other two coils.

40. The coil assembly of claim 39 wherein the first and second substrates are generally rectangular, and wherein the frame is a cuboid-like frame.

41. The coil assembly of claim 40 wherein the first coil is disposed around the plurality of spacing members such that the first coil is generally rectangular and extends around a periphery of the cuboid-like frame in an orientation generally parallel to the first and second substrates, and wherein each of the second and third coils is disposed around the first and second substrates such that the respective coil is generally rectangular and extends around the periphery of the cuboid-like frame.

42. The coil assembly of claim 39 wherein each of the first and second substrates has opposed notches, and wherein the first and second coils are disposed around the opposed notches such that portions of the respective coils cross one another on the outer faces of the respective substrates.

43. The coil assembly of claim 39 wherein at least one of the first and second substrates comprises a circuit board, wherein the coil assembly further includes transmitter circuitry disposed on the circuit board, and wherein the transmitter circuitry is electrically coupled to the first, second, and third coils for driving the respective coils.

44. The coil assembly of claim 43 further including a ferrite shield disposed between the outer face of at least one of the first and second substrates comprising the circuit board and the second and third coils adjacent thereto.

45. The coil assembly of claim 39 wherein the first and second substrates comprise first and second circuit boards, respectively, wherein each of the first and second circuit boards includes electronic circuitry disposed thereon, and wherein at least one of the first and second circuit boards includes transmitter circuitry electrically coupled to at least some of the first, second, and third coils.

46. The coil assembly of claim 39 wherein each of the first, second, and third coils includes a ribbon cable having first and second ends and a plurality of wires, wherein the first and second ends of the ribbon cable are interconnected such that the wires at the first end are connected to an adjacent wire at the second end to produce the respective coil, and wherein an unconnected wire on one side of the first end of the respective ribbon cable and an unconnected wire on the opposing side of the second end of the respective ribbon cable comprise ends of the respective coil.

47. The coil assembly of claim 39 wherein the first, second, and third coils each comprise a single wire wound around the frame.

48. A coil assembly with three orthogonal coils, comprising:

first, second, and third bobbins having first, second, and third coils disposed around the respective bobbins,

wherein the first bobbin is slidably disposed within a cooperative opening extending axially through the second bobbin so that the first bobbin and the first coil are nested within and generally orthogonal to the second bobbin and the second coil, and

wherein the nested first and second bobbins are slidably disposed through a cooperative opening extending axially through the third bobbin so that the first and second bobbins and the first and second coils are nested within the third bobbin, and each of the first, second, and third coils is orthogonal to the other two coils.

49. The coil assembly of claim 48 wherein the first, second, and third coils have a common center.

50. A method of calibrating a magnetic tracking system, the magnetic tracking system comprising a transmitter having a plurality of transmitter coils and at least one receiver having a plurality of receiver coils, the receiver being associated with a remote object, the plurality of transmitter coils for generating a plurality of electromagnetic fields in response to a plurality of excitation currents, respectively, thereby inducing a corresponding plurality of receiver voltages in each of the plurality of receiver coils, the plurality of excitation currents and the plurality of receiver voltages being substantially sinusoidal, the plurality of electromagnetic fields defining a reference coordinate system for the magnetic tracking system, the method comprising the steps of:

successively positioning the receiver and the remote object associated therewith at a plurality of random test locations within a 3-dimensional space, the plurality of electromagnetic fields producing corresponding pluralities of magnetic fields at the plurality of test locations, the plurality of magnetic fields at each test location being represented by three magnetic field vectors, the three magnetic field vectors at each test location defining an ellipsoid having an associated aspect ratio;

detecting, within a sampling window, the plurality of receiver voltages induced in each receiver coil by the plurality of excitation currents at each of the plurality of test locations, the sampling window having a start and a duration;

obtaining sine and cosine component amplitudes for each receiver voltage at each receiver coil;

using the sine and cosine component amplitudes for each receiver voltage, calculating a real phasor component and an imaginary phasor component for each receiver voltage induced in each receiver coil, each of the real and imaginary phasor components having an associated phase; and

using the real and imaginary phasor components for the plurality of receiver voltages detected at each of the plurality of test locations;

iteratively adjusting the phase associated with each of the real and imaginary phasor components for each receiver voltage to minimize the imaginary phasor component;

applying a calibration scale value to each real phasor component for each receiver voltage, the calibration scale value having an initial predetermined scale value; and

iteratively adjusting the calibration scale value applied to each real phasor component for each receiver voltage, wherein the magnetic tracking system is calibrated so as to adjust the aspect ratio of the ellipsoid defined by the three magnetic field vectors at the respective test location to at least approximately 2;

1;

1.

51. The method of claim 50 wherein the obtaining of the sine and cosine component amplitudes includes converting each of the plurality of receiver voltages induced in each receiver coil into a set of digital samples, and, for each set of digital samples, calculating an inner product of the respective set of digital samples and a predetermined set of basis functions, thereby obtaining the sine and cosine component amplitudes for each receiver voltage at each receiver coil.

52. The method of claim 50 further including:

deriving excitation frequencies for the respective excitation currents from a transmitter clock associated with the transmitter; and

deriving a sampling frequency for the receiver voltages induced in each receiver coil from a receiver clock associated with the receiver, the receiver clock being unsynchronized with the transmitter clock,

wherein the duration of the sampling window corresponds to an integer number of cycles of the sampling frequency,

wherein the plurality of excitation currents are periodic within the duration of the sampling window, and

wherein the duration of the sampling window represents a minimum time resolution for sensing a position and an orientation of the remote object using the magnetic tracking system.

53. The method of claim 50 further including:

deriving excitation frequencies for the respective excitation currents from a transmitter clock associated with the transmitter; and

deriving a sampling frequency for the receiver voltages induced in each receiver coil from a receiver clock associated with the receiver, the receiver clock being unsynchronized with the transmitter clock,

wherein the excitation frequencies correspond to different integer sub-multiples of a predetermined master clock frequency, the excitation frequencies having values that are in integer ratios to each other, and

wherein the sampling frequency corresponds to an integer sub-multiple of the predetermined master clock frequency.

54. The method of claim 50 further including calculating a time shift between a common zero crossing of the plurality of receiver voltages induced in each receiver coil by the plurality of excitation currents at each of the plurality of test locations and the start of the sampling window, and wherein the calculating of the real and imaginary phasor components includes calculating the real and imaginary phasor components for each receiver voltage induced in each receiver coil using the sine and cosine component amplitudes for each receiver voltage and taking into account the time shift.

55. The method of claim 54 wherein the calculating of the time shift includes calculating the time shift between the common zero crossing of the plurality of receiver voltages and the start of the sampling window at a time when the plurality of receiver voltages transition from negative to positive.

56. A coil assembly for use in a magnetic tracking system, comprising:

a generally flat transmitter housing having a top surface generally defining a top surface plane; and

first, second, and third transmitter coils fixedly positioned within the transmitter housing, wherein each of the transmitter coils has a center and coil turns that generally lie in a plane substantially parallel to the top surface plane, and wherein the center of each transmitter coil is displaced from the centers of the other two transmitter coils.

57. The coil assembly of claim 56 wherein the vertices of the first, second, and third transmitter coils form an equilateral triangle.

58. The coil assembly of claim 56 wherein the vertices of the first, second, and third transmitter coils form an isosceles triangle.

59. The coil assembly of claim 56 wherein the substrate has a home plate-like shape.

60. The coil assembly of claim 56 further including transmitter circuitry disposed within the housing and operative to drive the first, second, and third transmitter coils to produce three electromagnetic fields.

61. The coil assembly of claim 56 wherein the transmitter housing has beveled edges.

62. The coil assembly of claim 56 wherein each of the transmitter coils comprises wound coils.

63. The coil assembly of claim 56 wherein each of the transmitter coils comprises printed coils.

64. A method of sensing a position and orientation of a remote object, for use in a system comprising a transmitter having first, second, and third transmitter coils and at least one receiver having first, second, and third receiver coils, the receiver being associated with the remote object, the first, second, and third transmitter coils for generating a plurality of electromagnetic fields in response to a plurality of excitation currents, respectively, thereby inducing a corresponding plurality of receiver voltages in each of the first, second, and third receiver coils, the first, second, and third transmitter coils being modeled as a plurality of ideal dipoles generating a corresponding plurality of magnetic fields, the method comprising the steps of:

calculating a plurality of magnetic field vectors representing the plurality magnetic fields generated by the first, second, and third transmitter coils modeled as the plurality of ideal dipoles, wherein the first, second, and third transmitter coils are contained in a generally flat transmitter housing having a top surface generally defining a top surface plane, wherein each of the first, second, and third transmitter coils has a center and coil turns that generally lie in a plane substantially parallel to the top surface plane, and wherein the center of each of the first, second, and third transmitter coil is displaced from the centers of the other two transmitter coils; and

using the plurality of magnetic field vectors and a specified spatial triangulation technique, calculating at least an approximate position and orientation of the receiver and the remote object associated therewith.

65. The method of claim 64 wherein the center of each transmitter coil conceptually corresponds to a center of a sphere, and wherein the calculating of the at least an approximate position and orientation includes determining a point of intersection of the three spheres centered at the three centers of the respective first, second, and third transmitter coils, the point of intersection corresponding to the approximate position of the receiver and the remote object associated therewith.