Magnetic field position and orientation measurement system with dynamic eddy current rejection

• US 5,767,669 A
• Filed: 06/14/1996
• Issued: 06/16/1998
• Est. Priority Date: 06/14/1996
• Status: Expired due to Term

• Current Assignee: Ascension Technology Corporation (Roper Technologies, Inc.)
• Original Assignee: Ascension Technology Corporation (Roper Technologies, Inc.)
• Inventors: Ashe, Westley S., Hansen, Per Krogh
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US08/663,774
• Time in Patent Office: 732 Days
• Field of Search: 324/207.12, 324/207.17, 324/207.26, 324/225, 324/239, 324/243, 324/249, 324/326, 324/329, 342/450, 342/451, 342/453, 340/870.31, 364/559, 364/449.1
• US Class Current: 324/207.12
• CPC Class Codes: F41G 3/225 Helmet sighting systems
• CPC Class Codes: G01B 7/004 for measuring coordinates of points

Abstract

The position and orientation of remote sensors is determined using pulsed magnetic fields generated from a fixed location. Pulsed magnetic fields are sequentially generated from a plurality of spatially independent components defining a source coordinate frame. The pulsed magnetic fields are sensed by a remote sensor having a plurality of passive field sensing elements. Eddy current distortions are sensed separately and subtracted by the system. The system measures the effect of metallic objects present in the environment and dynamically adjusts the measured values accordingly. The sensed magnetic fields, free of eddy current distortion, are used in order to calculate the position and orientation of the remote object. The system shortens the duration of pulsed magnetic fields and operates correctly at much further distances for the same energy output as compared to known systems.

Claims

1. A position and orientation measurement system operating in a space and including a fixed source of successive magnetic field pulses and a magnetic sensor movable within said space and adapted to sense rate-of-change of a magnetic field generated by each of said pulses, and including:

a) eddy current distortion detection means for sensing when a magnetic field generated by eddy currents has dissipated and, responsive to said sensing, stopping transmission of each of said magnetic field pulses, eddy current distortion being caused by eddy currents generated due to presence of metallic material adjacent to and/or in said space;

b) compensation means for dynamically compensating for eddy current distortion including integration means for integrating a signal received from said magnetic sensor from a start of each said magnetic field pulse and reading means for reading an output of said integration means just after said eddy current distortion detection means has sensed dissipation of an eddy current; and

c) computer means for controlling receiving of signals, free of eddy current distortion, from said magnetic sensors and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

2. The system of claim 1, wherein integrated signals are proportional to said magnetic field, said integrator means supplying said integrated signals to a computer means.

3. The system of claim 2, further including an amplifier interposed between said magnetic sensor and said integration means.

4. The system of claim 2, further including a comparator connected between said magnetic sensor and said computer means, said comparator comparing voltage output of said sensor with a reference voltage and emitting a comparator signal to said computer means when said sensor voltage output reduces below said reference voltage.

5. The system of claim 4, wherein said sensor voltage output includes a component related to said eddy current distortion, said reference voltage being set at a level whereby when said sensor voltage output drops below said reference voltage, said component has substantially dissipated.

6. The system of claim 5, further wherein said computer means has a comparator signal port receiving said comparator signal, said comparator signal terminating when said sensor voltage output drops below said reference voltage, said computer means, in response to termination of said comparator signal, stopping emission of said magnetic field.

7. The system of claim 6, wherein said magnetic field includes a time component of rising energy followed by a time component of steady state energy followed by a time component of falling energy, stopping of said magnetic field operatively shortening said steady state energy time component.

8. The system of claim 1, wherein said source of magnetic field comprises three mutually orthogonal sources, said magnetic sensor comprises a first passive coil, and further including second and third passive coils, said coils being mutually orthogonal and mounted on a movable remote unit.

9. The system of claim 8, wherein said computer means, responsive to said sensing, adjusts duration of each said magnetic field.

10. The system of claim 8, wherein said sources are sequentially pulsed, said computer means, responsive to said sensing, adjusting time spacing between successive pulses.

11. The system of claim 1, wherein said compensation means includes means for compensating for electronic and rotation offsets.

12. A position and orientation measurement system operating in a space and including a fixed source of successive magnetic field pulses and a magnetic sensor movable within said space and adapted to sense rate-of-change of a magnetic field generated by each of said pulses, said magnetic field having a time component of rising energy followed by a time component of falling energy, said system comprising:

a) eddy current distortion detection means for sensing when a magnetic field generated by said eddy currents has dissipated and, responsive to said sensing, controlling a time between successive ones of said magnetic field pulses, said eddy currents being generated due to presence of metallic material adjacent to and/or in said space;

b) compensation means for dynamically compensating for eddy current distortion generated responsive to activation of said source of successive magnetic field pulses, including means for performing a first integration of a signal from said magnetic sensor from a start of a said magnetic pulse and reading a first output of said first integration when said magnetic pulse has reached its maximum value and performing a second parallel integration of said signal from said magnetic sensor from the start of said magnetic pulse and reading a second output of said second integration, when said magnetic pulse has ended; and

c) computer means for controlling receiving of said two outputs and subtracting said second output from said first output and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

13. The system of claim 12, further including an integrator connected to receive and integrate signals received from said magnetic sensor, said integrated signals being proportional to said magnetic field generated from said fixed source of magnetic pulses and/or the eddy current distortion, said integrator supplying said integrated signals to said computer means.

14. The system of claim 13, further including a comparator connected between said magnetic sensor and said computer means, said comparator comparing voltage output of said sensor with a reference voltage and emitting a comparator signal to said computer means when said sensor voltage output goes below said reference voltage.

15. The system of claim 14, wherein said sensor voltage output includes a component related to said eddy current distortion, said reference voltage being set at a level whereby when said sensor voltage output drops below said reference voltage, said component has substantially dissipated.

16. The system of claim 15, further wherein said computer means has a comparator signal port receiving said comparator signal, said comparator signal terminating when said sensor voltage output drops below said reference voltage, said computer means, in response to termination of said comparator signal, stopping emission of said magnetic field.

17. The system of claim 12, wherein said source of magnetic field pulses comprises three mutually orthogonal sources, said magnetic sensor comprises a first passive coil, and further including second and third passive coils, said passive coils being mutually orthogonal and mounted on a movable unit.

18. A method of determining the position and orientation of a magnetic sensor within a space including the steps of:

a) positioning said sensor within said space;

b) emitting successive magnetic field pulses within said space;

c) receiving signals from said sensor related to (i) rate-of-change of a magnetic field generated by each of said pulses, and (ii) eddy current distortion generated due to presence of metallic material adjacent to and/or in said space;

d) detecting when the magnetic field due to said eddy currents has dissipated and, responsive to said detection, stopping transmission of each of said magnetic field pulses;

e) dynamically compensating for said eddy current distortion by integrating a signal from said magnetic sensor from a start of each of said pulses and reading output of said integration just after said eddy current has dissipated; and

f) controlling receiving of said signals, free of eddy field distortion, from said magnetic sensors and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

19. The method of claim 18, further including the step of comparing said signals with a reference signal and stopping emission of each of said magnetic field pulses when amplitude of said signals falls below amplitude of said reference signal.

20. The method of claim 19, further including the step of detecting existence of metals in and/or adjacent said space when said amplitude of said signals falls below said amplitude of said reference signal.

21. A position and orientation measurement system operating in a space and including a fixed source of successive magnetic field pulses and a magnetic sensor movable within said space and adapted to sense rate-of-change of a magnetic field generated by each of said magnetic field pulses, each of said magnetic field pulses having a time component of rising energy followed by a brief time component of steady state energy followed by a time component of falling energy including:

a) eddy current distortion detection means for sensing when a magnetic field generated by said eddy currents has dissipated and, responsive to said sensing, controlling time spacing between successive ones of said magnetic field pulses, said eddy currents being generated due to presence of metallic material adjacent to and/or in said space;

b) compensation means for dynamically compensating for eddy current distortion including means for performing a first integration of a signal received from said magnetic sensor from a start of each of said pulses and reading a first output of said first integration at a start of said time component of steady state energy and means for performing a second parallel integration of said signal from said magnetic sensor from a start of the time component of steady state energy and reading a second output of said second integration at an end of the time component of steady state energy; and

c) computer means for controlling receiving of said first and second outputs, subtracting said second output multiplied by a proportionality factor from said first output and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

22. The system of claim 21, further including an integrator connected to receive and integrate signals received from said magnetic sensor, said integrated signals being proportional to said magnetic field generated from said fixed source of magnetic pulses, said integrator supplying said integrated signals to said computer means.

23. The system of claim 22, further including a comparator connected between said magnetic sensor and said computer means, said comparator comparing voltage output of said sensor with a reference voltage and emitting a comparator signal to said computer means when said sensor voltage output goes below said reference voltage.

24. The system of claim 23, wherein said voltage output includes a component related to said eddy current distortion, said reference voltage being set at a level whereby when said sensor voltage output drops below said reference voltage, said component has substantially dissipated.

25. The system of claim 24, further wherein said computer means has a comparator signal port receiving said comparator signal, said comparator signal terminating when said sensor voltage output drops below said reference voltage, said computer means, in response to termination of said comparator signal, adjusting spacing of emission of said magnetic field pulses.

26. The system of claim 25, wherein said computer means, responsive to said sensing, adjusts duration time between each of said magnetic field pulses.

27. The system of claim 26, wherein said source comprises a plurality of sources of magnetic field pulses, said sources being sequentially pulsed, said computer means, responsive to said sensing, adjusting time spacing between successive pulses.

28. The system of claim 24, further including a metal detector sub-circuit receiving signals from said sensor and integrating them to determine effects of eddy current distortion caused by adjacent metallic material.

29. The system of claim 21, wherein said compensation means includes a memory preprogrammed with equations concerning typical eddy current distortion decay curves and said proportionality factor for various metallic materials.

30. The system of claim 21, wherein said compensation means includes a memory preprogrammed with a look-up table listing decay times of eddy currents and said proportionality factor caused by presence of various metallic materials adjacent said space.

31. A position and orientation measurement system operating in a space and including a fixed source of successive magnetic field pulses and a magnetic sensor movable within said space and adapted to sense rate-of-change of a magnetic field generated by each of said pulses, said magnetic field having a time component of rising energy followed by a time component of falling energy including:

a) eddy current distortion detection means sensing when a magnetic field generated by eddy currents has dissipated and, responsive to said sensing, controlling a time interval between successive ones of said magnetic field pulses, said eddy currents being generated due to presence of metallic material adjacent to and/or in said space;

b) compensation means for dynamically compensating for eddy current distortion generated responsive to activation of said source of magnetic field pulses, said compensation means performing a first integration of a signal from said magnetic sensor from a start of each of said magnetic pulses and performing a second integration of an output of said first integration from the start of each of said magnetic pulses and reading an output of said second integration when said eddy current distortion detection means detects that eddy current distortion has dissipated; and

c) computer means for controlling receiving of signals free of eddy field distortion and, therefrom, calculating the position and orientation of said sensor relative to said fixed source.

32. The system of claim 31, further including an integrator connected to receive and integrate signals received from said magnetic sensor, integrated signals being proportional to said magnetic field generated from said fixed source of magnetic field pulses and/or said eddy current distortion, said integrator supplying said integrated signals to said computer means.

33. The system of claim 32, further including a comparator connected between said magnetic sensor and said computer means, said comparator comparing voltage output of said sensor with a reference voltage and emitting a comparator signal to said computer means when said sensor voltage output goes below said reference voltage.

34. The system of claim 33, wherein said sensor voltage output includes a component related to said eddy current distortion, said reference voltage being set at a level whereby when said sensor voltage output drops below said reference voltage, said component has substantially dissipated.

35. The system of claim 34, further wherein said computer means has a comparator signal port receiving said comparator signal, said comparator signal terminating when said sensor voltage output drops below said reference voltage, said computer means, in response to termination of said comparator signal, stopping emission of said magnetic field pulses.

36. The system of claim 31, wherein said source of magnetic field pulses comprises three mutually orthogonal sources, said magnetic sensor comprises a first passive coil, and further including second and third passive coils, said coils being mutually orthogonal and mounted on a movable unit.

37. The system of claim 31, further wherein said compensation means includes a first integrator for performing said first integration, said system further including a second integrator that receives integrated signals from said first integrator and integrates said integrated signals and supplies doubly integrated signals to said computer means.

38. The system of claim 31, wherein said magnetic field has no steady state component, said doubly integrated signals having no eddy current component.

39. A position and orientation measurement system operating in a space and including a fixed source of successive magnetic field pulses and a magnetic sensor movable within said space and adapted to sense rate-of-change of a magnetic field generated by each of said magnetic field Pulses, said magnetic field having a time component of rising energy followed by a time component of falling energy, said system including:

a) eddy current distortion detection means sensing when magnetic field generated by eddy currents has dissipated and, responsive to said sensing, controlling time between successive ones of said magnetic field pulses, said eddy currents being generated due to presence of metallic material adjacent to and/or in said space;

b) compensation means for dynamically compensating for eddy current distortion generated responsive to said source of magnetic field, said compensation means performing a first sample-and-hold of a signal received from said magnetic sensor just before each respective one of said magnetic field pulses reaches its maximum energy and performing a second sample-and-hold of said signal from said magnetic sensor just after said respective magnetic pulse has started to drop from its maximum energy; and

c) computer means for controlling receiving of data resulting from said first and second sampling and holding of said signal and subtracting data from said second sample and hold from data from said first sample and hold signal and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

40. The system of claim 39, further including a comparator connected between said magnetic sensor and said computer means, said comparator comparing voltage output of said sensor with a reference voltage and emitting a comparator signal to said computer means when said sensor voltage output goes below said reference voltage.

41. The system of claim 40, wherein said sensor voltage output includes a component related to said eddy current distortion, said reference voltage being set at a level whereby when said sensor voltage output drops below said reference voltage, said component has substantially dissipated.

42. The system of claim 41, further wherein said computer means has a comparator signal port receiving said comparator signal, said comparator signal terminating when said sensor voltage output drops below said reference voltage, said computer means, in response to termination of said comparator signal, stopping emission of said magnetic field pulses.

43. The system of claim 39, wherein said magnetic sensor signals are also supplied to a magnetic sensor sample-and-hold sub-circuit that measures a time derivative of said sensor signals and outputs a sensor time derivative signal to said computer means.

44. The system of claim 43, further including a metal detector sample-and-hold sub-circuit that receives said sensor signals and measures time derivative of a portion of the sensor signals related solely to eddy current distortion.

45. A position and orientation measurement system operating in a space and including a fixed source of successive magnetic field pulses and a magnetic sensor movable within said space and adapted to sense rate-of-change of a magnetic field generated by each of said magnetic field pulses, said magnetic field having a time component of rising energy followed by a time component of falling energy, said system comprising:

a) eddy current distortion detection means for sensing when a magnetic field generated by said eddy currents has dissipated and, responsive to said sensing, controlling a time interval between successive ones of said magnetic field pulses, said time interval occurring before said eddy current has fully dissipated;

b) compensation means for dynamically compensating for eddy current distortion generated responsive to said source of magnetic field, said compensation means performing a first integration of a signal from said magnetic sensor from a start of each of said magnetic field pulses and said compensation means reading an output of said first integration when said magnetic pulse has reached its maximum value and performing a second parallel integration of said signal from said magnetic sensor from the start of said magnetic pulse and reading an output of said second integration, when said magnetic pulse has concluded; and

c) computer means for controlling receiving of said two outputs and subtracting said second output from said first output and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

46. A method of determining the position and orientation of a magnetic sensor within a space including the steps of:

a) positioning said sensor within said space;

b) emitting successive magnetic field pulses having a time component of rising energy followed by a brief time component of steady state energy followed by a time component of falling energy;

c) receiving signals from said sensor related to (i) rate-of-change of a magnetic field generated by each of said magnetic field pulses, and (ii) eddy current distortion generated due to presence of metallic material adjacent to and/or in said space;

d) detecting when a magnetic field generated by said eddy current has dissipated and, responsive to said detection, stopping transmission of each of said magnetic field pulses;

e) compensating dynamically for eddy current distortion generated responsive to operation of said source of said magnetic field pulses, including the step of performing a first integration of a signal from said magnetic sensor from a start of each successive one of said magnetic pulses and reading a first output of said first integration at a start of the steady state and performing a second integration of said signal from said magnetic sensor from a start of steady state and reading a second output of said integration at an end of the steady state;

f) controlling receiving of said outputs resulting from said integrations, subtracting said second output multiplied by a proportionality factor from said first output and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

47. A method of determining the position and orientation of a magnetic sensor within a space including the steps of:

a) positioning said sensor within said space;

b) emitting successive magnetic field pulses having a time component of rising energy followed by a time component of falling energy, within said space;

c) receiving signals from said sensor related to (i) rate-of-change of a magnetic field generated by successive ones of said magnetic field pulses, and (ii) eddy current distortion generated due to presence of metallic material adjacent to and/or in said space;

d) detecting when a magnetic field portion generated by eddy currents has dissipated and, responsive to said detection, stopping transmission of each of said magnetic field pulses;

e) dynamically compensating for eddy current distortion generated responsive to operation of said source of magnetic field pulses, by performing a first integration of a signal from said magnetic sensor from a start of each of said magnetic field pulses and reading a first output of said first integration when each of said magnetic field pulses has reached its maximum value and performing a second integration of said signal from said magnetic sensor from a start of each of said magnetic field pulses and reading a second output of said second integration, when said magnetic pulse has concluded;

f) controlling receiving of said two outputs and subtracting said second output from said first output and thereby forming a signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

48. A method of determining the position and orientation of a magnetic sensor within a space including the steps of:

a) positioning said sensor within said space;

b) emitting successive magnetic field pulses each having a time component of rising energy followed by a time component of falling energy, within said space;

c) receiving signals from said sensor related to (i) rate-of-change of a magnetic field generated by each of said magnetic field pulses, and (ii) eddy current distortion generated due to presence of metallic material adjacent to and/or in said space;

d) detecting when a magnetic field generated by said eddy current has dissipated and, responsive to said detection, stopping transmission of each of said magnetic field pulses;

e) dynamically compensating for eddy current distortion generated responsive to said source of magnetic field, by performing a first integration of a signal from said magnetic sensor from a start of each of said magnetic field pulses and reading a first output, and performing a second integration of said first output of said first integration from said start of each of said magnetic field pulses and reading a second output of said second integration when said eddy current distortion has dissipated;

f) controlling receiving of said second output, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

49. A method of determining the position and orientation of a magnetic sensor within a space including the steps of:

a) positioning said sensor within said space;

b) emitting successive magnetic field pulses each having a time component of rising energy followed by a time component of falling energy, within said space;

c) receiving signals from said sensor related to (i) rate-of-change of each of said magnetic field pulses, and (ii) eddy current distortion generated due to presence of metallic material adjacent to and/or in said space;

d) detecting when a magnetic field generated by said eddy current has dissipated and, responsive to said detection, stopping transmission of each of said magnetic field pulses;

e) dynamically compensating for said eddy current distortion generated responsive to said source of magnetic field pulses, by performing a first sample-and-hold of a signal from said magnetic sensor just before a magnetic field pulse reaches its maximum energy and performing a second sample-and-hold of said signal just after said magnetic field pulse has started to drop from its maximum energy;

f) controlling receiving of said data resulting from said first and second sampling and holding of said signal and subtracting said data from said second sample-and-hold from data from said first sample-and-hold and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.

50. A method of determining the position and orientation of a magnetic sensor within a space including the steps of:

a) positioning said sensor within said space;

b) emitting successive magnetic field pulses having a time component of rising energy followed by a time component of falling energy, within said space;

c) receiving signals from said sensor related to (i) rate-of-change of a magnetic field generated by each of said magnetic field pulses, and (ii) eddy current distortion generated due to presence of metallic material adjacent to and/or in said space;

d) sensing when a magnetic field generated by eddy current has dissipated and, responsive to said sensing, controlling a time interval between successive ones of said magnetic field pulses, said time interval being before said eddy current has fully dissipated;

e) dynamically compensating for eddy current distortion generated responsive to said source of magnetic field pulses, by performing a first integration of a signal from said magnetic sensor from a start of a said magnetic field pulse and reading a first output of said first integration when said magnetic field pulse has reached its maximum value and performing a second integration of said signal from said magnetic sensor from a start of said magnetic field pulse and reading a second output of said second integration, when said magnetic field pulse has concluded;

f) controlling receiving of said outputs and subtracting said second output from said first output and thereby forming a net signal, free of eddy field distortion and, therefrom, calculating position and orientation of said sensor relative to said fixed source.