Inertial orientation tracker apparatus having automatic drift compensation for tracking human head and other similarly sized body

• US 5,645,077 A
• Filed: 06/16/1994
• Issued: 07/08/1997
• Est. Priority Date: 06/16/1994
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Foxlin, Eric M.
• Primary Examiner(s): RIMELL, SAMUEL G
• Application Number:
  US08/261,364
• Time in Patent Office: 1,118 Days
• Field of Search: 128/774, 128/782, 73/505, 364/453, 600/26, 33/333, 33/355 R
• US Class Current: 600/587
• CPC Class Codes: A61B 2562/0219 Inertial sensors, e.g. accelerometers, gyroscopes, tilt switches
• CPC Class Codes: A61B 5/1114 Tracking parts of the body
• CPC Class Codes: A61B 5/6814 Head
• CPC Class Codes: A61B 5/6825 Hand
• CPC Class Codes: A61B 5/7242 using integration

Abstract

A self contained sensor apparatus generates a signal that corresponds to at least two of the three orientational aspects of yaw, pitch and roll of a human-scale body, relative to an external reference frame. A sensor generates first sensor signals that correspond to rotational accelerations or rates of the body about certain body axes. The sensor may be mounted to the body. Coupled to the sensor is a signal processor for generating orientation signals relative to the external reference frame that correspond to the angular rate or acceleration signals. The first sensor signals are impervious to interference from electromagnetic, acoustic, optical and mechanical sources. The sensors may be rate sensors. An integrator may integrate the rate signal over time. A drift compensator is coupled to the rate sensors and the integrator. The drift compensator may include a gravitational tilt sensor or a magnetic field sensor or both. A verifier periodically measures the orientation of the body by a means different from the drift sensitive rate sensors. The verifier may take into account characteristic features of human motion, such as stillness periods. The drift compensator may be, in part, a Kalman filter, which may utilize statistical data about human head motion.

Claims

1. An apparatus to be attached to a body having a size between the sizes of a human finger and a human torso, said body having an orientation, relative to an external reference frame, said apparatus generating an orientation signal that corresponds to at least two degrees of freedom of said orientation of said body, said apparatus comprising:

a. a self contained sensor that generates first sensor signals that correspond to x^th order integrals of rotational accelerations of said body about at least two axes of said body, x being selected from the group consisting of 0, 1 and 2 and said first sensor signals being impervious to any interference from electromagnetic, acoustic, optical and mechanical sources;

b. a mechanism for mounting said sensor to said body; and

c. coupled to said sensor, a signal processor that generates an orientation signal that corresponds to said at least two degrees of freedom of said orientation of said body.

2. The apparatus of claim 1, said sensor comprising angular rate sensors and the order x being equal to 1 and said first sensor signals corresponding to rotational rates of said body.

3. The apparatus of claim 1, said sensor comprising gyroscopes and the order x being equal to 2 and said first sensor signals corresponding to said at least two degrees of freedom of said orientation of said body.

4. The apparatus of claim 2, said angular rate sensors comprising three angular rate sensors arranged to span three space.

5. The apparatus of claim 4, said signal processor comprising an integrator that integrates said rotational rate signals with respect to time and generates a signal that corresponds to orientation.

6. The apparatus of claim 3, said gyroscopes being arranged to comprise one vertical gyroscope and one directional gyroscope.

7. The apparatus of claim 5, further comprising a sensor error compensator, coupled to said angular rate sensors and said integrator, that compensates for errors such as bias and non-linearity in said rotational rate signals that may arise over time.

8. The apparatus of claim 5, further comprising a drift compensator, coupled to said angular rate sensors and said integrator, that compensates for any drift over time in said orientation signal.

9. The apparatus of claim 8, said drift compensator comprising at least one gravitational tilt sensor.

10. The apparatus of claim 8, said drift compensator comprising at least one magnetic field sensor.

11. The apparatus of claim 10, said magnetic field sensor comprising a compass.

12. The apparatus of claim 9, said drift compensator further comprising at least one magnetic field sensor.

13. The apparatus of claim 9, said gravitational tilt sensor comprising a two axis fluid inclinometer.

14. The apparatus of claim 4, said angular rate sensors comprising a vibrating piezoelectric apparatus.

15. The apparatus of claim 5, further comprising an orientation predictor, coupled to said integrator and to said angular rate sensors.

16. The apparatus of claim 2, said first sensor weighing less than 300 grams.

17. The apparatus of claim 2, said mounting means comprising a human head mount.

18. The apparatus of claim 2, said mounting means comprising a human hand mount.

19. The apparatus of claim 8, said drift compensator comprising a verifier that periodically measures said orientation of said body by a means different from using said rotational rate signal and generates an orientation drift compensation signal based on results of said verification measurement to reduce the effect of drift.

20. The apparatus of claim 19, said verifier comprising means for relying upon characteristic features of human motion.

21. The apparatus of claim 20, said means for relying upon characteristic features of human motion comprising an at rest sensor that identifies any occurrence of periods exceeding a predetermined duration when the body is substantially at rest relative to said reference frame.

22. The apparatus of claim 21, said at rest sensor comprising:

a. a timer;

b. a signal generator that generates signals that correspond to;

i. nongravitational acceleration and;

ii. rotation of the body;

c. means for storing and comparing said nongravitational acceleration and rotation signals at several points over time; and

d. a signal generator that generates an at rest signal if said nongravitational acceleration and rotation signals are both constant over a preselected time period.

23. The apparatus of claim 2, further comprising means for coupling said orientation signal to a receiver that is spaced away from said self contained sensor.

24. The apparatus of claim 23, said coupling means comprising a wireless coupling means.

25. The apparatus of claim 2, said signal processor being located spaced away from said self contained sensor, further comprising wireless means for coupling said self contained sensor to said signal processor.

26. The apparatus of claim 2, said body also having a position relative to said external reference frame, further comprising a position sensor that determines said position of said body.

27. The apparatus of claim 2, said angular rate sensors comprising spinning wheel rate gyros.

28. The apparatus of claim 4, said three angular rate sensors being selected from the group consisting of a silicon micro machined sensor assembly, a magneto-hydrodynamic device and a micro-optical gyroscope assembly.

29. The apparatus of claim 19, said drift compensator comprising a Kalman filter.

30. The apparatus of claim 9, said gravitational tilt sensor comprising at least 2 accelerometers.

31. An apparatus to be attached to a body having a size between the sizes of a human finger and a human torso, said body having an orientation, relative to a reference frame, said apparatus generating an orientation signal that corresponds to at least two degrees of freedom of orientation of said body, said apparatus comprising:

a. a self contained sensor that generates a rotational rate signal that corresponds to at least two degrees of freedom of rotational rate of said body and, which signal is impervious to interference from electromagnetic, acoustic, optical and mechanical sources;

b. a mounting mechanism for mounting said self contained sensor to said body; and

c. coupled with said self contained sensor, an integrator that integrates said rotational rate signal over time to generate said orientation signal.

32. The apparatus of claim 31, further comprising, coupled with said mounting mechanism, a gravity-referenced tilt sensor that generates a tilt orientation signal that corresponds to two degrees of freedom of said orientation of said body.

33. The apparatus of claim 32, further comprising, coupled with said mounting mechanism, a magnetic field sensor that generates a signal that corresponds to a third degree of freedom of orientation of said body.

34. The apparatus of claim 33, further comprising means for substituting said tilt orientation signal and said signal that corresponds to a third degree of freedom for said orientation signal generated by said integrator, at selected times.

35. The apparatus of claim 34, further comprising a verifier that determines the time when said body is at rest relative to said reference frame, and further comprising a processor that determines the occurrence of said selected times, based on said at rest time.

36. An apparatus to be attached to a body having a size between the sizes of a human finger and a human torso, said body having an orientation, relative to a reference frame, said apparatus generating an orientation signal that corresponds to at least two degrees of freedom of said orientation of said body, said apparatus comprising:

a. a self contained first sensor that generates a drift sensitive orientation signal that corresponds to said at least two degrees of freedom of orientation of said body and is impervious to interference from electromagnetic, acoustic, optical and mechanical sources and is subject to drift over time;

b. a second sensor that generates a drift compensating orientation signal that corresponds to said at least two degrees of freedom of orientation of said body and is relatively impervious to drift over time;

c. a mounting mechanism for mounting said first sensor and said second sensor to said body; and

d. coupled to said first sensor and said second sensor, a signal corrector that generates a corrected orientation signal based on said drift sensitive and drift compensating orientation signals.

37. The apparatus of claim 36, said signal corrector comprising means to take into account characteristic features of human motion.

38. The apparatus of claim 37, said means to take into account characteristic features of human motion comprising means to determine times when said body has been at rest for more than a predetermined period of time.

39. The apparatus of claim 38, said signal corrector further comprising means to apply said drift compensating signal generated at times when said body has been at rest for more than said predetermined period of time to correct said drift sensitive signal.

40. An apparatus for simulating a virtual environment that is displayed to a user by an appropriate display device, said apparatus comprising:

a. a self contained orientation sensor to be attached to a body member of said user that has an orientation and a position relative to a reference frame, said sensor generating an orientation signal that;

i. corresponds to at least two degrees of freedom of said orientation of said body member; and

ii. is impervious to interference from electromagnetic, acoustic, optical and mechanical sources;

b. a position sensor that generates a position signal that corresponds to said position of said body member;

c. a mechanism for mounting said position and orientation sensors to said body member;

d. a virtual environment means for generating virtual environment signals that define, in part, a virtual environment;

e. a display that displays said virtual environment signals to said user; and

f. means for coupling said position sensor and said orientation sensor to said virtual environment means.

41. The apparatus of claim 40, said orientation sensor comprising:

a. a first sensor that generates a drift sensitive orientation signal that corresponds to said at least two degrees of freedom of orientation and is subject to drift over time;

b. a second sensor that generates a drift compensating orientation signal that corresponds to said at least two degrees of freedom of orientation and which is relatively impervious to drift over time; and

c. coupled to said first sensor and said second sensor, a signal corrector that generates a corrected orientation signal based on said drift sensitive and drift compensating orientation signals.

42. The apparatus of claim 41, said signal corrector comprising means to take into account characteristic features of human motion.

43. The apparatus of claim 42, said means to take into account characteristic features of human motion comprising means to determine times when said body has been at rest for more than a predetermined period of time.

44. The apparatus of claim 43, said signal corrector further comprising means to apply said drift compensating signal generated at times when said body has been at rest for more than said predetermined period of time to correct said drift sensitive signal.