Inertial orientation tracker having automatic drift compensation using an at rest sensor for tracking parts of a human body

• US 20030023192A1
• Filed: 12/18/2001
• Published: 01/30/2003
• Est. Priority Date: 06/16/1994
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Foxlin, Eric M.
• Granted Patent: US 6,786,877 B2
• Time in Patent Office: Days
• US Class Current: 600/595
• 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 sate 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 for generating 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, said apparatus comprising:

a. a self contained sensor for generating first sensor signals that correspond to x^th order integrals of the rotational accelerations of said body about certain axes relative to said body, where x is between and includes 1 and 2;

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

c. coupled to said sensor, a signal processor for generating orientation signals relative to said external reference frame that correspond to said at least two orientational aspects from said x^th order integral signals,

wherein the first sensor signals are impervious to interference from electromagnetic, acoustic, optical and mechanical sources.

2. The apparatus of claim 1, wherein said sensor comprises angular rate sensors and the order x is equal to 1 and said sensor signals correspond to rotational rates of said body.

3. The apparatus of claim 1, wherein said sensor comprises gyroscopes and the order x is equal to 2 and said sensor signals correspond to aspects of rotational orientation of said body.

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

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

6. The apparatus of claim 3, wherein said gyroscopes are 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, for compensating 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 sensor and said integrator, for compensating for any drift with respect to time in said rotational 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 sensor 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 means for mounting said signal generator to a human head.

18. The apparatus of claim 2, said mounting means comprising means for mounting said signal generator to a human hand.

19. The apparatus of claim 8, said drift compensator comprising a verifier that periodically measures the rotational orientation of said body by a means different from using said rotational rate signal and generates a rotational orientation drift compensation signal based on the 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 for determining the occurrence of periods exceeding a predetermined time 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. means for generating 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;

d. means for generating an “

at rest”

signal if the 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 signal first sensor.

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

25. The apparatus of claim 2, wherein said signal processor is located spaced away from said signal generator, further comprising wireless means for coupling said first sensor to said signal processor.

26. The apparatus of claim 2, further comprising a position sensor for determining the location relative to said reference frame of said body.

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

28. The apparatus of claim 4, wherein said three angular rate sensors are selected from the group 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 for generating a signal that corresponds to the orientation of a human-scale body, relative to a reference frame, said apparatus comprising:

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

b. a mounting mechanism for mounting said sensing means to said body; and

c. coupled with said first sensor, an integrator for integrating said rotational rate signal over time to generate an orientation signal.

32. The apparatus of claim 31, further comprising, coupled with said mounting mechanism, a gravity-referenced tilt sensor for measuring the orientation of the body with respect to two orthogonal axes of said reference frame and generating a tilt orientation signal relative to said two orientation axes.

33. The apparatus of claim 32, further comprising a compass, coupled with said mounting mechanism, for measuring the orientation of the body with respect to a third axis of said reference frame, orthogonal to said two orthogonal axes, and generating a yaw orientation signal relative to said third orientation axis.

34. The apparatus of claim 33, further comprising means for substituting the tilt and yaw orientation signals for the orientation signal generated by said integrator at selected times.

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

36. An apparatus for generating a signal that corresponds to the orientation of a human-scale body, relative to a reference frame, said apparatus comprising:

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

b. a self contained second sensor for generating a drift compensating orientation signal that corresponds to the rotational orientation with respect to said at least two degrees of freedom of said body and is impervious to interference from electromagnetic, acoustic, optical and mechanical sources and which 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 means for generating a corrected rotational orientation signal based on said drift sensitive and drift compensating orientation signals.

37. The apparatus of claim 36 said signal corrector means 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 corrector means 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. A method for generating a signal that corresponds to the orientation of a human-scale body, relative to a reference frame, said method comprising the steps of:

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

b. using a second self contained sensor physically coupled to said body, generating a drift compensating orientation signal that corresponds to the rotational orientation with respect to said at least two degrees of freedom of said body and that is impervious to interference from electromagnetic, acoustic, optical and mechanical sources and which is relatively impervious to drift over time;

c. generating a corrected rotational orientation signal based on said drift sensitive and drift compensating orientation signals.

41. The method of claim 40, said step of generating a corrected rotational orientation signal comprising the step of taking into account characteristic features of human motion.

42. The method of claim 41, said step of taking into account characteristic features of human motion comprising the step of determining times when said body has been at rest for more than a predetermined period of time.

43. The method of claim 42, said step of generating a corrected rotational orientation signal comprising the step applying 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.

44. An apparatus for simulating a virtual environment that is displayed to a user, said apparatus comprising:

a. a self contained orientation sensor for generating an orientation signal that;

i. corresponds to at least two of the three orientational degrees of freedom of yaw, pitch and roll of a body member of said user, relative to a reference frame; and

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

b. a position sensor for generating a position signal that corresponds to the position of said body, relative to said reference frame;

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

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

e. a means for displaying said virtual environment signals to said user; and

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

45. The apparatus of claim 44, said orientation sensor comprising:

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

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

c. coupled to said first sensor and said second sensor, a signal corrector means for generating a corrected rotational orientation signal based on said drift sensitive and drift compensating orientation signals.

46. The apparatus of claim 45 said signal corrector means comprising means to take into account characteristic features of human motion.

47. The apparatus of claim 46, 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.

48. The apparatus of claim 47, said corrector means 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.