Posture sensor automatic calibration

• US 8,165,840 B2
• Filed: 04/16/2009
• Issued: 04/24/2012
• Est. Priority Date: 06/12/2008
• Status: Active Grant

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Hatlestad, John D., Lewicke, Aaron, Maile, Keith R.
• Primary Examiner(s): COSIMANO, EDWARD R
• Application Number:
  US12/425,195
• Publication Number:
  US 20090312973A1
• Time in Patent Office: 1,104 Days
• Field of Search: 73/10.1, 73/13.7, 73/13.8, 73/17.5, 73/17.7, 733/82.R, 733/82.G, 734/321, 738/658, 738/659, 702/1, 702/33, 702/56, 702/85, 702/92, 702/94, 702/104, 702/127, 702/141, 702/150, 702/187, 702/189
• US Class Current: 702/104
• CPC Class Codes: A61B 2560/0223 of calibration, e.g. protocols for calibrating sensors
• CPC Class Codes: A61B 2562/0219 Inertial sensors, e.g. accelerometers, gyroscopes, tilt switches
• CPC Class Codes: A61B 5/0031 Implanted circuitry
• CPC Class Codes: A61B 5/103 Detecting, measuring or recording devices for testing the shape, pattern, colour, size or movement of the body or parts thereof, for diagnostic purposes A61B5/08 takes precedence; measuring aids for tailors A41H1/00; measuring instruments specially adapted for dentistry A61C19/04
• CPC Class Codes: A61B 5/1038 Measuring plantar pressure during gait
• CPC Class Codes: A61B 5/1116 Determining posture transitions
• CPC Class Codes: A61B 5/686 Permanently implanted devices, e.g. pacemakers, other stimulators, biochips A61B5/6861 takes precedence
• CPC Class Codes: G01C 22/00 Measuring distance traversed on the ground by vehicles, persons, animals or other moving solid bodies, e.g. using odometers, using pedometers
• CPC Class Codes: G01C 25/00 Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass testing, calibrating or compensating compasses G01C17/38
• CPC Class Codes: G01D 18/00 Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00
• CPC Class Codes: G01P 13/00 Indicating or recording presence, absence, or direction, of movement electric switches H01H; counting moving objects G06M7/00
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups
• CPC Class Codes: G06F 17/40 Data acquisition and logging for input to computer G06F3/00
• CPC Class Codes: G16H 20/30 relating to physical therapies or activities, e.g. physiotherapy, acupressure or exercising
• CPC Class Codes: G16H 40/67 for remote operation
• CPC Class Codes: G16Z 99/00 Subject matter not provided for in other main groups of this subclass

Abstract

A system and method automatically calibrate a posture sensor, such as by detecting a walking state or a posture change. For example, a three-axis accelerometer can be used to detect a patient'"'"'s activity or posture. This information can be used to automatically calibrate subsequent posture or acceleration data.

Claims

1. A method comprising:

sensing an acceleration signal from a patient using an accelerometer;

detecting a walking state using the acceleration signal, the detecting the walking state including;

extracting a characteristic of an AC component of the acceleration signal;

performing a comparison between the characteristic and a template; and

using the comparison to distinguish the walking state from a non-walking activity state; and

using an orientation of the accelerometer in the walking state to perform a posture calibration of the acceleration signal of the accelerometer.

2. The method of claim 1, wherein performing a comparison between the characteristic and a template comprises performing the comparison between at least one of:

a frequency-domain characteristic and a frequency-domain template;

a time-domain characteristic and a time-domain template; and

a time-frequency-domain characteristic and a time-frequency-domain template.

3. The method of claim 1, comprising:

computing a DC component of the acceleration signal;

determining a gravity vector using the DC component of the acceleration signal;

computing a parallel component of the acceleration signal, the parallel component being in the direction of the gravity vector;

computing an orthogonal component of the acceleration signal, the orthogonal component being in a direction orthogonal to the gravity vector;

wherein the performing the comparison comprises comparing the parallel component to a parallel template and comparing the orthogonal component to an orthogonal template; and

wherein the using the comparison to distinguish the walking state from the non-walking activity state comprises using the computed parallel component of the acceleration signal and using the computed orthogonal component of the acceleration signal.

4. The method of claim 3, wherein the using the comparison to distinguish the walking state from the non-walking activity state comprises comparing the parallel comparison to a first threshold value and comparing the orthogonal comparison to a second threshold value.

5. The method of claim 3, wherein:

computing the DC component of the acceleration signal comprises computing a DC component of the acceleration signal in each of orthogonal x, y, and z directions of the accelerometer; and

determining a gravity vector comprises determining a three dimensional gravity vector using the DC component of the acceleration signal in each of x, y, and z directions of the accelerometer.

6. The method of claim 3, wherein using the comparison to distinguish the walking state from the non-walking activity state comprises:

determining a central tendency of the gravity vectors;

comparing the central tendency of the gravity vectors to a previously-determined central tendency of the gravity vectors; and

determining whether the gravity vectors are consistent using a difference between the central tendency of the gravity vectors and the previously-determined central tendency of the gravity vectors.

7. A method comprising:

sensing an acceleration signal from a patient using an accelerometer;

sensing a first steady state posture from the acceleration signal;

calculating a first gravity vector orientation of plurality of gravity vectors detected from the acceleration signal during a first time period and in the first steady state posture;

detecting a second gravity vector orientation that differs from the first gravity vector orientation by at least a threshold value;

detecting a second steady state posture associated with the second gravity vector orientation; and

upon detecting the second steady state posture associated with the second gravity vector orientation, automatically performing a calibration using a plurality of gravity vectors associated with only one of either the first or second steady state postures.

8. The method of claim 7, wherein:

sensing an acceleration signal from a patient using an accelerometer comprises determining a DC component of the acceleration signal; and

detecting a second steady state posture associated with the second gravity vector orientation comprises detecting a change in orientation between the first gravity vector orientation and the second gravity vector orientation, the change in orientation being above a specified threshold value for at least a specified second time period.

9. The method of claim 7, wherein:

calculating a first gravity vector orientation of plurality of gravity vectors detected from the acceleration signal during a first time period and in the first steady state posture comprises determining a consistency of the first gravity vector orientation during the first time period;

and detecting a second steady state posture associated with the second gravity vector orientation comprises determining a consistency of the second gravity vector orientation during the second time period;

and when the first and second gravity vector orientations are deemed consistent, using a central tendency of one of the consistent first and second gravity vector orientations to perform a posture calibration of the acceleration signal of the accelerometer.

10. The method of claim 9, wherein determining a consistency of the first or second gravity vector orientations during the first time period comprises:

determining a central tendency of the gravity vectors;

comparing the central tendency of the gravity vectors to a previously-determined central tendency of the gravity vectors; and

determining whether the gravity vectors are consistent using a difference between the central tendency of the gravity vectors and the previously-determined central tendency of the gravity vectors.

11. The method of claim 10, comprising, when the first or second gravity vector orientations are deemed inconsistent, performing the posture calibration without using the inconsistent gravity vectors.

12. The method of claim 10, wherein determining a consistency of the first or second gravity vector orientations during the first time period comprises:

determining a dispersion of the gravity vectors; and

determining whether the gravity vectors are consistent using a comparison of the dispersion to a specified dispersion threshold value.

13. The method of claim 12, comprising computing the specified dispersion threshold value using a previously-determined dispersion of the gravity vectors.

14. The method of claim 10, comprising identifying a posture during the first or second time period.

15. The method of claim 14, wherein identifying the posture during the first or second time period comprises using at least one of:

a time of day for the identifying;

a patient activity level for the identifying; and

a physiologic sensor, of a different type than the accelerometer, for the identifying.

16. A device comprising:

an accelerometer, configured to sense an acceleration signal from a patient; and

a signal processor circuit, coupled to the accelerometer, the signal processor circuit comprising or coupled to;

a walking state detection circuit, configured to detect a walking state of the patient using the acceleration signal, the walking state detection circuit comprising;

a highpass filter circuit, configured to extract an AC component of the acceleration signal, and further configured to extract a characteristic of the AC component of the acceleration signal;

a stored template;

a comparison circuit, coupled to the filter circuit and the template, the comparison circuit configured to perform a comparison between the characteristic and the template to distinguish the walking state from a non-walking activity state; and

a posture calibration circuit, coupled to the comparison circuit, the posture calibration circuit configured to use the orientation of the accelerometer in the walking state to perform a posture calibration of the acceleration signal of the accelerometer.

17. The device of claim 16, comprising a stored domain template, the domain being at least one of frequency, time, and time-frequency;

and wherein the comparison circuit is configured to perform a comparison between the characteristic and the domain template, wherein the characteristic comprises a domain characteristic.

18. The device of claim 16, comprising:

a lowpass filter circuit, configured to compute a DC component of the acceleration signal; and

wherein the signal processor circuit is configured to;

determine a gravity vector using the DC component of the acceleration signal;

compute a parallel component of the acceleration signal, the parallel component being in the direction of the gravity vector;

compute an orthogonal component of the acceleration signal, the orthogonal component being in a direction orthogonal to the gravity vector;

compare the parallel component to a parallel template and compare the orthogonal component to an orthogonal template; and

distinguish the walking state from the non-walking activity state using the computed parallel component of the acceleration signal and using the computed orthogonal component of the acceleration signal.

19. The device of claim 18, wherein the signal processor circuit configured to distinguish the walking state from the non-walking activity state comprises:

determining a central tendency of the gravity vectors;

comparing the central tendency of the gravity vectors to a previously-determined central tendency of the gravity vectors; and

determining whether the gravity vectors are consistent using a difference between the central tendency of the gravity vectors and the previously-determined central tendency of the gravity vectors.

20. A device comprising:

an accelerometer, configured to sense an acceleration signal from a patient; and

a signal processor circuit, coupled to the accelerometer, the signal processor circuit configured to;

sense a first steady state posture from the acceleration signal;

calculate a first gravity vector orientation of plurality of gravity vectors detected from the acceleration signal during a first time period and in the first steady state posture;

detect a second gravity vector orientation that differs from the first gravity vector orientation by at least a threshold value;

detect a second steady state posture associated with the second gravity vector orientation; and

upon detecting the second steady state posture associated with the second gravity vector orientation, automatically perform a calibration using a plurality of gravity vectors associated with only one of either the first or second steady state postures.

21. The device of claim 20 comprising:

a lowpass filter circuit, coupled to the accelerometer, configured to use the acceleration signal from the patient to determine a DC component of the acceleration;

wherein the signal processor circuit is configured to detect a second steady state posture associated with the second gravity vector orientation by detecting a change in orientation between the first gravity vector orientation and the second gravity vector orientation, the change in orientation being above a specified threshold value for at least a specified second time period.

22. The device of claim 20, wherein the signal processor circuit is configured to:

determine a consistency of the first gravity vector orientation during the first time period;

determine a consistency of the second gravity vector orientation during the second time period;

and when the first and second gravity vector orientations are deemed consistent, use a central tendency of one of the consistent first and second gravity vector orientations to perform a posture calibration of the acceleration signal of the accelerometer.

23. The device of claim 22, wherein the signal processor being configured to determine a consistency of the first or second gravity vector orientations during the first time period comprises the signal processor configured to:

determine a central tendency or a dispersion of the gravity vectors; and

determine whether the gravity vectors are consistent using a comparison of the dispersion to a specified dispersion threshold value.

24. The device of claim 22, wherein the signal processor is configured to identify a posture during the first or second time periods using at least one of:

a time of day;

a patient activity level; and

a physiologic sensor, of a different type than the accelerometer.

25. The device of claim 24, wherein the signal processor is configured to perform the posture calibration, when the first or second gravity vector orientations are deemed inconsistent, without using the inconsistent gravity vectors.