METHOD FOR ESTIMATING VELOCITIES AND/OR DISPLACEMENTS FROM ACCELEROMETER MEASUREMENT SAMPLES

• US 20120232823A1
• Filed: 11/22/2010
• Published: 09/13/2012
• Est. Priority Date: 11/25/2009
• Status: Active Grant

• Current Assignee: Koninklijke Philips N.V.
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Baggen, Constant Paul Marie Jozef, Chen, Ningjiang
• Granted Patent: US 9,456,771 B2
• Time in Patent Office: Days
• US Class Current: 702/104
• CPC Class Codes: A61B 2562/0219 Inertial sensors, e.g. accelerometers, gyroscopes, tilt switches
• CPC Class Codes: A61B 5/1117 Fall detection
• CPC Class Codes: G01P 15/0891 with indication of predetermined acceleration values G01P15/135 takes precedence
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 7/00 Measuring speed by integrating acceleration measuring travelled distance by double integration of acceleration G01C21/16
• CPC Class Codes: G08B 21/043 detecting an emergency event, e.g. a fall
• CPC Class Codes: G08B 21/0446 worn on the body to detect changes of posture, e.g. a fall, inclination, acceleration, gait

Abstract

There is provided a fall detector for use in detecting falls by a user, the fall detector comprising an accelerometer for producing a time series of measurement samples representing the acceleration acting on the fall detector; a processor for estimating a vertical velocity and/or vertical displacement of the fall detector from the measurement samples and using the estimated vertical velocity and/or vertical displacement to determine whether the user has suffered a fall; wherein the processor is configured to estimate a vertical velocity and/or vertical displacement of the fall detector from the measurement samples by estimating a corresponding time series of unit vectors representing acceleration due to gravity in the reference frame of the accelerometer from the time series of measurement samples; projecting each measurement sample onto the corresponding unit vector and subtracting acceleration due to gravity to give a series of estimates for the vertical acceleration of the fall detector; and integrating the series of estimates for the vertical acceleration over a time period to give a time series of values for the vertical velocity and/or vertical displacement of the fall detector.

Claims

1. A fall detector (2) for use in detecting falls by a user, the fall detector (2) comprising:

an accelerometer (4) for producing a time series of measurement samples representing the acceleration acting on the fall detector (2);

a processor (6) for estimating a vertical velocity and/or vertical displacement of the fall detector (2) from the measurement samples and using the estimated vertical velocity and/or vertical displacement to determine whether the user has suffered a fall;

wherein the processor (6) is configured to estimate a vertical velocity and/or vertical displacement of the fall detector (2) from the measurement samples by;

estimating a corresponding time series of unit vectors representing acceleration due to gravity in the reference frame of the accelerometer (4) from the time series of measurement samples (105);

projecting each measurement sample onto the corresponding unit vector and subtracting acceleration due to gravity to give a series of estimates for the vertical acceleration of the fall detector (107); and

integrating the series of estimates for the vertical acceleration over a time period to give a time series of values for the vertical velocity and/or vertical displacement of the fall detector (109, 111).

2. A fall detector (2) as claimed in claim 1, wherein the processor (6) is configured to integrate the series of estimates for the vertical acceleration over a time period during which the vertical acceleration and vertical velocity of the fall detector (2) are substantially non-zero to give the time series of values for the vertical velocity and/or vertical displacement of the fall detector (2).

3. A fall detector (2) as claimed in claim 2, wherein the processor (6) is configured to identify time periods during which the vertical acceleration and vertical velocity of the fall detector (2) are substantially non-zero (103) by:

computing an absolute acceleration value for each measurement sample in the time series (1031);

subtracting acceleration due to gravity from each of the computed absolute values to obtain a time series of normalized absolute accelerations (1033);

squaring each of the normalized absolute accelerations to obtain a time series representing the power of the normalized absolute accelerations (1035);

taking a moving average of the time series representing the power of the normalized absolute acceleration (1037); and

comparing the moving average filtered power with a non-zero threshold value to identify said time periods (1039).

4. A fall detector (2) as claimed in claim 1, wherein each measurement sample comprises three acceleration components, and wherein the processor (6) is configured to estimate the corresponding time series of unit vectors representing acceleration due to gravity in the reference frame of the accelerometer (4) from the time series of measurement samples by:

applying a median filter to each of the three acceleration components to obtain respective time series of median filtered components (1051); and

normalizing the length of each of the components in the respective time series to give the time series of unit vectors representing acceleration due to gravity (1053).

5. A fall detector (2) as claimed in claim 1, wherein, after projecting each measurement sample onto the corresponding unit vector and subtracting acceleration due to gravity to give the series of estimates for the vertical acceleration of the fall detector, the processor (6) is further configured to:

refine the series of estimates to correct for excess acceleration due to sampling errors and/or clipping in the measurement samples (109);

and wherein the processor (6) is configured to integrate the refined series of estimates for the vertical acceleration to give a time series of values for the vertical velocity and/or vertical displacement of the fall detector (111).

6. A fall detector (2) as claimed in claim 5, wherein the processor (6) is configured to refine the estimates to correct for excess acceleration due to sampling errors and/or clipping in the measurement samples that occur when the fall detector (2) has an impact.

7. A fall detector (2) as claimed in claim 5, wherein the processor (6) is configured to refine the estimates by:

integrating the series of estimates of the vertical acceleration over said time period, the resulting non-zero amount being denoted the excess vertical acceleration; and

dividing the excess vertical acceleration across each of the estimates of the vertical acceleration in said time period to refine those estimates of the vertical acceleration.

8. A fall detector (2) as claimed in claim 7, wherein the processor (6) is configured to divide the excess vertical acceleration across each of the estimates proportionally according to a computed filtered impact.

9. A fall detector (2) as claimed in claim 7, wherein the processor (6) is configured to divide the excess vertical acceleration across each of the estimates proportionally by:

computing the absolute value of each of the estimates of the vertical acceleration during said time period;

determining the sum of the absolute accelerations in said period; and

dividing the excess vertical acceleration among the estimates of the vertical acceleration in said time period where each estimate receives a fraction of its absolute value divided by the sum.

10. A fall detector (2) as claimed in claim 1, wherein the processor (6) is configured to integrate the series of estimates for the vertical acceleration over said time period using a non-rectangular integration window to give the time series of values for the vertical velocity and/or vertical displacement of the fall detector (2).

11. A fall detector (2) as claimed in claim 10, wherein the processor (6) is configured to integrate the series of estimates for the vertical acceleration over said time period using a raised cosine integration window.

12. A fall detector (2) as claimed in claim 1, wherein the fall detector (2) is in the form of a pendant to be worn around the neck of the user.

13. A method of estimating a vertical velocity and/or vertical displacement of an object comprising an accelerometer (4), the method comprising:

obtaining a time series of measurement samples from the accelerometer (4) representing the acceleration acting on the object (101);

estimating a corresponding time series of unit vectors representing acceleration due to gravity in the reference frame of the accelerometer (4) from the time series of measurement samples (105);

projecting each measurement sample onto the corresponding unit vector and subtracting acceleration due to gravity to give a series of estimates for the vertical acceleration of the object (107); and

integrating the series of estimates for the vertical acceleration over a time period to give a time series of values for the vertical velocity and/or vertical displacement of the object (109, 111).

14. A method for use in detecting falls by a user of a fall detector (2) comprising an accelerometer (4), the method comprising:

estimating a vertical velocity and/or vertical displacement from measurement samples from the accelerometer (4) as claimed in claim 13; and

using the estimated vertical velocity and/or vertical displacement to determine whether the user has suffered a fall.

15. A computer program product comprising computer program code that, when executed on a suitable computer or processor, is configured to cause the computer or processor to perform the method claimed in claim 13.