DEVICE FOR MONITORING A USER AND A METHOD FOR CALIBRATING THE DEVICE

• US 20140296660A1
• Filed: 10/08/2012
• Published: 10/02/2014
• Est. Priority Date: 10/17/2011
• Status: Active Grant

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips N.V.
• Inventors: Van Den Heuvel, Teun, Van De Laar, Jakob, Duric, Haris
• Granted Patent: US 9,895,086 B2
• Time in Patent Office: Days
• US Class Current: 600/301
• 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 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• CPC Class Codes: A61B 5/024 Detecting, measuring or recording pulse rate or heart rate A61B5/0205, A61B5/021 take precedence
• CPC Class Codes: A61B 5/0816 Measuring devices for examining respiratory frequency measuring frequency of electric signals G01R23/00
• CPC Class Codes: A61B 5/1116 Determining posture transitions
• CPC Class Codes: A61B 5/1118 Determining activity level
• CPC Class Codes: A61B 5/1121 Determining geometric values, e.g. centre of rotation or angular range of movement

Abstract

A method of calibrating a monitoring device to be attached to a user is provided. Prior to attachment of the device, the device is aligned with respect to the user such that the measurement reference frame of the device is substantially aligned with a reference frame of the user. A first measurement of the orientation of the device with respect to a world reference frame is obtained. After attachment of the device, a second measurement of the orientation of the device with respect to a world reference frame is obtained. A transformation matrix is determined for use in transforming subsequent measurements obtained by the device into the reference frame of the user. The matrix is calculated using the first and second measurements and information on the amount of rotation of the device relative to the user about a vertical axis in the world reference frame between the first and second measurements being taken.

Claims

1. A method of calibrating a device that is to be attached to a user and used to monitor the user, the method comprising the steps of:

(i) prior to attachment of the device to the user, aligning the device with respect to the user such that the measurement reference frame of the device is substantially aligned with a reference frame of the user and obtaining a first measurement of the orientation of the device with respect to a world reference frame using the device;

(ii) after attachment of the device to the user, obtaining a second measurement of the orientation of the device with respect to a world reference frame using the device; and

(iii) determining a transformation matrix for use in transforming subsequent measurements obtained by the device into the reference frame of the user, the transformation matrix being, calculated using the obtained first and second measurements and information on the amount of rotation of the device relative to the user about a vertical axis in the world reference frame between steps (i) and (ii).

2. A method according to claim 1, wherein the information on the amount of rotation of the device relative to the user about the vertical axis of the world reference frame between steps (i) and (ii) comprises information on an angle φ

_p between the projections of a basis vector β

_m in the reference frame of the user onto a horizontal plane H perpendicular to the direction in which gravity acts and the basis vector α

_p in the measurement reference frame of the device onto the horizontal plane H for some m,pε

{1, 2, 3}, where β

_m and α

_p do not lie in the direction of gravity.

3. A method according to claim 2, wherein the step of determining a transformation matrix comprises the steps of:

computing a first coordinate vector d that is orthonormal to the first measurement b;

computing a second coordinate vector f that is orthonormal to b and d;

computing support vectors s_i and radii r_i of the circles C₁, C₂ and C₃ on which the tips of the α

basis vectors lie for iε

{1, 2, 3};

if |a_k|=1 for some k then computing t_k;

=sign(α

_k)b;

computing the p-th column of the transformation matrix; and

computing one or more remaining resolvable angles and the corresponding one or more columns of transformation matrix.

4. A method according to claim 3, wherein the step of computing one or more remaining resolvable angles and the corresponding one or more columns of the transformation matrix comprises performing the following steps for iε

{1, 2, 3 \ {p, k}} and j;

={1, 2, 3 \ {i, p}};

computing the Levi-Civita symbol for the triple (p, i, j);

computing the angle φ

_i between the projections of the basis vector α

_i in the measurement reference frame of the device and the basis vector β

_m in the reference frame of the user onto the plane H; and

computing the corresponding column of the transformation matrix.

5. A method according to claim 1, further comprising the step of:

estimating the posture of the user;

wherein the step of determining the transformation matrix further uses the estimated posture of the user.

6. A method according to claim 5, wherein the step of estimating the posture of the user comprises:

estimating the posture of the user by analysing the first measurement of the orientation of the device with respect to the world reference frame or receiving an input from the user or an operator indicating the posture of the user.

7. A method as claimed in claim 5, the method further comprising the step of:

displaying information associated with the estimated posture of the user for use by the user or an operator in the attachment of the device to the user.

8. A method as claimed in claim 7, wherein the displayed information comprises information on permissible rotations of the device between steps (i) and (ii).

9. A method as claimed in claim 5, wherein the information on the amount of rotation of the device relative to the user about a vertical axis in the world reference frame between steps (i) and (ii) is assumed based on the estimated posture of the user.

10. A method as claimed in claim 1,

wherein the information on the amount of rotation of the device relative to the user about a vertical axis in the world reference frame between steps (i) and (ii) is provided by the user or an operator of the device.

11. A method of monitoring a user, the method comprising the steps of:

determining a transformation matrix according to the method in claim 1;

obtaining further measurements using the device;

converting the obtained further measurements into the reference frame of the user using the transformation matrix; and

processing the converted obtained further measurements to determine at least one of the posture of the user, the movements of the user, the activity of the user, the respiration rate of the user and/or the pulse rate of the user.

12. A computer program product comprising a plurality of program code portions for carrying out the following method steps when executed by a suitable computer or processor:

using a first acceleration measurement as an indication of the orientation of the reference frame of the user with respect to a world reference frame obtained prior to measurement of the orientation of the device with respect to a world reference frame following attachment of the device to the user,

determining a transformation matrix for use in transforming subsequent acceleration measurements obtained by the device into the reference frame of the user, the transformation matrix being calculated using the obtained first and second measurements and information on the amount of rotation of the device relative to the user about a vertical axis of the world reference frame between the first and second measurements being taken.

13. A device for monitoring a user, the device being suitable for attachment to a user, the device comprising:

a sensor configured to measure accelerations; and

a processor for processing the acceleration measurements;

wherein the processor is configured such that, when the device is operating in a calibration mode;

the processor uses a first acceleration measurement as an indication of the orientation of the reference frame of the user with respect to a world reference frame obtained prior to attachment of the device to the user and a second acceleration measurement as a measurement of the orientation of the device with respect to a world reference frame following attachment of the device to the user, and

the processor determines a transformation matrix for use in transforming subsequent acceleration measurements obtained by the device into the reference frame of the user, the transformation matrix being calculated using the obtained first and second measurements and information on the amount of rotation of the device relative to the user about a vertical axis of the world reference frame between the first and second measurements being taken.

14. A device as claimed in claim 13, wherein the processor is further configured such that, when the device is operating in a monitoring mode, the processor uses the calculated transformation matrix to convert further acceleration measurements obtained by the sensor into the reference frame of the user.

15. A device as claimed in claim 13, further comprising:

a user interface operable by the user or an operator of the device, wherein operation of the user interface causes the device to enter into the calibration mode and to selectively cause the sensor to obtain the first acceleration measurement and the second acceleration measurement.