Method to determine positions and states of an activity monitoring device

1. A personal activity monitor wearable by a user, such personal activity monitor comprising a motion sensor formed as an at least 3-axis accelerometer for providing acceleration signals, defining a first local spatial reference with same axis associated with the motion sensor and independent from a spatial geographical reference, and a processor unit, wherein the personal activity monitor may be worn by the user at various body locations, and configured to determine that the device is worn at the wrist location among other possible body locations,

wherein the personal activity monitor is further configured to:

apply a low pass filter to the acceleration signals, with a cut-off frequency less than 0.5 Hz, wherein such low pass filtering eliminates the acceleration induced by alternate inertia forces due to movements and keep only the DC component of the acceleration signals such that the filtered acceleration is substantially equal to gravity, reflecting an absolute orientation of the personal activity monitor,

Wa—identify at each instant which axis of the first local spatial reference senses the largest acceleration, the largest acceleration being provided by gravity,

Wb—determine a change of predominant axis when the largest acceleration changes from one axis to another in the first local spatial reference,

Wc—determine a number of changes of predominant axis per time period,

Wd—define a wrist worn condition, meaning that the personal activity monitor is worn at the wrist of the user, and,

infer that the wrist worn condition is true, based on a determination that the number of changes of predominant axis per time period exceeds a first predetermined threshold,

We—, infer that the wrist worn condition is or has become false, based on a determination that the number of changes of predominant axis per time period is below a second predetermined threshold; and

thereby, determine that the personal activity monitor is worn or is not worn at the wrist of the user, meaning that the personal activity monitor is worn at a body location different from a wrist or not worn at all.

2. The personal activity monitor of claim 1 , wherein acceleration signals consists of 3 linear acceleration signals.

3. The personal activity monitor of claim 1 , wherein the second predetermined threshold is lower than the first predetermined threshold.

4. The personal activity monitor of claim 1 , wherein the number of changes of predominant axis is computed periodically over the last elapsed time period.

5. The personal activity monitor of claim 1 , further comprising a temperature sensor enclosed in the personal activity monitor, wherein in step

Wd—, the wrist worn condition is maintained over a certain period of time based on a temperature sensed by the temperature sensor meeting a first predetermined temperature criterion over said certain period of time.

6. The personal activity monitor according to claim 5 , in which the first predetermined temperature criterion is a temperature interval comprised between a first predefined temperature threshold and a second predefined temperature threshold.

7. A personal activity monitor, comprising a motion sensor, a processor unit and a temperature sensor, wherein a set of at least three states are defined regarding the personal activity monitor together with the user, the set of states comprising at least NOT WORN, WORN&AWAKE, WORN&POTENTIAL SLEEPING, WORN&SLEEPING, the personal activity monitor being configured to:

Sa—determine continuously via the motion sensor a current level of activity, and compare the current level of activity to a predefined low level of activity, and to a second predefined level of activity,

Sb—determine via the temperature sensor a current temperature at the vicinity of the activity monitor, and a first temperature profile,

Sc—determine that the personal activity monitor is being worn by the user has entered a WORN POTENTIAL SLEEPING state, based on a determination that the current level of activity is below the predefined low level of activity for at least a first period of time and concurrently that the first temperature profile meets a first predetermined temperature criterion over the first period of time

based on a determination that the current level of activity remains below the second predefined level of activity and concurrently that the first temperature profile keeps meeting the first predetermined temperature criterion over a second period of time, determine that the personal activity monitor is being worn by the user and has entered a WORN&SLEEPING state, and then include a duration of the WORN&POTENTIAL SLEEPING state into effective sleep, and otherwise exclude the duration of the WORN&POTENTIAL SLEEPING state from effective sleep count,

Sd—determine that the user has emerged from sleep, based on a first condition at least requiring that the current level of activity is above the predefined low level of activity for at least a third period of time,

Se—determine that the user has emerged from sleep, based on a second condition at least requiring that the current level of activity is above the second predefined level of activity for at least a fourth period of time, and

Sf—calculate a sleep duration of the user, between instants of entering and emerging from sleep,

calculate a total sleep duration as the addition of several consecutive sleep phases.

8. The personal activity monitor according to claim 7 , wherein the first condition and/or the second condition further requires that the current temperature meets a second predetermined temperature criterion met when the current temperature is below a first predefined temperature threshold.

9. The personal activity monitor according to claim 8 , wherein the third predefined level of activity is greater than the predefined second level of activity which is greater than the predefined low level of activity; and

wherein the third period of time is greater than the fourth period of time which is also greater than the fifth period of time.

10. The personal activity monitor according to claim 7 , wherein the first predetermined temperature criterion is a temperature interval comprised between a first predefined temperature threshold and a second predefined temperature threshold.

11. The personal activity monitor according to claim 10 , wherein the processing unit is further configured to calculate a ratio of change of sensed temperature per time unit, and wherein the first condition and/or the second condition further requires that said ratio of change of sensed temperature per time unit exceeds a predefined temperature change threshold.

12. The personal activity monitor according to claim 7 , wherein the motion sensor is formed as a 3-axis accelerometer configured to provide raw acceleration signals, wherein each of the acceleration signals are divided into a first, DC component outputted after a low pass filter and a second, AC component obtained after suppression of the first DC component from the raw acceleration signals, and the current level of activity is determined by summing the average signals of the AC components of each of the 3 linear acceleration signals.

13. The personal activity monitor according to claim 7 , wherein the second predefined level of activity is greater than the predefined low level of activity, and

wherein the third period of time is, greater than the fourth period of time.

14. The personal activity monitor according to claim 7 , further configured to:

determine that the user has emerged from sleep, based on a third condition at least requiring that the current level of activity is above a third predefined level of activity for at least a fifth period of time.