FIELD OF THE INVENTION

The present invention relates to a measurement apparatus for calculating one or more features characterizing an activity of a subject. The present invention further relates to a correction method for calculating one or more features characterizing an activity of a subject. The present invention additionally relates to a computer program for executing the steps of said method.

BACKGROUND OF THE INVENTION

Wearable measurement devices including an accelerometer for measuring a motion of a subject during sport activities and other actions are generally known. The accelerometer provides accelerometer data from which several feature are calculated such as activity counts, an activity type, a walking or running speed, energy expenditure and cardiorespiratory fitness. A corresponding measurement device is e.g. known from US

2008/0281234 Al .

WO 2015/183193 Al discloses a method for classifying the activity and/or counting steps of a user. Said method comprises measuring accelerometer data for a plurality of axes; identifying a most active one of the plurality of axes based on the accelerometer data; and classifying the activity of the user based on a signal amplitude of the accelerometer data for the most active axis and one or more threshold values.

The position of the measurement device at the body of the user can change over time during the use and between different users. In the case the user wears the wearable device at the wrist, the user can wear a device during each use in a slightly different position and each user has a different arm length so that the wearable device including the

accelerometer can be worn in different positions. This change of the wearing position of the measurement device including the accelerometer influences the measurement data and correspondingly the calculation of the features which relate to the magnitude of the acceleration signal directly, such as activity counts, or indirectly, such as energy expenditure.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a measurement apparatus for measuring an activity of a subject, wherein the measurement results are independent of the position of the measurement device at the body of the subject.

It is a further object of the present invention to provide a corresponding correction method for correcting a measurement of an activity of a subject.

According to one aspect of the present invention, a measurement apparatus for calculating one or more features characterizing an activity of the subject is provided, comprising:

- a wearable base part including an accelerometer adapted to measure a motion of the base part and to provide an acceleration signal,

- a memory device adapted to store reference acceleration data, and

- a correction unit adapted:

(i) to select a correction formula depending on an activity type which is determined based on the acceleration signal or defined by the subject,

(ii) to determine a correction coefficient based on a frequency component of the measured acceleration signal and on the stored reference acceleration data;

(iii) to correct a magnitude component of the measured acceleration signal or one or more features derived from the magnitude componentbased on the selected correction formula using the determined correction coefficient; and

(iv) to calculate the one or more features characterizing the activity of the subject based on the corrected magnitude component of the measured acceleration signal.

According to another aspect of the present invention, a correction method for calculating one or more features characterizing an activity of a subject is provided comprising the steps of:

- storing a reference acceleration data in a memory device,

- measuring a motion of the subject and providing an acceleration signal corresponding to the motion,

- selecting a correction formula depending on an activity type which is determined based on the acceleration signal or defined by the subject,

- determining a correction coefficient based on a frequency component of the measured acceleration signal and on the stored reference acceleration data;

- correcting a magnitude component of the measured acceleration signal or one or more features derived from the magnitude component based on the selected correction formula using the determined correction coefficient; and - calculating the one or more features characterizing the activity of the subject based on the corrected magnitude component of the measured acceleration signal.

According to another aspect of the present invention, a computer program is provided comprising program code means for causing a computer to carry out the steps of the method according to the present invention when said computer program is carried out on a computer.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method has similar and or identical preferred

embodiments as a claimed device and as defined in the dependent claims.

The present invention is based on the idea to measure an acceleration of a wearable base part of the measurement apparatus by means of an accelerometer and to provide a corresponding acceleration signal including a magnitude component and a frequency component. A correction unit corrects the magnitude component or one or more features derived from the magnitude component, which is highly dependent on a wearing position of the wearable base part, wherein the correction unit utilizes reference acceleration data to correct the magnitude component or the feature and, further, a frequency component of the measured acceleration signal for correcting the magnitude component or the feature derived from the magnitude component. The so corrected magnitude component of the acceleration data or the corrected feature allows to provide global consistent activity measurements which merely differ based on the motion of the subject and not based on the wearing position of the wearable base part.

Hence, a measurement of the motion of the subject can be provided independent of the wearing position of the accelerometer.

The correction unit is adapted to select the correction formula depending on the activity type performed by the subject, and to determine the correction coefficient used in said correction formula based on the frequency component of the measured acceleration signal and on the stored reference acceleration data. This is a possibility to consider a wearing position of the accelerometer for correcting the magnitude component or the feature, since the wearing position influences the frequency component of the measured acceleration signal. It is also a possibility to consider a calibration position of the wearable base part and to correct the magnitude component or the feature with respect to the calibration position, since the calibration position influences the stored reference acceleration data. In a preferred embodiment, the measurement apparatus further comprises an activity classifying unit adapted to determine the activity type of the subject based on the measured acceleration signal. This is a possibility to correct the magnitude component of the measured acceleration signal or the feature precisely and individually for different activity types of the subject.

In a further preferred embodiment, the correction unit is adapted to determine the correction coefficient for correcting the magnitude component of the measured acceleration signal based on the magnitude component and the frequency component of the measured acceleration signal. This is a further possibility to consider the wearing position of the wearable base port, since the ratio of the magnitude component and the frequency component correspond to the respective wearing position. In a further preferred embodiment, correction unit is adapted to determine the correction coefficient for correcting the magnitude component of the measured acceleration signal based on a ratio of the magnitude component and the frequency component of the measured acceleration signal.

In a further preferred embodiment, the correction unit is adapted to determine a position of the wearable base part at the subject based on a reference value of the magnitude component and the frequency component. This is a possibility to further improve the determination of the position of the wearable base part.

In a further preferred embodiment, the measurement device further comprises a calibration unit adapted to determine the reference acceleration data including a magnitude component and a frequency component. This is a possibility to provide reference data for the correction of the magnitude component or the feature derived therefrom.

In a further preferred embodiment, the calibration unit is adapted to determine a reference acceleration data based on a measurement of the accelerometer. The reference acceleration data is measured preferably with the measurement apparatus itself as an initial measurement during the first use by a user. This is a possibility to determine the reference acceleration data comfortable with low effort.

In a further preferred embodiment, the calibration unit includes an input device adapted to receive information of a calibration position of the wearable base part at the subject as a user input. This is a possibility to calibrate the calibration position of the wearable base part precisely with low technical effort.

In a further preferred embodiment, the correction unit is adapted to determine a position of the wearable base part at the subject based on the calibration position and the measured acceleration signal. This is a possibility to precisely determine a position of the wearable base part at the subject.

As mentioned above, the measurement device allows to normalize the magnitude component of the acceleration signal or the feature derived from the magnitude component so that the measurement results are only based on the movement of the subject and not based on the wearing position of the wearable base part. Hence, the precision of the measurement apparatus can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings

Fig. 1 shows a schematic representation of a measurement apparatus for measuring an activity of a subject in use;

Fig. 2 shows a schematic diagram of an embodiment of the measurement apparatus shown in Fig. 1;

Fig. 3 shows a timing diagram of activity counts of the measurement apparatus worn at different positions at the subject;

Fig. 4 shows a further timing diagram of activity counts measured by the measurement apparatus at different wearing positions and for different activities;

Fig. 5 shows a flow diagram of a calibration of the measurement device; and

Fig. 6 shows a schematic flow diagram of a correction of the measured acceleration signal.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a schematic illustration of a measurement apparatus 10 for measuring an activity of a subject 12. The measurement apparatus 10 comprises a wearable base part 14, which is wearable at a wrist or a forearm of the subject 12. The wearable base part 14 comprises an accelerometer for measuring a motion of the arm of the subject 12. The accelerometer provides an acceleration signal corresponding to the motion of the arm of the subject 12 and the measurement apparatus 10 calculates several features based on the acceleration signal such as activity counts, activity type, movement speed of the subject 12, energy expenditure and cardio -respiratory fitness or the like. The position of the wearable base part 14 at the body of the subject 12 may change over time during the use and between different subjects 12 so that the measured motion and the corresponding acceleration signal changes due to different accelerations based on the different wearing position of the wearable base part 14. Hence, the measurement apparatus 10 comprises corresponding circuits in order to normalize the components of the acceleration signal in order to provide calculated features which are independent of the wearing position of the wearable base part 14 as described in the following.

Fig. 2 shows a detailed schematic diagram of the measurement apparatus 10 shown in Fig. 1. The wearable base part 14 comprises an accelerometer 16, which is adapted to measure a motion of the arm of the subject 12 and provide an acceleration signal including a magnitude component and a frequency component. The measurement apparatus further comprises a control unit 18 connected to the accelerometer 16, which receives the acceleration signal. The measurement apparatus 10 further comprises a memory device 20 which is connected to the control unit 18. The memory device 20 is adapted to store reference acceleration data which is used to correct the acceleration signal or the features derived from the magnitude component as described in the following. The control unit 18 is further connected to a correction unit 22, which corrects or normalizes the magnitude component of the measured acceleration signal in order to provide a normalized measurement, which forms a basis for calculating several features by means of the control unit 18 or corrects the features derived from the magnitude component directly. The correction unit 22 corrects the magnitude component on the basis of the reference acceleration data stored in the memory device 20 and the frequency component of the measured acceleration signal or one or more features derived from the frequency component. The normalized or recalculated magnitude component is provided to the control unit 18 for calculating features of the subject 12 which are provided to an interface 24 or a display unit or an output unit (not shown).

The measurement apparatus further comprises a calibration unit 26 for calibrating the measurement apparatus 10 and a position of the wearable base part 14. The calibration unit 26 is connected or connectable to an input interface or an input device 28, which allows the user to specify the position of the wearable base part 14 during a first time of the use, wherein the control unit 18 stores the measured acceleration data measured during a first time of the use including magnitude components and frequency components in the memory device 20 as the reference acceleration data depending on the respective activity performed by the subject 12.

The measurement apparatus 10 may further comprise an activity classifying unit for determining the activity type of the subject based on the measured acceleration signal. During the calibration, the reference acceleration data is stored in the memory device 20 based on the activity type determined by the activity classifying unit. Alternatively, the activity type may be provided by the user to the calibration unit 26 via the input device 28.

During the first use of the measurement apparatus 10, the measured acceleration signal including the magnitude component and the frequency component is stored in the memory device dependent on the respective activity type of the subject.

During the following use of the measurement apparatus 10, the control unit 18 selects a correction formula based on the activity type and a correction coefficient is selected based on the frequency component of the measured acceleration signal and based on the reference acceleration data and the correction unit 22 corrects the magnitude component or the feature correspondingly based on the correction formula and the selected coefficient. The so corrected magnitude component is provided to the control unit 18 and the control unit 18 calculates the features of the subject 12 during the activity and provides it to the output interface 24.

The control unit 18 may further comprise a comparison system which compares the magnitude component and/or the frequency component with a certain threshold level and decides whether a correction of the magnitude component is necessary or not and activates correspondingly the correction unit 22.

Fig. 3 shows a timing diagram of an activity counts of the accelerometer 16 measured at the wrist and the forearm. As shown in Fig. 3, the activity counts at the wrist is larger than the activity count at the fore arm due to the larger distance from the elbow of the subject 12. This magnitude component which depends on the position of the wearable base part 14 at the arm is corrected by the correction unit 22 in order to achieve a normalized magnitude component of the measured acceleration signal.

In Fig. 4 a further time diagram of activity counts is schematically shown. The timing diagram shows during a time frame from 0 to ti the activity counts of a running activity and in a time frame between t ₂ and t ₃ the activity counts of a walking activity. As shown in Fig. 4, the activity counts during the running activity is higher than the activity counts during the walking activity so that the different activities of the subject 12 can be detected to be based on the measured acceleration signal.

Fig. 5 shows a schematic flow diagram of a calibration method of the measurement apparatus 10. The calibration method is generally denoted by 30.

At step 32 it is checked whether the measurement apparatus 10 is the first time in use and/or the memory device 20 is empty for the respective activity. If the measurement apparatus 10 is used for the first time for this activity, the position of the wearable base part 14 is manually provided by the user via the input device 28. The position is e.g. provided to the measurement apparatus 10 as a distance from the shoulder.

At step 36, the position is stored in the memory device 20.

At step 38, the accelerometer measures the motion of the wearable base part 14 and the motion of the subject 12 correspondingly and provides the acceleration signal including the magnitude component and the frequency component. At step 40, the activity type is determined by the activity classifying unit or alternatively provided by the user via the input device 28 and at step 42 it is checked whether the acceleration signal comprises new features for this activity type. If the acceleration signal comprises new features, the acceleration signal including the magnitude component and the frequency component is stored in the memory device 20 as shown at step 44.

If the features of the acceleration signal are already stored on the memory device 20 for this activity type, the normal use of the measurement apparatus 10 starts as shown at step 46.

Fig. 6 shows a schematic flow diagram of a method for correcting the measurement of an activity of the subject 12. The method is generally denoted by 50. At step 52, the activity type is determined or received via the input device 28 and at step 54 a correction formula is selected for the determined activity type. At step 56, the frequency component of the measured acceleration signal is extracted and at step 58 a correction coefficient for correcting the magnitude component or the feature is selected or determined based on the frequency component and based on the reference acceleration data stored in the memory device 20. At step 60, the magnitude component of the measured acceleration signal is determined and at step 62, the magnitude component is corrected by the correction unit 22 based on the selected correction formula (step 54) and the selected correction coefficient (step 58). At step 64, the new magnitude component is provided to the control unit 18 for determined features of the activity of the subject 12 as explained above.

alternative embodiment, the control unit 18 determines the feature of the activity of the subject 12 at step 62 and the feature is corrected by the correction unit 22 at step 64 based on the selected correction formula (step 54) and the selected correction coefficient (step 58).

The correction of the magnitude component for a walking activity can be calculated by: wherein AC X2P1 corresponds to a calculated activity count as a sum of magnitude features at a speed X2 and a position of the wearable base part 14 at PI, AC X2P2 corresponds to measured activity counts as a sum of magnitude features of the measured acceleration signal at a speed X2 and an (unknown) position of the wearable base part 14 at P2, LE Pl is a position of the wearable base part 14 during the calibration procedure (distance from the shoulder in cm) and LE P2 is calculated as an unknown position by the formula:

, _Π „ CA XIPl AC X\P\ * LE PI

LE P2 = = * = ⁵¹ =—

CA _ X2P2 AC _ X2P2

wherein CA X1P1 corresponds to a speed XI at the wearing position PI during a calibration procedure and CA X2P2 corresponds to the measured speed at the unknown position P2.

Hence, the activity counts as the magnitude component can be calculated for a not known position of the wearable base part 14 based on the calibration data and the measured data. For different activity types, other formulas can be used.

The calibration procedure may be performed at the same subject 12 or at a different subject 12 and stored in the memory device 20.

Further, a relationship between the magnitude components and the frequency components of the measured acceleration signal may be calculated. The frequency component related to an ambulatory speed should have a linear relationship with the magnitude component. If a sufficient amount of variability is recorded in the frequency component data, the ratio between the magnitude component and the frequency component includes information on the wearing position of the wearable base part 14. An appropriate correction coefficient can be applied to the magnitude component according to predetermined reference ratios of the magnitude component and the frequency component representing preferred wearing positions of the wearable base part 14.

Hence, the wearing position can be determined based on different measurement values and based on different calculation methods.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope.