Patent Publication Number: US-2016228744-A1

Title: Device and method for the classification and the reclassification of a user activity

Description:
FIELD OF THE INVENTION 
     The present invention relates to portable devices capable of monitoring and registering the activities of its user More specifically, the present invention relates to the way a portable activity monitoring device classifies the activities of its user 
     BACKGROUND PRIOR ART 
     An activity monitoring device is a device configured for the evaluation of the user&#39;s daily activities Such a device is capable of detecting the types of activity the user is performing and may be able to quantify the detected activities. For example, the device could be used to detect when a user is walking and then count the number of steps and/or stairs the user takes, and determine the daily calorie burn of the user These devices are usually portable, and can take for example the form of a bracelet or a (belt) clip. In these forms, they can be worn by the user all day long and continuously monitor his/her activity. Portable activity monitoring devices usually contain one or more sensors, for example accelerometers that measure the accelerations of the device due to the movements of the user Nowadays, smart phones also include many types of sensors, including motion sensors, and can therefore also be used for this type of monitoring. 
     One important aspect of activity monitoring is the separation of activities into classes, or the classification of activities The principle of a classification is to assign a class of activity such as “walk”, “run”, “rest”, “play tennis” to certain periods in the user&#39;s day This classification of activity may be used to determine the percentage of time that a user spends performing his/her different activities. A correct determination and classification of activities also allows the activation of activity-specific monitoring For example, if the current detected activity of the user is “climb stairs”, a specific procedure can be launched for counting steps and/or floors. 
     A simple classification system, where one is not interested in the type of activity, but rather the amount of activity, classifies the activities of the user in various classes representing a predetermined amount of activity by measuring the level of activity of the user for a short time window, less than 1 minute for example This system may comprise e g 3 different classes: “rest”, “low activity” and “high activity” Although this classification provides a hint about the user&#39;s activity, it fails to determine the precise activity of the user. For example, an activity identified as “high activity” may represent the activity of running as well as biking 
     Moreover, such instantaneous classification fails when interpreting more complex or varying activities For example, if the user is resting but changes his/her rest posture while still resting, a time window centered on the posture change may interpret it as “low activity”, while the user is globally still resting. 
     In this application, we will refer as “primary activity” any activity which can be determined by monitoring the user for a very short time window Many primary activities are basic and can be cyclic or repetitive. For example, walking is an activity that is very basic and repetitive and can be detected by observing the movement of the user in a very short time (e g the time it takes the user to take a few steps) On the other hand, hiking, while including the primary activity of walking, can only be determined by observing the user over a longer time period, and may also include other primary activities such as resting, standing or sitting Indeed, it is impossible for a device to determine that the user is hiking with the only information that it is walking as concluded from a short term analysis Such activities that cannot be determined using a short term analysis will be referred to as “secondary activity”, and may contain a variation or repetition of one or more primary activities. 
     US 2013/0190903 (BALAKRISHNAN and al), Jul. 25, 2013 discloses an activity classification that can operate on a small amount of time windows If, in a small amount of time, the vast majority of time windows detect an activity type, for example resting, all time windows can be set to this activity type, ignoring any outliers. This method improves short-term activity classification Indeed, in the example of a sudden move during the rest, this classification will apply the activity type of the vast majority of time windows, which still is “rest”, to all time windows including the one that was initially detected as “low activity”. 
     Detection of secondary activities like sleeping or hiking is a desirable feature for the monitoring of these specific activities. For example, the determination of the periods when the user is sleeping may allow the monitoring of data, such as sleep time, sleep quality, sleep cycles, or other feedback about his/her sleep that may be of interest to a user. However, a short time classification of activities is only able to detect primary activities, and is unable to understand the secondary activity of the user. For example, a short term analysis will not be able to detect the difference between a small rest and sleeping, or between a small utilitarian walk and a real hike 
     Some devices overcome this issue by requiring the user to manually start and stop specific activity monitoring procedure. This is for instance the case with the Jawbone Up, the Fitbit Flex, or the Withings Pulse devices, where the sleep analysis function has to be started manually by the user 
     Although achieving the desirable goal of using functions associated with secondary activities, such manual activation is very constraining for the user They also may cause a loss of accuracy in the duration of activity monitoring For example, a user may not likely manually start a sleep monitoring function at the exact time he/she is falling asleep. 
     One objective of the invention is to provide an activity monitoring device which is able, not only to automatically classify primary activities, but also to automatically classify secondary activities, as well as starting and stopping monitoring functions associated with secondary activities 
     SUMMARY OF THE INVENTION 
     To this effect, the invention discloses a portable device to monitor at least an activity of a user, said portable device comprising at least: a first sensor configured to capture signals representative of at least a physical parameter induced by at least an activity of the user; a channel of communication to processing capabilities, said processing capabilities configured to: perform a first classification algorithm to classify the signals representative of the at least a physical parameter over a first period of time into a first set of classes of activities of the user; perform a second classification algorithm to classify the at least an activity of the user at the output of the at least first classification algorithm over a second period of time which includes the first period of time using a second set of classes which is different from the first set of classes. 
     Advantageously, the second classification algorithm evaluates the activities of the user according to at least an analysis of the output of the first classification algorithm during the second period of time, which is larger than the first period of time. 
     In an embodiment of the invention, the classification of activities into a second set of classes is at least based on a data from a second sensor 
     In an embodiment of the invention, the second sensor is packaged with the portable device 
     In an embodiment of the invention, the second sensor is packaged with a second device 
     In an embodiment of the invention, at least parts of the processing capabilities are packaged with a second device in communication with the portable device 
     In an embodiment of the invention, at least parts of the processing capabilities are packaged with a third device in communication with the portable device 
     In an embodiment of the invention, the processing capabilities are further configured to first label the at least an activity with a first class attributed by the first classification algorithm and then replace this first label by a second label corresponding to a second class attributed by the second classification algorithm. 
     In an embodiment of the invention, the processing capabilities are further configured to first label the at least an activity with a first class attributed by the first classification algorithm and then add to this first label a second label corresponding to a second class attributed by the second classification algorithm 
     Advantageously, the processing capabilities are further configured to, if the second algorithm attributes at least one class of activity, launch a procedure in relation to said class of activity 
     Advantageously, at least one sensor is one of accelerometer, gyrometer, magnetometer, pressure sensor, temperature sensor, gps sensor, audio sensor, optical sensor 
     Advantageously, at least a physical parameter relates to the motion of the device imparted to the device by the movement of the user, and the first set of activities relates to the quantity of motion determined by the first classification algorithm. 
     In an embodiment of the invention, the second set of classes comprises at least a class of activity in relation to the sleep activity of the user. 
     Advantageously, the attribution of a class from the at least one class of activity in relation to the sleep of the user, starts an procedure for monitoring at least one parameter of the sleep of the user. 
     The invention also relates to a method to monitor at least an activity of a user, said method comprising at least capturing signals representative of at least a physical parameter induced by the at least an activity of the user; performing a first classification algorithm to classify the signals representative of the at least a physical parameter over a first period of time using a first set of classes of activities of the user; performing a second classification algorithm to classify at least an activity of the user at the output of at least the first classification algorithm over a second period of time which includes at least the first period of time using a second set of classes which is different from the first set of classes. 
     The invention also relates to a computer program product, stored on a computer readable medium, configured for, when it is executed by a processor, monitoring at least an activity of a user, said program comprising at least code means for capturing signals representative of at least a physical parameter induced by at least an activity of the user; code means for performing a first classification algorithm to classify the signals representative of the at least a physical parameter over a first period of time using a first set of classes of activities of the user; code means for performing a second classification algorithm to classify at least an activity of the user at the output of at least the first classification algorithm over a second period of time which includes at least the first period of time using a second set of classes which is different from the first set of classes 
     The invention also relates to a system to monitor at least an activity of a user, said system comprising at least: a first device comprising a first sensor configured to capture signals representative of at least a physical parameter induced by at least a an activity of the user; a portable device configured to: perform a first classification algorithm to classify the signals representative of the at least a physical parameter over a first period of time using a first set of classes of activities of the user; perform a second algorithm to classify at least an activity of the user at the output of at least the first classification algorithm over a second period of time which includes at least the first period of time using a second set of classes which is different from the first set of classes 
     A device according to the invention has the ability to classify secondary activities, and is therefore able to provide information about a larger number of activities than an activity monitoring device that operates only with short time information 
     A device according to the invention also provides a finer activity classification, since it is able to classify activities using activity history, and also, possibly a plurality of internal or external sensors and connected devices 
     A device according to the invention is also able to automatically monitor a large number of activities with activity-specific procedures without any intervention from the user. 
     A device according to the invention offers a greater flexibility for setting the parameters of activity classifications, since it provides parameters for the first classification and the secondary reclassification 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and its various features and advantages will emerge from the following description of a number of exemplary embodiments and its appended figures in which: 
         FIG. 1  displays an example of a portable device configured to monitor the activities of the user and display an activity report; 
         FIG. 2  displays an example of a functional architecture of a portable activity device configured to monitor the activities of the user; 
         FIG. 3  displays an example of a method of classification and reclassification of activities in a number of embodiments of the invention; 
         FIGS. 4 a , 4 b , 4 c  and 4 d    display respectively an example of sensor output, activity classification, activity reclassification and specific procedure launch in the exemplary case of Automatic Sleep Mode Activation; 
         FIGS. 5 a  and 5 b    display an example of activity classification and activity reclassification using an internal sensor, in the exemplary case of a mountain hike; 
         FIGS. 6 a , 6 b , and 6 c    display an example of activity classification and activity reclassification using an external sensor and specific procedure launch in the exemplary case of tennis; 
         FIG. 7  displays an example of a 2 level reclassification process in the example of a user performing a 200 m swim medley; 
         FIG. 8  displays an example of activity classification and reclassification; in the exemplary case of public transportation; 
         FIG. 9  displays an example of an architecture of a portable device configured to monitor the activities of the user. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  displays an example of a portable device that can be worn on the wrist of the user and is configured to monitor the activities of the user and display an activity report. The device is equipped with MEMS accelerometers to detect the movement of the device (e.g Invensense MPU9250, Freescale MMA8451Q) The acquired accelerometer signals are processed by a microcontroller (e.g. STM32f4) with an incorporated flash memory which contains the processing algorithms that determine the activity based on the accelerometer signals. The resulting activity logs that are calculated in the microcontroller are stored in a second flash memory linked to the microcontroller Communication with the device, for example to retrieve the produced activity reports, is possible through a USB connector or by a wireless bluetooth protocol (e g. Texas Instruments CC2540) In addition, the device features a small display where information related to the user&#39;s activities can be shown, for example the amount of steps 
       FIG. 2  displays an example of a functional architecture of a portable device configured to monitor the activities of the user The portable device  200  comprises a sensing module  210 , an analysis module  220 , a storage module  230  and a communication module  240 . The sensing module  210  contains the sensors to monitor the user&#39;s activities The analysis module  220  analyses the information from sensing module  210 . In particular, it is able to determine, according to information from sensing module  210 , the activity of a user during a given period of time Storage module  230  is used to store the output of analysis module  220 . Information previously stored in module  230  might also be reused in the analysis module  220 . The type of information that is stored depends on the analysis module and might vary from raw sensor signals to a final activity histogram of the user. Finally, communication module  240  is used to communicate information to the user, either by displaying the information directly to the user, in which case the device has to be equipped with some type of display (or audio) means, or transmitting the information to another device which is able to present the data to the user, such as for example a smartphone. This transmission can be performed by means of e.g. a USB connection, or a wireless connection such as e g Bluetooth. 
     The device may also be able to communicate with other devices which might include additional sensors, for example to retrieve additional information about user&#39;s activity. Such devices can be for example a tennis racket connected to a portable device to monitor tennis activity, a pair of shoes connected to portable device to monitor running activity, or any other device able to provide additional information about user activity 
     Alternatively, the sensing module and the analysis module might not be in the same device For example, the sensing module might be in a device worn by the user (e g a bracelet), which then communicates the sensor data to a second device (e.g. smartphone) which includes the analysis module and determines the activity of the user This second device may also acquire additional info from other sensors or devices. 
     The device is able to translate the signals from the sensors in the sensing module into information relating to the activity of the user. Different types of sensors can be used depending on the activities that the device should be able to classify. In many cases the detection of the movement of the device is important, which can be done using accelerometers, gyrometers, and magnetometers to detect linear accelerations, angular velocities, and heading changes respectively. Pressure sensors can in addition be used to detect a change in elevation Sound/audio sensors (microphone) and image sensors (camera) may also be used to get information on the activity of the user, directly or indirectly, for example by analyzing the surroundings of the user A GPS sensor can be added if the absolute position of the device needs to be known. The sensors mentioned here are not meant to represent an exhaustive list, but should merely serve as examples. More generally, a first sensor shall be configured to capture signals representative of at least a physical parameter induced by at least an activity of the user. The person skilled in the art will be able to determine which sensors are needed in sensing module  210  depending on the activities that the device should be able to detect, the physical parameters related to these activities, and on the classification performed in analysis module  220   
     In a first embodiment, a sensor detects the movements of the whole body, for example the translation/rotation of the whole body of the user, and the classification of activities is made according to these global movements 
     In a second embodiment, a sensor is attached to a e g limb of the user and detects the movement of a specific limb For example, a sensor attached to the ankle of the user is able to classify activities related to walking or running with greater accuracy 
     According to the embodiments, the classification can be performed according to data from one sensor, or by fusing data from a plurality of sensors. In an embodiment with a plurality of sensors, sensors may be localized in the same device or in separate ones. Devices that contain sensors for activity classification may all be worn by the user, or at least one may not be worn by the user 
       FIG. 3  displays an example of a method of classification and reclassification of an activity in a number of embodiments of the invention. 
     In a number of embodiments of the invention, a first classification algorithm  320  analyses signals representative of the activity of the user from at least one sensor  310  over a first period of time This algorithm is performing the first analysis of the sensor signals, and detects the primary class of activity of the user It will be therefore referred to as “primary classification algorithm” or “first classification algorithm”. Primary classes of activities are therefore the classes of activities that can often be instantly determined by analysis within a short time window. Based on the analysis, the activity of the user is attributed a class selected from a first set of classes of activities of the user. The available classes in the first set of classes depend on the details and capabilities of the first classification algorithm  320 . 
     In an embodiment, a device according to the invention is equipped with a (3-axis) accelerometer The device might, for example, be connected to the wrist of the user, or attached at the waist By measuring the acceleration, a primary activity of the user is determined and divided into classes The type of activity classes depends on the analysis algorithm For example, in a basic embodiment we can determine the average acceleration for a certain time period (e g of the order of 30 seconds) and attribute classes by comparing the acceleration to certain thresholds: ‘rest’ in the (quasi-) absence of acceleration, ‘low activity’ for a medium average acceleration, and ‘high activity’ for high average accelerations 
     In another embodiment, with a more advanced analysis of the accelerometer signal, it is possible to determine if the user is lying, sitting, standing, walking, or running and attribute the corresponding classes. 
     First classification algorithm  320  classifies the activity of the user for each period of time into activity blocks  330  An activity block represents the activity performed by the user over a certain period of time. In a number of embodiments, an activity block contains information about the beginning and duration of an activity, and at least one activity class Activity blocks may have a fixed predefined duration, or a variable duration. For example, if the analysis algorithm analyzes the sensor signals for successive periods of 10 seconds and then attributes an activity class, the result will be a sequence of activity blocks, wherein each block represents the activity of the user for a period of 10 seconds The duration of the activity blocks might be constant, which means that if the user is performing an activity for a period longer then a single block, it will result in a sequence of identical blocks The typical duration of an activity block might vary between a few seconds and a few minutes depending on the activities that are of interest. For example, to determine if a user is standing, walking or running, an activity block might be quite short, such as 5 seconds, because this covers at least a few steps of the person On the other hands, to analyze the sleep of the user, the activity blocks might be much longer, of the order of 1 minute or more 
     Alternatively, the duration of the activity blocks might be variable. For example, an activity blocks might have the duration of the actual activity meaning an activity blocks start when there is a change in activity, and ends at the next activity change. In this case the activity block contains the activity and the duration. In the device  200 , activity blocks  330  are stored in the storage module  230 . Alternatively, the communication module  240  might be used to transfer the activity blocks  330  to an external storage device such as e g a smartphone 
     The first classification algorithm  320  analyses the sensor signals over a relatively short time window, which means that the activity blocks  230  represent a short term view of the users activity In addition, the device may perform a second classification algorithm  340  at the output of at least the first classification algorithm. This second classification takes place over a second period of time and classifies the activities into a second set of classes of activities which is different from the first set of classes, where the second period of time includes at least the first period of time The second classification algorithm  340  may be applied to the activity blocks in order to obtain more detailed knowledge of the activity of the user. For example, this second classification algorithm can be performed over a longer period of time by analyzing a sequence of activity blocks, or by combining the information of the activity blocks with more information from other sensors  350  from the same device or from external devices  360 . 
     If the second classification algorithm yields more information, i e if the second classification algorithm classifies the activities of the user over said first period of time in a class of activity different from the output of the first classification algorithm over said first period of time, the classes attributed to the activity blocks after the first classification might need to be changed The device can thus perform a reclassification  370  of the activities of the user over the first period of time into said class of activity different from the output of the first classification algorithm 
     The step of reclassification  370  updates activity blocks according to the output of second classification algorithm  340 . Activities that belong to the second set of classes will be referred to as “secondary activities” 
     The step of reclassification  370  may have a number of different objectives:
         Partition a list of activities into a second set of classes, different from the first set of classes, for which the classification cannot be based only on instantaneous or short term analysis of the sensor signals used for first classification For example, ‘sleep’ is a secondary activity because in a short term analysis it is not possible to differentiate between sleep and rest or momentary absence of movement. The ‘sleep’ classification will be discussed in detail in the first example below;   Combine the measurements of a plurality of sensors of different types or in different devices, possibly on different timelines, to combine two or more different classifications of primary activities into a third set of classes of activities where the reclassification is based on a list of secondary activities which are a combination of the plurality of primary activities or states. For example, a first device worn on the wrist might be classifying the activities of the user as walking and running, and a second device in the form of an electronic tennis racket is classifying different types of tennis swings By combining the two different first classifications, the secondary algorithm classifies the activity as ‘playing tennis’. The tennis classification will be discussed in detail in the third example below;   Disambiguate a class of activities where the first classification algorithm was not able to attribute a specific activity In this case the first classification algorithm might have attributed, for example, ‘unknown’ or ‘inconclusive’, and the second classification algorithm might be able to determine the actual class of activities over time or with other sensors       

     Reclassification can be performed a number of times in succession or in an iterative manner, where the output of the second classification algorithm can serve again as the input in a next reclassification cycle using a longer time period or additional sensors In specific embodiment of the invention, classes of states can be organized in a tree structure By way of example: “Practicing sport”&lt;-“Practicing athletics”&lt;-“Running”&lt;-“Running 100 m” In this tree structure, over a given period of time, the activity “practicing sport” is attributed by the first classification algorithm Within this period of time, the second classification algorithm can classify an activity which was “practicing sport” into “practicing athletics”, which is a first level of reclassification The same second algorithm can classify activities that contained at least “practicing athletics” into “running” This is a second level of classification, because the attribution of the class “running” requires the class “practicing athletics” to have been attributed at least once, in a first level of classification The same reasoning applies to the attribution of class “running 100 m”, which imply that the class “running” has been attributed at least once, and is therefore a third level of reclassification. 
     In an embodiment, reclassification by reclassification algorithm  370  is done by replacing the original value representative of the class of activity in an activity block by a new value as determined by the secondary analysis. In another embodiment, activities are labels of the activity blocks. A label corresponding to a class attributed by the second classification algorithm can replace a label attributed by the first reclassification algorithm, or be added to it An activity block can thus have more than one activity label Therefore, in this embodiment an activity block is labeled with at least a primary activity from a first set of classes, and may be labeled with at least one secondary activity from a second set of classes, if it has been reclassified. In this case, the reclassification adds at least one class from the second set of classes of activities to a class from the first set of classes of activities 
     The second classification algorithm  340  uses the activity blocks  330  produced by the first classification algorithm  320  When specific patterns of classes or a repetition of the same activity class occur, it is able to detect the occurrence of an additional class of activity, and classify a set of activity blocks into that class If no specific pattern is found, the activity blocks keep their original classes. In an embodiment, the second classification algorithm  340  only uses the content of activity blocks to perform the classification. In another embodiment, the second classification algorithm  340  also uses information from an additional sensor  350  to perform its classification. In another embodiment, the second classification algorithm  340  also uses information from external devices  360  to perform its classification Examples provided hereinafter will make apparent to the skilled man these different choices for the reclassification algorithm and their respective benefits 
     The available classes in the first and second set of classes depend on the primary and second classification algorithm, respectively, and may depend on the device A very basic first classification algorithm might only have a set a classes including ‘no activity’, ‘low activity’, and ‘high activity’, while an advanced first classification algorithm may have a set a classes including ‘lying’, ‘sitting’, ‘standing’, ‘walking’, and ‘running’. Therefore, the complexity of the designation of the different classes depends on the sophistication of the primary algorithm, i e the activities that the algorithm can recognize, and the specifications of the device In addition, the set of classes might also depend on the requirements or demands of the user. 
     A similar reasoning is valid for the second classification algorithm; the classes in the second set of classes depend on the algorithm and the specifications of the device 
     Once the reclassification step  370  has successfully classified one or a plurality of activity blocks into an additional activity class, it also may start an activity specific algorithm  380  to provide, for example, additional information about the activity to the user. For example, if the second classification algorithm  340  has detected that the user was sleeping, it may further launch a sleep quality monitoring algorithm In another example, if the second classification algorithm  340  has detected that the user was jogging, reclassification step  370  may, while updating the content of activity blocs, launch a specific procedure to monitor the duration and speed of the jogging, the distance crossed, and the heart rate of the user 
     The activity blocks can be used to present an activity summary  390  to the user This information may comprise the list of activities performed by the user according to activity blocks  230 , as well as any information provided by the activity specific algorithm  370  In an embodiment, the device presents the activity information directly to the user by means of a display incorporated in the device (see e g  FIG. 1 ). In another embodiment, for example when the device is not equipped with a display, the activity summary is transmitted to a second device where the user can look at his activity summary on the display 
     In a situation where the user accesses the information in the activity summary after the reclassification has taken place, the user does not necessarily need to be aware that such a reclassification has taken place. Of course, the occurrence of the reclassification might be reported to the user However, if the user has instant access to the real time information of the activity blocks and the activity summary, the user might be confused in changes in the activity report if he or she is not made aware of the reclassification 
     The examples below will present a number of embodiments of the invention, and will make its various features and benefits apparent to the person skilled in the art 
     First Example 
       FIGS. 4 a , 4 b , 4 c  and 4 d    display respectively an example of sensor output, activity classification, activity reclassification and specific algorithm launch in the exemplary case of Automatic Sleep Mode Activation 
     In a device, the Automatic Sleep Mode Activation is used to automatically detect when the user is sleeping, in order to perform a sleep analysis. This means that the device determines when the user is falling asleep, and when the user is waking up, and analyses the total sleep time and sleep quality in this time period This Automatic Sleep Mode Activation is explained by means of example only, and runs on a portable device that can be worn on the wrists and contains at least an accelerometer to measure the motion of the device imparted to the device by the movement of the user 
       FIG. 4 a    displays the normalized output of the accelerometer of the device as a function of the time of the day, in this case during the night starting from about midnight The accelerometer signals as shown in  FIG. 4 a    are representative of the quantity of activity and the higher the signal the more the (wrist of the) user moves. For example, the figure shows a lot of movement  420   a  before 01:00 as the user goes to bed During the night there are long periods of no movement at all  430   a  (e g 01:00 to 01:40), intersected with periods of small activity  440   a  (e.g. 02:10) This is typical behavior of the various sleep cycles a person experiences during the night A typical sleep cycle is of the order of 90 to 120 minutes, and contains a period of deep sleep where muscle activity is slowed down, and a period of REM or paradoxical sleep where the brain is extremely active. After the REM and between two cycles, a brief awake phase occurs which is most of the time forgotten and is sent to the subconscious. During this wake phase some movement or repositioning takes place 
       FIG. 4 b    shows a schematic representation of the activity blocks  330  at the output of a first classification algorithm  320  for an extracted time window from 00:40 until 02:40 Note that the widths of the activity blocks are exaggerated for presentation purposes In this example, the first classification algorithm  320  computes the average value of the accelerometer output over the duration of a small time window, and classifies, according to the average level of activity, the activity blocks into 3 classes representing a quantity of activity/motion: ‘high activity’ when the level of activity is above a first threshold; ‘rest’ when the level of activity if below a second threshold, lower than the first threshold, and ‘low activity’ when the average level of activity is between first and second thresholds According to the data displayed in  FIG. 4 a   , the first classification algorithm  320  classified activity blocks  420   b  before 01:00 into ‘high activity’ class, which is as expected with the user being active as he or she is preparing to go the bed. Activity block  430   b  corresponds to ‘low activity’ as the user is starting to fall asleep, followed by a series of activity blocks  440   b  where no activity is detected and which are thus classified as ‘rest’. After a first sleep cycle, the user moves a bit and activity blocks  450   b  are classified as ‘low activity’, which is followed by another series of activity block  460   b  classified as ‘rest’ 
     The first classification algorithm has converted the signals from  FIG. 4 a    into a series of activity blocks and attributed a class from the first set of classes to each block (‘high activity’, ‘low activity’, ‘rest’). Using the first classification algorithm  320  it is not possible to distinguish between activity blocks that have been classified as ‘rest’ because the user was sleeping during the night or because there was a momentarily period of inactivity during the day. If we want to be able to monitor the activities during the day and monitor the sleep quality during the night we have to be able to distinguish between a rest block during the day when the user is resting but is awake, from a rest block during the night when the user is sleeping. This is exactly the goal of the second classification algorithm  340   
       FIG. 4 b    shows a complete sequence of activity blocks  440   b  that are classified as rest because no activity is detected. Such a sequence is very logical when the user is sleeping, but highly unlikely during the day when the user is active and wearing the device Therefore, it is possible to define a rule that stipulates that after a sequence of N rest blocks in a row, the user is considered to be sleeping, and not merely resting Alternatively, the rule could be that for N activity blocks the percentage of blocks classified as blocks should be above a predefined threshold e g. 90% The second classification algorithm is thus designed to analyze the sequence of activity blocks that is output from the first classification algorithm, and detect if a sequence of N consecutive rest block is present. If this is the case it is determined that the user is sleeping since the start of the sequence, and the reclassification algorithm  370  is launched  460   b  N might depend on the user, and can be pre-set, adjusted by the user, or adjusted automatically depending on the habits of the user 
     At least one of the first or second classification algorithm should preferably also be able to detect false positives, meaning detecting when to user is not actually wearing the device so as to not to confuse this with the user sleeping For example, if absolutely no movement is detected for an extended period of time, larger than the typical sleep cycle, it is more likely that the user is not wearing the device than that the user is sleeping. If the orientation of the device can be determined, this information can also be used as the typical orientation when wearing the device may be different from when the device is placed on a hard surface If an internal clock is present, the time of day might be used to help determine the probability that the user is sleeping If the device is equipped with a temperature sensor, the measured temperature can also help to determine when the user is wearing the device as in general the temperature of the device is higher when worn due to the radiation of the body temperature of the user 
       FIG. 4 c    displays the activity blocks  330  from  FIG. 4 b    at the output of the reclassification  370 , using the first set of classes (rest, low activity, high activity) and the second set of classes (deep sleep, light sleep, restless) 
     At the start of the reclassification  460   b , the reclassification algorithm will reclassify all the blocks of the current sequence of blocks that were classified as rest  440   b  by the first classification algorithm into the new class of ‘deep sleep’ (DS)  440   c . The activity blocks immediately before the rest sequence that were previously classified as ‘low activity’  430   b  might be reclassified as ‘light sleep’  430   c  as the user was most likely in the process of falling asleep The high activity block  420   b , might be classified as restless  420   c , but might also be considered as being not part of the sleep process and therefore not be reclassified In addition to reclassifying the sequence of the N rest blocks in deep sleep, the start of the reclassification also launches the activity specific algorithm  380 , in this case the ‘sleep mode’ As a consequence the future activity blocks with no activity will be directly classified as deep sleep ( 450   c ,  470   c ), and any block with low activity will be classified as light sleep  460   c . In other words, since it is detected that the user is sleeping, the future classification will be performed using classes from the second set of classes corresponding to the sleeping activity. This might be done as immediate reclassification by the second classification algorithm, or instructions might be passed to the first classification algorithm to temporarily apply classes from the second set of classes 
       FIG. 4 d    displays the result of the reclassification where a class is attributed from the second set of classes throughout the night 
     In the sleep mode, the activity specific algorithm or procedure can also be used to measure the actual sleep time, the sleep quality, the number of sleep cycles, the duration of the cycles etc 
     Once in the sleep mode, the algorithm should also detect when the user is waking up, in order to de-activate the sleep mode and stop the automatic reclassification in the set of classes corresponding to the sleep mode. This detection can be similar as the start of the sleep mode, where it is determined that the user is awake after a certain sequence of consecutive activity block with a high activity. 
     In the embodiments described above the reclassification starts as soon as the sequence of N consecutive rest block is detected. In this case the reclassification takes place in the device and the activity blocks that are output from the device have the correctly attributed classes according to the primary and second classification algorithms In an alternative embodiment the reclassification might not be instantaneous. The device might only perform the first classification algorithm, and then output the sequence of activity blocks A second device, e g. a smartphone, might communicate with the first device through the communication module  240  and obtain the sequence of activity blocks, and run the second classification on the second device. In this case the reclassification is identical: look for a sequence of N rest blocks which is considered the start of the sleep, and perform the reclassification according to the second set of classes until another sequence of M high activity blocks which is considered being caused by the user waking up 
     Second Example 
       FIGS. 5 a  and 5 b    display an example of activity classification and activity reclassification using an internal sensor, in the exemplary case of a hike in the mountains. 
     Using a portable activity monitoring device and a first classification it is possible to divide the daily activities of the user in e g. lying, sitting, standing, walking During a normal day people walk from point A to B, but in general these are short walks for the purpose of transportation On the other hand, people also walk for leisure or health purposes A first classification algorithm is not able to distinguish between walking for transportation and for leisure 
     In this example the user is performing a hike for leisure, for example in the mountains. The portable device comprises a gyrometer with a least two axes and a GPS and the first classification algorithm  320  is similar to the one used in the first example This device is able, by performing a frequency analysis of the gyrometer signals on the different axes, to determine that the user is walking according to e g. the method disclosed in EP2400889 
       FIG. 5 a    displays the activity blocks at the output of a first classification algorithm  320  during the hike in an embodiment of the invention. The first classification algorithm classifies the activity of the user into 3 classes, for example: “rest”, “walk” and “run” according to the frequency of the movement of the user. The activity blocks  510   a  show that the user is walking (note that the width of the blocks are exaggerated for presentation purposes) The user takes a small rest  520   a  in the middle of the hike, after which the user continues to walk  530   a.    
       FIG. 5 b    displays the activity blocks at the output of the reclassification algorithm  370  during the hike in the mountains in an embodiment of the invention In this example, the second classification algorithm  340  analyses the activity blocks ( 510   a ,  520   a ,  530   a ) output by the first classification algorithm and can use in addition information from the GPS sensor data Similar to the sleep analysis, the algorithm can look at the duration of the walking activity In general, walking for transport purposes has a short duration, while walking for leisure on a hike has a longer duration (e g &gt;20-30 minutes). The walking does not have to be continuous as in general people take small rests while on a long hike For example, during a hike that lasts 2 hours, 90% of activity blocks may be classified as ‘walk’ and 10% as ‘rest’ In addition, the GPS data indicates that the user has travelled 8 kilometers and climbed 450 meters of altitude The GPS location can also help determine the type of walk, e g. in this example the GPS location can indicated the hike was in the mountains When the second classification algorithm determines that the user is hiking, it can start the reclassification algorithm  370  It therefore adds the ‘hiking’ labels (‘H’) to the activity blocks ( 510   b ,  520   b , and  530   b ). By adding the hiking label even to the rest period  520   b ) the amount of rest during the hike can also be determined, and this rest can be separated from the resting or in-active periods during the other activities of the day It also can launch a hike monitoring procedure  370  to evaluate the total duration, speed or distance of the hike. 
     Third Example 
       FIGS. 6 a  and 6 b    display an example of activity classification and activity reclassification using an external sensor and specific procedure launch in the exemplary case of tennis. 
     In this example, the user is playing tennis The portable device comprises motion sensors including at least an accelerometer, and the first classification algorithm is capable of detecting at least the activities: standing, walking, running. The user is using an electronic tennis racket, meaning a racket equipped with motion sensors and able to detect the different types of swings, effects, and the impact locations (e.g. Play Pure Drive racket of Babolat) The racket may be equipped with piezoelectric sensors to detect the impact location, and a combination of accelerometer and gyrometer to determine the type of swing and effect An example of such a racket is disclosed in detail in WO2013/167395 
     During the tennis game, the user alternates periods of standing, walking and running.  FIG. 6 a    shows an example of the first classification in a game in tennis, where initially the user is standing  620   a  to serve or receive a service During the game the user is running  630   a  from one tennis swing to the next When the game has finished the user is walking  640   a  back to the baseline to serve again or receive the next serve  650   a    
     When the electronic racket is able to communicate with the activity monitoring device, the second classification algorithm  340  can link the activity of the racket with the results of the first classification.  FIG. 6 b    displays the output of a reclassification algorithm  370 , which reclassifies activity blocks from  FIG. 6 a    ( 620   a - 650   a ) and adds the activity label ‘tennis’ (‘T’) to them ( 620   b - 650   b ) 
     In this example the tennis racket is communicating with the portable activity monitoring device and the second classification algorithm running on the device can perform the reclassification immediately during the tennis if a real-time communication link is established, or the reclassification can be performed after the tennis if the device and the racket can only communicate once the play has finished In an alternative embodiment, at least parts of processing capabilities are comprised within a third device in communication with the device as well as the tennis racket This third device, for example a smartphone, combines the information from both device and performs the reclassification 
     The reclassification algorithm  340  may also start an activity specific procedure  370  to monitor the tennis game or any interesting information about this activity For example, the effective duration of the point can be determined by detecting the start of the point when the racket detects a serve, and the end of the point when the user stops running, and starts walking back to the baseline In addition the (average) amount, and types of swing, during the points can be determined If the racket or portable device is equipped with a localization sensor, the monitoring procedure may also compute the distance travelled during the tennis game. If the portable device comprises a sensor or function to compute the calorie burn, it may also compute calorie burn during the tennis game 
     The activity specific algorithm  370  that is started once it has been detected that the user is playing tennis can also start a multi-level reclassification process  FIG. 6 c    shows an example of the multi-level process in the present case of tennis. In level 1, the first classification  620   c  is combined with the information from the racket  630   c  as discussed in  FIGS. 6 a  and 6 b    A point always starts with a serve (SV), and is followed, if the serve is not an ace, by one or more backhands (BH) or forehand (FH) swings As mentioned above, during the point the user is running, and at the end of the point the user is walking (back to the baseline) All the activity blocks from the start (serve) to the end (walking) belong to the same point and can be classified as ‘point  1 ’ ( 640   c ). The same player serves for the next points, and the same process of classifying the ‘points’ is repeated until point n  650   c  when the game is won by one of the two players. 
     After the game has been decided, the other player is performing the serve, which means that the player who was serving in the previous point will now be the one receiving the serve and will start the points by either a forehand  660   c  or backhand swing. By detecting that the other player is performing the serve, it can be deduced that the previous point was the last point of the previous set, and that the point currently being played is the first point of the next game Now in the level 2 reclassification, the output of the level 1 reclassification serves as the input for the level 2 reclassification and the activity label ‘game  1 ’  670   c  can be added to the points for the game in question ( 640   c - 650   c ) 
     Fourth Example 
       FIG. 7  shows an example of a 2 level reclassification process in the example of a user performing a 200 m swim medley. 
     In this example the user is wearing a portable activity monitoring device with algorithms designed for swimming. The device is able to detect the different swim strokes of the user (breaststroke, backstroke, front crawl and butterfly), and can detect when the user is turning in order to count the number of lanes. The first sensor may capture the signals representative of the motion of the swimmer Alternatively, the first sensor is a pressure sensor that detects pressures variations when swimming. Some examples of such methods are disclosed in US2012/072165 (swim stroke) and US 2012/0283856 (changes of directions) In this case, the first classification algorithm analyses the output of sensor by successive short periods According to detection methods such as those disclosed in the references above, the first classification algorithm determines that the user is successively performing butterfly  710 , backstroke  711 , breaststroke  712 , front crawl  713 , and then rest  714  In parallel, the device uses a method such as those described in US 2012/0283856 to detect the turns, marked ‘T’  720   
     Over a longer period of time, a level 1 secondary classification algorithm analyses the output  710 ,  711 ,  712 ,  713 ,  714  of the first classification algorithm, along with information ‘T’ about turns. The length of the pool may have been entered manually by the user, determined from the displacement of the user, or by GPS. Using that information, the secondary classification algorithm may determine that the user has successfully performed 200 m butterfly  730 , 200 m backstroke  731 , 200 m breaststroke  732 , 200 m front crawl  733 , and rest  714   
     In addition to the second classification where the separate swim strokes and distances are classified, a level 2 second classification algorithm can also be used. The level 1 second classification has determined that the user has done 4 different swim strokes, each for 200 m, after which the user is a rest  714 . The level 2 classification can be programmed to recognize this pattern as a ‘200 m medley’, using the results from the level 1 second classification as an input Thus, after the 200 m front crawl, the reclassification  370  can add the ‘200 m medley’ classification label  740   
     This example demonstrates the ability of the invention to classify activities on several levels It is also able to mix activities from several levels, for example 200 m medley  740  (level 2 reclassification) and rest  714  (first classification) It is also possible to reclassify classes of activity over only a part of the time window For example, the second level of reclassification has reclassified some of the activity blocks in the time window into 200 m medley  740 , but let others in the rest activity  714  which is issued from the first classification algorithm It is possible to run concurrently a plurality of second classification algorithms  340 . In addition, the second classification algorithm may be able to classify activities according to the output of the first classification algorithm  320 , a second classification algorithm  340  or a mix of activity blocks from both the first classification algorithm  320  and a second classification algorithm  340 . 
     Fifth Example 
       FIG. 8  displays an example of activity classification and activity reclassification using an internal sensor, in the exemplary case of the user using means of public transportation 
     In this example the user is using a means of public transportation, for example a bus, metro, or train The activity blocks of the first classification  320  in  FIG. 8  shows that the user is initially walking (‘w’)  820  to the bus stop At the bus stop, the user is standing (‘std’)  830  and waiting for the bus to arrive. Once the bus has arrived, the user enters the bus and walks to a seat  840  During the trajectory the user is sitting  850  Arrived at the bus stop close to his or her destination, the user gets up and walks out of the bus and continues walking to the destination  860   
     The second classification algorithm  340  receives the activity blocks, and in addition can receive information from the microphone and the GPS It infers, according to the sequence of activities  820 ,  830 ,  840 ,  850 ,  860  and the information from GPS, that the user moved while he was sitting  850  and must thus be using some means of transportation. When the user is in the bus (or metro, train, .) the microphone can pick up the background noise, and use a (frequency) analysis to determine the type of transport An example of such method is disclosed by A Rakotomamonjy, G Gasso, “ Histogram of gradients of Time - Frequency Representations for Audio scene  detection”. The GPS might also provide information on the speed and the position where the user gets on and off the bus The positions can be used in combination with lookup tables to help determine or confirm what type of transportation stops at these particular stops and which line he or she took. 
     Once the second classification algorithm has determined that the user is using public transportation, and which type of transportation, the reclassification process  370  can start. The period that the user was standing still  830  at the bus stop can be reclassified as waiting  880  for the bus to arrive, and the activity blocks ‘walking’  840  and ‘sitting’  850  can be regrouped into ‘public transportation’. If the type of transportation can be determined, an additional label, in this case ‘bus’ can be added In addition, the activity specific algorithm can be started that can keep track of e g. the actual time the bus left, and the total transportation time This data can be compared to the advertised times and used to optimize future public transportation trips 
       FIG. 9  displays an example of an architecture of a portable device  900  configured to monitor the activities of the user. The device  900  may be similar to host device  200  as described with reference to  FIG. 2  The device  900  may include a display  902 , an application processor  904 , application memory  906  and a sensor processing unit  908  (further referred to as SPU) Communication may be done over a device bus  910 . The sensors  914  in the SPU  908  may be implemented using the MEMS technique All the modules of the SPU (processor, memory, sensors) may be integrated into a single chip. 
     SPU  908  may include a processor  912 , one or more sensors  914 , and a memory  916  All communication within the SPU may be done over a SPU bus  918  One or more external sensors  920  may communicate with the SPU  908  over link  922  Data portion  924  of memory  916  may be used to store permanent and transient values generated during the operation of the SPU  908  The first and second classification algorithms  926  may be stored in memory  916 . The processing of the algorithms  926  may take place using processor  912 , and the resulting activity blocks and labels may be stored in the data portion  924  of memory  916  If all the processing, classification, and reclassification is done within SPU  908 , the output of the SPU may be the activity blocks/log including the primary and secondary classes of activity. Alternatively, part of the data or algorithms may be stored in application memory  906 , and part of the processing may be done by application processor  904  For example, the first classification algorithm might be stored on memory  916  and processes the sensor data  924  on processor  912 . The resulting activity block with the primary classes of activity may be stored in memory  916  or application memory  906 . Next, the secondary classification algorithm, which may be stored on application memory  906 , may be run on application processor  904 , and the resulting activity block with the secondary classes of activity may be stored in application memory  906 . In this case, the output of the SPU after the first classification algorithm may be the activity blocks/log with the primary classes of activity 
     Device  900  may be a smartphone or a smartwatch including a display where the results of the activity (re-)classification may be shown Alternatively, the device  900  may be a fitness tracker in the form of a bracelet or a (belt) clip without a display, but with communication means to output the results to another device that does include a display, such as a smartphone 
     The examples described above are given as illustrations of embodiments of the invention They do not in any way limit the scope of the invention which is defined by the following claims.