Abstract:
There is provided a bed monitoring system comprising a sensor for detecting and measuring the pressure or force exerted on a bed; and a processor configured to process signals from the sensor to identify a pressure or force pattern that is characteristic of a state of a user relative to the bed; wherein the pressure or force pattern is identified by examining the measured pressure or force over time.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to a bed monitoring system and method, and in particular relates to a bed monitoring system and method that monitors the pressure or force exerted on the bed to determine the state of a user relative to the bed. 
       BACKGROUND TO THE INVENTION 
       [0002]    It is important in several applications to monitor whether a user or patient is in their bed, or whether the user or patient has got up and left their bed. The most prominent applications are the care of the elderly, care of children and care of patients with specific conditions or circumstances, for example post-surgery. 
         [0003]    There are several prior art devices available that can detect when a patient has left the bed. Usually, these devices use either pressure switches or pressure sensing mats to measure the pressure or load on the bed. In general, an alarm is triggered when the measured pressure falls below a predefined threshold. 
         [0004]    Usually, carers are interested in two events when a bed monitoring system is being used. Firstly, they want to be alerted when the patient or user starts to leave the bed, and secondly, the alert should be cancelled once the user or patient is safely in bed again (i.e. they are laying down in bed and have come to rest). 
         [0005]    Between these two events, the monitored user or patient may return to bed, but, for example, only sit on the edge of the bed, or may lie down but remain very active. Both situations often precede the user or patient getting out of bed again, so it is desirable for the alarm to remain active in these cases. Likewise, there should be no spurious activations or cancellations of the alarm when someone makes up the bed, or undertakes similar actions. 
         [0006]    Therefore, it is an object of the invention to provide a bed monitoring system and method that can provide an indication of the state of the user relative to the bed, and in which this indication can be used to improve the reliability of the activation and deactivation of the alarm condition. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore, according to a first aspect of the invention, there is provided a bed monitoring system comprising a sensor for detecting and measuring the pressure or force exerted on a bed; and a processor configured to process signals from the sensor to identify a pressure or force pattern that is characteristic of a state of a user relative to the bed; wherein the pressure or force pattern is identified by examining the measured pressure or force over time. 
         [0008]    According to a second aspect of the invention, there is provided a method of monitoring a bed, the method comprising detecting and measuring the pressure or force exerted on a bed using a sensor; and processing signals from the sensor to identify a pressure or force pattern that is characteristic of a state of a user relative to the bed; the pressure or force pattern being identified by examining the measured pressure or force over time. 
         [0009]    According to a third aspect of the invention, there is provided a computer program product, comprising computer program code that, when executed on a computer or processor, is configured to process signals from a pressure or force sensor to identify a pressure or force pattern that is characteristic of a state of a user relative to a bed, the pressure or force pattern being identified by examining the measured pressure or force over time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various embodiments of the invention will now be described, by way of example only, with reference to the following drawings, in which: 
           [0011]      FIG. 1  is an illustration of a bed monitoring system in accordance with the invention; 
           [0012]      FIG. 2  is a block diagram of a bed monitoring system in accordance with the invention; 
           [0013]      FIG. 3  shows plots of the signal amplitude, autocorrelation coefficients and variance for an exemplary set of samples from a sensor; and 
           [0014]      FIG. 4  is a flow chart illustrating a method in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    In  FIG. 1 , a bed  2  is shown that has a mattress  4 . A bed monitoring system  5  is provided for monitoring the state of a user (not shown) relative to the bed  2 . In this illustrated embodiment, the bed monitoring system  5  comprises a sensor unit  6  that is provided beneath the mattress  4 , but in alternative embodiments, the sensor unit  6  may be provided on top of the mattress  4 , or as an integral part of the bed  2 , or integrated into the mattress  4 . 
         [0016]    The bed monitoring system  5  also comprises a base station  8 , which can be located near to the bed  2  or further away at a monitoring station (such as a nurse&#39;s station, for example), that communicates with the sensor unit  6 , and which can sound an alarm or warning that a person on the bed  2  is getting out of the bed  2 , or has got out of the bed  2 . 
         [0017]      FIG. 2  shows the bed monitoring system  5  in more detail. The sensor unit  6  comprises a sensor  10  that generates signals indicating the pressure or forces exerted on the bed  2 . A processor  12  is provided that receives the signals from the sensor  10  and processes the signals to determine the state of the user relative to the bed  2 . 
         [0018]    The sensor unit  6  can comprise a single sensor  10  that is placed in a generally central position on or in the bed  2  or on or in the mattress  4 , or in a position on the bed  2  or mattress  4  that corresponds to the position of the user when they are lying correctly on the bed  2 . Alternatively, the sensor unit  6  can comprise a number of sensors  10  that are distributed over or in the bed  2  so that the pressure or force can be measured at different points. 
         [0019]    The sensor  10  can be a force sensor for measuring the forces exerted on the bed  2  by the user, a pressure sensor for measuring the pressure exerted on the bed  2  by the user, or a strain gauge for measuring the strain on the bed  2  caused by the user. In this latter embodiment, the measurements from the strain gauge are used to determine the pressure being exerted on the bed  2  by the user. In preferred embodiments, the sensor  10  is selected from a piezoelectric switch, a piezoelectric foil, an electret foil or a strain gauge. Those skilled in the art will appreciate that alternative types of sensors can be used to perform the method in accordance with the invention. 
         [0020]    It will be appreciated that in the embodiments in which the sensor  10  is placed under the mattress  4 , some component of the measured signal will represent the weight of the mattress  4  (and any bedding) acting on the bed  2 . In accordance with embodiments of the invention discussed further below, the algorithm used to analyze the measurements from the sensor  10  can be adapted to compensate for these components. 
         [0021]    As described in more detail below, the bed monitoring system  5  can determine a number of different states of the user relative to the bed  2  by identifying corresponding pressure or force patterns from changes in the measured pressure or forces over time. 
         [0022]    The different “states” of the user relative to the bed  2  distinguishable by the bed monitoring system  5  according to the invention can include one or more of the following: the user lying on the bed  2  without substantial movement that is indicative of the user getting out of bed, the user lying on the bed  2  but being quite active or restless, the user starting to get out of the bed  2 , the user sitting on the edge of the bed  2 , the bed  2  being empty and the bed being manipulated in some way (for example the bed  2  is being made up). 
         [0023]    If the processor  12  does determine that the user is likely to get out of bed  2  (i.e. the user is lying on the bed  2  but is quite active or restless or is sitting on the edge of the bed  2 ), the user is getting out of bed  2  or if the bed  2  is empty, a warning or alert signal can be provided to transmitter  14  that transmits the warning or alarm signal to the base station  8 . 
         [0024]    In some embodiments, the warning or alarm signal can include information identifying the particular state of the user relative to the bed  2 . 
         [0025]    The base station  8  comprises a receiver  20  for receiving the warning or alarm signal from the sensor unit  6 , a processor  22  and an alarm  24  for issuing an alarm or warning that the user is about to get out of the bed  2 , or has got out of the bed  2 . 
         [0026]    In further embodiments, the processor  12  can provide a signal to the transmitter  14  indicating that the user is lying on the bed  2  without substantial movement (i.e. the user is lying on the bed  2  asleep or without any indication that they are going to get out of the bed  2 ). In this case, the base station  8  can issue an indication that the user is on the bed  2 . 
         [0027]    It will be appreciated that, in alternative embodiments, the processing of the signals from the sensor  10  to determine the state of the user relative to the bed  2  can be performed by the processor  22  in the base station  8  rather than the processor  12  in sensor unit  6 , in which case the sensor unit  6  transmits the signals from the sensors  10  to the base station  8  for analysis. 
         [0028]    It will be further appreciated that the functions of the sensor unit  6  and base station  8  can be integrated into a single device. 
         [0029]    The operation of the invention will now be illustrated with reference to  FIGS. 3 and 4 . 
         [0030]      FIG. 3(   a ) shows a typical signal obtained from a single pressure or force sensor  10  covering a period of time in which the user and bed  2  are in several different states relative to each other. Two threshold values for the signal amplitude, a in  and a out , are indicated in  FIG. 3(   a ). As described further below, in preferred embodiments of the invention, the threshold values can be dynamically set during the operation of the bed monitoring system  5  in order to adapt to changes in the ambient conditions and/or in the event that the signals from the sensor  10  show drift. 
         [0031]      FIG. 3(   b ) is a plot of the autocorrelation coefficient, xcor, of the signal in  FIG. 3(   a ), which is calculated by the processor  12  (or the processor  22  in the alternative embodiment in which the processing is performed in the base station  8 ). In particular, a moving window of length d is used, and in each window the autocorrelation coefficient xcor (delayed by one data point) is computed. 
         [0032]      FIG. 3(   c ) is a plot of the variance, var of the signal in  FIG. 3(   a ), which, again, is calculated by the processor  12  (or processor  22  in the alternative embodiment), and in which a moving window of size d is used. 
         [0033]    Initially, the bed  2  is occupied by a user that is resting calmly, and this is represented by the first segment, from t=1.5 to t=1.57. At the point marked by “ 1 ” (at approximately t=1.57) the user has sat up, but is still on the edge of the bed  2 . Since, in this position, the upper body of the user hardly imposes any load onto the pressure sensor  10 , the signal amplitude falls below the lower threshold a out . 
         [0034]    While the user is still sitting on the bed  2 , some small pressure changes are detected by the sensor  10  (as shown by the point “ 2 ”). These small changes are more clearly evident from the corresponding part of the plot shown in  FIG. 3(   b ), which shows that the autocorrelation coefficients at this time have relatively high values. 
         [0035]    After the user has eventually left the bed (indicated by point “ 3 ”), the value of the autocorrelation coefficients fall and remain below a threshold xcor adj  (shown in  FIG. 3(   b )). Therefore, an empty bed  2  is characterized by both a low signal amplitude (i.e. below threshold a out ) as well as a low autocorrelation coefficient of the signal (i.e. below threshold xcor adj ). 
         [0036]    Alternatively, in situations in which there is a high signal-to-noise ratio, the autocorrelation criterion could be replaced by considering the variance, as shown in  FIG. 3(   c ). In this case, an empty bed  2  would then be characterized by a low signal amplitude (below threshold a out ) together with a variance smaller than a threshold var in  (not shown in  FIG. 3(   c )). 
         [0037]    In order to determine whether the user has returned to bed properly (i.e. they are laying down asleep or they are laying down without being active), the amplitude of the signal is analyzed along with the corresponding variance. Once the user has lain down on the bed  2 , the signal amplitude will exceed the upper threshold a in  (as shown by point “ 4   b ” in  FIG. 3(   a )). At this point, the variance of the signal is high since the user is still moving around in bed  2 . The variance of the signal falls to a low or negligible value once the user has stopped moving and has settled down (as shown by point “ 5 ” in  FIG. 3(   c )). 
         [0038]    At point “ 4   a ” in  FIG. 3(   a ), it can be seen that the signal amplitude is briefly above the upper threshold a in . However, as the signal amplitude drops below the upper and lower thresholds (a in  and a out ) a short time later, this is not indicative of the user returning to bed  2 , but instead is indicative of the bed  2  being manipulated, such as the bed  2  being made up. Furthermore, the high variance of the signal also indicates that the user has not lain down calmly in the bed  2 . 
         [0039]    Thus, it can be seen from the above that an evaluation of the amplitude of the signal from the sensor  10  in combination with an evaluation of the autocorrelation coefficient of the signal or the variance of the signal allows various different states of the user and the bed  2  to be determined. In particular, it is possible to distinguish between the states of: the user lying on the bed  2  asleep or at rest, the user lying on the bed  2  but being quite active or restless, the user sitting on the edge of the bed  2 , the bed  2  being empty or the bed  2  being manipulated in some way (for example being made up). 
         [0040]      FIG. 4  is a flow chart illustrating the processing path of the processor  12  in the sensor unit  6  (or processor  22  in the base station  8  in the alternative embodiment). 
         [0041]    The method starts in step  101 , at time t=t start , and it is assumed that the bed  2  is occupied by the user. For example, an operator of the system (for example a care provider for the user) may only activate the bed monitoring system  5  once the user is in the bed  2 . It will be appreciated, however, that the initial assumption can be that the bed  2  is unoccupied. As a further alternative (not shown), part of the initialization process in the method can determine whether the bed  2  is initially occupied or unoccupied. 
         [0042]    In step  103 , various parameters are initialized. In particular, the status of the bed (in terms of whether the user is in bed or out of bed) is set to “in bed”, the upper threshold for the signal amplitude a in  is set to b+α in , where b is value initially set equal to an operator defined value b 0 , the lower threshold for the signal amplitude a out  is set to b+α out , the time, t in , that the signal amplitude has exceeded the upper threshold a in  is set to 0, the time, t out , that the signal amplitude has been below the lower threshold a out  is set to 0, a further time parameter, t adj , is set to 0, and a number of samples n is set equal to the product of t adj  (initially 0) and the sampling frequency, f sampl , used by the sensor  10 . The parameters α in , α out , τ in , τ out  and xcor adj  are predefined and remain constant throughout the operation of the method. 
         [0043]    Two threshold values for the signal amplitude are provided in order to avoid, or reduce the risk of, false alarms. The lower threshold, a out  is preferably set to a small value such that the signal amplitude should only fall below this value when the user has sat up or left the bed  2 , not when the user is simply lying close to the edge of the bed  2 . 
         [0044]    The upper threshold, a in , is set high enough such that the signal amplitude will not exceed this value if the user is sitting on the edge of the bed  2 . The signal amplitude can fall below this value when the user is lying on the edge of the bed  2 . 
         [0045]    In step  105 , the processor  12 / 22  reads the next sample, a, from the sensor  10 , and compares it to the current value of the lower threshold, a out  (step  107 ). 
         [0046]    If the amplitude of the sample a is lower than the lower threshold, a out , the method proceeds to step  109  in which the value of t out  is increased by an amount dt, where dt=1/f sampl , and the value of t in  is set to 0 (or reset if t in  was previously non-zero). 
         [0047]    Next, in step  111 , the updated value of t out  is compared to a threshold value τ out . If the value of t out  is less than the threshold value τ out , the method returns to step  105  and the next sample a is read and processed. 
         [0048]    If the value of t out  is greater than the threshold value τ out , the method moves to step  113 . In step  113 , it is determined whether the Status is “in bed”. 
         [0049]    If the Status is “in bed”, the method moves to step  115  in which an alarm signal is sent to the base station  8  of the system  5 . If the Status is not “in bed” (i.e. the status is “out of bed”, which means that the alarm has already been raised), the method returns to step  105  and the next sample a is read and processed. 
         [0050]    After step  115 , the method passes to step  117  in which the Status is set to “out of bed”, and the method returns to step  105  and the next sample a is read and processed. 
         [0051]    Thus, this branch of the overall method, as indicated by dashed box  119 , is concerned with detecting when the user has got out of the bed  2 . In particular, the effect of step  111  and the value of threshold τ out  is to ensure that the alarm is not activated inadvertently when the user is just moving around or turning over on the bed  2  (which can result in the amplitude of the signal temporarily dropping below the lower threshold a out ). Thus, the alarm condition is only activated when the signal amplitude falls below the threshold a out  for a period of time greater than τ out . 
         [0052]    Returning to step  107 , if the signal amplitude is greater than the lower threshold a out , the method passes to step  121  in which the signal amplitude a is compared to the upper threshold, a in . 
         [0053]    If the signal amplitude is below the upper threshold, a in  (i.e. the signal amplitude is between the upper and lower threshold values following from step  107 ), the method passes to step  123  in which t in  and t out  are set or reset to 0. The method then returns to step  105  and the next sample a is read and processed. 
         [0054]    If the signal amplitude is above the upper threshold, a in , the method moves to step  125  in which t in  is incremented by an amount dt, and t out  is set or reset to 0. 
         [0055]    Next, in step  127 , the updated value of t in  is compared to a threshold value τ in . If the value of t in  is less than the threshold value τ in , the method returns to step  105  and the next sample a is read and processed. 
         [0056]    If the value of t in  is greater than the threshold value τ in , the method moves to step  129 . In step  129 , it is determined whether the variance is below a variance threshold, var in  and whether the Status is “out of bed”. 
         [0057]    If one or both of these criteria are not satisfied, the method returns to step  105  and the next sample a is read and processed. 
         [0058]    If both of these criteria are satisfied, the method moves to step  131  in which the alarm is cancelled. 
         [0059]    In the following step, step  133 , the Status is set to “in bed” and then the method returns to step  105  and the next sample a is read and processed. 
         [0060]    Thus, this branch of the overall method, as indicated by dashed box  135 , is concerned with detecting when the user has got back into the bed  2  and is at rest. In particular, the effect of steps  127  and  129  is to ensure that the alarm is only deactivated when the user has properly got back into bed  2 , and that the alarm is not deactivated when the bed  2  is being manipulated in some way (for example the bed  2  is being made up). Thus, the alarm condition is only deactivated once the signal amplitude has been above the threshold a in  for a period of time greater than τ in  and the variance is below the variance threshold var in . 
         [0061]    In an embodiment of the invention, the method can further comprise the steps shown in dashed box  137 , which relate to the dynamic adjustment of the thresholds a in  and a out . As described above, this adjustment can be used to make sure that the measurements from the sensor  10  are calibrated and interpreted correctly, and to counteract drift in the measurements. Thus, it is necessary to determine when the bed  2  is empty. Therefore, after a positive response to the decision in step  111 , the value of the autocorrelation coefficient is compared with a threshold value, xcor adj . 
         [0062]    If the autocorrelation coefficient xcor is greater than the threshold value, the method moves to step  141  in which the time t adj  is set or reset to 0. The method then returns to step  105  and the next sample a is read and processed. 
         [0063]    If the autocorrelation coefficient xcor is less than the threshold value, the method moves to step  143  in which the time t adj  is incremented by an amount dt. 
         [0064]    The incremented time is compared with a threshold time value, τ adj , in step  145 . If the incremented time t adj  is less than the threshold value τ adj  then the method returns to step  105  and the next sample a is read and processed. 
         [0065]    If the incremented time t adj  is greater than the threshold value τ adj  then the method passes to step  147  in which the value of the sensor baseline parameter b is set to be equal to the mean of the last n samples. The values of the upper threshold a in  and lower threshold a out  are adjusted based on the new value for b. 
         [0066]    The method then returns to step  105  and the next sample a is read and processed. 
         [0067]    In the exemplary situation shown in  FIG. 3 , the upper and lower thresholds will only be adapted if both the amplitude of the signal is lower than a out  and if the autocorrelation criterion is met. In this way, it should be possible to avoid the situation in which the thresholds are modified when a heavy object is placed onto the bed. It is possible, in further embodiments, for the bed monitoring system to provide an indication to the care provider about abnormal changes in the baseline b by issuing a notification once the signal amplitude is larger than a out  and the autocorrelation criterion is met. 
         [0068]    In further embodiments of the invention, the operation of the bed monitoring system  5  can be enhanced by providing further sensors to detect vital signs of the user (such as heart rate, heart rate variability or breathing rate). The measurements that these additional sensors provide can be used to reduce the time taken to confirm that a bed departure event has occurred, and can be used to improve the determination of whether the user is resting in bed, or asleep in bed. This method has been described in European patent application number EP08164045.0, filed on 10 Sep. 2008 in the name of Koninklijke Philips Electronics N.V., the content of which is hereby incorporated by reference. 
         [0069]    There is therefore provided a bed monitoring system and method that can provide an indication of the state of the user relative to the bed, and in which this indication can be used to improve the reliability of the activation and deactivation of the alarm condition. 
         [0070]    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. 
         [0071]    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 processor 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 measured 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.