Patent Publication Number: US-8979761-B2

Title: Sleepiness assessment apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a sleepiness assessment apparatus for determining the degree of sleepiness of a subject. 
     BACKGROUND ART 
     Hitherto, an apparatus that detects a sleepy state of a driver in a vehicle has been proposed as this type of sleepiness assessment apparatus (for example, see PTL 1). In this apparatus, by utilizing the fact that activities of the sympathetic nervous system increase in a case where a person is in a state in which the person struggles against sleepiness, a time series of a cardiac cycle is acquired from a cardiac signal detected by a heartbeat sensor. Additionally, the amplitude spectral power of a low-frequency component of heart rate fluctuation is acquired from the time series of the cardiac cycle. When the amplitude spectral power of the low-frequency component of the heart rate fluctuation is larger than a predetermined value, it is determined that the degree of arousal is reduced. Note that, when it is determined that the degree of arousal is reduced, a stimulus, for example, a sound such as a buzzer, light such as meter illumination, or vibration, is applied in order to increase the degree of arousal of the driver. 
     DISCLOSURE OF INVENTION 
     As described above, detection of a sleepy state of a driver in a vehicle at an early stage to prevent the driver from falling asleep at the wheel is considered to be a very significant safety issue. However, because there are large differences among individuals in a case of estimating autonomic nervous activities using the above-mentioned heart rate fluctuation, there occur cases in which detection of a sleepy state cannot be stably performed. 
     It is a main object of a sleepiness assessment apparatus according to the present invention to accurately determine the degree of sleepiness of a person. 
     The sleepiness assessment apparatus according to the present invention employs the following means in order to achieve the above-mentioned main object. 
     The sleepiness assessment apparatus according to the present invention is a sleepiness assessment apparatus for determining whether or not a subject is sleepy, the sleepiness assessment apparatus comprising:
         a respiration-interval acquisition unit for acquiring a respiration interval of the subject; and   a sleepiness assessment unit for determining, in a case where a variation in which the acquired respiration interval increases occurs, by using a degree of the variation as a sleepiness assessment parameter, whether or not the subject is sleepy.       

     In the sleepiness assessment apparatus according to the present invention, the respiration interval of the subject is acquired, and, in the case where a variation in which the acquired respiration interval increases occurs, whether or not the subject is sleepy is determined by using a degree of the variation as the sleepiness assessment parameter. The sleepiness assessment parameter is based on the fact that, when a person feels sleepy, because the person yawns or breathes deeply more often, the respiration interval varies by a large amount, compared with that in a case where the person is sufficiently awake. Accordingly, whether or not the subject is sleepy can more accurately be determined. 
     In the above-described sleepiness assessment apparatus according to the present invention, the sleepiness assessment parameter may be a parameter that is, in the case where a variation in which the acquired respiration interval increases occurs, obtained on the basis of a ratio obtained by the variation. In this manner, whether or not the subject is sleepy can be further accurately determined. 
     Additionally, in the sleepiness assessment apparatus according to the present invention, the sleepiness assessment parameter may be a parameter that is obtained by calculating a standard deviation of the acquired respiration interval. In this manner, whether or not the subject is sleepy can be further accurately determined. 
     Moreover, in the sleepiness assessment apparatus according to the present invention, the sleepiness assessment parameter may include a plurality of sleepiness assessment parameters, and the sleepiness assessment unit may determine, by using the plurality of sleepiness assessment parameters and weights, each of the weights being set for a corresponding one of the plurality of sleepiness assessment parameters, whether or not the subject is sleepy. In this manner, even when it is supposed that a reaction of a person in a case where the person feels sleepy varies with individuals, whether or not the subject is sleepy can be more accurately determined. 
     Additionally, the sleepiness assessment apparatus according to the present invention may further include a heartbeat-interval acquisition unit for acquiring a heartbeat interval of the subject. The sleepiness assessment unit may calculate an average value of the acquired heartbeat interval, and determine, in a case where a variation in which the acquired heartbeat interval decreases occurs, by using a frequency of the variation and the calculated average value of the heartbeat interval as the sleepiness assessment parameter, whether or not the subject is sleepy. In the sleepiness assessment apparatus according to this aspect of the present invention, the sleepiness assessment parameter may be a parameter that is, every time a variation in which the acquired heartbeat interval decreases occurs, obtained by multiplying the calculated average value of the heartbeat interval by a value which is obtained by dividing the acquired heartbeat interval by the number of heartbeats per unit time. In this case, the sleepiness assessment parameter may be a parameter that is, every time a variation in which the acquired heartbeat interval decreases occurs, obtained by integrating a square of the calculated average value of the heartbeat interval to obtain an integrated value, and by dividing the integrated value by a unit time. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram schematically illustrating a configuration of an anti-dozing system including a sleepiness assessment apparatus  20  that is provided as an embodiment of the present invention. 
         FIG. 2  is an explanatory diagram for explaining a heartbeat interval RRI of an electrocardiogram. 
         FIG. 3  is an explanatory diagram illustrating an example of a waveform of a signal detected by a respiration sensor  24 . 
         FIG. 4  is a flowchart illustrating an example of a sleepiness assessment processing routine that is performed by a controller  30 . 
         FIG. 5  is an explanatory diagram illustrating an example of a waveform of the heartbeat interval RRI. 
         FIG. 6  is an explanatory diagram illustrating an example of a waveform of a respiration interval RespI. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, preferred embodiments of the present invention will be described.  FIG. 1  is a configuration diagram schematically illustrating a configuration of an anti-dozing system including a sleepiness assessment apparatus  20  that is provided as an embodiment of the present invention. The sleepiness assessment apparatus  20  according to the embodiment is configured as an apparatus that is mounted in an automobile and that is used to determine the degree of sleepiness of a driver. As illustrated in the figure, the sleepiness assessment apparatus  20  includes a pulse wave sensor  22  that detects a pulse wave of the driver, a respiration sensor  24  that detects breathing movement of the driver, and a controller  30  that performs overall control of the apparatus. 
     The pulse wave sensor  22  is attached to a grip portion of a steering wheel  14 . The pulse wave sensor  22  is an optical sensor that is constituted by a light emitting element which emits near infrared light so that the driver is irradiated with the near infrared light from the skin surface and a light receiving element which receives transmitted light or reflected light. The pulse wave sensor  22  detects, as change in the amount of light, change in the volume flow of blood flowing through peripheral blood vessels. It is considered that a peak-to-peak interval of an acceleration pulse wave which is obtained by a signal detected by the pulse wave sensor  22  corresponds to an interval (heartbeat interval) between R waves of an electrocardiogram illustrated in  FIG. 2 . The signal detected by the pulse wave sensor  22  is output, in a wireless or wired manner, to the controller  30  via an amplifier and a filter for reducing noise. 
     The respiration sensor  24  is a strain gauge that is attached to an area which comes into contact with the thoracicoabdominal part of the driver when a seat belt  12  is fastened. The respiration sensor  24  detects, as change in an electric resistance, movement of the chest or the abdomen of the driver when the driver breathes. In  FIG. 3 , an example of a waveform of a signal detected by the respiration sensor  24  is illustrated. The signal detected by the respiration sensor  24  is output, in a wireless or wired manner, to the controller  30  via an amplifier and a filter for reducing noise. 
     The controller  30  is configured as a microprocessor in which a CPU  32  serves as a central unit. In addition to the CPU  32 , the controller  30  includes a ROM  34  in which a processing program is stored, a RAM  36  that temporarily stores data, and an input/output port that is not illustrated. The pulse wave signal that is output from the pulse wave sensor  22 , the respiration signal that is output from the respiration sensor  24 , and so forth are input to the controller  30  via the input port. Furthermore, drive signals that are to be provided to vibration applying actuators (motors)  42 ,  44 , and  46 , which are embedded in a backrest portion of a seat  10 , a footrest  16 , and the steering wheel  14  and which apply vibration, and so forth are output from the controller  30  via the output port. 
     Next, an operation of the sleepiness assessment apparatus  20  having the above-described configuration will be described.  FIG. 4  is a flowchart illustrating an example of a sleepiness assessment processing routine that is performed by the controller  30 . This routine is repeatedly performed on a predetermined-time-by-predetermined-time basis. 
     When the sleepiness assessment processing routine is performed, first, the CPU  32  of the controller  30  inputs data that is necessary for processing, such as the pulse wave signal that is output from the pulse wave sensor  22  and the respiration signal that is output from the respiration sensor  24  (step S 100 ). Then, the CPU  32  extracts a peak (a maximal value) of the input pulse wave signal, and calculates, as a heartbeat interval RRI, the time difference between a currently extracted peak and the peak extracted immediately prior to the currently extracted peak (step S 110 ). An example of a waveform of the heartbeat interval RRI is illustrated in  FIG. 5 . Then, the CPU  32  calculates an average heart rate HR (times/min) for one minute (60000 msec) by using Equation (1), which is given below, on the basis of the calculated heartbeat interval RRI (step S 120 ), and sets a sleepiness assessment value P 1  to be the calculated average heart rate HR (step S 130 ). Here, the sleepiness assessment value P 1  is one of parameters for determining whether or not the driver is sleepy, and is based on the fact that, in a situation in which a person cannot fall asleep, such as during driving, because the person is consciously trying to remain awake, the heart rate shows a tendency to temporarily increase.
 
HR=60000 /RRI   (1)
 
     When the CPU  32  sets the sleepiness assessment value P 1 , next, the CPU  32  extracts a peak (a maximal value) of the input respiration signal, and calculates, as a respiration interval RespI, the time difference between a currently extracted peak and the peak extracted immediately prior to the currently extracted peak (step S 140 ). An example of a waveform of the respiration interval RespI is illustrated in  FIG. 6 . Then, the CPU  32  computes a standard deviation σ of the calculated respiration interval RespI for one minute (step S 150 ). The CPU  32  sets a sleepiness assessment value P 2  to be the computed standard deviation σ of the respiration interval RespI (step S 160 ). Here, the sleepiness assessment value P 2  is one of the parameters for determining whether or not the driver is sleepy, and is based on the fact that, when a person feels sleepy, because the person yawns or breathes deeply more often, a large variation occurs in the respiration interval RespI, compared with that in a case where the person is sufficiently awake. 
     When the CPU  32  sets the sleepiness assessment value P 2 , the CPU  32  calculates an average heartbeat interval RRIavg from the heartbeat interval RRI, which has been calculated in step S 110 , by using Equation (2) given below (step S 170 ), and sets an RRI count value Xi by using Equation (3) given below (step S 180 ). The CPU  32  sets a sleepiness assessment value P 3  on the basis of the set RRI count value Xi and the average heartbeat interval RRIavg by using Equation (4) given below (step S 190 ). Here, “n” in Equation (2) denotes the number of heartbeats per minute. Furthermore, as represented by Equation (3), when, for one minute, a heartbeat interval RRI (i) that is currently calculated is smaller than a heartbeat interval RRI (i−1) that was calculated immediately prior to the heartbeat interval RRI (i), the RRI count value Xi is incremented by a value of one (see  FIG. 5 ). The sleepiness assessment value P 3  is based on the fact that many examples are confirmed, in which, when a person feels sleepy, because a variation in which the heartbeat interval decreases frequently occurs, the frequency of the variation shows a tendency to gradually increase. 
     
       
         
           
             
               
                 
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     When the CPU  32  sets the sleepiness assessment value P 3 , the CPU  32  sets a RespI count value Yi by using Equation (5) given below (step S 200 ), and sets a sleepiness assessment value P 4  on the basis of the set RespI count value Yi by using Equation (6) given below (step S 210 ). Here, as represented by Equation (5), when, for one minute, a respiration interval RespI (i) that is currently calculated is larger than a respiration interval RespI (i−1) that was calculated immediately prior to the respiration interval RespI (i), the RespI count value Yi is incremented by a value of one (see  FIG. 6 ). As is clear from Equation (6), the sleepiness assessment value P 4  is a value obtained by averaging a variation ratio in a case where a variation in which the respiration interval RespI increases occurs, and shows a tendency to increase as the frequency of an action of causing longer breathing, such as a deep breath or a yawn, increases. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     When the CPU  32  sets the individual sleepiness assessment values P 1  to P 4  in this manner, the CPU  32  sums values that are obtained by multiplying the individual sleepiness assessment values P 1  to P 4  by coefficients a to d, each of which is set in advance for a corresponding one of the sleepiness assessment values P 1  to P 4 , thereby setting a sleepiness assessment value P (step S 220 ). The CPU  32  compares the set sleepiness assessment value P and a value of zero with each other (step S 230 ). When the sleepiness assessment value P is smaller than the value of zero, the CPU  32  determines that the driver is in a state in which the driver does not feel sleepy, i.e., a state in which the driver is highly alert (step S 240 ). When the set sleepiness assessment value P is equal to or larger than the value of zero, the CPU  32  determines that the driver is in a state in which the driver feels sleepy, i.e., a state in which the driver is less alert (step S 250 ). The CPU  32  finishes the present routine. Here, the coefficients a to d are weight coefficients that are set for the sleepiness assessment values P 1  to P 4 , respectively. Specific values of the coefficients a to d may be determined on the basis of experiments or the like. The reason why the sleepiness assessment value P for determining whether or not the driver feels sleepy is set using the multiple sleepiness assessment values P 1  to P 4  and the coefficients a to d as described above is that, because indicators indicating a reaction of a person in a case where the person feels sleepy vary with individuals, stable determination cannot be performed using single physiological indicator in most cases. Note that, in the embodiment, when it is determined in the sleepiness assessment processing routine that the driver is less alert, the controller  30  controls the vibration applying actuators so as to apply vibration at predetermined time intervals (for example, one-second intervals), thereby applying a stimulus to improve the degree of arousal. As a matter of course, the scheme for applying a stimulus to the driver is not limited to thereto. Various schemes including provision of a sound such as a buzzer, provision of light such as room light, and application of a pressure using an air bag or the like may be employed. 
     In the sleepiness assessment apparatus  20  according to the embodiment described above, the sleepiness assessment value P 4  is set by averaging a variation ratio in a case where a variation in which the respiration interval RespI increases occurs, and whether or not the driver is sleepy is determined using the set sleepiness assessment value P 4  as one of the parameters. Thus, whether or not the driver is sleepy can be accurately determined using the indicator indicating the respiratory interval. Additionally, the sleepiness assessment value P is set using the four sleepiness assessment values P 1  to P 4  and the weights a to d, each of which is set for a corresponding one of the sleepiness assessment values P 1  to P 4 . Thus, even when it is supposed that that indicators indicating a reaction of a person in a case where the person feels sleepy vary with individuals, more stable determination of whether or not the driver is sleepy can be performed. Furthermore, the average heart rate HR is used as the sleepiness assessment value P 1 . The standard deviation σ of the respiration interval RespI for one minute is used as the sleepiness assessment value P 2 . A value, per unit time, of the square of the average heartbeat interval RRIavg in a case where the RRI count value Xi is the value of one is used as the sleepiness assessment value P 3 . Thus, whether or not the driver is sleepy can be more accurately determined. 
     In the sleepiness assessment apparatus  20  according to the embodiment, determination of whether or not the driver is in the state in which the driver is less alert or the state in which the driver is high alert, i.e., determination using two steps, is performed on the basis of the sleepiness assessment value P. However, determination performed in the sleepiness assessment apparatus  20  is not limited thereto. Determination of the level of arousal using multiple steps that are equal to or more than three steps may be performed, for example, as follows: two thresholds Pref 1  and Pref 2  are set; when the sleepiness assessment value P is equal to or larger than the threshold Pref 1 , it is determined that the level of arousal is a high level; when the sleepiness assessment value P is equal to or larger than the threshold Pref 2  and smaller than the threshold Pref 1 , it is determined that the level of arousal is a median level; and when the sleepiness assessment value P is smaller than the threshold Pref 2 , it is determined that the level of arousal is a low level. 
     In the sleepiness assessment apparatus  20  according to the embodiment, whether or not the driver is sleepy is determined using the four sleepiness assessment values P 1  to P 4 . However, some of the sleepiness assessment values may be omitted, if at least either the sleepiness assessment value P 3  or P 4  is included as an indicator regarding variation in the respiration interval RespI. Whether or not the driver is sleepy may be determined using five or more sleepiness assessment values. 
     In the sleepiness assessment apparatus  20  according to the embodiment, the heartbeat interval RRI is calculated using the pulse wave sensor  22  as a heartbeat sensor. However, the pulse wave sensor  22  is not limited thereto. Any heartbeat sensor may be used if the heartbeat sensor is a sensor that can detect something corresponding to the cardiac signal, for example, a sensor in which an electrode is attached to each of grip portions of the steering wheel  14  on the left and right sides, and which detects a potential difference in a case where both hands are in contact with the electrodes on the left and right sides. 
     In the sleepiness assessment apparatus  20  according to the embodiment, a strain gauge is used as the respiration sensor  24 . However, the respiration sensor  24  is not limited thereto. Any respiration sensor may be used if the respiration sensor is a sensor that can detect breathing movement, for example, a sensor that detects, by using laser ranging, movement of the thoracicoabdominal part which is made along with respiration. 
     The relationships between main elements in the embodiment and main elements, which are described in Means for Solving the Problems, of the invention will be described. In the embodiment, the respiration sensor  24  and the controller  30  that performs, in step S 140  of the sleepiness assessment processing routine, the process of calculating the respiration interval RespI on the basis of the respiration signal that is output from the respiration sensor  24  correspond to “a respiration-interval acquisition unit”. The controller  30  that performs, in steps S 130 , S 150 , and S 160  to S 250 , the process of setting the sleepiness assessment value P 4  by averaging the variation ratio in the case where a variation in which the respiration interval RespI increases occurs, of setting the sleepiness assessment value P using the sleepiness assessment values P 1  to P 4 , which include the set sleepiness assessment value P 4 , and the weight coefficients a to d, of determining, when the set sleepiness assessment value P is smaller than the value of zero, that the driver is highly alert, and of determining, when the sleepiness assessment value P is equal to or larger than the value of zero, that the driver is less alert corresponds to “a sleepiness assessment unit”. Furthermore, the controller  30  that performs, in step S 110 , the process of calculating the heartbeat interval RRI on the basis of the pulse wave signal which is output from the pulse wave sensor  22  and the pulse wave sensor  22  corresponds to “a heartbeat-interval acquisition unit”. 
     The embodiments of the present invention are described above. However, the present invention is not limited to the above-described embodiments at all. As a matter of course, various modifications may be made without departing from the gist of the present invention. 
     The present application claims priority on the basis of Japanese Patent Application No. 2009-137551 filed on Jun. 8, 2009. The entire contents of the application noted above are hereby incorporated by reference. 
     INDUSTRIAL APPLICABILITY 
     The present invention may be utilized in industries for producing sleepiness assessment apparatuses, automobile industries, and so forth.