Abstract:
A heart beat sensor ( 10 ) comprises signal processing means ( 15 ) for performing frequency analysis of the output signal of a piezoelectric sensor ( 1 ) installed in a seat ( 2 ) and judging means ( 16 ) judges on the basis of the results of the frequency analysis that if the amplitude of the signal component in a first preset frequency range included in the output signal is in a preset amplitude range, the signal component includes a human body seating waveform indicating a person is seated in the seat ( 2 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heartbeat detecting apparatus that detects a heartbeat of a person seated on a seat based on output signals of piezoelectric sensors provided at the seat. 
     2. Description of the Invention 
     There is a heartbeat detecting apparatus for detecting a heartbeat of a person seated on a seat of a vehicle, such as an automobile. Heartbeat detecting apparatuses are categorized into an apparatus of constrained type, in which a sensor for detecting the heartbeat of the person is attached at an arm of the person, or the like, and an apparatus of unconstrained type, in which a sensor for detecting the heartbeat of the person is not attached at the person. The heartbeat detecting apparatus of constrained type surely detects the heartbeat when the sensor is attached at a predetermined position of the person. However, because a portion of the person is constrained, the person is prevented from moving freely and a drawback may occur that a subject feels that his/her heartbeat is being detected. On the other hand, the heartbeat detecting apparatus of unconstrained type is preferable in respect that the person is allowed to move freely and the subject does not feel that his/her heartbeat is being detected. 
     In a heartbeat detecting apparatus of unconstrained type according to patent publication 1, a piezoelectric sensor is provided at a seat of a vehicle. The piezoelectric sensor detects a pulse (i.e. a heartbeat) of a blood flow of a person seated on the seat. More specifically, a frequency analysis is executed on an output signal of the piezoelectric sensor and a power spectrum is calculated. The heartbeat of the person seated on the seat is detected by extracting a signal component corresponding to a heartbeat frequency, from the power spectrum.
     Patent publication 1: JP2004-345617A   

     Such drawback may occur in the heartbeat detecting apparatus of the unconstrained type that the piezoelectric sensor outputs a signal based on a factor other than the heartbeat because the heartbeat detecting apparatus of unconstrained type does not constrain the person. For example, signal components resulted from, such as a movement of the person on the seat (a body movement) and a placement of an object on the seat, may be included in a frequency range of the heartbeat. Therefore, the heartbeat detecting apparatus according to patent publication 1 determines all the signal components among the output signals of the piezoelectric sensor corresponding to the heartbeat frequency to be the heartbeats. However, as described above, signals resulted from, such as the movement of the person and the placement of the object may be included in the signal components of the heartbeat. In particular, signals resulted from the placement of the object and a vibration of a vehicle in an idling state may be determined as the heartbeat signals. In other words, the heartbeat detecting apparatus according to patent publication 1 has a drawback that the heartbeat signal of the person is not precisely obtained because the heartbeat detecting apparatus determines the heartbeat signal without determining whether the occupant is seated on the seat or not. 
     An objective of the present invention is to provide a heartbeat detecting apparatus that detects whether an occupant is seated on a seat or not, based on an output signal of a piezoelectric sensor. A further objective of the present invention is to provide the heartbeat detecting apparatus that precisely obtains a signal component including a waveform of a heartbeat of the person, from the output signal of the piezoelectric sensor. 
     SUMMARY OF THE INVENTION 
     According to an aspect of a heartbeat detecting apparatus of the present invention for achieving the above-described objectives, the heartbeat detecting apparatus includes a signal processing device executing a frequency analysis on an output signal of a piezoelectric sensor provided at a seat and a determining device determining that the signal component includes a waveform of a seated person indicating that a person is seated on the seat, when an amplitude of a signal component, which is included in the output signal and which exists within a first predetermined frequency range, exists within a predetermined amplitude range, based on a result of the frequency analysis of the signal processing device. 
     According to the above-described aspect, the frequency analysis and an amplitude analysis are executed on the output signal of the piezoelectric sensor. In other words, by executing the frequency analysis on the output signal of the piezoelectric sensor, the signal component corresponding to the waveform of the seated person is obtained. The waveform of the seated person corresponds to the frequency resulted from, for example, a pulse of a blood flow (i.e. the heartbeat) or a breath of the person seated on the seat. Further, by considering that amplitudes of signal components differ from each other when the piezoelectric sensor detects the heartbeat of the person seated on the seat, a movement of the person on the seat and a placement of an object on the seat, the amplitude analysis is executed on the signal component existing within the first predetermined frequency range. Thus, signals generated by the placement of the object on the seat, by the movement of the person on the seat, and by a vibration of an idling vehicle are eliminated. Therefore, the heartbeat detecting apparatus is provided that surely obtains the signal component, including the waveform of the seated person indicating that the person is seated on the seat, from the output signal of the piezoelectric sensor. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, when the determining device determines that the signal component includes the waveform of the seated person, the determining device determines that a potential heartbeat signal extracted from the output signal of the piezoelectric sensor is the heartbeat signal of the person. 
     According to the above-described aspect, the person is determined to be seated on the seat. Therefore, the potential heartbeat signal extracted form the output signal of the piezoelectric sensor is surely the heartbeat signal of the person. Therefore, the signal component including the waveform of the heartbeat of the person is accurately obtained form the output signal of the piezoelectric sensors. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, the piezoelectric sensor is a plurality of piezoelectric sensors provide at the seat. 
     According to the above-described aspect, the plurality of piezoelectric sensors is provided on the seat. Therefore, in whatever state the person is seated on the seat, the waveform of the heartbeat of the person is more likely to be detected by any one of the plurality of piezoelectric sensors. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, the signal processing device executes the frequency analysis separately on the respective output signals of the plurality of piezoelectric sensors, and when each of equal to or more than two of the potential heartbeat signals extracted from each of equal to or more than two of the signal components, which is included in each of the output signals of equal to or more than two of the piezoelectric sensors among the plurality of piezoelectric sensors and which exists within the second predetermined frequency range, exists during a predetermined time frame, and when each of the signal components, which is included in each of the output signals of equal to or more than two of the piezoelectric sensors and which exists within the first predetermined frequency range, is determined to include the waveform of the seated person, based on the result of the frequency analysis of the signal processing device on each of the output signals, the determining device is configured to determine that at least one of equal to or more than two of the potential heartbeat signals is the heartbeat signal of the person. 
     According to the above-described aspect, when the person is determined to be seated on the seat and when the potential heartbeat signals obtained by the plurality of piezoelectric sensors exist within the predetermined time frame, the determining device determines that at least one of the potential heartbeat signals is not to be a noise but to be the heartbeat signal resulted from the pulse of the blood flow (i.e. the heartbeat) of the person seated on the seat. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, the signal processing device executes the frequency analysis separately on the respective output signals of plurality of piezoelectric sensors, and, when each of equal to or more than two of the potential heartbeat signals extracted from each of equal to or more than two of the signal components, which is included in each of the output signals of equal to or more than two of the piezoelectric sensors among the plurality of piezoelectric sensors and which exists within the second predetermined frequency range, exists during a predetermined time frame, and when each of the signal components, which is included in each of the output signals of equal to or more than two of the piezoelectric sensors and which exists within the first predetermined frequency range, is determined to include the waveform of the seated person, based on the result of the frequency analysis of the signal processing device on each of the output signals, the determining device is configured to determine that each of equal to or more than two of the potential heartbeat signals is the heartbeat signal of the person. 
     According to the above-described aspect, when the person is determined to be seated on the seat and when the potential heartbeat signals obtained by the plurality of piezoelectric sensors exist within the predetermined time frame, the determining device determines that each of the potential heartbeat signals is not to be the noise but to be the heartbeat signal resulted from the pulse of the blood flow (i.e. the heartbeat) of the person seated on the seat. Thus, by comparing the potential heartbeat signals obtained by the piezoelectric sensors, whether the potential heartbeat signals are the heartbeat signals of the person or not is accurately determined. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, the signal processing device executes the frequency analysis separately on the respective output signals of plurality of piezoelectric sensors, and when a second potential heartbeat signal extracted from a second signal component, which is included in the output signal of a second piezoelectric sensor among the plurality of piezoelectric sensors and which exists within a second predetermined frequency range, exists during the predetermined time frame before and/or after a peak time of a first potential heartbeat signal, which is extracted from a first signal component included in the output signal of the first piezoelectric sensor among the plurality of piezoelectric sensors and existing within the second predetermined frequency range, and when the signal components, which are included in the output signals of the first and second piezoelectric sensors and which exist within the first frequency range, includes the waveform of the seated person, based on a result of the frequency analysis of the signal processing device executed on each of the output signals of the plurality of piezoelectric sensors, the determining device is configured to determine that the first potential heartbeat signal is the heartbeat signal of the human body. 
     According to the above-described aspect, when the person is determined to be seated on the seat and when the second potential heartbeat signal obtained by the second piezoelectric sensor exists within the predetermined time frame after the peak time of the first potential heartbeat signal obtained by the first piezoelectric sensor among the plurality of piezoelectric sensors, the determining device determines that the first potential heartbeat signal is not to be the noise but to be the heartbeat signal resulted from the pulse of the blood flow (i.e. the heartbeat) of the person seated on the seat. Thus, by comparing the potential heartbeat signals obtained by the plurality of piezoelectric sensors, whether the potential heartbeat signals are the heartbeat signals of the person or not is accurately determined. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, the signal processing device executes the frequency analysis separately on the respective output signals of plurality of piezoelectric sensors, and when a second potential heartbeat signal extracted from a second signal component which is included in the output signal of a second piezoelectric sensor among the plurality of piezoelectric sensors and which exists within a second predetermined frequency range, exists during the predetermined time frame before and/or after a peak time of a first potential heartbeat signal, which is extracted from a first signal component included in the output signal of the first piezoelectric sensor among the plurality of piezoelectric sensors and existing within the second predetermined frequency range, and when the signal components which are included in the output signals of the first and second piezoelectric sensors and which exist within the first frequency range, includes the waveform of the seated person, based on a result of the frequency analysis of the signal processing device executed on each of the output signals of the plurality of piezoelectric sensors, the determining device is configured to determine that the second potential heartbeat signal is the heartbeat signal of the human body. 
     According to the above-described aspect, when the person is determined to be seated on the seat and when the second potential heartbeat signal obtained by the second the piezoelectric sensor exists within the predetermined time frame after the peak time of the first potential heartbeat signal obtained by the first piezoelectric sensor among the plurality of piezoelectric sensors, the determining device determines that the second potential heartbeat signals is not to be the noise but to be the heartbeat signal resulted from the pulse of the blood flow (i.e. the heartbeat) of the person seated on the seat. Thus, by comparing the potential heartbeat signals obtained by the plurality of piezoelectric sensors, whether the potential heartbeat signals are the heartbeat signals of the person or not is accurately determined. 
     According to another aspect of the heartbeat detecting apparatus of the present invention, the signal processing device executes the frequency analysis separately on the respective output signals of plurality of piezoelectric sensors, when each of equal to or more than two of each of the signal components, which is included in each of the output signals of equal to or more than two of the piezoelectric sensors among the plurality of piezoelectric sensors and which exists within a second predetermined frequency range, exists during a predetermined time frame, and when each of the signal components, which is included in each of the output signals of equal to or more than two of the piezoelectric sensors and which exists within the first predetermined frequency range, is determined to include the waveform of the seated person, based on the results of the frequency analysis of the signal processing device on each of the output signals, the determining device is configured to determine that each of equal to or more than two signal components is the heartbeat signal of the person, and the determining device is configured so that, when the heartbeat signal detected by another piezoelectric sensor exists during a predetermined time frame before and/or after a peak time of the heartbeat signal detected by the base piezoelectric sensor, the determining device provides a point to the base piezoelectric sensor and the other piezoelectric sensor so that the point is accumulated in the piezoelectric sensors, and the determining device outputs the heartbeat signal based on the output signal of the piezoelectric sensor in which the highest score is accumulated. 
     According to the above-described aspect, the highest score is accumulated in the piezoelectric sensor that detects the heartbeat signals of the person for a great number of times. By executing scoring of each of the piezoelectric sensors, it is determined which piezoelectric sensor among the plurality of piezoelectric sensors frequently detects the heartbeat signals of the person. Therefore, when one piezoelectric sensor among the plurality of piezoelectric sensors needs to be used, by using the piezoelectric sensor in which the highest score is accumulated, the heartbeat signals of the person is more likely to be detected. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a function of a heartbeat detecting apparatus according to a first embodiment; 
         FIG. 2A  is a diagram illustrating an output signal of a piezoelectric sensor; 
         FIG. 2B  is a diagram illustrating a result obtained by executing a seating condition determination; 
         FIG. 3A  is a diagram illustrating an output signal of a piezoelectric sensor; 
         FIG. 3B  is a diagram illustrating a result obtained by executing a seating condition determination; 
         FIG. 4A  is a diagram illustrating waveforms of signal components after a signal processing (a frequency analysis) is executed on an output signal of the piezoelectric sensor by a second filter portion and a third filter portion; 
         FIG. 5  is a block diagram illustrating a function of a heartbeat detecting apparatus according to a second embodiment; 
         FIG. 6A  is a diagram illustrating an output signal of a piezoelectric sensor; 
         FIG. 6B  is a diagram illustrating a result obtained by executing a seating condition determination; 
         FIG. 6C  is a diagram illustrating a potential heartbeat signal obtained by executing a signal processing by a second filter portion and a fourth filter portion; 
         FIG. 7  is a diagram of data on potential heartbeat signals included in output signals of the plurality of piezoelectric sensors provided at the seat, illustrated by each of the piezoelectric sensors; and 
         FIG. 8  is a diagram of data on potential heartbeat signals included in output signals of the plurality of piezoelectric sensors provided at the seat, illustrated by each of the piezoelectric sensors. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A heartbeat detecting apparatus  10  according to the present invention will be described hereinbelow with reference to the attached drawings. An objective of the heartbeat detecting apparatus  10  according to the present invention is to detect a heartbeat of a person seated on a seat  2 . A sensor of unconstrained type is adapted at the heartbeat detecting apparatus  10  in order to allow a subject to move freely and in order for the subject not to feel that his/her heartbeat is detected. More specifically, piezoelectric sensors  1  ( 1 A- 1 D) of unconstrained type illustrated in  FIG. 1  are provided at a seating portion of the seat  2 . A plurality of piezoelectric sensors  1 A- 1 D are provided at the seat  2 , and therefore wherever the person is seated on the seat  2 , the heartbeat of the person is detected. The piezoelectric sensors  1 A- 1 D may be provided at a backrest portion of the seat  2 , or the like, as long as the heartbeat of the person is detected. 
       FIG. 1  is a block diagram illustrating a function of the heartbeat detecting apparatus  10  according to the first embodiment. The heartbeat detecting apparatus  10  includes a signal processing device  15  and a determining device  16 . The signal processing device  15  executes a frequency analysis on output signals of the piezoelectric sensors  1  provided at the seat  2 . When amplitude of a signal component, which is included in the output signals and which exists within a predetermined frequency range (which serves as a first predetermined frequency range of the invention), exists within a predetermined amplitude range, the determining device  16  determines that the signal component include a waveform of a seated person indicating that the person is seated on the seat  2  (the person seated on the seat  2  is in a substantially still state), based on a result of the frequency analysis of the signal processing device  15 . Further, the signal processing device  15  may be modified to execute an amplification processing on the output signals of the piezoelectric sensors  1 . Furthermore, the signal processing device  15  and the determining device  16  may separately process the output signals of the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D. 
       FIG. 2A  is a diagram illustrating the output signal of the single piezoelectric sensor  1  that is inputted into the signal processing device  15 .  FIG. 3A  is a diagram illustrating the output signal of another piezoelectric sensor  1  that is inputted into the signal processing device  15 . The diagrams in  FIG. 2A  and  FIG. 3A  are obtained at the same time frame but waveforms of signals differ from each other. The output signals are obtained when the person is seated on the seat  2 . 
     The piezoelectric sensors  1  are provided at the seat  2 . Therefore, when the seat  2  is vibrated and the vibration is transmitted to the piezoelectric sensors  1 , output signals corresponding to a frequency and an amplitude of the vibration are outputted from the piezoelectric sensors  1 . The seat  2  is vibrated when there is the pulse of a blood flow of the person (i.e. the heartbeat) seated on the seat  2 , when the person sits on the seat  2 , when the person stands up from the seat  2 , when the person moves while seated, when an object is placed on the seat  2 , and the like. In a case where the seat  2  is adapted on a vehicle, such as an automobile, a vibration of the vehicle itself (for example, a vibration of the vehicle in an idling state) may cause the vibration of the seat  2 . 
     The signal processing device  15  of the heartbeat detecting apparatus  10  executes the frequency analysis on the output signals of the piezoelectric sensors  1  in order to distinguish signal components, which exist within the predetermined frequency range corresponding to a frequency range of the waveform of the seated person, from the output signals of the piezoelectric sensors  1 . 
     Signal components resulted from the person sits on the seat  2 , the person Stands up from the seat  2 , the person moves while seated, the object is placed on the seat  2 , and the vehicle in the idling state vibrates, may be included within the predetermined frequency range. However, when the signal components have different origins, strength of the signal components, that is, amplitudes of the signal components differ. For example, an amplitude of the signal component resulted from a vibration generated when the person sits on the seat  2 , an amplitude of the signal component resulted from a vibration generated when the person stands up from the seat  2 , and an amplitude of the signal component resulted from a vibration generated when the person moves while seated are extremely high. On the other hand, an amplitude of the signal component corresponding to the waveform of the seated person indicating the person is seated on the seat  2 , is relatively low. When the amplitude of the signal component, which is included in the output signals of the piezoelectric sensors  1  and which exists within the predetermined frequency range, exists within the predetermined amplitude range, the determining device  16  determines that the signal component includes the waveform of the seated person indicating that the person is seated on the seat  2  based on the result of the frequency analysis of the signal processing device  15 . Thus, the determining device  16  executes the amplitude analysis based on the output signals of the piezoelectric sensors  1 . 
     As illustrated in  FIG. 1 , the signal processing device  15  executes a signal processing on the output signals of the piezoelectric sensors  1  by means of a first filter portion  11  in parallel with signal processing on the output signals of the piezoelectric sensors  1  by means of a second filter portion  12  and a third filter portion  13 . Based on a result of the signal processing by means of the first filter portion  11  and on results of the signal processing by means of the second and third filter portions  12  and  13 , the determining device  16  determines whether the person is seated on the seat  2  or not. 
     The first filter portion  11  executes the frequency analysis for filtering and eliminating a signal component existing within a frequency range of an idling of the vehicle where the seat  2  is adapted (for example, equal to or more than 10 Hz) from the output signals. When neither the person nor the object exists on the seat  2 , amplitudes of the signal component, after the signal processing executed by the first filter portion  11 , are extremely low. On the other hand, when the person, the object, or the like exists on the seat  2 , the amplitudes of the signal component after the signal processing executed by the first filter portion  11  is high to some extent. Therefore, based on the result of the signal processing of the first filter portion  11 , by analyzing the amplitudes of the signal components after the signal processing of the filter portion  11 , the determining device  16  determines whether the seat  2  is empty or the seat  2  is occupied by the person, the object, or the like. 
     As described above, the frequency analysis and the amplitude analysis are executed on the output signals of the piezoelectric sensors  1  by means of the first filter portion  11  of the signal processing device  15  and the determining device  16 . 
     The second filter portion  12  executes the frequency analysis in order to filter and extract a signal component existing within a predetermined frequency range corresponding to a frequency range of the heartbeat, from the output signals. The third filter portion  13  executes the frequency analysis in order to filter and extract a signal component existing within a predetermined frequency range that is lower than the frequency range of the heartbeat. Therefore, the signal component outputted from the third filter portion  13  is the signal component whose frequency range is lower than the frequency range of the heartbeat. In such frequency range, a frequency of the waveform of the seated person is included which indicates that the person is seated on the seat  2 , such as a breathing of the person. The signal component outputted after processing of the second and third filter portions  12  and  13  is the signal component that includes the frequency existing within the first predetermined frequency range of the present invention. 
     As described above, the frequency analysis and the amplitude analysis are executed on the output signals of the piezoelectric sensors  1  by means of the second and third filter portions  12  and  13  of the signal processing device  15  and the determining device  16 . 
     A detailed description of the processing executed by the heartbeat detecting apparatus  10  according to the first embodiment will be provided hereinbelow. The determining device  16  determines that nothing exists on the seat  2  when the amplitude of the signal component after the signal processing of the first filter portion  11  is less than a first threshold value. In other words, the determining device  16  determines that the piezoelectric sensors  1  do not detect vibrations other than the vibration of the frequency eliminated by the first filter portion  11  and resulted from the idling. That is, the determining device  16  detects that the seat  2  is in an empty state. On the other hand, the determining device  16  determines that the person, the object, or the like exits on the seat  2  when the amplitudes of the signal component after the signal processing of the first filter portion  11  is equal to or more than the first threshold value. As will be described hereinbelow, the second and third filter portions  12  and  13  execute the signal processing in order to analyze what exists on the seat  2 , for example the person or the object. 
       FIGS. 4A and 4B  are diagrams illustrating waveforms of the signal components after the signal processing (the frequency analysis) executed on the output signal outputted form the piezoelectric sensor  1  of  FIG. 2A  by means of the second and third filter portions  12  and  13 .  FIG. 4A  and  FIG. 4B  differ from each other only in a scale of longitudinal axes. 
     When the amplitude of the signal component after the signal processing of the second and third filter portions  12  and  13  exists within a predetermined amplitude range whose upper limit is defined by a second threshold value Th 2 , the determining device  16  determines that (executes the amplitude analysis so that) the object exists on the seat  2 . Further, when the amplitude of the signal component after the signal processing of the second and third filter portions  12  and  13 , exists within a predetermined amplitude range whose lower limit is defined by a third threshold value Th 3 , the determining device  16  determines that (executes the amplitude analysis so that) the person moves on the seat  2 . Furthermore, when the amplitude of the signal component after the signal processing of the second and third filter portions  12  and  13  exists within a predetermined amplitude range whose lower limit is defined by the second threshold value Th 2  and whose upper limit is defined by the third threshold value Th 3 , the determining device  16  determines that (executes the amplitude analysis so that) the person is seated on the seat  2 . 
     The conventional heartbeat detecting apparatus does not determine whether the person is seated on the seat  2  based on the output signal of the piezoelectric sensor  1 . However, as described above, the heartbeat detecting apparatus  10  according to the present invention determines whether the person is seated on the seat  2  based on the output signals of the piezoelectric sensors  1 . Therefore, a signal component that includes the waveform of the seated person is accurately obtained from the output signals of the piezoelectric sensors  1 . 
       FIG. 2B  and  FIG. 3B  are diagrams illustrating results of a seating condition determination executed by the determining device  16  on the signal components after the signal processing (the frequency analysis) of the first, second and third filter portions  11 ,  12  and  13  (the waveforms illustrated in  FIGS. 4A and 4B ). The determining device  16  determines, whether the object is detected on the seat  2 , whether the seating of the person on the seat  2  is detected, whether the movement of the person on the seat  2  is detected or whether the seat  2  is empty, based on the results of the frequency analysis and the amplitude analysis on the output signals of the piezoelectric sensors  1 . While the seating is always detected from the output signal of the piezoelectric sensor  1  in  FIG. 2A , the seating is discontinuously detected from the output signal of the piezoelectric sensor  1  in  FIG. 3A . Therefore, the seating of the person is not appropriately detected from the output signal of the piezoelectric sensor  1  in  FIG. 3A . Accordingly, even when attempting to detect the heartbeat signals from the piezoelectric sensor  1 , the heartbeat is not accurately detected. 
     Second Embodiment 
     A heartbeat detecting apparatus  20  according to a second embodiment is different from the heartbeat detecting apparatus  10  according to the first embodiment in that the heartbeat detecting apparatus  20  according to the second embodiment executes a processing in which output signals of plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D are compared.  FIG. 5  is a block diagram illustrating a function of the heartbeat detecting apparatus  20  according to the second embodiment. The heartbeat detecting apparatus  20  includes a signal processing device  25  and a determining device  26 . The signal processing device  25  executes a frequency analysis on output signals of the piezoelectric sensors  1  ( 1 A,  1 B,  1 C,  1 D) provided at a seat  2 . The determining device  26  determines whether a person is seated on the seat  2  and whether potential heartbeat signals, extracted from the output signals of the piezoelectric sensors  1 , are a heartbeat of the person, based on a result of the frequency analysis of the signal processing device  25 . As well as the first embodiment, the signal processing device  25  includes first, second and third filter portions  21 ,  22  and  23 . Functions of the first, second and third filter portions  21 ,  22 , and  23  are the same as functions of the first, second and third filter portions  11 ,  12 , and  13  according to the first embodiment. The signal processing device  25  executes the frequency analysis on each of the output signals of the piezoelectric sensors  1 . The determining device  26  determines whether an object is detected on the seat  2 , whether a seating of the person on the seat  2  is detected, whether a movement of the person on the seat  2  is detected, or whether the seat  2  is empty, based on results of the frequency analysis and the amplitude analysis on the output signals of the piezoelectric sensors  1 . A fourth filter portion  4  according to the present embodiment executes a smoothing and a zero cross point extraction on signal components existing within a frequency range corresponding to a waveform of the heartbeat and obtained by the frequency analysis of the second filter portion  22 . Signal components, extracted by signal processing of the second filter portion  22  and the fourth filter portion  24 , are the signal components that include a frequency within a second predetermined frequency range of the present invention. 
       FIG. 6A  is a diagram illustrating the output signal of the single piezoelectric sensor  1  that is inputted into the signal processing device  25 .  FIG. 6B  is a diagram illustrating a result of a seating condition determination, executed in the same manner as the first embodiment, on the output signal after the signal processing (the frequency analysis) by means of the second and third filter portions  22  and  23 . These diagrams are the same as the diagrams in  FIGS. 2A and 2B . 
       FIG. 6C  is a diagram illustrating potential heartbeat signals obtained by the signal processing (the frequency analysis) by means of the second and fourth filter portions  22  and  24 . More specifically, the fourth filter portion  24  executes the smoothing on the signal components after the frequency analysis of the second filter portion  22  and then extracts the zero cross point (i.e. extracts the potential heartbeat signals). In other words,  FIG. 6C  is a diagram illustrating the potential heartbeat signals included in the output signal of the piezoelectric sensor  1 . 
     The output signals of the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D provided at the seat  2  are inputted into the signal processing device  25 . The same number of data of potential heartbeat signals as illustrated in  FIG. 6C  as the number of the piezoelectric sensors  1 A,  1 B,  1 C and  1 D are obtained. More specifically, the signal processing device  25  of the heartbeat detecting apparatus  20  according to the second embodiment executes the frequency analysis on each of the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D, and the data of the potential heartbeat signals after the signal processing are inputted into the determining device  26 . 
     The above-described potential heartbeat signals are extracted regardless of the results of the determination of seating of the person on the seat  2 . Therefore, the signals may be resulted from a movement generated on the seat  2  by the person or from a vibration of the object generated on the seat  2 . In the same manner described above, according to the second embodiment, when the output signal of the piezoelectric sensor  1 , by which the potential heartbeat signal is detected, is determined to include a waveform of the seated person, the determining device  26  determines that the potential heartbeat signal extracted from the piezoelectric sensor  1  is the heartbeat signal, based on results of the frequency analysis and the amplitude analysis of the second filter portion  22 , third filter portion  23 , and the determining device  26 . Therefore, the heartbeat signal is more accurately detected. 
     Further, a manner described hereinbelow may be used in order to determine that the potential heartbeat signal is the heartbeat signal. 
       FIG. 7  is a diagram of data on the potential heartbeat signals included in the output signals of the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D provided at the seat  2 , illustrated by each of the piezoelectric sensors  1  ( 1 A,  1 B,  1 C and  1 D). In  FIG. 7 , data only on the piezoelectric sensors,  1 A,  1 B and  1 C are illustrated. The data on the potential heartbeat signals is inputted from the signal processing device  25  into the determining device  26 . In order to extract the heartbeat signals from the output signals of the piezoelectric sensors  1  more accurately, the manner of comparing the output signals of the piezoelectric sensors  1  may be used. In other words, in a case where the potential heartbeat signals, resulted from the same heartbeat (i.e. the same pulse of a blood flow), are detected by the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D, the potential heartbeat signals are determined to be the heartbeat signals. 
     Therefore, according to the heartbeat detecting apparatus  20  of the second embodiment, when equal to or more than two of the potential heartbeat signals extracted from the signal components, which are included in the output signals of equal to or more than two of the piezoelectric sensors  1  among the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D and which exist within a predetermined frequency range (which serves as a second predetermined frequency range of the present invention), exist during a predetermined time frame, and when the signal components, which are included in equal to or more than two of the output signals of the piezoelectric sensors  1  and which exist within a predetermined frequency range (which serves as a first predetermined frequency range according to the present invention), include the waveform of the seated person, the determining device  26  determines that at least one of two potential heartbeat signals is the heartbeat, based on the results of the frequency analysis of the signal processing device  25  on each of the output signals. 
     More specifically, as illustrated in  FIG. 7 , the piezoelectric sensor  1 A detects a potential heartbeat signal PA 1  and a potential heartbeat signal PA 2 . The piezoelectric sensor  1 B detects a potential heartbeat signal PB 1  and a potential heartbeat signal PB 2 . The piezoelectric sensor  1 C detects a potential heartbeat signal PC 1 . The determining device  26  uses the piezoelectric sensor  1 A, which outputs the potential heartbeat signal PA 1  detected earliest in terms of time, as a base. In a case where another potential heartbeat signal is detected by another piezoelectric sensor  1  during a predetermined time frame: tA 1  after a peak time of the potential heartbeat signal PA 1 , the determining device  26  determines that the potential heartbeat signals are the heartbeat signals. The potential heartbeat signals are detected by the piezoelectric sensors  1  outputting the signal components that are determined to include the waveform of the seated person based on the results of the frequency analysis and the amplitude analysis by means of the second and third filter portions  22  and  23  and the determining device  26 . 
     When each of peak times of equal to or more than two of waveforms of each of the signal components which are included in the each of output signals of equal to or more than two of the piezoelectric sensors  1  among the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D and which exist within the predetermined frequency range (the second predetermined frequency range) (i.e. the potential heartbeat signals), exists during the same time frame: tA 1 , and when each of the signal components, which is included in each of the output signals equal to or more than two piezoelectric sensors  1  and which exists within the predetermined frequency range (the first predetermined frequency range), is determined to include the waveform of the seated person, the determining device  26  determines that the each of equal to or more than two signal components includes the heartbeat signal of the person (i.e. the determining device  26  determines that the potential heartbeat signals are the heartbeat signals). In regard to the potential heartbeat PA 1  in  FIG. 7 , the potential heartbeat signal PC 1  and the potential heartbeat signal PB 1  exist during the time frame: tA 1 . Therefore, the determining device  26  determines that the potential heartbeat signals PA 1 , PB 1  and PC 1  are the heartbeat signals. Further, how many potential heartbeat signals need to exist during the time frame: tA 1  for the determining device  26  to determine that the potential heartbeat signals are the heartbeat signals may be modified. 
     Subsequently, the determining device  26  executes the same analysis on the potential heartbeat signal PA 2  that is subsequently detected by the base piezoelectric sensor  1 A and determines whether the potential heartbeat signal PA 2  is assumed to be the heartbeat signal. In the example illustrated in  FIG. 7 , both of the potential heartbeat signals PA 2  and PB 2  are determined to be the heartbeat signals. 
     As described above, in a condition where the seating of the person on the seat  2  is determined, whether the potential heartbeat signals are the heartbeat signals or not is determined objectively by comparing time when the potential heartbeat signals, which are detected by the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D are appeared. The time frame: tA 1  is a time interval in which the potential heartbeat signals can be regarded to be outputted based on the same blood flow. In other words, the time frame: tA 1  is defined within a difference of time in which the pulse of the blood flow is transmitted to several piezoelectric sensors  1 . 
     The determining device  26  executes scoring of each of the piezoelectric sensors  1  in order to determine which piezoelectric sensor  1  detects the heartbeat signals of the person in a high frequency among the plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D provided on the seat  2 . More specifically, the determining device  26  provides a point with the piezoelectric sensor  1  that detects the heartbeat signals determined in the manner described above. The heartbeat signal PC 1  detected by another piezoelectric sensor  1 C exists during a predetermined time frame: tA 2  after the peak time of the heartbeat signal PA 1  detected by the base piezoelectric sensor  1 A. Therefore, each of the piezoelectric sensors  1 A and piezoelectric sensor  1 C scores the point. In a range illustrated in  FIG. 7 , the piezoelectric sensor  1 A scores one point, the piezoelectric sensor  1 B scores zero point, and the piezoelectric sensor  1 C scores one point. The scored points are accumulated in each of the piezoelectric sensors. The time frame may be set as follows. The time frame: tA 2  ≦tA 1  When the heartbeat signal, based on the output signal of one piezoelectric sensor  1  among plurality of piezoelectric sensors  1 A,  1 B,  1 C and  1 D, needs to be outputted, the determining device  26  is configured to output the heartbeat signal from the piezoelectric sensor  1  in which the highest score is accumulated. 
     [Other embodiments][1] In the heartbeat detecting apparatus  20  according to the second embodiment, the determining device  26  determines whether the peak time of another potential heartbeat signal exists during the predetermined time frame after the peak time of the base potential heartbeat signal.However, the heartbeat detecting apparatus  20  may be  modified so that the determining device  26  determines whether the peak time of the potential heartbeat signal extracted from the output signal of another piezoelectric sensor exists during a predetermined time frame before and after the peak time of the potential heartbeat signal extracted from the output signal of the base piezoelectric sensor. For example,  FIG. 8  is a diagram of data on the potential heartbeat signals included in the output signals of the plurality of piezoelectric sensors provided at the seat, illustrated by each of the piezoelectric sensors. The determining device  26  determines whether the peak time of another potential heartbeat signal exists during the predetermined time frame:  2 ×tAl (the sum of the time frame: tAl before the peak time and time frame: tAl after the peak time. The same applies hereinbelow) before and after the peak time of the potential heartbeat signal of the base piezoelectric sensor  1 A. In the example illustrated in  FIG. 8 , all of the potential heartbeat signals PA 1  PB  1 , and PC 1  are determined to be the heartbeat signals. 
     Further, scoring of each of the piezoelectric sensors  1  is executed in the same manner. 
     More specifically, when the piezoelectric sensor lA is used as the base, the heartbeat signal PC 1 , detected by another piezoelectric sensor  1  C, exists during the predetermined time frame: 2×tA 2  (the sum of the time frame before the peak time: tA 2  and the time frame after the peak time: tA 2 . The same applies hereinbelow) before and after the peak time of the heartbeat signal PAL Therefore, each of the piezoelectric sensors  1 A and  1 C scores the point. Further, when the piezoelectric sensor  1 B is used as the base, the heartbeat signal PC 1 , detected by another piezoelectric sensor  1  C, exists during the predetermined time frame: 2×tB 2  before and after the peak time of the heartbeat signal PB  1 . Therefore, the piezoelectric sensors  1 B and  1 C score the point. Furthermore, when the piezoelectric sensor  1 C is used as the base, the heartbeat signals PA 1  and PB 1 , detected by other piezoelectric sensors  1  A and  1 B, exists during the predetermined time frame: 2×tC 2  before and after the peak time of the heartbeat signal PC 1 . Therefore, the piezoelectric sensors  1  A,  1 B and  1 C score the point. 
     [2] 
     The determining device  26  may be modified so that the determining device  26  executes the above-described determination not only on the peak time of the potential heartbeat signals outputted from the base piezoelectric sensor, but also on the potential heartbeat signals outputted from other piezoelectric sensors. In other words, as illustrated in  FIG. 8 , the determining device  26  determines whether the peak time of the potential heartbeat signal outputted from another piezoelectric sensor exists during a predetermined time frame: 2×tB 1  before and after the peak time of the potential heartbeat signal PB 1  of the piezoelectric sensor  1 B. When the peak time of potential heartbeat signals outputted from other piezoelectric sensors exist during the time frame: 2×tB 1 , the potential heartbeat signals PA 1  and PC 1  existing during the time frame: 2×tB 1  are determined to be the heartbeat signals. In the same manner, the determining device  26  determines whether the peak time of the potential heartbeat signal outputted from another piezoelectric sensor exists during predetermined time frame: 2×tC 1  before and after the peak time of the potential heartbeat signal PC 1  of the piezoelectric sensor  1 C. When the peak time of potential heartbeat signals outputted from other piezoelectric sensors exist during the time frame: 2×tC 1 , the potential heartbeat signals PA 1  and PB 1  existing during the time frame: 2×tC 1  are determined to be the heartbeat signals. 
     [3] 
     The determining device  26  may be modified to determine only the potential heartbeat signal outputted from the base piezoelectric sensor to be the heartbeat signal instead of determining all the heartbeat signals existing during the above-described predetermined time to be the heartbeat signals. 
     In other words, when a second potential heartbeat signal extracted from a second signal component, which is included in the output signal of a second piezoelectric sensor among the plurality of piezoelectric sensors and which exists within the second predetermined frequency range, exists during the predetermined time frame before and after the peak time of a first potential heartbeat signal extracted from a first signal component, which is included in the output signal of the first piezoelectric sensor among the plurality of piezoelectric sensors and which exists within the second predetermined frequency range, and when the signal components, which are included in the output signals of the first and second piezoelectric sensors and which exist within the first frequency range, includes the waveform of the seated person, the determining device  26  may be modified to determine that the first potential heartbeat signal is the heartbeat signal of the person, based on a result of the frequency analysis of the signal processing device executed on each of the output signals of the plurality of piezoelectric sensors. 
     More specifically, as illustrated in  FIG. 8 , when the potential heartbeat signal PA 1  is used as the base, peak times of other potential heartbeat signals (the second potential heartbeat signals) PB 1  and PC 1  exist during the predetermined time frame: 2×tAl before and after the peak time of the potential heartbeat signal (the first potential heartbeat signal) PAL The potential heartbeat signals are detected by the piezoelectric sensors  1  that output the signal components determined to include the waveform of the seated person based on the results of frequency analysis and amplitude analysis of the second filter portion  22 , the third filter portion  23  and the determining device  26 . 
     The determining device  26  determines that the base potential heartbeat signal PA 1  is the heartbeat signal and does not determine that the other potential heartbeat signals PB 1  and PC 1  are the heartbeat signals. The same applies when the potential heartbeat signal PB 1  or PC 1  is used as the base. Further, how many potential heartbeat signals need to exist during the predetermined time frame for the determining device  26  to determine that the potential heartbeat signals are the heartbeat signals may be modified. 
     Further, the determining device  26  may be modified to determine another potential heartbeat signal to be the heartbeat signal instead of determining all of the potential heartbeat signals existing during the predetermined time frame to be the heartbeat signals. 
     In other words, when a second potential heartbeat signal, extracted from a second signal component which is included in the output signal of second piezoelectric sensor among the plurality of piezoelectric sensors and which exists within the second predetermined frequency range, exists during the predetermined time frame before and after the peak time of a first potential heartbeat signal extracted from a first signal component, which is included in the output signal of the first piezoelectric sensor among the plurality of piezoelectric sensors and which exists within the second predetermined frequency range, and when the signal components, which are included in the output signals of the first and second piezoelectric sensors and which exist within the first frequency range, include the waveform of the seated person, the determining device  26  may be modified to determine that the second potential heartbeat signal is the heartbeat signal of the person, based on a result of the frequency analysis of the signal processing device executed on each of the output signals of the plurality of piezoelectric sensors. 
     More specifically, as illustrated in  FIG. 8 , when the potential heartbeat signal PA 1  is used as a base, other potential heartbeat signals (the second potential heartbeat signals) PB  1  and PC 1  exist during the predetermined time frame: 2×tAl before and after the peak time of the potential heartbeat signal (i.e. the first potential heartbeat signal) PAL The potential heartbeat signals are detected by the piezoelectric sensors  1  that output the signal components that are determined to include the waveform of the seated person based on the results of the frequency analysis and amplitude analysis of the second filter portion  22 , the third filter portion  23  and the determining device  26 . The determining device  26  determines that the potential heartbeat signals PB 1  and PC 1 , other than the base potential heartbeat signal, to be the heartbeat signals and does not determine that the base potential heartbeat signal PAl to be the heartbeat signal. The same applies when the potential heartbeat signal PB 1  or PC 1  is used as the base. Further, how many potential heartbeat signals need to exist during the predetermined time frame for the determining device  26  to determine that the potential heartbeat signals are the heartbeat signals may be modified. 
     [4] 
     The number of piezoelectric sensors  1  provided relative to the seat  2  and positions of the piezoelectric sensors  1  provided relative to the seat  2  may be modified. For example, the plurality of piezoelectric sensors  1  may be arranged at a portion of the seating portion of the seat  2  where thigh portions of the person contact. 
     [5] 
     In the embodiments described above, the first and second frequency ranges may be the same frequency range or a portion of the first frequency range and a portion of the second frequency range may overlap with each other. 
     INDUSTRIAL APPLICABILITY 
     The heartbeat detecting apparatus according to the present invention may be used in order to measure the heartbeat without constraining the person. 
     EXPLANATION FOR REFERENCE NUMBER 
     
         
           1 . piezoelectric sensor 
           2 . seat 
           10 . heartbeat detecting apparatus 
           15 . signal processing device 
           16 . determining device 
           20 . heartbeat detecting apparatus 
           25 . signal processing device 
           26 . determining device