Patent Publication Number: US-2023157613-A1

Title: Electrocardiographic detection device for vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to an electrocardiographic detection device for a vehicle that detects an electrocardiographic waveform of a vehicle occupant. 
     BACKGROUND ART 
     There is known a technique in which electrodes are provided at a steering wheel and/or a vehicle seat, and the electrocardiographic waveform of a vehicle occupant is detected. 
     Japanese Patent Application Laid-Open (JP-A) No. 2013-212311 discloses the invention of an electrocardiographic detection device for a vehicle having a steering wheel electrode that is disposed at the steering wheel of a vehicle and contacts the skin of a vehicle occupant and detects the potential of the body of the vehicle occupant, and a first capacitive coupling type electrode and a second capacitive coupling type electrode disposed at the backrest of a vehicle seat and detecting the potential of the body of the vehicle occupant without contacting the skin of the vehicle occupant, wherein the electrocardiographic waveform of the vehicle occupant is measured on the basis of the difference between the potential difference between the potential at the steering wheel electrode and the potential at the first capacitive coupling type electrode, and the potential difference between the potential at the steering wheel electrode and the potential at the second capacitive coupling type electrode. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the technique disclosed in JP-A No. 2013-212311, a grasped portion that is covered by a hand (body) by being grasped by the vehicle occupant, and a non-grasped portion that is not grasped by the vehicle occupant and is not covered by the body, arise at the steering wheel electrode. Of the steering wheel electrode, the non-grasped portion generates noise due to vibration of the vehicle, and there is the problem that, due to this noise being superposed on the electrocardiographic signal, it is difficult to detect the electrocardiographic signal. 
       FIG.  8    is an explanatory drawing that illustrates the path of generation and entry of noise at the electrocardiographic detection device for a vehicle. As illustrated in  FIG.  8   , in a structure such as that disclosed in JP-A No. 2013-212311, the following two types of noise that are caused by vibration of the vehicle body can be mentioned. (In  FIG.  8   , the arrows indicate the direction of the flow of current (noise).) One is common mode noise  130  that enters from capacitor CN 2  formed by the charges generated at a body  12  side due to vibration, and flows via a seat electrode  122 , which is the first capacitive coupling type electrode or the second capacitive coupling type electrode, to a ground region (GND). Another one is normal mode noise  132  that enters from a non-grasped portion  118  of a steering wheel electrode  116  that functions as a capacitor CN 1  due to vibration, and flows via the seat electrode  122  to GND. At the time of detecting an electrocardiographic signal by the steering wheel electrode  116 , the latter normal mode noise  132  is particularly problematic. 
     The normal mode noise  132  is current that is generated accompanying vibration of the electrode at the non-grasped portion  118  of the steering wheel electrode  116 , and flows via the body  12  to GND, and flows also via a buffer circuit  140  to GND. The potential difference between GND and the potential at the steering wheel electrode is outputted from the output end of the buffer circuit  140 . Therefore, there is the problem that accurate detection of an electrocardiographic signal is difficult due to the effects of the normal mode noise  132  that flows via the body  12  to GND. 
     The present disclosure provides an electrocardiographic detection device for a vehicle in which the S/N ratio of an electrocardiographic signal is high due to noise caused by vibration being suppressed. 
     Solution to Problem 
     A first aspect of the present disclosure is an electrocardiographic detection device for a vehicle, comprising: a first electrode portion provided at a lower layer of a covering material that covers a surface of a steering wheel, a second electrode portion provided at a lower layer of the covering material of the steering wheel and an insulating material of a predetermined thickness; a first detecting portion detecting a differential voltage between the first electrode portion and a ground region; a second detecting portion detecting a differential voltage between the second electrode portion and the ground region; and a signal processing section that generates an electrocardiographic signal based on the differential voltage detected at the first detecting portion and the differential voltage detected at the second detecting portion. 
     In accordance with the first aspect, an electrocardiographic signal of a high S/N ratio and in which noise due to vibration is suppressed can be obtained by the processing of outputting the difference between the electrocardiographic signal detected at the first electrode portion and the signal detected at the second electrode portion and in which the normal mode noise  132  is seen markedly, or the like. 
     A second aspect of the present disclosure is an electrocardiographic detection device for a vehicle, comprising: a first electrode portion provided at a lower layer of a covering material that covers a surface of a steering wheel; a second electrode portion provided at a lower layer of the covering material of the steering wheel and an insulating material of a predetermined thickness; a third electrode portion provided at a seat in which a vehicle occupant sits; a first detecting portion detecting a differential voltage between the first electrode portion and a ground region; a second detecting portion detecting a differential voltage between the second electrode portion and a ground region; a third detecting portion detecting a differential voltage between the third electrode portion and a ground region; and a signal processing section that generates an electrocardiographic signal based on the differential voltage detected at the first detecting portion, the differential voltage detected at the second detecting portion, and the differential voltage detected at the third detecting portion. 
     In accordance with the second aspect, an electrocardiographic signal of a high S/N ratio and in which noise due to vibration is suppressed can be obtained by the processing of outputting the difference between the electrocardiographic signal detected at the first electrode portion, the signal detected at the second electrode portion and in which the normal mode noise  132  is seen markedly, and the signal detected at the third electrode portion and in which the common mode noise  130  is seen markedly, or the like. 
     Further, the first electrode portion and the second electrode portion respectively may be disposed so as to be symmetrical as seen from a radial direction of the steering wheel. 
     Further, as seen from the radial direction of the steering wheel, the first electrode portion and the second electrode portion respectively are adjacent to one another, respective shapes thereof are substantially congruent, and the first electrode portion and the second electrode portion may have a geometrically equivalent relationship in which respective surface areas thereof are substantially the same. 
     Further, the first electrode portion and the second electrode portion respectively may be disposed parallel as seen from a radial direction of the steering wheel. 
     Further, the first electrode portion and the second electrode portion may be disposed in forms of dots as seen from a radial direction of the steering wheel. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a drawing illustrating the schematic structure of an electrocardiographic detection device for a vehicle relating to a first embodiment. 
         FIG.  2 A  is a schematic drawing illustrating an example of an arranged pattern of steering wheel electrodes. 
         FIG.  2 B  is a schematic drawing illustrating another example of an arranged pattern of the steering wheel electrodes. 
         FIG.  3 A  is an enlarged drawing of the arranged pattern of the steering wheel electrodes illustrated in  FIG.  2   . 
         FIG.  3 B  is a cross-sectional view at line A-A of  FIG.  3 A . 
         FIG.  4 A  is a schematic drawing illustrating an example of an electrocardiographic signal that is detected at the steering wheel electrode and outputted from a waveform generator. 
         FIG.  4 B  is a schematic drawing illustrating an example of an electrocardiographic signal that is detected at the steering wheel electrode, which is separated from the surface of a steering wheel by an insulating material, and is outputted from a waveform generator. 
         FIG.  4 C  is a schematic drawing illustrating an example of an electrocardiographic signal that is outputted from a signal processing section. 
         FIG.  5 A  is a schematic drawing illustrating a modified example of the arranged pattern of the steering wheel electrodes. 
         FIG.  5 B  is a schematic drawing illustrating yet another example of a modified example of the arranged pattern of the steering wheel electrodes. 
         FIG.  6    is a drawing illustrating the schematic structure of an electrocardiographic detection device for a vehicle relating to a second embodiment. 
         FIG.  7 A  is a schematic drawing illustrating an example of an electrocardiographic signal that is detected at the steering wheel electrode and outputted from the waveform generator. 
         FIG.  7 B  is a schematic drawing illustrating an example of an electrocardiographic signal that is detected at the steering wheel electrode, which is separated from the surface of the steering wheel by an insulating material, and is outputted from a waveform generator. 
         FIG.  7 C  is a schematic drawing illustrating an example of an electrocardiographic signal that is detected at a seat electrode and outputted from a waveform generator. 
         FIG.  7 D  is a schematic drawing illustrating an example of an electrocardiographic signal that is outputted from a signal processing section. 
         FIG.  8    is an explanatory drawing illustrating a path of generation and entry of electrocardiographic noise at an electrocardiographic detection device for a vehicle. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Examples of electrocardiographic detection devices for a vehicle relating to embodiments of the present disclosure are described in detail hereinafter with reference to the drawings. 
     First Embodiment 
     First, an electrocardiographic detection device for a vehicle relating to a first embodiment is described.  FIG.  1    is a drawing illustrating the schematic structure of an electrocardiographic detection device  10  for a vehicle relating to the first embodiment. 
     The electrocardiographic detection device  10  for a vehicle relating to the present embodiment has steering wheel electrodes  16 A,  16 B, a seat electrode  22 A, buffer circuits  30 A,  30 B, waveform generators  40 A,  40 B, an A/D converter  50 , and a signal processing section  60 . 
     The steering wheel electrodes  16 A,  16 B and the seat electrode  22 A are provided as electrodes that are provided at positions at which a vehicle occupant can contact them. 
     The steering wheel electrodes  16 A,  16 B are provided along the entire peripheral direction region of a steering wheel  14  that is for carrying out steering operation of the vehicle. When the vehicle occupant grasps the steering wheel  14 , the hands of the vehicle occupant contact the steering wheel electrodes  16 A,  16 B, and capacitive coupling arises between the hands of the vehicle occupant and the steering wheel electrodes  16 A,  16 B, and electrostatic capacitive coupling type capacitors are formed. Further, the steering wheel electrodes  16 A,  16 B are electrically connected to the waveform generators  40 A,  40 B via the buffer circuits  30 A,  30 B. The waveform generators  40 A,  40 B detect the ion current changes (alternating current), which accompany the electrical activity of the heartbeat of the heart, from the steering wheel electrodes  16 A,  16 B as current signals (electrocardiographic signals). As will be described later, the steering wheel electrode  16 A is an electrode that detects mainly an electrocardiographic signal, and the steering wheel electrode  16 B is an electrode that detects mainly noise that enters the electrocardiographic signal. 
     The seat electrode  22 A is provided at a seat cushion  20 A of a vehicle seat  20 , which is further toward the vehicle lower side than the position of the heart of the vehicle occupant seated in the vehicle seat  20 , and is covered by a seat cover (not illustrated). Due to the vehicle occupant sitting on the vehicle seat  20 , the seat electrode  22 A contacts the buttocks of the vehicle occupant via the clothing of the vehicle occupant and the seat cover, and forms an electrostatic capacitive coupling type capacitor that is capacitively coupled with the vehicle occupant. The seat electrode  22 A is grounded together with the inverting input terminal (−) of a differential amplifier  42 B that is described later, so that the static electricity generated by the clothes of the vehicle occupant seated in the vehicle seat  20  and the like flows to GND. The seat electrode  22 A may be disposed at a seatback  20 B of the vehicle seat  20  provided that it is at the lower side of the position of the heart of the vehicle occupant. 
     The buffer circuit  30 A is provided between the steering wheel electrode  16 A and the waveform generator  40 A, and outputs the signal from the steering wheel electrode  16 A to the waveform generator  40 A as a buffer. The buffer circuit  30 A is structured by a positive feedback circuit called a bootstrap circuit that includes an operational amplifier  32 A, resistors  36 A 1 ,  36 A 2 , and a capacitor  34 A. More specifically, the steering wheel electrode  16 A is electrically connected to the non-inverting input terminal (+) of the operational amplifier  32 A, and is grounded via the resistors  36 A 1 ,  36 A 2 . The resistors  36 A 1 ,  36 A 2  structure a type of voltage divider. The signal from the steering wheel electrode  16 A, which is divided in accordance with the resistance values of the resistors  36 A 1 ,  36 A 2 , is inputted via the capacitor  34 A to the inverting input terminal (−) of the operational amplifier  32 A. 
     The waveform generator  40 A is structured to include a differential amplifier  42 A that serves as a detecting portion, and a filter amplification section  44 A, and generates an electrocardiographic waveform of the vehicle occupant on the basis of the current signal inputted from the steering wheel electrode  16 A. 
     Due to the non-inverting input terminal (+) of the differential amplifier  42 A being connected to the steering wheel electrode  16 A via the buffer circuit  30 A, and the inverting input terminal (−) being grounded, the differential voltage (differential signal) between the signal inputted to the non-inverting input terminal (+) and the potential of GND is detected, and is outputted to the filter amplification section  44 A. Specifically, the differential amplifier  42 A amplifies, by a predetermined factor, the difference between the input signal from the steering wheel electrode  16 A and the potential of GND, and outputs a signal. 
     The filter amplification section  44 A amplifies the output signal of the differential amplifier  42 A, and carries out the processing of converting the resultant signal into a signal of a frequency of a predetermined range by using a predetermined filter, or the like, and outputs the results of processing via the A/D converter  50  to the signal processing section  60 . A filter such as, for example, a high pass filter, a low pass filter, a bandpass filter or the like can be used appropriately as the predetermined filter. The signal that is outputted by the filter amplification section  44 A is converted from an analog signal into a digital signal by the A/D converter  50 , and is inputted to the signal processing section  60 . 
     The buffer circuit  30 B is provided between the steering wheel electrode  16 B and the waveform generator  40 B, and outputs the signal from the steering wheel electrode  16 B to the waveform generator  40 B as a buffer. In the same way as the buffer circuit  30 A, the buffer circuit  30 B is structured by a positive feedback circuit that includes an operational amplifier  32 B, resistors  36 B 1 ,  36 B 2 , and a capacitor  34 B. 
     The waveform generator  40 B is structured to include the differential amplifier  42 B that serves as a detecting portion, and a filter amplification section  44 B, and, on the basis of the current signal inputted from the steering wheel electrode  16 B, generates an electrocardiographic waveform of the vehicle occupant that includes noise much more markedly than the signal generated by the waveform generator  40 A. 
     Due to the non-inverting input terminal (+) of the differential amplifier  42 B being connected to the steering wheel electrode  16 B via the buffer circuit  30 B, and the inverting input terminal (−) being grounded together with the seat electrode  22 A, the differential voltage (differential signal) between the signal inputted to the non-inverting input terminal (+) and the signal inputted to the inverting input terminal (−) is detected, and is outputted to the filter amplification section  44 B. Specifically, the differential amplifier  42 B amplifies, by a predetermined factor, the difference between the input signal from the steering wheel electrode  16 B and GND, and outputs a signal. 
     The filter amplification section  44 B amplifies the output signal of the differential amplifier  42 B, and, in the same way as the above-described filter amplification section  44 A, carries out the processing of converting the resultant signal into a signal of a frequency of a predetermined range by using a predetermined filter, or the like, and outputs the results of processing via the A/D converter  50  to the signal processing section  60 . 
     The signal processing section  60  is structured by a computer including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and a non-volatile memory such as a flash memory or the like. The CPU executes signal processing by executing a program that is stored in advance in the memory. 
     In the present embodiment, the processing of outputting the difference between a signal, in which the waveform of the electrocardiographic signal outputted from the waveform generator  40 A is marked, and a signal, which markedly includes the normal mode noise  132  such as illustrated in  FIG.  8    in the electrocardiographic signal outputted from the waveform generator  40 B, or the like, is carried out. Due to this processing, the component relating to the normal mode noise  132  that is due to vibration is removed from the electrocardiographic signal outputted from the waveform generator  40 A, and an electrocardiographic signal having a high S/N ratio is outputted. 
       FIG.  2 A  is a schematic drawing illustrating an example of the arranged pattern of the steering wheel electrodes  16 A,  16 B, and  FIG.  2 B  is a schematic drawing illustrating another example of the arranged pattern of the steering wheel electrodes  16 A,  16 B. The steering wheel electrodes  16 A,  16 B illustrated in  FIG.  2 A  are respectively disposed parallel to the peripheral direction of the steering wheel  14 . The steering wheel electrodes  16 A,  16 B illustrated in  FIG.  2 B  are respectively disposed orthogonal to the peripheral direction of the steering wheel  14 . In both cases of  FIG.  2 A  and  FIG.  2 B , the aforementioned first electrode portion and the aforementioned second electrode portion are respectively disposed parallel and present an arranged pattern that is a striped pattern, as seen from the radial direction of the steering wheel  14 . 
     As described above, the steering wheel electrodes  16 A,  16 B are provided over the entire peripheral direction region of the steering wheel  14 , and the steering wheel electrode  16 A and the steering wheel electrode  16 B respectively are disposed so as to be symmetrical as seen from the radial direction of the steering wheel  14 . More specifically, the steering wheel electrodes  16 A,  16 B respectively are adjacent to one another, and the respective shapes thereof are substantially congruent, and the steering wheel electrodes  16 A,  16 B have a geometrically equivalent relationship in which the respective surface areas thereof are substantially the same. Accordingly, as illustrated in both  FIG.  2 A  and  FIG.  2 B , the respective surface areas of the steering wheel electrode  16 A and the steering wheel electrode  16 B at a region where the steering wheel  14  is grasped by a hand (the body  12 ) of the vehicle occupant are substantially the same. Further, the arranged pattern of the steering wheel electrodes  16 A,  16 B is such that, the more narrow the respective pitches of the steering wheel electrodes  16 A,  16 B, the greater the correlation with noise. 
       FIG.  3 A  is an enlarged drawing of the arranged pattern of the steering wheel electrodes  16 A,  16 B illustrated in  FIG.  2   , and  FIG.  3 B  is a cross-sectional view at line A-A of  FIG.  3 A . As illustrated in  FIG.  3 B , the steering wheel electrodes  16 A,  16 B are respectively disposed on a core material  14 C, which is made of resin or the like, of the steering wheel  14 , and the steering wheel electrodes  16 A,  16 B are covered by a covering material  14 L of leather or the like that structures the surface of the steering wheel  14 . As illustrated in  FIG.  3 B , the steering wheel electrodes  16 A,  16 B respectively do not contact the body  12  directly, and are grasped by the vehicle occupant via the covering material  14 L. 
     The steering wheel electrode  16 A is disposed on the core material  14 C via an insulating material  161  that has a predetermined thickness, and buffer materials  18 A 1 ,  18 A 2 . The steering wheel electrode  16 B is disposed on the core material  14 C via buffer materials  18 B 1 ,  18 B 2 . The insulating material  161  is disposed on the steering wheel electrode  16 B that is disposed on the buffer materials  18 B 1 ,  18 B 2 . 
     As illustrated in  FIG.  3 B , because the distance of the steering wheel electrode  16 B from the hands (the body  12 ) of the vehicle occupant is greater than that of the steering wheel electrode  16 A, the charge that is generated between the hand of the vehicle occupant and the steering wheel electrode  16 B is lower than that of the steering wheel electrode  16 A. As a result, the electrocardiographic signal detected at the steering wheel electrode  16 B is smaller than the electrocardiographic signal detected at the steering wheel electrode  16 A. 
       FIG.  4 A  is a schematic drawing illustrating an example of the electrocardiographic signal that is detected at the steering wheel electrode  16 A and outputted from the waveform generator  40 A.  FIG.  4 B  is a schematic drawing illustrating an example of the electrocardiographic signal that is detected at the steering wheel electrode  16 B and outputted from the waveform generator  40 B.  FIG.  4 C  is a schematic drawing illustrating an example of the electrocardiographic signal that is outputted from the signal processing section  60 . 
     An R wave  72 A is seen markedly in the electrocardiographic signal that is detected at the steering wheel electrode  16 A and illustrated in  FIG.  4 A , and normal mode noise  74 A that is due to vibration of the steering wheel electrodes  16 A,  16 B is suppressed relatively, and the S/N ratio of the signal is high. In the electrocardiographic signal that is detected at the steering wheel electrode  16 B and illustrated in  FIG.  4 B , although an R wave  72 B is seen, normal mode noise  74 B that is due to vibration of the steering wheel electrodes  16 A,  16 B is marked, and as a result, the S/N ratio of the signal is low as compared with the electrocardiographic signal detected at the steering wheel electrode  16 A. 
     In the present embodiment, the processing of outputting the difference between the electrocardiographic signal detected at the steering wheel electrode  16 A and the signal, which is detected at the steering wheel electrode  16 B and in which noise is marked, or the like is carried out by the signal processing section  60 . Due to this processing, the component relating to the normal mode noise  74 A is removed, and an electrocardiographic signal having a high S/N ratio is outputted. 
       FIG.  5 A  is a schematic drawing illustrating a modified example of the arranged pattern of the steering wheel electrodes  16 A,  16 B, and  FIG.  5 B  is a schematic drawing illustrating another example of a modified example of the arranged pattern of the steering wheel electrodes  16 A,  16 B. The steering wheel electrodes  16 A,  16 B illustrated in  FIG.  5 A  are respectively disposed so as to be inclined with respect to the peripheral direction of the steering wheel  14 . The steering wheel electrodes  16 A,  16 B illustrated in  FIG.  5 B  are disposed in forms of dots. 
     In both the cases of  FIG.  5 A  and  FIG.  5 B , as seen from the radial direction of the steering wheel  14 , the steering wheel electrode  16 A and the steering wheel electrode  16 B respectively are symmetrical and are adjacent to one another, and the respective shapes thereof are substantially congruent, and the steering wheel electrodes  16 A,  16 B have a geometrically equivalent relationship in which the respective surface areas thereof are substantially the same. Further, the finer the arranged pattern of the steering wheel electrodes  16 A,  16 B, the higher the correlation with the noise that is detected. 
     In addition to the forms illustrated in  FIG.  5 A  and  FIG.  5 B , arranged patterns of other forms may be used provided that the steering wheel electrode  16 A and the steering wheel electrode  16 B respectively have a geometrically equivalent relationship in the plane that is parallel to the surface of the steering wheel  14 . 
     As described above, in accordance with the present embodiment, due to the steering wheel electrode  16 B being placed further away from the hands of the vehicle occupant than the steering wheel electrode  16 A, the normal mode noise  74 B that enters from the non-grasped portion of the steering wheel electrode  16 B is marked, and the R wave  72 B that is obtained from the steering wheel electrode  16 B is smaller than the R wave  72 A that is obtained from the steering wheel electrode  16 A. As a result, the electrocardiographic signal detected at the steering wheel electrode  16 B has a lower S/N ratio than the electrocardiographic signal detected at the steering wheel electrode  16 A. In the present embodiment, an electrocardiographic signal having a high S/N ratio can be obtained by the processing of outputting the difference between the electrocardiographic signal detected at the steering wheel electrode  16 A and the signal, which is detected at the steering wheel electrode  16 B and in which noise due to vibration is marked, or the like. 
     Note that the first electrode portion in the claims corresponds to the steering wheel electrode  16 A in the present embodiment, the second electrode portion therein corresponds to the steering wheel electrode  16 B in the present embodiment, the first detecting portion therein corresponds to the waveform generator  40 A in the present embodiment, the second detecting portion therein corresponds to the waveform generator  40 B in the present embodiment, and the signal processing section therein corresponds to the signal processing section  60  in the present embodiment. 
     Second Embodiment 
     An electrocardiographic detection device  100  for a vehicle relating to a second embodiment is described next.  FIG.  6    is a drawing illustrating the schematic structure of the electrocardiographic detection device  100  for a vehicle relating to the second embodiment. The electrocardiographic detection device  100  for a vehicle relating to the present embodiment differs from the electrocardiographic detection device  10  for a vehicle relating to the first embodiment with regard to the points of additionally having a seat electrode  22 B, a buffer circuit  30 C and a waveform generator  40 C, and that an A/D converter  52  converts the analog signals outputted from the waveform generators  40 A,  46 B,  40 C respectively into digital signals, and a signal processing section  62  generates an electrocardiographic signal of a high S/N ratio by using the signals outputted from the waveform generators  40 A,  46 B,  40 C respectively, and the seat electrode  22 A is grounded together with the inverting input terminal (−) of a differential amplifier  42 C of the waveform generator  40 C. Because the other structures are the same as those of the electrocardiographic detection device  10  for a vehicle relating to the first embodiment, the structures that are the same as the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. 
     In the present embodiment, the steering wheel electrodes  16 A,  16 B, the buffer circuits  30 A,  30 B and the waveform generator  40 A are the same as in the first embodiment. At the waveform generator  46 B relating to the present embodiment, the filter amplification section  44 B is the same as in the first embodiment, but a differential amplifier  48 B is different from the differential amplifier  42 B of the first embodiment with regard to the point that the seat electrode  22 A is not connected to the inverting input terminal (−) of the differential amplifier  48 B and is grounded. 
     The buffer circuit  30 C is provided between the seat electrodes  22 A,  22 B and the waveform generator  40 C, and outputs the signals from the seat electrodes  22 A,  22 B to the waveform generator  40 C as a buffer. In the same way as the buffer circuit  30 A, the buffer circuit  30 C is structured by a positive feedback circuit that includes an operational amplifier  32 C, resistors  36 C 1 ,  36 C 2 , and a capacitor  34 C. 
     The waveform generator  40 C is structured to include the differential amplifier  42 C that serves as a detecting portion, and a filter amplification section  44 C, and, on the basis of the current signal inputted from the seat electrode  22 B, generates an electrocardiographic waveform of the vehicle occupant that includes the common mode noise  130 . 
     Due to the non-inverting input terminal (+) of the differential amplifier  42 C being connected to the seat electrode  22 B via the buffer circuit  30 C, and the inverting input terminal (−) being grounded together with the seat electrode  22 A, the differential voltage (differential signal) between the signal inputted to the non-inverting input terminal (+) and the signal inputted to the inverting input terminal (−) is detected, and is outputted to the filter amplification section  44 C. Specifically, the differential amplifier  42 B amplifies, by a predetermined factor, the difference between the input signal from the seat electrode  22 B and GND, and outputs a signal. 
     The filter amplification section  44 C amplifies the output signal of the differential amplifier  42 C, and, in the same way as the above-described filter amplification section  44 A, carries out the processing of converting the resultant signal into a signal of a frequency of a predetermined range by using a predetermined filter, or the like, and outputs the results of processing via the A/D converter  52  to the signal processing section  62 . 
     The signal processing section  62  is structured by a computer including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and a non-volatile memory such as a flash memory or the like. The CPU executes signal processing by executing a program that is stored in advance in the memory. 
     In the present embodiment, the processing of outputting the difference between a signal, in which the waveform of the electrocardiographic signal outputted from the waveform generator  40 A is marked, and a signal, in which the common mode noise  130  and the normal mode noise  132  such as illustrated in  FIG.  8    are markedly included in the electrocardiographic signal outputted from the waveform generator  46 B, and a signal, in which the common mode noise  130  is markedly included in the electrocardiographic signal outputted from the waveform generator  40 C, or the like, is carried out. Due to this processing, the components relating to the common mode noise  130  and the normal mode noise  132  are removed from the electrocardiographic signal outputted from the waveform generator  40 A, and an electrocardiographic signal having a high S/N ratio is outputted. 
       FIG.  7 A  is a schematic drawing illustrating an example of the electrocardiographic signal that is detected at the steering wheel electrode  16 A and outputted from the waveform generator  40 A.  FIG.  7 B  is a schematic drawing illustrating an example of the electrocardiographic signal that is detected at the steering wheel electrode  16 B and outputted from the waveform generator  46 B.  FIG.  7 C  is a schematic drawing illustrating an example of the electrocardiographic signal that is detected at the seat electrode  22 B and outputted from the waveform generator  40 C.  FIG.  7 D  is a schematic drawing illustrating an example of the electrocardiographic signal that is outputted from the signal processing section  62 . 
     An R wave  82 A is seen markedly in the electrocardiographic signal that is detected at the steering wheel electrode  16 A and illustrated in  FIG.  7 A , and normal mode noise  84 A that is due to vibration of the steering wheel electrodes  16 A,  16 B is suppressed relatively. However, common mode noise  86 A that is due to charges generated at the body  12  side is seen, and as a result, the S/N ratio of the signal is lowered. In the electrocardiographic signal that is detected at the steering wheel electrode  16 B and illustrated in  FIG.  7 B , although an R wave  82 B is seen, normal mode noise  84 B that is due to vibration of the steering wheel electrodes  16 A,  16 B is marked, and moreover, common mode noise  86 B that is due to charges generated at the body  12  side is seen. An R wave  82 C and normal mode noise  84 C that is due to vibration of the steering wheel electrodes  16 A,  16 B are not seen in the electrocardiographic signal that is detected at the seat electrode  22 B and illustrated in  FIG.  7 C . However, common mode noise  86 C that is due to charges generated at the body  12  side is seen. 
     In the present embodiment, the processing of outputting the difference between the electrocardiographic signal detected at the steering wheel electrode  16 A, the signal detected at the steering wheel electrode  16 B, and the signal detected at the seat electrode  22 B, or the like is carried out at the signal processing section  62 . Due to this processing, the components relating to the normal mode noise  84 A and the common mode noise  86 A are removed, and an electrocardiographic signal having a high S/N ratio is outputted. 
     As described above, in the present embodiment, an electrocardiographic signal of a high S/N ratio can be obtained by the processing of outputting the difference between the electrocardiographic signal detected at the steering wheel electrode  16 A, the signal detected at the steering wheel electrode  16 B and in which the normal mode noise  84 B is seen markedly, and the signal detected at the seat electrode  22 B and in which the common mode noise  86 C is seen markedly, or the like. 
     Although the present embodiment describes a form in which electrocardiographic waveforms are detected by using the steering w % heel electrodes  16 A,  16 B and the seat electrodes  22 A,  22 B, the present invention is not limited to this. For example, a pair of seat electrodes that are provided at the seatback  20 B may be used instead of the seat electrodes  22 A,  22 B. 
     Note that the first electrode portion in the claims corresponds to the steering wheel electrode  16 A in the present embodiment, the second electrode portion therein corresponds to the steering wheel electrode  16 B in the present embodiment, the third electrode portion therein corresponds to the seat electrode  22 B in the present embodiment, the first detecting portion therein corresponds to the waveform generator  40 A in the present embodiment, the second detecting portion therein corresponds to the waveform generator  46 B in the present embodiment, the third detecting portion therein corresponds to the waveform generator  40 C in the present embodiment, and the signal processing section therein corresponds to the signal processing section  62  in the present embodiment. 
     The processings carried out at the signal processing sections  60 ,  62  in the above-described embodiments may be software processings, or may be processings carried out by hardware. Or, these processings may be processings that combine both hardware and software. 
     Further, the processings carried out at the signal processing sections  60 ,  62  in the above-described embodiments may be stored as programs on storage media and distributed. 
     Moreover, the present invention is not limited to the above, and, other than the above, can of course be implemented by being modified in various ways within a scope that does not depart from the gist thereof. 
     The disclosure of Japanese Patent Application No. 2020-055102 filed on Mar. 25, 2020 is, in its entirety, incorporated by reference into the present specification.