Abstract:
A double-electrode capacitive sensor for detecting a detection object includes: an electrode assembly having first and second electrodes and an insulation substrate; and a detection circuit. The first and second electrodes are disposed on first and second surfaces of the substrate, respectively. The detection circuit applies an alternating voltage between a ground and the first electrode, and detects an electric potential of the second electrode, or controls the electric potential of the second electrode to follow the alternating voltage. The detection circuit detects a capacitance change between the first electrode and the ground when the detection object approaches the first electrode for determining whether the detection object approaches the first electrode. A periphery of the second electrode is substantially opposite to a periphery of the first electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on Japanese Patent Application No. 2007-28548 filed on Feb. 7, 2007, the disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a double-electrode capacitive sensor, a passenger detector, and a passenger protection system. 
   BACKGROUND OF THE INVENTION 
   JP-A-2006-201129 corresponding to US Patent Application Publication No. 2006-164254 proposes a vehicular seating detection sensor using a double-electrode capacitance sensor having an electrode assembly in which a front electrode is provided on the front major surface of a resin film buried in a seat, and a back electrode which is smaller than the front electrode is provided on the back major surface of the resin film. The double-electrode capacitance sensor also has a detection circuit section for detecting whether someone is seated or not using the electrode assembly. 
   The detection circuit section applies an AC voltage between one electrode and the vehicle body and supplies an AC current to the other electrode via an operational amplifier so that the potential of the other electrode becomes equal to that of the one electrode. The detection circuit section judges whether someone is seated or not on the basis of a variation of the AC current. 
   Although the above double-electrode capacitance sensor is higher in detection sensitivity than single-electrode capacitance sensors, there is a market requirement that its detection sensitivity be increased further. 
   SUMMARY OF THE INVENTION 
   In view of the above-described problem, it is an object of the present disclosure to provide a double-electrode capacitive sensor. It is another object of the present disclosure to provide a passenger detector. It is further another object of the present disclosure to provide a passenger protection system. 
   According to a first aspect of the present disclosure, a double-electrode capacitive sensor for detecting a detection object made of dielectric material includes: an electrode assembly having a first electrode, a second electrode and an insulation substrate; and a detection circuit electrically coupled with the first and second electrodes. The first electrode faces the detection object. The first and second electrodes are overlapped, and the second electrode is coupled with a ground. The insulation substrate has a first surface and a second surface. The first electrode is disposed on the first surface, and the second electrode is disposed on the second surface, so that the second electrode faces the first electrode in parallel to the first electrode through the insulation substrate. The detection circuit applies an alternating voltage between the ground and one of the first and second electrodes. The detection circuit detects an electric potential of the other one of the first and second electrodes, or controls the electric potential of the other one of the first and second electrodes to follow the alternating voltage of the one of the first and second electrodes. The detection circuit detects a capacitance change between the first electrode and the ground when the detection object approaches the first electrode. The detection circuit determines whether the detection object approaches the first electrode based on the capacitance change. The second electrode has a periphery, which is substantially opposite to a periphery of the first electrode. 
   In the above sensor, since the periphery of the second electrode is opposite to the periphery of the first electrode, detection accuracy of the detection object is improved. 
   According to a second aspect of the present disclosure, a double-electrode capacitive sensor for detecting a detection object made of dielectric material includes: an electrode assembly having a first electrode, a second electrode and an insulation substrate; and a detection circuit electrically coupled with the first and second electrodes. The first electrode faces the detection object. The first and second electrodes are overlapped, and the second electrode is coupled with a ground. The insulation substrate has a first surface and a second surface. The first electrode is disposed on the first surface, and the second electrode is disposed on the second surface, so that the second electrode faces the first electrode in parallel to the first electrode through the insulation substrate. The detection circuit applies an alternating voltage between the ground and one of the first and second electrodes. The detection circuit detects an electric potential of the other one of the first and second electrodes, or controls the electric potential of the other one of the first and second electrodes to follow the alternating voltage of the one of the first and second electrodes. The detection circuit detects a capacitance change between the first electrode and the ground when the detection object approaches the first electrode. The detection circuit determines whether the detection object approaches the first electrode based on the capacitance change. The detection circuit includes an oscillation circuit and a differential amplifier circuit. The oscillation circuit applies the alternating voltage between the ground and the one of the first and second electrodes. The differential amplifier circuit alternatingly energizes the other one of the first and second electrodes so that the electric potential of the other one of the first and second electrodes follows the electric potential of the one of the first and second electrodes. The differential amplifier outputs a signal corresponding to an energizing current to the other one of the first and second electrodes. The differential amplifier includes a voltage follower circuit for energizing the other one of the first and second electrodes with the electric potential of the other one of the first and second electrodes. 
   In the above sensor, the following performance of the electric potential is improved, and oscillation of the differential amplifier circuit is prevented. Thus, a usable frequency of the sensor can be increased. 
   According to a third aspect of the present disclosure, a passenger detector includes: the double-electrode capacitive sensor according to the first aspect. The double-electrode capacitive sensor is mounted on a seat of a vehicle. The detection object is a passenger in the vehicle so that the detection circuit determines whether the passenger sits down on the seat based on the capacitance change. The detection circuit further identify the passenger based on the capacitance change. 
   In the above detector, detection accuracy of the passenger is improved. 
   According to a fourth aspect of the present disclosure, a passenger protection system includes: the passenger detector according to the third aspect; a passenger protection device for protect the passenger in case of collision of the vehicle; and a controller for controlling the passenger protection device based on determination of the passenger detector. 
   In the above system, the passenger can be protected with much safer. 
   According to a fifth aspect of the present disclosure, a double-electrode capacitive sensor for detecting a detection object made of dielectric material includes: an electrode assembly having a first electrode, a second electrode and an insulation substrate; and a detection circuit electrically coupled with the first and second electrodes. The insulation substrate has a first surface and a second surface. The first electrode is disposed on the first surface, and the second electrode is disposed on the second surface. The first electrode faces the detection object. The first and second electrodes are overlapped so that a periphery of the second electrode is substantially opposite to a periphery of the first electrode. The second electrode is coupled with a ground. The detection circuit applies an alternating voltage between the ground and the second electrode. The detection circuit controls an electric potential of the first electrode to follow the alternating voltage. The detection circuit detects a capacitance change between the first electrode and the ground when the detection object approaches the first electrode. The detection circuit determines whether the detection object approaches the first electrode based on the capacitance change. The detection circuit includes an oscillation circuit and a differential amplifier circuit. The oscillation circuit applies the alternating voltage between the ground and the second electrode. The differential amplifier circuit alternatingly energizes the first electrode so that the electric potential of the first electrode follows the electric potential of the second electrode. The differential amplifier outputs a signal corresponding to an energizing current to the first electrode. The differential amplifier includes a voltage follower circuit for energizing the first electrode with the electric potential of the first electrode. A deviation between the periphery of the first electrode and the periphery of the second electrode is in a range between −10% and +10%. 
   In the above sensor, since the periphery of the second electrode is opposite to the periphery of the first electrode, detection accuracy of the detection object is improved. Further, the following performance of the electric potential is improved, and oscillation of the differential amplifier circuit is prevented. Thus, a usable frequency of the sensor can be increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIG. 1  is a schematic vertical sectional view of a vehicular seat apparatus according to an example embodiment of the present disclosure; 
       FIG. 2  is a circuit diagram of a seating detection apparatus using the double-electrode capacitance sensor shown in  FIG. 1 ; 
       FIG. 3  is a circuit diagram of a circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 4  is a circuit diagram of another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 5  is a circuit diagram of further another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 6  is a circuit diagram of another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 7  is a circuit diagram of further another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 8  is a circuit diagram of another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 9  is a circuit diagram of further another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 10  is a circuit diagram of another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 11  is a circuit diagram of further another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 12  is a circuit diagram of another circuit example of the detection circuit section shown in  FIG. 2 ; 
       FIG. 13  is a vertical sectional view showing the electrode arrangement of a mat electrode assembly according to the embodiment; 
       FIG. 14  is a vertical sectional view showing the electrode arrangement of a mat electrode assembly according to a related art; 
       FIG. 15  is a characteristic diagram showing a relationship between the detection sensitivity and the deviation between two electrodes; and 
       FIG. 16  is a block diagram of a vehicular passenger protection system using the double-electrode capacitance sensor according to the embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A vehicular seating detection sensor using a double-electrode capacitance sensor according to a preferred embodiment will be hereinafter described. The double-electrode capacitance sensor can be used in seating detection sensors for uses other than the vehicular use and can be used broadly in industrial or home dielectric proximity sensors for detecting a proximate dielectric. 
   The embodiment will be described below with reference to  FIGS. 1 and 2 .  FIG. 1  is a schematic vertical sectional view of a vehicular seat apparatus on which a passenger is seated. 
   Configuration of vehicular seat apparatus  1  will be explained as follows. 
   As shown in  FIG. 1 , a vehicular seat apparatus  1  is installed on a floor  2  of a vehicle body. The vehicular seat apparatus  1  is equipped with a metal frame  3  which is attached to the floor  2 , a seat  4  which is a seating portion of the vehicular seat apparatus  1  and is fixed to the frame  3 , and a seat back (backrest)  5  which erects obliquely upward (inclined backward) from the rear end of the seat  4 . 
   The seat  4  is equipped with a cushion pad  6  which is mainly made of an elastic, electrically insulation material such as a hard polyurethane foam and is fixed to the frame  3 , a cushion cover  7  which is mainly made of, for example, a beautiful electrically insulation material such as a woven cloth and covers the top surface (seat surface) of the cushion pad  6 , a seat heater  8  which is disposed approximately parallel with the seat surface of the cushion pad  6 , and a mat electrode assembly  9  as a seating detection electrode assembly which is interposed between the seat heater  8  and the cushion cover  7  and buried so as to be approximately parallel with the seat surface. 
   The seat heater  8  has a heater  81  and a waterproof film  82 . The mat electrode assembly  9  has an electrically insulation resin film  10  having prescribed relative permittivity, a layer-like top electrode (corresponds to the term “front electrode” used in the claims)  11  extending so as to be in close contact with the top surface of the resin film  10 , and a layer-like bottom electrode (corresponds to the term “back electrode” used in the claims)  12  extending so as to be in close contact with the bottom surface of the resin film  10 . The top electrode  11  and the bottom electrode  12  are protected being covered with resin. The resin film  10 , the top electrode  11 , and the bottom electrode  12  can be realized by an ordinary flexible circuit board. 
   Circuit configuration will be explained as follows. 
   Next, the circuit configuration of the double-electrode capacitance sensor and the mat electrode assembly  9  will be described with reference to  FIG. 2 . 
   The two electrodes  11  and  12  are connected to a detection circuit section  20  (described later). The detection circuit section  20  incorporates an oscillation circuit which oscillates at a prescribed frequency f. The oscillation circuit applies an AC voltage to one of the electrodes  11  and  12 . The detection circuit section  20  outputs a detection signal voltage Vs to a wave detector  22  via its output end and a band pass filter  21  which passes a signal having the above-mentioned frequency f. A wave-detected detection signal voltage Vs is smoothed by a smoothing circuit  23  and compared with a threshold voltage Vref by a comparator  24 . A microcomputer (not shown) judges whether or not a passenger having very large relative permittivity is seated on the basis of a comparison result. Here, a passenger includes a driver and a person in a vehicle. 
   Capacitance equivalent circuit including electrodes  11  and  12  will be explained as follows. 
   An ideal equivalent circuit including the electrodes  11  and  12  will be described below with reference to  FIG. 2 . 
   Symbol Cg represents capacitance between the bottom electrode  12  and the combination of the heater  81  and the vehicle body (ground), Co represents capacitance between the top electrode  11  and the bottom electrode  12 , and Cs represents capacitance between the top electrode  11  and the combination of the heater  81  and the vehicle body (ground). More specifically, Cs represents the capacitance between the top electrode  11  and the vehicle body (ground) in a state that no passenger is seated on the seat and Ch represents an increment of the capacitance between the top electrode  11  and the vehicle body (ground) due to seating of a passenger. Symbol Cx represents capacitance between the top electrode  11  and the ground which is the sum of Cs and Ch and varies depending on whether a passenger is seated. 
   To simplify the description, the resistances and the inductances of the interconnections and the electrodes will be omitted. And it is assumed that the capacitance Co between the top electrode  11  and the bottom electrode  12  and the capacitance Cg between the bottom electrode  12  and the ground are kept constant irrespective of whether a passenger is seated or not. 
   However, in actuality, the internal resistance of an oscillation circuit section and the frequency characteristic of an operational amplifier  200  need to be taken into consideration, and the resistances and the inductances of the interconnections and the electrodes are not negligible. Each of the capacitance Co and the capacitance Cg varies when a passenger is seated, that is, when a dielectric approaches the top electrode  11  directly from above. However, these variations are smaller than a variation of the capacitance Cx and hence can be disregarded in a circuit analysis. 
   Circuit example 1 of detection circuit section  20  will be explained as follows. 
   Circuit example 1 of the detection circuit section  20  will be described below in a specific manner with reference to a circuit diagram of  FIG. 3 . The circuit configuration of this detection circuit section  20  is described in the above-mentioned publication JP-A-2006-201129. 
   Reference numeral  200  denotes an operational amplifier and numeral  201  denotes its feedback resistor having a resistance value rf. They constitute a differential amplifier circuit section (this term is used in the claims). The top electrode  11  is connected to the inverting input end of the operational amplifier  200 , and the bottom electrode  12  is connected to the non-inverting input end of the operational amplifier  200 . Reference numeral  202  denotes an oscillation circuit section which oscillates at a prescribed frequency (preferably, several hundreds of hertz to several megahertz). The oscillation circuit section  202  having a low output impedance applies an AC voltage Vac between the bottom electrode  12  and the ground. 
   The operation of this circuit will be described below in detail. The two input ends of the operational amplifier  200  are imaginarily short-circuited and hence the potentials of the top electrode  11  and the bottom electrode  12  can be regarded as identical. This means that the operational amplifier  200  charges and discharges the detection subject capacitance Cx via the feedback resistor  201 . Since the top electrode  11  and the bottom electrode  12  have the same potential, the operational amplifier  200  does not charge and discharge the capacitance Co. Since the capacitance Co is not charged and discharged, it can be considered that the oscillation circuit section  202  charges and discharges only the capacitance Cg. 
   That is, the AC current Ix flowing through the feedback resistor  201  and the capacitance Cx and the voltage drop Vs across the feedback resistor  201  are given by 
   
     
       
         
           
             
               
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   Here, Vac is the output voltage of the oscillation circuit section  202 , rf is the resistance of the feedback resistor  201 , and Zx is the AC impedance of the capacitance Cx. Parameter V 1 =rf×Vac×jωCs and ΔV is the variation component of Vs that is caused by seating/non-seating. 
   It is seen from the above equations that if the voltage drop Vs across the feedback resistor  201  is regarded as a signal voltage, the signal voltage Vs varies in proportion to seating. It is important that the current Ix includes no parameter relating to the capacitance Cg between the bottom electrode  12  and the ground. Since the signal voltage Vs varies approximately in proportion to the capacitance variation Ch which is caused by seating/non-seating, high detection sensitivity can be expected. 
   It is seen that the absolute value of the variation component ΔV of the signal voltage Vs which is caused by seating/non-seating can be increased by increasing the resistance rf of the feedback resistor  201 , the output voltage Vac of the oscillation circuit section  202 , or its frequency f. However, this also increases the voltage V 1  which is obtained when no passenger is seated. The resistance rf cannot be increased beyond a certain limit because the increase of the resistance rf causes deterioration of the phase characteristic etc. It should be noted that the above description is directed to the ideal case the internal resistance of the oscillation circuit section  202  is regarded as zero and the above-mentioned various parasitic impedances are ignored. 
   For example, although in the above description the capacitance Co between the two electrodes  11  and  12  is regarded as constant irrespective of seating/non-seating, it has been found that in actuality it varies depending on whether a passenger is seated or not because in particular the electric field lines extending from the top major surface of the top electrode  11  to the bottom major surface of the bottom electrode  12  occupy a wide space, and that this results in a problem that the signal voltage Vs is modulated and the detection sensitivity is lowered. 
   In the above embodiment, the operational amplifier  200  supplies AC current to the top electrode  11  via the feedback resistor  201  so that the two electrodes  11  and  12  have the same potential. A first important point of the detection circuit section  20  of  FIG. 3  is that since the oscillation circuit section  202  supplies power to the parasitic capacitance Cg which is originally connected parallel with the detection subject capacitance Cx, it is sufficient for the operational amplifier  200  (which has a circuit function as a power supply circuit whose potential follows the potential of the oscillation circuit section  202 ) to supply power to only the detection subject capacitance Cx. This realizes sensitivity increase. 
   A second important point of the detection circuit section  20  of  FIG. 3  is that the capacitance Co between the electrodes  11  and  12  should not be charged and discharged. It is understood that this requirement is satisfied by causing the potential of the top electrode  11  to follow a potential variation of the bottom electrode  12 . 
   A modification of the circuit of  FIG. 3  will be described below. This modification can also be made as appropriate in circuits of  FIG. 4  onward. 
   In this modification, the two electrodes  11  and  12  have a DC potential difference. That is, the parasitic capacitance Cg can substantially be separated from the detection subject capacitance Cx by causing the potential of the operational-amplifier- 200 -side electrode to follow a potential variation of the oscillation-circuit-section- 202 -side electrode. 
   However, where the operational amplifier  200  is used as a differential amplifier circuit section, it is preferable to provide a passive element circuit upstream of the operational amplifier  200  so that the two input ends of the operational amplifier  200  are short-circuited imaginarily. 
   Circuit example 2 of detection circuit section  20  will be explained as follows. 
   Circuit example 2 of the detection circuit section  20  will be described below in a specific manner with reference to a circuit diagram of  FIG. 4 . This circuit example is such that the circuit of  FIG. 3  is modified in such a manner that the oscillation circuit section  202  applies an AC voltage Vac to the top electrode  11  and an AC current Iy is fed back from the operational amplifier  200  to the bottom electrode  12  via the feedback resistor  201 . The circuit configuration of this detection circuit section  20  is described in above-mentioned publication JP-A-2006-201129. 
   The operation of this circuit will be described below briefly. The two input ends of the operational amplifier  200  are imaginarily short-circuited and hence the potentials of the top electrode  11  and the bottom electrode  12  can be regarded as identical. This means that the operational amplifier  200  charges and discharges the capacitance Cg via the feedback resistor  201 . Since the top electrode  11  and the bottom electrode  12  have the same potential, operational amplifier  200  does not charge and discharge the capacitance Co. Since the capacitance Co is not charged and discharged, it can be considered that ideally the oscillation circuit section  202  charges and discharges only the capacitance Cx. 
   Assume that ri represents the impedance from the output end of the oscillation circuit section  202  to the top electrode  11  (or the internal output resistance of the oscillation circuit section  202 ), Vac represents the output voltage of the oscillation circuit section  202 , ΔV represents the voltage drop across the impedance ri, V 2  represents the voltage drop across the capacitance Cx, Ix represents the current flowing through the impedance ri and the capacitance Cx, rf represents the resistance of the feedback resistor  201 , Vs represents the output signal voltage of the operational amplifier  200 , Zx represents the AC impedance of the capacitance Cx, and Iy represents the AC current flowing through the feedback resistor  201 . The oscillation circuit section  202  charges and discharges the capacitance Cx by the current Ix, and the operational amplifier  200  charges and discharges the capacitance Cg via the feedback resistor  201  by the current Iy.
 
 ΔV=ri×Ix  
 
 Ix=Vac /( Zx+ri )
 
 V 2= Vac−ΔV  
 
 Vs=V 2− Iy×rf  
 
 Iy=V 2 ×jωCg  
 
   It is seen from the above equations that when the capacitance Cx varies, the voltage drop across the impedance ri varies and the potential of the top electrode  11  varies accordingly, as a result of which the output signal voltage Vs varies. 
   The above description assumes that the capacitance Cg between the bottom electrode  12  and the ground and the capacitance Co between the two electrodes  11  and  12  do not vary depending on whether a passenger is seated or not. However, as mentioned above, since electric field lines develop widely in a space around the two electrodes  11  and  12 , the above parameters are not constant but vary in a strict sense. The output signal voltage Vs is thereby varied. 
   Circuit example 3 of detection circuit section  20  will be explained as follows. 
   Circuit example 3 of the detection circuit section  20  will be described below in a specific manner with reference to a circuit diagram of  FIG. 5 . This circuit is characterized in that the circuit of  FIG. 3  is modified in such a manner that the feedback resistor  201  is omitted to make the differential amplifier circuit section a voltage follower circuit and that the output end of the differential amplifier circuit section is connected to the top electrode  11  by a current detection resistor  203  having a resistance value r. 
   The operation of this circuit will be described below. The two input ends of the operational amplifier  200  are imaginarily short-circuited and hence the potentials of the top electrode  11  and the bottom electrode  12  can be regarded as identical. The output voltage of the voltage follower circuit (i.e., the differential amplifier circuit section without the feedback resistor  201 ) can follow a potential variation of the bottom electrode  12  with a good phase characteristic to a high frequency range. That is, the operational amplifier  200  charges and discharges the capacitance Cx at high speed without intervention of the feedback resistor  201  shown in  FIG. 3 . An output signal voltage Vs is taken from the top electrode  11 . This is equivalent to detection of a voltage drop across the current detection resistor  203 . 
   In the circuit of  FIG. 5 , the following equation holds ideally:
 
 Ix=Vs/Zx =( Vac−Vs )/ r  
 
   It is seen from this equation that the output signal voltage Vs varies in link with the detection subject capacitance Cx. 
   Since the feedback resistor  201  is omitted, in the circuit of  FIG. 5  the voltage across the capacitance Cx can follow a variation of the AC voltage Vac better than in the circuit of  FIG. 3 . 
   Circuit example 4 of detection circuit section  20  will be explained as follows. 
   Circuit example 4 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 6 . This circuit is characterized in that the circuit of  FIG. 4  is modified in such a manner that the feedback resistor  201  is omitted to make the differential amplifier circuit section a voltage follower circuit and that the oscillation circuit section  202  is connected to the top electrode  11  by a current detection resistor  204  having a resistance value r. 
   The operation of this circuit will be described below. The two input ends of the operational amplifier  200  are imaginarily short-circuited and hence the potentials of the top electrode  11  and the bottom electrode  12  can be regarded as identical. This means that the operational amplifier  200  charges and discharges the capacitance Cg at high speed without intervention of the feedback resistor  201  shown in  FIG. 4 . Since the top electrode  11  and the bottom electrode  12  have the same potential, the operational amplifier  200  does not charge and discharge the capacitance Co. Since the capacitance Co is not charged and discharged, the oscillation circuit  202  charges and discharges only the capacitance Cg via the current detection resistor  204 . 
   Let Vac represent the output voltage of the oscillation circuit section  202 ; then, the following equations hold:
 
 Ix=Vs/Zx=Vac /( r+Zx )
 
 Vs={Vac /( r+Zx )}× Zx  
 
   It is seen from the above equations that when the AC impedance Zx of the capacitance Cx is varied due to seating/non-seating, the output signal voltage Vs varies accordingly. Therefore, whether a passenger is seated or not can be detected on the basis of the magnitude of the output signal voltage Vs. 
   Since the feedback resistor  201  is omitted, in the circuit of  FIG. 6  the voltage across the capacitance Cg can follow a variation of the AC voltage Vac better than in the circuit of  FIG. 4 . 
   Circuit example 5 of detection circuit section  20  will be explained as follows. 
   Circuit example 5 of the detection circuit section  20  will be described below in a specific manner with reference to a circuit diagram of  FIG. 7 . This circuit is characterized in that the circuit of  FIG. 3  is modified in such a manner that the feedback resistor  201  is omitted to make the differential amplifier circuit section a voltage follower circuit and that the output end of the differential amplifier circuit section is connected to the top electrode  11  by a capacitor  205  having a capacitance value C 1 . 
   The operation of this circuit will be described below. The two input ends of the operational amplifier  200  are imaginarily short-circuited and hence the potentials of the top electrode  11  and the bottom electrode  12  can be regarded as identical. The output voltage of the voltage follower circuit (i.e., the differential amplifier circuit section without the feedback resistor  201 ) can follow a potential variation of the bottom electrode  12  with a good phase characteristic to a high frequency range. That is, the operational amplifier  200  charges and discharges the capacitances Cx and C 1  at high speed without intervention of the feedback resistor  201  shown in  FIG. 3 . The output signal voltage Vs is taken from the top electrode  11 . This is equivalent to detection of a voltage drop across the capacitor  205 . 
   In the circuit of  FIG. 7 , the following equations hold ideally:
 
 Ix=Vs/Zx=Vac /( Zc 1 +Zx )
 
 Vs={Vac /( Zc 1 +Zx )}× Zx  
 
   Here, Zc1 is the AC impedance of the capacitor  205 . It is seen from the above equations that the output signal voltage Vs varies in link with the detection subject capacitance Cx. 
   Since the feedback resistor  201  is omitted, in the circuit of  FIG. 7  the voltage across the capacitance Cx can follow a variation of the AC voltage Vac better than in the circuit of  FIG. 3 . Since the current detection impedance element is the capacitor  205 , good balance is obtained between the detection subject capacitance Cx and the current detection impedance element and hence the circuit of  FIG. 7  is given a superior phase characteristic. Furthermore, the circuit of  FIG. 7  is superior in safety because the inverting input end of the operational amplifier  200  can be DC-insulated from the mat electrode assembly  9  by means of the capacitor  205 . 
   Circuit example 6 of detection circuit section  20  will be explained as follows. 
   Circuit example 6 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 8 . This circuit is such that capacitors  206  and  207  are added to the circuit of  FIG. 7 . The capacitor  206  is a connection capacitor which is provided between the bottom electrode  12  and the non-inverting input end of the operational amplifier  200  and which has the same capacitance as the capacitor  205 . The capacitor  207  is provided between the non-inverting input end of the operational amplifier  200  and the ground and whose capacitance is set the same as the non-seating capacitance Cs of the detection subject capacitance Cx. 
   The operation of this circuit will be described below. The two input ends of the operational amplifier  200  are imaginarily short-circuited and hence the potentials of the top electrode  11  and the bottom electrode  12  can be regarded as identical. This means that the operational amplifier  200  charges and discharges the capacitances Cx and C 1  at high speed without intervention of the feedback resistor  201  shown in  FIG. 3 . 
   The output voltage Vac′ of the operational amplifier  200  is a voltage obtained by dividing the AV voltage Vac by the capacitances of the capacitors  206  and  207 . Therefore, a current Ix flowing through the detection subject capacitance Cx can be detected by detecting a voltage drop across the capacitor  205 . The detection subject capacitance Cx can be detected from the current Ix and a voltage drop across the capacitance Cx. 
   In addition to the advantages of the circuit of  FIG. 7 , this circuit provides an advantage that the two input ends of the operational amplifier  200  can be DC-insulated from the mat electrode assembly  9 , which increases the safety of the operational amplifier  200 . Furthermore, an optimum DC bias voltage can be applied to the non-inverting input end of the operational amplifier  200 . 
   In this circuit, the potential of the top electrode  11  is not equal to that of the bottom electrode  12 . However, the potential of the bottom electrode  12  varies so as to follow the potential of the top electrode  11 . Also in this circuit, since the operational amplifier  200  supplies power to only the detection subject capacitance Cx, the parasitic capacitance Cg can be separated from the detection subject capacitance Cx and hence the detection sensitivity can be increased. 
   Circuit example 7 of detection circuit section  20  will be explained as follows. 
   Circuit example 7 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 9 . This circuit is such that the circuit of  FIG. 6  is modified in such a manner that the current detection resistor  204  is replaced by a capacitor  207 . As a result, the output signal voltage Vs of this circuit is given a superior phase characteristic as in the circuits of  FIGS. 4 and 6 . Furthermore, this circuit increases the safety because the oscillation circuit section  202  is DC-separated from the mat electrode assembly  9 . 
   Circuit example 8 of detection circuit section  20  will be explained as follows. 
   Circuit example 8 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 10 . This circuit is such that the circuit of  FIG. 3  is modified in such a manner that the feedback resistor  201  is replaced by a coil (inductance element)  208 . This circuit provides the same advantages as the circuit of  FIG. 3 . 
   Circuit example 9 of detection circuit section  20  will be explained as follows. 
   Circuit example 9 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 11 . This circuit is such that the circuit of  FIG. 5  is modified in such a manner that the current detection resistor  203  is replaced by a coil (inductance element)  209 . This circuit provides the same advantages as the circuit of  FIG. 5 . 
   Circuit example 10 of detection circuit section  20  will be explained as follows. 
   Circuit example 10 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 12 . This circuit is such that the circuit of  FIG. 6  is modified in such a manner that the current detection resistor  204  is replaced by a coil (inductance element)  210 . This circuit provides the same advantages as the circuit of  FIG. 6 . 
   Circuit example 11 of detection circuit section  20  will be explained as follows. 
   Circuit example 11 of the detection circuit section  20  will be described below with reference to a circuit diagram of  FIG. 12 . This circuit is such that the oscillation circuit section  202  shown in  FIG. 12  is a resonance circuit which operates at a resonance frequency of the series connection of the inductance element  210  and the combined capacitance of the mat electrode assembly  9  (i.e., the sum of the capacitance of the capacitance Cx and the combined capacitance of the series connection of the capacitances Co and Cg). That is, referring to  FIG. 12 , the frequency of the AC power source  202  is set equal to this series resonance frequency. 
   In actuality, it is not necessary to charge and discharge the capacitance Co because the capacitance Cg is charged and discharged by the voltage follower  200 . Therefore, the capacitance of the mat electrode assembly  9  as seen from the oscillation circuit section  202  is equal to only the capacitance Cx of the above-mentioned combined capacitance of the mat electrode assembly  9 . 
   Therefore, the above-mentioned series resonance frequency is proportional to the square root of the product of the detection subject capacitance Cx and the inductance L of the inductance element  210  and is varied very much due to a variation of the detection subject capacitance Cx. As a result, whether a passenger is seated or not can be detected with a steep characteristic curve by detecting a frequency variation of an amplitude variation of the output signal voltage Vs. 
   Electrode arrangement will be explained as follows. 
   Next, a preferred electrode arrangement of the above-described double-electrode capacitance sensor will be described with reference to  FIG. 13 .  FIG. 13  is a sectional view, taken perpendicularly to its extending direction, of the band-like mat electrode assembly  9  of the vehicular seating sensor. 
   In  FIG. 13 , reference numeral  10  denotes a resin film;  11 , a top electrode;  12 , a bottom electrode;  13  and  14 , cover films;  15 , adhesives, and  16 , water detection electrodes. The top electrode  11 , the bottom electrode  12 , and the water detection electrodes  16  are long in the paper thickness direction of  FIG. 13  and extend in the same direction in a band-like manner. 
   The water detection electrodes  16  are provided on the top surface of the resin film  10  so as to be distant from the right and left ends of the top electrode  11  and to extend parallel with the top electrode  11 . Each of the top electrode  11  and the bottom electrode  12  is composed of a carbon electrode layer and a silver electrode layer. However, the details of the top electrode  11  and the bottom electrode  12  will not be described because they are not important features of the embodiment. The cover film  13  covers the top electrode  11  and the water detection electrodes  16 , and the cover film  14  covers the bottom electrode  12 . 
   This electrode arrangement is characterized in that the side end faces of the top electrode  11  are located approximately at the same positions as those of the bottom electrode  12 . As a result, the width W 11  of the top electrode  11  is approximately equal to the width W 12  of the bottom electrode  12  and the two electrodes  11  and  12  have approximately the same area. In this embodiment, the bottom electrode  12  is disposed in such a manner that no parts of it coextend with the water detection electrodes  16 . The water detection electrodes  16  are electrode lines for detecting exposure to water and are given a prescribed potential (low impedance). Naturally, the same result is obtained by laying lines other than the water detection electrodes  16  at the same positions. 
   It has been found that the above electrode arrangement can attain higher detection sensitivity than a comparison electrode arrangement shown in  FIG. 14 . 
     FIG. 15  is a characteristic diagram showing a relationship between the detection sensitivity and the projection length of the bottom electrode  12  from the side end faces of the top electrode  11 . It has been found that the detection sensitivity can be made higher in the case where the side end faces of the two electrodes  11  and  12  are located at the same positions (S 0 ) than in the case where their side end faces are deviated from each other (S 1 -S 4 ). Symbol S 1  denotes a case that W 11  is greater than W 12  by 20%, S 2  denotes a case that W 11  is shorter than W 12  by 20%, S 3  denotes a case that W 11  is greater than W 12  by 10%, S 4  denotes a case that W 11  is shorter than W 12  by 10%. 
   The circuit-related aspect of the above electrode arrangement according to this embodiment will be described below. 
   In an AC sense, the water detection electrodes  16  can be regarded as ground electrodes like the heater  81  etc. are. Therefore, if parts of the bottom electrode  12  coextend with the water detection electrodes  16 , the ground capacitance Cg of the bottom electrode  12  is increased and the sensitivity is lowered. 
   Next, a description will be made of the relationship between the detection sensitivity and the deviations between the two electrodes  11  and  12  in the width direction. 
   In the electrode arrangement of  FIG. 14  which is described in the above-mentioned publication JP-A-2006-201129, the bottom electrode  12  is smaller than the top electrode  11 . The double-electrode capacitance sensor is characterized in that the ground capacitance Cs of the top electrode  11  is reduced by electrostatically shielding the top electrode  11  from the ground by the bottom electrode  12 , whereby the variable capacitance component Ch of the detection subject capacitance Cx is increased relatively and the detection sensitivity is increased. 
   When the bottom electrode  12  is smaller than the top electrode  11 , parts of the major surface, not opposed to the detection subject, of the top electrode  11  are opposed to the underlying ground without intervention of the bottom electrode  12 . Therefore, the capacitance Cs is large and the detection sensitivity is low. 
   When the bottom electrode  12  is larger than the top electrode  11 , parts of the major surface, opposed to the detection subject, of the bottom electrode  12  are opposed to the detection subject (e.g., a passenger) located above without intervention of the top electrode  11 . As a result, the capacitance Cg between the bottom electrode  12  and the ground varies being influenced by a seated passenger as a dielectric, that is, it varies depending on whether a passenger is seated or not. Referring to  FIG. 3 , if the wiring impedances and the internal resistance of the oscillation circuit section  202  are taken into consideration, a variation of the capacitance Cg causes a variation of the potential of the bottom electrode  12 . This results in reduction in detection sensitivity. This problem also occurs in the circuit examples other than the circuit example of  FIG. 3 . 
   It is concluded that the detection sensitivity can be increased by locating the side end faces of the two electrodes  11  and  12  approximately at the same positions (tolerance: less than 5%) in the direction parallel with the electrode surfaces irrespective of whether or not the water detection electrodes  16  or other lines extend close to the side end faces of one of the two electrodes  11  and  12 . 
   Vehicular passenger protection system will be explained as follows. 
   Next, a vehicular passenger protection system using the above-described double-electrode capacitance sensor will be described with reference to  FIG. 16 . 
   Reference numeral  101  denotes the above-described double-electrode capacitance sensor, which outputs an analog signal voltage that varies depending on whether or not a passenger is seated on the vehicle seat. Reference numeral  102  denotes a judgment device for judging whether a passenger is seated or not on the basis of the analog signal voltage. For example, the judgment device  102  is a comparator which compares an input analog signal voltage with a prescribed threshold voltage. The judgment device  102  can also judge, for example, the kind of seated passenger (i.e., an adult or a child) on the basis of the level of an analog signal voltage. The double-electrode capacitance sensor  101  and the judgment device  102  constitute a vehicular passenger detection apparatus  100 . A judgment result, that is, a signal indicating whether a passenger is seated or not, that is output from the judgment device  102  is input to a control device  200 . The control device  200  controls operation of a passenger protection apparatus  300  such as an airbag. When receiving a vehicle collision detection signal from a collision sensor (not shown), for example, the control device  200  activates the passenger protection apparatus  300  only if judging that a passenger is seated on the basis of a judgment result from the judgment device  102 . Also, the control device  200  changes the operation form of the passenger protection apparatus  300  according to the build of a passenger that is judged by the judgment device  102 . 
   As such, the vehicular passenger protection system which is composed of the vehicular passenger detection apparatus  100 , the control device  200 , and the passenger protection apparatus  300  exhibits superior passenger protection performance because of increased detection accuracy of the above-described double-electrode capacitance sensor  101 . 
   While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.