Abstract:
An electrode unit for detecting whether a seat is occupied or not (occupancy conditions) is embedded in a seat of an automotive vehicle together with an electrically powered seat heater. Both of the electrode unit and the seat heater are flat, and the electrode unit is laminated on the heat heater. The electrode unit is composed of an upper electrode, a lower electrode and an insulating film disposed between both electrodes. The occupancy conditions are detected based on changes in a capacitance between the upper electrode and a member grounded to a vehicle body. To reduce a capacitance between the upper electrode and seat heater and to thereby improve sensitivity of the detection of the occupancy conditions, an alternating voltage that alternates in accordance with an alternating voltage supplied to the upper electrode is supplied to the lower electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority of Japanese Patent Application No. 2006-190848 filed on Jul. 11, 2006, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a passenger seat device that includes a detector for detecting whether a seat is occupied, and to a method of manufacturing the same. 
     2. Description of Related Art 
     JP-A-2003-80989 proposes a device for detecting whether a passenger seat of an automotive vehicle is occupied or not. In this device, a passenger on the seat is detected based on changes in an alternating current electromagnetic field in the vicinity of the seat, particularly at a portion above the seat. This type of the detector is referred to as a capacitance-type detector. The detector includes a transmitting electrode embedded in the seat for forming the electromagnetic field and a receiving electrode embedded in a seat-back. A capacitive impedance between the transmitting electrode and receiving electrode decreases when a passenger occupies the seat. Thus, whether the seat is occupied or not is detected. 
     The capacitance-type detector described in JP-A-2003-80989 also includes a seat heater embedded in the seat at a position under the transmitting electrode (also referred to as a detecting electrode). When both of the seat heater and the occupant detector are embedded in a seat as in this example, the seat heater having a substantially constant potential under alternating current is positioned in the vicinity and under the detecting electrode. 
     In this detector, there is a problem that a large capacitance (referred to as a seat heater capacitance) is formed between the detector electrode and the seat heater because both are positioned close to each other, facing each other in a large area. An alternating current impedance for detecting a passenger is a compound impedance (mostly capacitance) composed of an impedance between the detecting electrode and a passenger and another impedance between the passenger and a vehicle body (a ground potential), both impedances being connected in series. Accordingly, the compound impedance becomes considerably large compared with an impedance due to the seat heater capacitance. 
     Since the compound impedance and the impedance due to the seat heater capacitance are connected in parallel to the detector electrode, detection sensitivity of the detector is considerably decreased due to presence of the heater impedance. In addition, the seat heater hinders flow of electric flux lines between a lower surface of the detecting electrode and a passenger due to a shielding effect of the seat heater. JP-A-2003-80989 proposes, to cope with these problems, to dispose a flat sheet electrode having a floating potential between the detecting electrode and the seat heater. However, it has been found out that the detecting sensitivity is hardly improved because the seat heater capacitance is little decreased by the presence of the sheet electrode having a floating potential. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved seat device having an occupant detector that has a high sensitivity even though it is embedded in a seat together with a seat heater. Another object of the present invention is to provide a method of manufacturing such a seat device. 
     The heater device according to the present invention includes an electrode unit embedded in a passenger seat in parallel to a seat heater. Both of the electrode unit and the seat heater are flat and laminated on each other. The electrode unit composed of an upper electrode, a lower electrode and an insulating film disposed between the upper and lower electrodes are positioned above the seat heater. 
     An alternating voltage having a predetermined frequency is supplied to the upper electrode, and a voltage altering according to the voltage supplied to the upper electrode is supplied to the lower electrode to reduce a capacitance between the upper electrode and the seat heater. Occupancy conditions, i.e., whether a passenger is seated on the seat and whether the seated person is an adult, a child or a child seat, are detected based on changes in a capacitance between the upper electrode and a member grounded to a vehicle body. 
     Since the capacitance between the upper electrode and the seat heater is reduced by supplying to the lower electrode the voltage changing according to the voltage supplied to the upper electrode, the occupancy conditions are accurately and surely detected based on changes in the capacitance between the upper electrode and the member grounded to the vehicle body. An alternating voltage may be supplied between the upper electrode and the lower electrode. 
     A heater-electrode unit that includes the electrode unit and the seat heater may be integrally formed, and then the heater-electrode unit may be embedded in the seat. In this manner, the seat device is efficiently manufactured. The seat heater may be embedded also in a seat-back (a vertical portion) in addition to a horizontal portion of the seat. 
     According to the present invention, the occupancy conditions are accurately detected with a high sensitivity without being disturbed by the capacitance between the upper electrode and the seat heater. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing a seat device having an occupant detector according to the present invention; 
         FIG. 2  is a block diagram showing electric connections in the occupant detector as a first embodiment of the present invention; 
         FIG. 3  is a block diagram showing electric connections in the occupant detector as a second embodiment of the present invention; 
         FIG. 4  is a schematic cross-sectional view showing a heater-electrode unit used in the occupant detector as a third embodiment of the present invention; 
         FIG. 5  is a circuit diagram showing a power supply system in the seat device as a fourth embodiment of the present invention; 
         FIG. 6  is a schematic view showing a passenger seat having an occupant detector as a fifth embodiment of the present invention; and 
         FIG. 7  is a schematic view showing a passenger seat having an occupant detector as a sixth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . First, referring to  FIG. 1 , a seat device  1  having an occupant detector will be described. The seat device  1  is positioned on a metallic frame  3  of a floor  2  of an automotive vehicle. The passenger seat is composed of a seat  4  and a seat-back  5  that supports a passenger&#39;s back. 
     The seat  4  includes a cushion pad  6  made of a material such as hard foam-urethane fixed to the frame  3 , a cushion cover  7  covering an upper surface of the cushion pad  6 , a seat heater  8  embedded in the cushion pad substantially in parallel to the seat surface, and an electrode unit  9  disposed between the cushion cover  7  and the seat heater  8 . The seat heater  8  is made of a sheet having a plane electric resistor or plural resistor wires and is disposed substantially in parallel to the seat surface. The seat heater  8  may be extended to the seat-back  5 . Heating power is supplied to the seat heater  8  from a direct current power source such as an on-board battery. It is also possible to heat the seat heater  8  with alternating current such as three-phase alternating current generated in an alternator driven by the engine. 
     The electrode unit  9  is composed of an insulating film  10 , an upper electrode  11  disposed on an upper surface of an insulating film  10  and a lower electrode  12  disposed on a lower surface of the insulating film  10 . The insulating film  10  may be made of a resin material such as polyethylene-terephthalate (PET) having a thickness of about 1 mm. The electrode unit  9  is disposed immediately underneath the cushion cover  7 , and the seat heater  8  is disposed under the electrode unit  9  several millimeters apart therefrom. As a distance between the seat heater  8  and the electrode unit  9  becomes larger, a parasitic capacitance formed between them becomes smaller. However, a distance between the seat surface and the seat heater  8  increases. 
     With reference to  FIG. 2 , an electric circuit in the seat device  1  will be explained. An alternating voltage (AC-voltage) Vac having a proper frequency, e.g., several tens to several hundreds kHz, is supplied between the upper electrode  11  and the lower electrode  12  from an AC power source  13 . The AC power source  13  is composed of an oscillator  13   a  and an insulating transformer  13   b . The electric circuit shown in  FIG. 2  is an example, and this may be variously changed. 
     An occupant detector  14 , as shown in  FIG. 2 , includes a resistor  15  for detecting current flowing therethrough, a rectifying and smoothing circuit  16 , an amplifier  17 , an analog to digital converter (A-D converter)  18  and a microcomputer  19 . An equivalent electric circuit in the seat device  1  when the alternating voltage Vac is supplied between the upper electrode  11  and the lower electrode  12  will be explained with reference to  FIG. 2 . C 1  represents a capacitance between the upper electrode  11  and the lower electrode  12 , and C 2  represents a capacitance between the lower electrode  12  and the seat heater  8 . 
     The seat heater  8  can be regarded as being substantially grounded (for alternating current). An impedance Z 1  is an impedance between the upper electrode  11  and the ground on the vehicle body. The impedance Z 1  is substantially a capacitance component, but it includes a small amount of resistance component when a passenger is seated on the seat. An impedance Z 2  is an impedance between the lower electrode  12  and the ground. The impedance Z 2  is substantially a capacitance component, but it includes a small amount of resistance component when a passenger is seated. The impedances Z 1 , Z 2  change according to occupancy conditions, i.e., whether the seat is occupied or not. An impedance Z 3  between the upper electrode  11  and the lower electrode  12  also changes depending on whether the seat is occupied or not. 
     When the seat is occupied by a passenger, the impedances Z 1 , Z 2  and Z 3  decrease. Since the upper electrode  11  is the closest to the occupant, the lower electrode  12  is next to the upper electrode  11 , and the seat heater  8  is farthest from the occupant, an amount of impedance decrease in Z 1  and Z 2  is larger than that in Z 3 . This means that an amount of current increase in i 1  and i 2  due to seat occupancy is larger than that in i 3 . According to impedance decrease in Z 1  and Z 2 , the current i 1  and i 2  flowing out of the upper electrode  11  increase. This causes an increase in current i 3  flowing through the resistor  15 , resulting in increase in a voltage drop Vs (=r×i 3 ) across the resistor  15 . 
     The voltage drop Vs is a signal voltage indicating occupancy conditions. The signal voltage Vs is rectified and smoothed in the circuit  16 , amplified in the amplifier  17  and converted into a digital signal in the A-D converter  18 . The digital signal is fed to the microcomputer  19 . The microcomputer  19  determines that the seat is occupied if the digital signal exceeds a predetermined level. It is also possible to determine whether the occupant is an adult or a child, or whether a child seat is positioned on the seat according to the level of the digital signal. 
     A second embodiment of the present invention will be described with reference to  FIG. 3 . In this embodiment, the alternating power source  13  of the first embodiment is replaced with a power source  130 . The power source  130  is composed of a oscillator  13   a  and a voltage follower circuit  13   c . Other structures are the same as those of the first embodiment. An alternating voltage Vac is supplied to the upper electrode  11 , and a voltage, which is made substantially equal to the voltage Vac through the voltage follower circuit  13   c , is supplied to the lower electrode  12 . In this manner, the impedance Z 3  can be neglected. When the seat is occupied, the impedance Z 1  decreases, causing increase in current i 1 . The increase in i 1  results in increase in i 3  and increase in a signal voltage Vs. In the second embodiment, an amount of changes in the signal voltage Vs due to occupancy of the seat can be made larger than that in the first embodiment. 
     A third embodiment of the present invention will be described with reference to  FIG. 4 . In this embodiment, a heater-electrode unit  100  is embedded in the seat. Other structures and functions are the same as those of the first embodiment. The heater-electrode unit  100  includes: the electrode unit  9  composed of the upper electrode  11 , the insulating film  10  and a lower electrode  12 ; a seat heater  8 ; and an insulating layer  20  disposed between the electrode unit  9  and the seat heater  8 . All of these components are integrally formed in the heater-electrode unit  100  as a single body. The insulating film  10  and the insulating layer  20  may be made of the same material having the same thickness. However, it is preferable to form the insulating layer  20  with a material having a lowest possible relative dielectric constant and a thickness thicker than that of the insulating film  10 . This results in decrease in capacitance C 2  and increase in an amount of changes in the signal voltages Vs due to occupancy. The heater-electrode unit  100  formed as an integral body is assembled in the seat. This makes a process of manufacturing the seat device  1  simple. 
     A fourth embodiment of the present invention will be described with reference to  FIG. 5 . In this embodiment, the heater-electrode unit  100  and an electric circuit block  200  including an occupant detector  201  and a heater controller  202  are electrically connected through a wiring harness  300 . The occupant detector  201  includes the alternating power source  13  or  130  and the detector circuit  14 . The wiring harness  300  is composed of four insulated wires integrally bound. A grounding wire may be separately provided from the wiring harness  300 . By making electrical connections in this manner, a process of making the seat device  1  is further simplified. Other structures are the same as those of the foregoing embodiments. 
     A fifth embodiment of the present invention is shown in  FIG. 6 . A hip-point where a center of a passenger is positioned is denoted by  400 . The seat heater  8  is made smaller than the electrode unit  9  in this embodiment. In other words, the seat heater  8  is completely covered by the electrode unit  9 . In this manner, suppression of a potential change in the electrode unit  9  due to the seat heater  8  acting as a substantially constant potential body can be reduced. The electrode unit  9  and the seat heater  8  may be separately formed. The upper electrode  11  and the lower electrode  12  may be separately made. 
     A sixth embodiment of the present invention is shown in  FIG. 7 . This embodiment is a modified form of the fifth embodiment. In this embodiment, the seat heater  8  is positioned not to overlap the electrode unit  8 . In this manner, the parasitic capacitance between the electrode unit  9  and the seat heater  8  can be considerably reduced. It is preferable to position the electrode unit  9  exactly underneath the hip-point  400  to obtain a larger change in signal voltage due to seat occupancy. 
     The present invention is not limited to the embodiments described above, but it may be variously modified. For example, though the load of the alternating voltage source is formed as a resistance-capacitance circuit (CR circuit) in the foregoing embodiments, it is possible to form the load as a circuit including an inductance (LCR circuit). The inductance may be formed by a coil connecting the upper electrode  11  to the lower electrode  12 . In the LCR circuit, a larger change in the signal voltage Vs can be obtained at a vicinity of a series or parallel resonant frequency. 
     While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.