Abstract:
The purpose of this invention is to sense the presence of a seated occupant in a vehicle such as an automobile, plane, train or bus, or in a room or location where it is desirable to detect if seats are occupied. The occupant presence detection device consists of a single seat-mounted electrode, an oscillator circuit, a bridge circuit, a detection circuit and a circuit for processing the detected signals. The oscillator circuit excites the electrode. If an occupant is present on the seat, additional capacitance from the human body is introduced into the bridge via the electrode. This created differences in the voltage and phase of the waveform in each arm of the bridge circuit which are amplified by a differential amplifier. The signal is then converted to a DC voltage that, when above a predetermined threshold, causes the device to outputs a signal that indicates the presence of an occupant. Using a bridge configuration and a differential amplifier allows the circuit to be operated over a wide range of supply voltages. It also reduces the need for high precision components and the need to regulate the amplitude of the waveform produced by the oscillator. The net result is a capacitive occupant sensing device that is less complex and less expensive that previous capacitive occupant sensing devices, yet is tolerant of power supply fluctuations, is able to function over a wide range of operating voltage and still provides failsafe functionality.

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/341,123, filed Dec. 13, 2001. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to safety systems and more particularly to capacitive occupancy detection devices. Occupant detection devices can be used to enable or disable a safety restraint device, such as an airbag, or to determine how many occupants are present in a vehicle or a room. These devices can be used to detect the absence of an occupant in the passenger seat of a vehicle, and thereby disable the deployment of the passenger&#39;s airbag. The number of occupants in a vehicle may also be monitored prior to an accident in order to provide a telematic unit, such as Onstar®, with an occupant count in order to dispatch an adequate amount of emergency response. Having an accurate occupant count prior to an accident can also help emergency response personnel to determine if one or several of the occupants may have been ejected from the vehicle during the collision. In the transportation industry, occupant presence detection devices can provide a quickly count of the number of passengers in a plane, train or bus. They can also show which seats are occupied and which are not. This can also apply to theaters or halls where it is desirable to know how many occupants are present and where newly arriving customers can find empty seats. 
   Various technologies have been proposed to sense the presence of an occupant in a vehicle. Early detection apparatus utilized one or more mechanical switches, which are actuated by the weight of the body upon the seat. Some systems use infrared or ultrasonic transmitters and receivers, which generate signals that are reflected off of the occupant and then received and processed. Capacitive sensors have also been used as a means of detecting the presence of an occupant. 
   Other capacitance-based systems exist that consist of only one electrode mounted between the seat foam and the seat coverings. These systems also rely on the occupant adding capacitance to the system, and thus causing a change in the voltage, current, or phase of the oscillator signal, which can be detected. However, many of these devices, which claim to be inexpensive, use circuitry that is far more complex than the circuitry of the device described herein. Some or these devices, such as the device described in U.S. Pat. No. 6,161,070, require precision power supplies and amplitude control of the waveform generated by their oscillators. They may require precision components and may only function over a small range of supply voltages. In addition, in order to provide better noise rejection, these devices must have additional circuitry to filter out noise. This adds a great deal of cost and complexity to these devices in comparison with this invention. 
   Furthermore, some devices, such as the device described in U.S. Pat. No. 4,796,013, cannot accurately detect whether the electrode is disconnected or damaged and will determine this situation to be an empty seat regardless of whether an occupant is present or not. This is because a disconnected electrode reduces the capacitance of the system and a capacitance below a certain threshold is assumed to mean an empty seat. This could prove to be fatal when the device is being used to provide logic that enables or disable a safety restraint device, such as an airbag. 
   SUMMARY OF THE INVENTION 
   The purpose of the present invention is to provide an occupant detection device, which avoids the use of mechanical sensing apparatuses, and is less expensive and more reliable than existing capacitive based occupant sensing systems. The present invention includes a single conductive electrode which, in conjunction with its surroundings, forms a capacitor which is a part of a bridge circuit. The device includes an oscillator for continuous excitation of the bridge, a differential amplifier to determine if the bridge is unbalanced, an AC-DC converter circuit to convert the output of the amplifier to a DC signal, and a threshold circuit for triggering the output signal once the output of the AC-DC converter exceeds a predetermined threshold. 
   One arm of the bridge circuit is used as a reference for the arm of the bridge that contains the electrode. Each arm of the bridge is essentially a low-pass filter. The reference arm of the bridge is tuned to have the same filter characteristics as the arm that contains the electrode. The change in attenuation and phase of the waveform passing through the electrode arm of the bridge is measured with respect to the reference arm of the bridge. Since both arms of the bridge are receiving the same waveform, it does not matter if the amplitude varies slightly. 
   If an occupant is present on the seat, additional capacitance from the human body is introduced into the bridge via the electrode. This creates differences in the voltage and phase of the waveform in each arm of the bridge circuit. These changes are then amplified by a differential amplifier. The signal is then converted to a DC voltage that, when above a predetermined threshold, causes the device to output a signal that indicates the presence of an occupant. Using a bridge configuration and a differential amplifier allows the circuit to be operated over a wide range of supply voltages. It also reduces the need for high precision components and the need to regulate the amplitude of the waveform produced by the oscillator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
       FIG. 1  illustrates an occupant detection system according to the present invention, as installed in a vehicle. 
       FIG. 2  shows the block diagram of the basic detection circuit. 
       FIG. 2   a  shows the basic model for a capacitor. 
       FIG. 2   b  shows the sources of capacitance when the seat is empty. 
       FIG. 2   c  shows the sources of capacitance when the seat is occupied. 
       FIG. 3  shows the output of the differential amplifier before and after the AC-DC conversion when the seat is empty, occupied and when the electrode is disconnected or damaged. 
       FIG. 4  is a graph of the output voltage of the AC-DC converter versus the capacitance detected on the electrode for an empty seat, an occupied seat and a disconnected or damaged electrode. 
       FIG. 5  shows the block diagram for the detection circuit of the second embodiment of the invention. 
       FIGS. 6   a  and  6   b  show the occupant presence detection device with child-seat detection in accordance with the third embodiment of the invention.  FIG. 6   a  shows the TOP and SIDE view of the electrode.  FIG. 6   b  shows the block diagram of the basic detection circuit. 
       FIG. 6C  shows a child-seat on top of the electrode according to the third embodiment of the invention. 
       FIG. 7  shows an alternate detection circuit. 
       FIG. 8  is a graph of the output frequency of the detection circuit of  FIG. 7  as a function of capacitance. 
       FIG. 9  shows one way for protecting the electrode in the prior figures from false detection due to moisture. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates an occupant presence detection system  10  for determining the presence of an occupant  12  in a vehicle seat  14 . For illustrative purposes, the occupant presence detection system  10  of the present invention will be described as it is used in a vehicle seat  14  installed in a vehicle passenger compartment  16  in conjunction with an occupant safety system including an automatic safety restraint, such as an airbag  18 ; however, the occupant presence detection system  10  could be used in other applications to determine the presence of an occupant. Although a steering wheel mounted airbag  18  is illustrated as an example, it should also be understood that the present invention is also useful for side airbags, seatbelt pre-tensioners, deployable knee bolsters, and any other automatic safety restraint actuators. A crash detector  19 , such as a crash sensor of any known type, is used to determine the occurrence of a vehicle crash and to determine the crash severity. A telematic unit  20  of the type described above may also be provided. 
   The system  10  includes a control unit  24  generally comprising a CPU  31  having memory  32 . The CPU  31  is suitably programmed to perform the functions described herein and a person of ordinary skill in the art could program the CPU  31  accordingly and supply any additional hardware not shown but needed to implement the present invention based upon the description herein. In operation, the control unit  24  communicates with the crash detector  19  to determine the occurrence and severity of a crash of the vehicle and activates an appropriate safety system, such as air bag  18 , in response. 
   The present invention provides a detector circuit  27  to determine the presence of the occupant  12  in the vehicle seat  14  and communicate the presence or absence of the occupant  12  in the seat  14 . As will be described in more detail, the detector circuit  27  generally includes a seat electrode  34  mounted adjacent the area to be occupied by the occupant  12 , in this case in the vehicle seat  14 . The seat electrode  34  comprises a sheet of non-conductive fabric  36  with a pattern sewn on using special conductive thread  38 , such as Dupont Araconâ. The electrode  34  can be made of any conductive material and can be of any size or shape. It does not have to form the same pattern as the conductive thread  38  shown and it could be made from a continuous sheet of conductive material; however, conductive thread  38  is preferred since it can be sewn directly into the non-conductive fabric  36 , which could be the seat  14  cover, or a layer of material below the seat cover. Although a crown-shaped pattern for the thread  38  is shown in  FIG. 1 , the pattern of the thread  38  does not have to be the same as the crown shaped pattern depicted in  FIG. 1 . The detection circuit can be tuned for any pattern that covers the desired sensing area. The larger the area covered by the electrode, the more sensitive it will be for all occupant seating positions. Although only one detector circuit  27  is shown, it is preferred that a detector circuit  27 , or at least a different seat electrode  34 , would be provided for each available seat in the vehicle. Alternatively, the electrode  34  could be solid, flat electrode instead of the conductive thread  38 . 
   This invention uses the detection circuit  27 , shown in  FIG. 2 , which can use a single differential amplifier  40  and AC-DC conversion circuit  42  to detect changes in the voltage, current and phase of the waveform produce by the oscillator  44 . A single threshold circuit  46  determines if these changes indicate the presence of an occupant. The two inputs to the differential amplifier  40  are each connected to one of a pair of arms in a bridge circuit  48 . One arm of the bridge circuit  48  is used as a reference arm, including R ref , C ref  and reference wire  52 . The other arm of the bridge circuit  48  contains the electrode  34  and R occ . An oscillator  50  is connected to both arms. Each arm of the bridge circuit  48  is essentially a low-pass filter. The reference arm of the bridge circuit  48  is tuned to have the same filter characteristics as the arm that contains the electrode  34 . The change in attenuation and phase of the waveform passing through the electrode arm of the bridge circuit  48  is measured with respect to the reference arm of the bridge circuit  48 . Since both arms of the bridge circuit  48  are receiving the same waveform, it does not matter if the amplitude varies slightly. 
   Noise rejection is accomplished by providing a second wire  52  that is connected to the reference arm of the bridge circuit  48  and twisted together with a wire  54  that connects the electrode  34  to the bridge circuit  48 . Since both wires  52 ,  54  pick up the same noise, the noise is not amplified because it is common to both arms of the bridge circuit  48  and both inputs to the differential amplifier  40 . All thresholds and signals in the device vary in proportion to the power supply voltage. As such, the device is tolerant to sudden changes in the supply voltage and will function over a wide range of supply voltages. Wire  54  may also be a coaxial cable in order to avoid noise and interference problems. 
   The virtual capacitor C v , created by electrode  34  is connected in series with the resistor R occ  to form one arm of the bridge circuit  48 . These are connected in parallel with the resistor R ref  and the capacitor C ref  which form the reference arm of the bridge circuit  48 . Each arm of the bridge circuit  48  is essentially a low pass filter. The product RC determines the characteristic of each low pass filter. When RC changes, the phase and the amplitude of output of the filter changes. The RC value for the reference low pass filter is chosen so the bridge circuit  48  is balanced when the seat is empty. When there is an occupant present in the seat, C v  increases and the RC value changes in only one arm of the bridge circuit  48 . The outputs of the two low pass filters are no longer the same. The unbalance in the bridge circuit  48  is detected by amplifying the differences between the two signals. The amplified signal is an AC signal representing the voltage difference between the two filters multiplied by the gain of the amplifier. The difference in phase shifts between the two filters are detected because the leading and lagging portion of each waveform overlap each other causing a voltage differences between theses signals. The AC signal is then passed through the AC-DC conversion circuit  42  to produce a DC signal that is then compared to a predetermined threshold in threshold detection circuit  46  to determine if an occupant is present or if a failure has occurred that causes the output to default to occupant present. 
   Both an increase and decrease in capacitance can cause a debalance in the bridge circuit  48 . An increase in capacitance indicates the presence of an occupant, while a decrease in capacitance indicates a disconnected or damaged electrode  34 . Both situations will cause the output to indicate “occupied.” This means that if the electrode  34  is damaged, the device will fail in a safe mode that will allow the safety restraints system to revert to a first generation configuration where the safety restraints device is always deployed in the event of a serious accident. However, other embodiments of the invention described below provide detection of these faults allowing for alternative measures to be taken in the event of a device failure. 
     FIG. 2   a  shows the basic model of a capacitor. The formula for a parallel capacitor is, C=∈A/d, where C is capacitance, ∈ is the permittivity, A is area of the plates and d is the distance between the plates. The values of these variables determine the capacitance of the capacitor. Therefore, a change in one or more of these variables causes a change in capacitance. The permittivity and the area of the plates are proportional to the capacitance while the distance between the plates is inversely proportional to the capacitance. This means that an increase in permittivity or area causes an increase in capacitance while a decrease in permittivity or area causes a decrease in capacitance. The opposite is true for the distance between the plates. An increase in the distance between the plates causes a decrease in capacitance while a decrease in the distance between the plates causes an increase in capacitance. The electrode acts as one plate, while the surrounding environment acts as the second plate. 
     FIG. 2   b  shows a model of the sources of capacitance in a typical vehicle. The value C v  is the sum of virtual capacitors formed between the electrode  34  and the portion of the chassis beneath the seat (C v1 ), the seat frame (C v2 ), the roof (C v3 ) and the floor pan (C v4 ). However, the invention does not require a grounded frame to function, any type of structure including walls, ceilings, floors and the earth beneath one&#39;s feet can act as the second plate of the capacitor. The capacitance of the virtual capacitor C v  changes depending on the medium between the electrode  34  and its surroundings. 
     FIG. 2   c  shows the same model with an occupant present. Assuming that we have a capacitor with constant area and distance between the plates, then the capacitance will be altered by the medium put between the plates. When the seat is empty the medium adjacent the electrode  34  is air. Water has a higher permittivity than air and the seat foam and the human body consists of approximately 65% water. Hence, putting a human body between the electrodes and its surroundings will increase the permittivity and, in turn, will increase the capacitance between the electrode and its surroundings (C v2 , C v3 , C v4 ). The weight of the body will also cause the distance between the electrode and the portion or the chassis beneath the seat to decrease, causing an increase in the capacitance C v3 . Therefore, the capacitance of an occupied seat (C′ v ) will be larger than the capacitance of an empty seat (C v ). 
     FIG. 3  shows the output of the differential amplifier  40  ( FIG. 2 ) before and after AC-DC conversion by AC-DC converter  42  ( FIG. 2 ). When the seat  14  is empty, the difference between the outputs of the two low pass filters will be small and the output of the differential amplifier will be almost flat and will be centered around half-supply. Once it is converted to a DC signal it will be below the predetermined threshold V thresh  and the device will output an empty signal. When an occupant is present in the seat  14  or when the electrode  34  is disconnected or damaged, the difference between the outputs of the two low pass filters will be large and the output of the differential amplifier will be a waveform centered around half-supply. Shorting the electrode  34  to the grounded chassis will also have this effect. Once the signal is converted to a DC signal, it will be above the predetermined threshold V thresh  and the device will output an occupied signal. 
   Note that the AC signals for an occupied seat  14  and for a damaged electrode  34  are of opposite phases. This is because when an occupant is present, the capacitance C v  increases causing the output signal coming from the sensing arm of the bridge circuit  48  to have a smaller peak-to-peak value than the output signal coming from the reference arm of the bridge circuit  48 . When the electrode  34  is disconnected or damaged, the capacitance C v  decreases causing the output signal coming from the sensing arm of the bridge circuit  48  to have a larger peak-to-peak value than the output signal coming from the reference arm of the bridge circuit  48 . When the electrode  34  is shorted to the grounded chassis, the signal on negative input of the differential amplifier will always be much smaller than the signal on the positive input and the output of the amplifier will saturate high and will always produce a DC signal above V thresh . 
     FIG. 4  shows the plot of the DC output of the differential amplifier versus the value of the virtual capacitance C v  for different configurations. Region B corresponds to an empty seat and at least a fairly balanced bridge circuit  48 . C bal  indicates the point of the graph that corresponds to a perfectly balanced bridge circuit  48 . Region C of the graph corresponds to an occupied seat. Region A of the graph corresponds to a disconnected or damaged electrode  34 . Regions A and C in  FIG. 4  both correspond to a debalanced bridge circuit  48 . The circuit is tuned for a given environment as follows: The position of the MINIMUM of the curve is set by the value or the components in the bridge circuit  48  R occ , R ref  and C ref . These values are tuned so that the MINIMUM point on the curve occurs at the value of C v  that corresponds to and empty seat (C bal ). The sensitivity of the device to changes in the virtual capacitance C v  is tuned by changing the gain of the differential amplifier and the predetermined threshold value V thresh . V thresh  must be situated between the MINIMUM of the curve and the saturation voltage of the differential amplifier less a diode drop. 
   In the second embodiment of the invention, shown in  FIG. 5 , a fault detection circuit  60  is incorporated to detect the most common failure modes of a capacitance based system. These include; failure of the oscillator  50  disconnected or damaged electrode  34 , and the electrode  34  being shorted to the grounded vehicle chassis. This allows for the device reading the occupant presence detection device to take alternative actions in the event of a failure. This device utilizes the electrode  34  shown in  FIG. 1 . 
   The fault detection circuit  60  is divided into two independent modules; an oscillator failure detection module  62  and a damaged/grounded electrode detection module  64 . The output of the oscillator  50  is coupled to an AC-DC converter  66  via the capacitor C which only allows an alternating signal to pass. Regardless of the voltage at which the oscillator  50  fails, the signal will not be passed to the AC-DC converter  66  once there is no oscillation. This will cause the DC signal to fall below a predetermined threshold as determined by threshold circuit  68 , triggering the FAULT signal to be output. 
   The damaged/grounded electrode detection module  64  works by measuring the voltage drop over the resistor R sense  using a differential amplifier  72  and converting the resulting AC signal to DC. The voltage drop across R sense  varies proportionally with the current drawn by the bridge circuit  48 . A damaged or disconnected electrode  34  will draw less current than an empty seat or occupied seat. Thus, the peak voltage across R sense  will be smaller than the peak voltage across R sense  when the seat is empty or occupied. A grounded electrode  34  will draw more current that an empty seat or occupied seat. Thus, the peak voltage across R sense  will be larger than the peak voltage across R sense  when the seat is empty or occupied. Therefore, the DC signal of the AC-DC converter  74  in the damaged/grounded electrode detection module  64  must be compared with both HI and LO thresholds by threshold detection circuit  76  to detect these faults. All thresholds and waveforms in the device vary in proportion to the power supply voltage. As such, the device is tolerant to sudden changes in the supply voltage and will function over a wide range of supply voltages. 
   The outputs of these modules  62 ,  64  are coupled together using a wire OR circuit  78  to provide a generic FAULT signal. However, two individual signals could be output instead of one generic FAULT signal. It is also possible to provide three individual fault signals: oscillator failure, electrode damaged, and electrode grounded if that information is even desired. Implementation of these variations will be apparent to those skilled in the art and are considered to be within the scope of the invention. 
   In the third embodiment of the invention, shown in  FIGS. 6   a  and  6   b , a second capacitive sensor  80 , configured to detect pressure, is used in conjunction with the original electrode in order to detect the presence of a child-seat.  FIG. 6   a  shows the TOP and SIDE view of the sensor. It consists of a basic occupant sensing electrode  34  (as described above) working in conjunction with the pressure-sensing capacitive sensor  80 . The capacitive sensor  80  comprises a sensing electrode  82 , a grounded plate  84  and a compressible material  86 . 
   As mentioned previously, a decrease in the distance between the electrodes  82 ,  84  causes an increase in capacitance. Therefore, the weight of a body, or of a child seat will cause the distance between the sensing electrode  82  and grounded electrode  84  to decrease, causing an increase in capacitance. This will be detected by the second detection circuit  90  as shown in  FIG. 6   b.    
   The second detection circuit  90  is identical to the first, only it is configured to detect a change in pressure due to a compression force causing the material  86  between the sensing electrode  82  and the grounded electrode  84  to compress. The compressible material  86  can be made from any foam, rubber, plastic or fabric that is compressible and retains its height after being compressed. The outputs may be connected to logic circuits, such as the AND gates  90 ,  92  shown (with the inverted input on the child seat presence AND gate  92 ). 
     FIG. 6C  shows a child-seat  98  on top of the electrode according to the third embodiment of the invention. In this situation the occupant-detecting electrode  34  would not detect the child seat since it is not conductive. However, the pressure sensor  80  would detect that an object with weight above a predetermined threshold is present. This object could be something other than a child seat. In both cases, however, it would not be desirable to deploy an airbag. When the seat is empty, both outputs would indicate empty. When a seated occupant is present, both outputs would indicate a presence since an occupant is both conductive and has weight above the predetermined threshold. TABLE 1 is a summary of the operation of this embodiment. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Presence 
               Child seat 
             
             
                 
               Object in Seat 
               Sensor Output 
               Sensor Output 
             
             
                 
                 
             
           
           
             
                 
               Empty 
               0 
               0 
             
             
                 
               Seated Occupant 
               1 
               1 
             
             
                 
               Child in Child seat 
               0 
               1 
             
             
                 
                 
             
           
        
       
     
   
   Of course, it is also contemplated as part of the present invention to implement the fault detection of  FIG. 5  in addition to child-seat detection of  FIGS. 6   a  and  6   b . The fault detection circuitry shown in  FIG. 5  would be connected to both bridges of the device shown in  FIG. 6   b . This allows for the device reading the occupant presence detection device to take alternative actions in the event of a failure. This device would utilize the electrode shown in  FIG. 6   a.    
     FIG. 7  shows an alternate detection circuit in which capacitance is used indirectly as the means of presence detection. The electrode  34  becomes a capacitor, C, in an oscillator circuit also including an op-amp  102  and resistor  104 . The frequency at which the oscillator functions is dependent on several parameters including the capacitance C. In an empty state (no human presence) the system will oscillate at a given frequency based on these parameters so long as they remain constant. When an occupant is present, the C value increases. If, for example an RC oscillator is used, an increase in capacitance C results in a decrease in oscillating frequency. This phenomenon can be used to determine the presence of an occupant. Other oscillator configurations may have an output in which an increase in capacitance results in an increase in frequency. It should be apparent to anyone skilled in the art that this will not change the intent of the invention. 
   A control unit  106  is used to measure the oscillator&#39;s frequency. The control unit  106  will compare the incoming frequency to a set threshold frequency. If the incoming frequency has crossed this threshold (meaning capacitance has decreased) the control unit will output an occupied signal. If the frequency has not crossed the threshold, the control unit will output an empty signal. This threshold must be tuned based on the application of the presence detector and the surrounding environment. 
   This capacitance results in an oscillating frequency of ω 1 . The control unit is tuned so that ω threshold  is less than the unoccupied frequency ω 1 . In this configuration, the control unit  106  will output an “unoccupied” signal. 
   With an occupant in the seat, the occupied capacitance, C, (due to the presence of the occupant) is higher than the empty capacitance. If the resulting frequency is lower than the threshold frequency, the control unit will output an “occupied” signal. 
   In addition, the control unit  106  can monitor the rate of change of the oscillator&#39;s frequency. This allows the control unit  106  to ignore slow changes in frequency which would tend not to represent an occupant sitting on the seat or leaving the seat. 
   Hysteresis can also be added to the control unit  106  to eliminate flickering of the output signal when the frequency is hovering around the threshold.  FIG. 8  shows that in the RC oscillator, the operating frequency of the oscillator must cross ω thresholdoccupied  in order for the circuit to output an “occupied” signal.  FIG. 8  shows ω thresholdoccupied  is the frequency that must be crossed prior to outputting an “empty” signal. These two thresholds can be tuned in the control unit  106 . Hysteresis can also be applied to the first embodiment of this invention by tuning V threshold  in the first embodiment as ω threshold  is tuned in the second embodiment. When applying hysteresis to the first embodiment a similar output would be shown as in  FIG. 8  with the oscillating frequency replaced by the output voltage. 
     FIG. 9  illustrates one possible technique for avoiding interference from moisture with the capacitance measurement by the electrode  34 . This technique is applicable to any of the embodiments described in any of the preceding Figures. An elastically deformable spacer  110 , preferably comprising foam similar to that used in seats, is positioned on top of the electrode  34 . The electrode  34  is then sealed against water by a seal  112 , such as EPDM rubber. A second spacer  114  (again, possibly foam) and a second seal  116  (again, possibly EPDM rubber) may be positioned below the electrode  34 . The spacers  110 ,  114  are preferably larger than the electrode  34  on all sides, preferably by about 0.5 inches. The seals  112 ,  116  may be coated with an adhesive on the side facing the spacer  110 ,  114 , respectively. The seals  112 ,  116  are preferably larger than the spacers  110 ,  114  on all sides, preferably by about 1 inch, and thus adhere to the spacers  110 ,  114  and to one another  112 ,  116  at the overlapping edges. The location where the wires (not shown) exit the electrode  34  is also sealed. Adhesive may also be used between the electrode  34  and spacers  110 ,  114 . The lower seal  116  may optionally include a hole  118  to permit air to escape to prevent ballooning. 
   The arrangement in  FIG. 9  mitigates the effect of water on the seat because it requires the occupant to exert a force on the electrode assembly. If enough force is exerted on the upper seal  112 , the spacer  110  will deform and the occupant&#39;s body will approach the electrode  34 , thus changing the capacitance and the system will indicate that an occupant has been detected. 
   If only water is applied to the seat, there is not sufficient force to deform the spacer  110  and the spacer  110  prevents the water from approaching the electrode  34  and therefore prevents the system from falsely detecting an occupant. The arrangement of  FIG. 9  could be implemented by placing the electrode  34  inside existing foam of a seat, wherein the spacers  110  and  114  would be the existing foam in the seat. 
   Alternatively, a moisture detector could be used in conjunction with a presence detector to notify the system when the seat is wet. When a significant amount of moisture is detected, the system could output a signal to indicate that the seat is wet and that the presence detection is currently unreliable or has been deactivated. 
   Again, although the present invention has been described for use in a vehicle, it would be useful in any seating application, such as those described in the Background of the Invention. Further, the present invention could also be used in non-seating applications to determine the presence of a person. It should be noted that the embodiments described above have been described for purpose of illustration and are not intended to limit the scope of the claimed invention, which is set forth in the claims. Claim terms below are intended to carry their ordinary meaning unless specifically defined otherwise in the claims. Alphanumeric identifiers on method steps are provided for ease of reference in dependent claims and are not intended to dictate a particular sequence for performance of the method steps unless otherwise indicated in the claims.