Abstract:
A vehicle occupancy sensing system provides a mechanism for determining whether a child is present in a vehicle. The sensing system may determine that an occupied front or rear facing child seat is present in an automobile, for example. To that end, the sensing system may employ a reliable electrode sensing system that may be conveniently installed in one or more locations in the vehicle seats. The sensing system thereby helps reduce occurrences of the potentially devastating consequences of unintentionally leaving a child in a vehicle.

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to vehicle occupant sensing. More specifically, the present invention relates to automatic detection of the presence of an occupant on a vehicle seat and occupant characteristics such as age or facing, with a special focus on detecting children in child safety restraint devices, such as safety seats. 
   2. Background Information 
   Child safety has always been an important societal focus. In recent years, for example, great progress has been made in the design of child safety seats for automobiles. In fact, in many instances, hospitals require new parents to have a properly configured child safety seat waiting in their car before they can even take their own child home. 
   The child safety seat does not protect the child from all dangers, however. In particular, when young children are left unattended in an automobile, the consequences can be tragic. Each year, multiple children suffocate because they were left unintentionally in the back seat of a car, minivan, or other vehicle. 
   While several passenger detection systems have been proposed for controlling air bag activation, they have not been entirely suitable for widespread and cost effective implementation. For example, weight sensors may incorrectly detect or classify unusually light or heavy children. As another example, optical sensors are typically expensive and require complex optical processing equipment. 
   Thus, interest remains strong in overcoming the problems noted above and arriving at a reliable sensing system that may be conveniently installed in one or more locations in vehicle seats, and that is particularly adapted to detecting occupied child safety devices. 
   BRIEF SUMMARY 
   As an introduction, the occupant sensing systems described below are adept at determining whether an occupied or unoccupied child restraint device (e.g., a child safety seat) is present on a vehicle seat. In addition, in some implementations, the occupant sensors may determine additional characteristics about the restraint device, including its facing (e.g., either front facing or rear facing), and an approximate age of a child occupying the restraint device. In the same or other implementations, the sensors determine characteristics of occupants free of any child safety seat. 
   To that end, systems consistent with the present invention implement a vehicle occupancy sensing system. The sensing systems may include a first electrode connection and a second electrode connection. A circuit parameter sensor operates in conjunction with a controller to provide a first parameter reading for the first electrode connection, and a second parameter reading for the second electrode connection. The controller determines occupant presence based, for example, on a ratio or product of the first and second parameter readings and a pre-selected threshold. 
   In another implementation, three or more electrodes are positioned in a seat. One of the electrodes is non-switchably connected to virtual or relative ground. The remaining electrodes are used to sense an occupant or characteristics of the occupant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a vehicle seat that incorporates electrodes that operate in conjunction with an occupant sensing system of one embodiment. 
       FIG. 2  shows one embodiment of a block diagram of an occupant sensing system. 
       FIG. 3  shows a detailed view of the occupant sensing system shown in  FIG. 2 . 
       FIG. 4  shows a diagram of seat electrode positioning with respect to a front facing and a rear facing child safety seat in one embodiment. 
       FIG. 5  shows a diagram of one embodiment of an occupancy test in which a product of loading currents is compared against an upper threshold and a lower threshold. 
       FIG. 6  shows a flow diagram one embodiment of the acts that the occupant sensing system shown in  FIG. 2  or another system may take to determine occupant presence. 
   

   DETAILED DESCRIPTION 
   The discussion below presents exemplary implementations of an occupant sensing system. The discussion is therefore not limiting, but explanatory in nature. The occupant sensing systems generally incorporate capacitive arrangements of multiple electrodes driven by a signal source, with a controller that interprets resultant loading or received current readings. In this regard, the sensing systems may employ the circuits and techniques described in U.S. Pat. No. 6,329,913, U.S. Pat. No. 6,329,914, or U.S. Pat. Pub. No. 2003-0090376 (application Ser. No. 10/033,585). The &#39;913 patent, &#39;914 patent, and the &#39;585 application are incorporated herein by reference in their entireties. Sensing systems using capacitive bridges, phase detection, capacitance measurement, frequency changes or other techniques for detection with transmitted signals may be used. 
   With regard to first to  FIG. 1 , that figure shows a vehicle seat  100 . Although  FIG. 1  illustrates the vehicle seat  100  as a longer bench seat (e.g., particularly suited for a minivan), the vehicle seat  100  may be a shorter seat, including a front or rear single passenger seat. The seat  100  includes a seat base  102 , a seat back  104 , and a seat bite  106  where the seat base  102  meets the seat back  104 . 
   Ground electrodes  108  and  110  are present inside or on the seat  100 . As shown, the ground electrodes  108  and  110  are placed in the seat base  102 , but may be positioned in the seat back  104  in other embodiments. Furthermore, signal electrodes  112 ,  114 ,  116 ,  118 ,  120 , and  122  are also present in pairs in the seat  100 . The electrode pairs are disposed across the seating positions  124 ,  126 , and  128 . The seating positions  124 – 128  may correspond, for example, to lawful placement positions for a child safety seat on the seat  100 . In other embodiments, two or more electrodes in the seat back extend across multiple seating positions. 
   Any of the electrodes  112 – 122  may be separated by at least a vehicle cabin feature distance. For example, the electrodes  112 – 122  may be separated by the largest dimension of a seat belt buckle, in order to prevent the seat belt buckle from shorting the two elements or interfering with electrical operation of the signal electrodes  112 – 122 . Other cabin features may also be taken into consideration, for example, the largest dimension of a radio or DVD remote control, or the like, may influence the electrode  112 – 122  separation. In one embodiment, the upper and lower electrodes (e.g.,  112 ,  114 ) are separated by approximately 25 to 50 mm. 
   The electrode pair  112  and  114 , in conjunction with the ground plane  108 , form one sensing position on the seat. Similarly, the electrode pair  116  and  118 , in conjunction with the ground plane  110  form a second sensing position on the seat. Although the ground plane  108  may span multiple sensing positions, a separate ground plane may support any given sensing position. As an example, the electrode pair  120  and  122  operate in conjunction with the separate ground plane  110  to form a third sensing position for the seat  100 . 
   In one implementation, the ground planes  108 ,  110  are connected to the vehicle chassis ground  130 . While the ground planes  108 ,  110  may be connected permanently through non-switchable connections (as shown in  FIG. 1 ) to the chassis ground, the ground planes  108 ,  110  may also be connected to a switch matrix that allows them to disconnect from the chassis ground and take the role of a signal electrode. Note that the ground planes  108 ,  110  or signal electrodes  112 – 122  may be slotted, cut, or otherwise shaped to impart additional flexibility or breathability for the seat  100 . 
   The ground planes  108 ,  110  or signal electrodes  112 – 122  may be formed, for example, from a conductively coated sheet of polyester disposed in the seat  100 . The conductive coating may include a layer of Nickel, a layer of Copper, and a layer of Nickel to protect the copper from corrosion. In other implementations, conductive paint, tape, or sewn metal fibers may form the ground planes  108 ,  110  and signal electrodes  112 – 122 . 
   The size, orientation, and relative position of the ground planes  108 ,  110  and signal electrodes  112 – 122  may vary according to the design of the seat  100 , or in response to empirical studies of occupied and unoccupied child restraint devices. Consequently, in one implementation, the signal electrodes  112 – 122  are shaped to fit in the sewing line groves  132  of the seat  100 . 
   In one implementation, the electrode  122  may be 280 mm wide and 220 mm high, and may be located 140 mm above the seat bite  106 . The electrode  120  may then be 280 mm wide and 200 mm high, and may be located 30 mm above the electrode  122 . The ground plane  110  may be 450 mm wide and 320 mm long, and may be located 140 mm in front of the seat bite  106 . Other sizes, shapes, positions, and orientations for the electrodes  112 – 122  are also suitable however. 
   Turning next to  FIG. 2 , that figure shows a block diagram of an occupant sensing system  200  that may be part of a larger vehicle electronics system. The occupant sensing system  200  includes a signal source  202  that presents a signal on a detection signal output  204  to a load sensor  206 . The load sensor  206  connects to a switching circuit  208  that connects or disconnects the detection signal output  204  (and therefore the signal source  202 ) to one or more electrode connections E 1 , E 2 , E 3 , E 4 , E 5 , and E 6 . 
   The switching circuit  208  connects the electrode connections E 1 –E 6  to the current-to-voltage conversion circuit  210 . Both the conversion circuit  210  and the load sensor  206  connect to the detecting circuit  212 . The load sensor  206  and/or the converting circuit  210  may generally be considered circuit parameter sensors. Accordingly, in one implementation, the load sensor  206  provides a voltage or current output indicative of load current flowing from the signal source  202  through the electrode connections E 1 –E 6 . Similarly, the converting circuit  210  provides a voltage or current output indicative of return current arriving through the electrode connections E 1 –E 6 , regardless of origin. While the discussion below proceeds with regard to current and voltage sensing, the sensing system  200  may alternatively or additionally employ a wide variety of circuit parameter sensors that measure, as examples, capacitance, phase, frequency, or Q, or that measure combinations of such circuit parameters. 
   The detecting circuit  212 , in turn, connects to an amplification circuit  214  connected to the controller  216 . The controller  216  applies controls signals to the offset converting circuit  218 , and to the air bag device  220  or a warning device. The controller  216  may operate under control of an occupancy sensing program  222  stored in a memory  224 . Consequently, the controller  216  may operate as explained below to determine whether any of the seating positions  124 – 128  are occupied and, optionally, one or more characteristics of the occupant. 
   Example implementations for the sensing system  200  are described in detail below, and in conjunction with  FIG. 3 . Other circuits may be substituted, or circuit parameters modified, however, depending on the particular implementation desired. 
   In one implementation, the signal source  202  may be a 100–120 kHz oscillator that outputs a 10–12 volt signal on the detection signal output  204 . The switching circuit  208  may incorporate a multiplexer, switches or other devices that selectively connect the electrodes E 1 –E 6  to the signal source  202 . 
   The conversion circuit  210  includes a resistor network and generates voltage signals indicative of the current returning from or transmitting from the electrode connections E 1 –E 6  through the switching circuit  208 . The conversion circuit  210  may also amplify the voltage signals before outputting them to the detection circuit  212 . The detection circuit  212  may include a demodulation circuit including a band pass filter that eliminates noise coupled to an AC-DC converter to provide a DC signal output to the converting circuit  210 . 
   The controller  216  may be an ASIC, processor, digital signal processor or other circuitry for evaluating the signals obtained from the amplifying circuit  214 . The controller  216  may be a PD78052CG(A) microprocessor manufactured by NEC Corporation of Japan that substitutes its own AC-DC conversion circuitry for that noted above in the detection circuit  212 . The controller  216 , after evaluating the signals, may set an occupancy indicator that specifies whether a particular seating position is occupied. The occupancy indicator may be an internal register or memory setting, or may be an external indicator such as a warning lamp, dashboard indicator, speaker alarm, voice prompt, or another indicator. 
     FIG. 3  is a circuit diagram  300  that shows the occupant sensing system  200  in additional detail. While four channels are shown (E 1 –E 4 ), the circuitry shown in  FIG. 3  may be extended to more channels (e.g., 6 channels E 1 –E 6 ) by replicating the sensing and detection circuitry described below. In other implementations, the sensing system  200  may use fewer channels, such as where connected with a single passenger seat, or on a larger multiple passenger seat for which fewer sensing positions are desirable. Note also that the sensing system  200  may determine occupant presence across multiple physically separate seats. The circuitry  300  connects to the six electrodes  112 – 122  to provide occupancy sensing across multiple positions in the seat  100 . The controller  216  may then scan across the seat sequentially or at random to determine whether the sensors for any seating position  124 – 128  indicate that an occupant is present or a characteristic of the occupant. 
   As shown, the amplification circuit  214  includes a relatively low gain (e.g., a gain of approximately 1) amplifier  302  and a relatively high gain (e.g., a gain of approximately 100) amplifier  304 . An analog switch  306  selectively connects the outputs of the low gain amplifier  302  or the high gain amplifier  304  to the controller  216  through the switching elements  308  and  310  as directed by the controller  216 . 
   The load sensor  206  may be implemented as an impedance/resistance element  312  and an amplifier  314  connected between the signal source  202  and the switching circuit  208 . The load sensor  206  thereby provides a voltage signal to the detection circuit  212  that indicates the amount of current flowing to the switching circuit  208  (and thus to one of the electrodes  112 – 122 ). 
   The switching circuit  208  includes switching elements  324 ,  326 ,  328 , and  330  and switching elements  332 ,  334 ,  336 , and  338 . The switching elements  324 – 330  selectively connect, in response to a control signal  320  from the controller  216 , an electrode connection E 1 –E 4  to the signal source  202 . The signal source  202  thereby drives the electrode connected to the electrode connection E 1 –E 4 . The load sensor  206  responsively measures the load current flowing to the electrode, with respect to the ground planes  108 – 110  and other grounding provided through any occupant to the vehicle. The controller  216  may open or close the switches  332 – 338  in order to obtain measurements of return currents present in the remaining electrodes. In other words, in alternative embodiments, the load sensor  206 , the converting circuit  210 , or other circuit parameter sensors measure parameters associated with electrodes different from the electrode used to transmit. 
   The converting circuit  210  may include an impedance/resistance element  318  that converts current flowing in the receiver electrodes to voltage signals, and an amplifier  316  that amplifies the converted voltage signals. The impedance/resistance elements  318  shunts high frequency noise from the input of the amplifier  316  to ground. 
   The detecting circuit  212  may include an impedance or resistance element and a differential amplifier (or other amplifier) whose output is coupled to the controller  216  through the amplifier circuit  214 . One such impedance/resistance element may be connected between the output of an amplification control circuit and the electrode connections E 1 –E 4 . The differential amplifier may be connected across the impedance/resistance element to generate a current signal based on the voltage differential across the impedance/resistance element. In particular, the current differential amplifier compares the voltage level of the oscillation circuit output signal with the voltage level generated on an electrode, and generates a current signal representative of the difference. 
   The detecting circuit  212  thereby receives the output signal from the load sensor  206  as well as the signals from the receiving electrodes and couples them to the high gain amplifier  304  and the low gain amplifier  302 . The high gain amplified outputs couple to the switches  308 , while the low gain amplified outputs couple to the switches  310 . In response to the control signal  322 , the analog switch  306  couples either the high gain or low gain output to input pins on the controller  216 . 
   With regard next to  FIG. 4 , that figure shows a sensor configuration  400  adapted to determine whether the seat  100  is occupied. More specifically the sensor configuration  400  may determine whether an occupied or unoccupied child safety seat is present. The child safety seat may be a rear facing child safety seat  402  or a front facing child safety seat  404 . 
   As noted above, the seat  100  includes an electrode pair  112 ,  114 , and a ground plane  108 . The first electrode  112  is disposed in the seat  100  as an upper body electrode. In this case, the upper body electrode is a head electrode disposed at a pre-selected head height, Hh, for an occupied child safety seat. 
   The second electrode  114  is disposed in the seat  100  as a lower body electrode. In one embodiment, the lower body electrode is a foot electrode disposed at a pre-selected foot height, Fh, for an occupied child safety seat. The electrode heights may be empirically determined through examination, measurement, and testing of multiple models of child safety seats, in conjunction with statistical data (e.g., dimensional measurements) of young children that occupy such safety seats. Although other heights are suitable, in one implementation, the height Fh is approximately 140 mm above the seat bite  106 , while the height Hh is approximately 390 mm above the seat bite  106 . 
   The head electrode  112  may be connected to the sensing system  200  through an electrode connection E 1 –E 6 . As an example, the electrode connection E 1  may serve as a seat back head electrode connection. Similarly, the electrode connection E 2  may serve as a seat back foot electrode connection. In conjunction with the ground electrode  108 , the head electrode  112  and foot electrode  114  form an electric field sensing circuit. 
   The controller  216  may determine occupant presence or absence in many ways. For example, the controller  216  may measure the loading current, HL, to the head electrode  112  and the loading current, FL, to the foot electrode  112 . Then, the controller may apply an occupancy test according to HL&lt;T 1  and FL&lt;T 2 , where T 1  and T 2  are pre-determined thresholds based on prior studies of occupied and unoccupied child safety seats. If HL&lt;T 1  and FL&lt;T 2 , then the controller  216  may determine that no occupant is present on the seat  100 . In one implementation, T 1  and T 2  may be approximately 6 bits, although other threshold settings may also be suitable based on the particular design. 
   If, however, the controller  216  determines that an occupant is present, the controller  216  may apply additional occupancy tests. For example, the controller  216  may next determine whether FL−HL&lt;T 3  and (FL/HL)&lt;T 4 , where T 3  and the ratio threshold T 4  are pre-determined thresholds based on prior studies of occupied and unoccupied child safety seats. Thus, when the foot electrode load current is greater than the head electrode load current (e.g., when the child&#39;s feet are closer to the foot electrode  114  than the head is to the heat electrode  112 ), and when their ratio is less than a threshold, the controller  216  may determine that a rear facing child safety seat  402  is present. For the purposes of this occupancy test, T 3  may be approximately zero bits, while T 4  may be approximately 1. Other thresholds may be employed, however. 
   Similarly, the controller  216  may determine whether FL−HL&gt;T 5  and (HL/FL)&gt;T 6 , where T 5  and the ratio threshold T 6  are pre-determined thresholds based on prior studies of occupied and unoccupied child safety seats. The test succeeds when an occupied front facing child safety seat is present in the seat  100 . While other thresholds may be used, the threshold T 5  may be approximately zero bits, while the threshold T 6  may be approximately 1. 
   Note that the thresholds may also be determined by empirical study not only of an occupied or unoccupied child safety seat, but also according to age of the occupant. Thus, empirical studies may be undertaken to obtain characteristic thresholds for children of one or more ages (e.g., 1 year old, and 3 years old) that occupy front or rear facing child safety seats or the seats without child safety seats. For that reason, the controller  216  may repeat the occupancy tests noted above with different thresholds, in order to obtain an age estimation for the occupant based on the load current readings. 
   In another implementation, the controller  216  determines occupant presence based on a product of loading current readings with regard to one or more thresholds. As one example, the controller  216  may sample and store unloaded (e.g., no occupant) current measurements for the head electrode  112  and the foot electrode  114 . Then, the controller  216  may determine a sum that represents the total load impact on both the head electrode  112  and the foot electrode  114 . The load impact is the difference between the unloaded condition (e.g., no occupant), and measurements taken to determine a loaded condition (e.g., with occupant). Similarly, the controller  216  may also determine a difference that represents the difference in load impact between the head electrode  112  and the foot electrode  114 . 
   The controller  216  then forms the product of the sum and the difference. In conjunction with one or more thresholds, the controller may then determine whether the seat  100  is occupied. 
   For example,  FIG. 5  shows a diagram of an occupancy test  500  based on loading currents  502  and a product of the loading currents  504 . The loading currents  502  include the head electrode loading current  506 , and the foot electrode loading current  508 .  FIG. 5  shows the loading currents  506 ,  508  measured in terms of raw bits across multiple samples. The loading currents  506 ,  508  change in response to the presence or absence of child safety seats on the seat  100 . 
   The occupancy test  500  includes a product of loading currents  504  compared against upper thresholds  512  and  514 , and lower thresholds  516  and  518 . As explained below, the thresholds  512 – 518  may distinguish between occupant age, but, in alternative embodiments, the thresholds may instead distinguish between other occupant characteristics. The thresholds  512 – 518  may be empirically determined according to studies of occupied and unoccupied child safety seats. 
   More specifically, the product  504  is a product of the sum and the difference of load impact explained above. When the product  504  crosses a product threshold, the controller  216  accordingly determines that the seat  100  is occupied, and optionally determines additional occupant characteristics. For example, when the product  504  crosses the upper threshold  512 , the controller  216  may determine that the seat  100  holds an occupied rear facing child safety seat with a 1–3 year old child. If the product  504  further crosses the upper threshold  514 , the controller  216  may determine that the occupant is approximately 3 years old. If the product  504  crosses the threshold  512  but not the threshold  514 , the controller  216  may determine that the occupant is approximately 1 year old. 
   Similarly, when the product  504  crosses the lower threshold  516 , the controller  216  may determine that the seat  100  holds an occupied front facing child safety seat with a 1–3 year old child. If the product  504  further crosses the lower threshold  518 , the controller  216  may determine that the occupant is approximately 3 years old. If the product  504  crosses the lower threshold  516  but not the lower threshold  518 , the controller  216  may determine that the occupant is approximately 1 year old. 
   The controller  216  performs the occupancy tests noted above using the circuitry  300  shown in  FIG. 3 , optionally expanded to include six channel connections E 1 –E 6 . The signal electrodes  112 ,  116 , and  120  may therefore correspond to head electrodes connected to the electrode connections E 1 , E 3 , and E 5 , for example. The signal electrodes  114 ,  118 , and  122  may then be foot electrodes that may be connected to the electrode connections E 2 , E 4 , and E 6 . The loading currents are sensed with respect to the ground plane  108  for the signal electrodes  112 – 118  and with respect to the ground plane  110  for the signal electrodes  120 – 122 . 
   When the controller  216  checks the seating position  124  for occupancy, the controller  216  may first assert the control signal  320  to couple the head electrode  112  to the signal source  202  through the switch  324  and electrode connection E 1 . The controller  216  then determines the head electrode load as sensed by the load sensor  206 . Next, the controller  216  opens the switch  324  and closes the switch  326 . As a result, the signal source  202  drives the foot electrode  114  through the electrode connection E 2 . The controller  216  may then monitor the foot electrode load as sensed by the load sensor  206 . Alternatively, the controller  216  may measure circuit parameters, such as load current, by transmitting from one electrode and receiving at another. 
   The controller  216  thereby connects one electrode  112 – 122  at a time to the signal source  202 . The remaining switches in the switch  208  remain open. However, in other embodiments, the controller  216  may close one or more switching elements  324 – 338  in the switch  208  to connect additional electrodes to the signal source  202  or to connect one or more electrodes to the converting circuit  210  and the detecting circuit  212 . Thus, the currents present in multiple electrodes may contribute to the occupancy analysis. 
   Once the controller  216  has obtained the head and foot electrode loads for the seating position  124 , the controller  216  may then proceed to perform the occupancy tests explained above. The controller  216  thereby arrives at an occupancy determination for the seating position  124 . The occupancy determination may indicate a front facing or rear facing child safety seat, or an age approximation, as examples. 
   The controller  216  may proceed to check the additional seating positions  126  and  128  as well. Thus, with regard to the seating position  126 , the controller  216  may connect the head electrode  116  to the signal source  202  through the switch  328  through the electrode connection E 3 , and obtain a head electrode load measurement for the seating position  126 . The controller  216  may then disconnect the head electrode  116  from the signal source, and connect the foot electrode  118  to the signal source  202  through the switch  330  and the electrode connection E 4 . After obtaining the foot electrode load measurement, the controller  216  may apply an occupancy test to determine whether the seating position  126  has an occupant. 
   Similarly, the controller  216  may then check the seating position  128  for occupancy. To that end, the controller  216  may connect the head electrode  120  to the signal source  202  through the electrode connection E 5 . After obtaining a head electrode load measurement, the controller  216  may then connect the foot electrode  122  to the signal source  202  through the foot electrode connection E 6 . A foot electrode load measurement results. The controller  216  then applies an occupancy test to determine whether the seating position  128  has an occupant. 
   The controller  216  may determine which occupancy test to apply in many ways. For example, the controller  216  may be pre-configured to employ the product or ratio test first unless the result are inconclusive, then employ the remaining test. Alternatively, the controller  216  may employ both tests, select one at random, select one according to a pre-set parameter in the memory  224 , or the like. In yet another alternative, the controller  216  may use a different function than the ratio or product, or may employ different formulations of a ratio or a product. Accordingly, the ratio or the product may include additional variables, including measured circuit parameters representative of additional electrodes. Alternatively, the ratio or product may be ratios or products of more than two variables, for example, a product of three or four circuit parameter measurements obtained from three or four different electrodes. Furthermore, the circuit parameter measurements included in the ratio or product may be arithmetic averages, modes, normalizations, or other forms of the circuit parameters measurements. 
   Turning next to  FIG. 6 , that figure presents a flow diagram of the acts that the occupant sensing system  100  may take to determine occupant presence. The controller  216  individually switches a head electrode connection and a foot electrode connection to a detection signal output (Act  602 ). The controller  216  then determines the head electrode load and a foot electrode load (Act  604 ). Given the head electrode and foot electrode loads, the controller  216  then determines an occupancy test to apply (Act  606 ). 
   When the controller  216  selects a load product occupancy test, the controller  216  proceeds as explained above to form a product of a difference and a sum of load impacts (Act  608 ). The controller  216  also compares the product to upper and lower thresholds to determine seat  100  occupancy, including a front or rear facing child safety seat, and optionally an age characteristic (Act  610 ). 
   On the other hand, when the controller  216  selects a ratio occupancy test, the controller  216  proceeds as explained above to form one or more ratios of the electrode loads (Act  612 ). Based on the result of comparisons against thresholds, the controller  216  may thereby determine vehicle occupancy, including a front or read facing child safety seat, and optionally age information (Act  614 ). 
   When the controller  216  determines that the seat  100  is occupied, the controller  216  may responsively set occupancy indicators in the vehicle (Act  616 ). Thus, for example, the controller  216  may illuminate a warning lamp on the dashboard. As another example, the controller  216  may issue a voice or sound alarm. As yet another example, the controller  216  outputs a signal to an air bag activation or control system, such as the airbag device  220 , to disable airbag activation if appropriate. 
   The vehicle occupancy sensing system  200  provides a mechanism for determining whether a vehicle is occupied. The sensing system  200  may determine that an occupied front or rear facing child seat is present in an automobile and issue appropriate reminders, warnings, and the like. The sensing system  200  may thereby helps reduce occurrences of children unintentionally left in vehicles. 
   Either or both of the ratio and product occupancy tests may be based upon parameter readings (e.g., load current) for a single upper body electrode (e.g., electrode  120 ) and a single lower body electrode (e.g., electrode  122 ). For example, the ratio (FL/HL) may be formed with respect to loading currents obtained solely from the head electrode  120  (HL) and solely from the foot electrode  122  (FL). Alternatively, either of the ratio and product occupancy tests may employ parameter readings obtained over multiple electrodes. Thus, for example, the ratio (FL/HL) may be formed with respect to loading currents obtained from two or more of the electrodes  108 – 122 . For example, the parameter reading HL may be determined from a combination of parameters associated with any two or three of the head electrodes  112 ,  116 , and  120 , while the parameter reading FL may be determined from a combination of parameters associated with any two or three of the foot electrodes  114 ,  118 , and  122 . 
   Furthermore, the product and ratio approaches explained above exemplary approaches that may be taken to determine occupant presence or occupant characteristics. For that reason, alternative tests may be employed, including modified, derivative, or alternate forms of the ratio and product tests. As examples, the product and ratio tests may both be evaluated to determine if they are in agreement, may be extended to include additional variables (e.g., parameter readings other than load current) and functions (such as differences) of the variables, or may be extended to evaluate contributing effects of multiple electrodes or combinations of electrodes or to incorporate readings obtained from humidity, temperature, or other types of sensors. 
   The occupant sensing system  200  determines occupancy characteristics. Thus, in the manner explained above, the sensing system  200  may determine any of occupant presence, occupant facing, and occupant age. The sensing system  200  may, however, be extended to determine additional occupancy characteristics. Thus, for example, the sensing system  200  may be employed to determine occupant size, weight, or other characteristics. 
   It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.