CA2246510C - Vehicle passenger sensing system and method - Google Patents

Abstract

A vehicle passenger sensing system and method in which the presence and position of a passenger is determined by transmitting an electric field (410) from a first electrode (30) and measuring currents induced by the electric field (420, 430) in a plurality of receiver electrodes (40, 50). The induced currents at the various receiver electrodes (40, 50) are then compared to determine the presence of an adult size passenger, to distinguish between a child in a front-facing child safety seat and a child in a rear-facing infant seat, and to detect when a passenger is out of position. A switching circuit (620) is provided to selectively transmit the electric field from each of the plurality of electrodes (511, 512, 516/517, 521, 522).

Description

CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 VEHICLE PASSENGER SENSING SYSTEM AND METHOD

BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention generally relates to vehicle passenger restraint systems, and in particular to a passenger sensing system for detecting the presence and position of passengers within a vehicle.

2. Description of the Prior Art Since their introduction in the late 1980's, airbags have saved lives and reduced the severity of countless injuries resulting from automobile accidents.
However, airbags have also taken lives and caused injuries to very young children when the force of a deploying airbag struck a rear-facing child sa~ety (infant) seat. In response to these tragic events, the National Transportation Safety Board (NTSB) issued a recommendation to the automotive industry to develop llsmart" passenger restraint systems which can detect a rear-facing child safety seat and automatically de-activate adjacent airbags. The present invention isprovided in response to the NTSB's recommendation.
Several passenger sensing systems are currently known. A first sensing system employs a load-dependant switch is mounted under the seat cushion of a vehicle seat. When the seat is occupied by an object of suf~icient weight, the switch is activated to arm the passenger restraint system. However, a disadvantage of this system is that it cannot distinguish between, for example, an occupied child safety seat and a small passenger. Such a system is disclosed in U.S. Patent No. 3,863,209.

CA 02246~10 1998-08-17 W097/30864 PCT~S97102797 A second passenger sensing system uses optical or ultrasonic transmitters and sensors to detect the position and motion o~ a passenger. However, the optical and ultrasonic transmitters and sensors are expensive and dif~icult to install properly within a vehicle cabin. Such a system is disclosed in U.S.
Patent No. 5,446,661.
A third type o~ passenger sensing system utilizes a pair o~ electrodes mounted on a vehicle seat to detect a passenger. With this system, the presence or absence o~ a passenger changes the capacitance between the electrodes. The electrodes are connected to a variable oscillator circuit whose ~requency is changed in response to the electrode capacitance. By measuring the ~requency o~ the variable oscillator circuit and comparing the measured frequency with a threshold value, the presence or absence o~ the passenger is determined. The threshold value is selected such that a passenger restraint system is activated when the detected passenger is larger than a predetermined size, and is de-activated when the detected passenger is smaller than the predetermined size. This capacitive passenger sensing system is disclosed in PCT
Application No. WO 95/21752.
A problem with the ~oregoing variable oscillator passenger sensing system is that airbags are de-activated even in "sa~e" situations, such as when a ~orward-facing child safety seat is used.
Further, with this variable oscillator sensing method, the e~ect o~ an object on the capacitance between the electrodes is substantially independent of its orientation. There~ore, it is dif~icult to use this method to distinguish between, ~or example, a child riding in a ~ront-~acing child seat and a child riding in a rear-~acing infant seat. That is, a child will produce the same variable oscillator ~requency CA 02246~10 1998-08-17 WO97/308G~ PCT~S97/02797 whether the child is riding in a rear-facing infant seat or in a forward-facing child safety seat.
In view of the shortcomings of the prior art passenger sensing systems, what is needed is an economical passenger sensing system and method which can distinguish between a front-facing child seat and a rear~facing infant seat.

SUMMARY OF THE INVENTION
In accordance with the present invention, a vehicle passenger sensing system is disclosed in which the presence and position of a passenger is determined by transmitting an electric field signal from a transmitter electrode and measuring the resulting currents at a plurality of receiver electrodes. The magnitude of the receiver current is affected by the presence of a passenger in the "path" between the transmitter and receiver electrodes. By measuring and comparing the receiver currents at a plurality of receiver electrodes, it is possible to distinguish between a child in a front-facing child safety seat and a child in a rear-facing infant seat, thereby providing "smart" control of a passenger restraint system.
In accordance with a first embodiment of the present invention, first and second electrodes are mounted on a base portion of a vehicle seat. The first electrode is connected to an alternating current signal generator such that an electric field signal is generated in the vicinity of the seat. A third electrode is mounted on a back support portion of the seat. The electric field produces receiver currents at the second and third electrodes which are amplified, then measured and compared by a controller. When the magnitudes of the measured receiver currents indicate the presence of a child in a rear-facing infant seat, the passenger sensing system generates command signals CA 02246~10 1998-08-17 f W097/30864 PCT~S97tO2797 which are used by a passenger restraint system to, for example, de-activate an airbag located adjacent the safety seat such that the airbag does not deploy in the event of a collision. When the magnitudes of the measured electric fields indicate the presence of an a~ult passenger or a fro~t-facing child seat, the passenger restraint system is controlled to activate the airbag.
In accordance with another aspect of the first embodiment, the first, second and third electrodes include a conductive layer separated ~rom a ground plane by an insulating layer. In a first embodiment, the conductive layer includes a woven conductive fabric mounted on a knitted non-conductive fabric, which in turn is mounted on a foam layer. The conductive fabric is connected either to the signal generator or the controller. A metal supporting structure of the vehicle seat (i.e., a metal frame or spring) acts as the ground plane. In a second embodiment, each electrode includes a pair of metal layers separated by an insulating layer. One of the metal layers is connected either to the signal generator or the controller, and the other metal layer is connected to ground.
In accordance with a second embodiment of the present invention, a plurality of electrodes mounted on a vehicle seat and connected to a switching circuit.
The switching circuit connects a selected electrode to a signal generator such that the selected electrode generates an electric field. The switching circuit also connects a first set of the r~ining electrodes to a controller for determining the magnitudes of the electric field at these electrodes. Subsequently, the switching circuit connects another electrode to the signal source and measures the resulting electric ~ield magnitudes at a second set of electrodes. This process CA 02246~10 1998-08-17 is continued in a cycle such that all o~ the electrodes are used as the transmitter electrodes. The passenger restraint system is activated by the controller when the magnitudes o~ the electric ~ields measured during the cycle meet predetermined criteria.
A method for sensing a passenger in a vehicle seat in accordance with the present invention includes transmltting an electrlc field ~rom a ~irst location on the seat, detecting and measuring strengths o~ the electric ~ield at two or more locations on the seat, then comparing the measured strengths to determine the presence and position of a passenger.

BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects o~ the present invention are described in greater detail in the ~ollowing description which re~erences the appended drawings, in which:
Fig. 1 shows a diagram o~ a passenger sensing system in accordance with a ~irst embodiment o~ the present invention.
Fig. 2 shows a cross sectional side view of a ~irst electrode used in accordance with the present invention.
Figs. 3(A) and 3(B) respectively show plan and cross sectional side views of a second electrode used in accordance with the present invention.
Fig. 4 shows a method ~or detec~ting the presence of a passenger in accordance with the ~irst embodiment.
Fig. 5 shows an exploded perspective view o~ a passenger sensing system in accordance with a second embodiment o~ the present invention.
Fig. 6 shows a simplified diagram showing connections between the electrodes and the controller of the passenger sensing system in accordance with the second embodiment.

CA 02246~10 1998-08-17 W097/30864 PCT~S97/02797 Fig. 7 shows a circuit diagram of the controller in accordance with the second embodiment.
Figs. 8(A), 8(~), 8(C) and 8(D) show circuit diagrams of various subsystems of the controller shown in Fig. 7;
Fig. 9 shows a flow diagram of process $or sensing passengers in accordance with the second embodiment of the present invention; and Figs. l0(A), l0(B), lO(C) and lO(D) show simplified diagrams indicating optional electrode arrangements.

DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Fig. l, a passenger sensing/restraint system in accordance with a ~irst embodiment of the present invention is shown in association with a vehicle l0. Although the shape of the vehicle lO may suggest an automobile, the passenger sensing/restraint system of the present invention may also be incorporated into any type of passenger vehicle, including buses, aircraft and boats.
The vehicle l0 includes a seat 20 which is attached to the vehicle l0 using known techniques. The - seat 20 includes a base portion 22 and a back support portion 24. A transmitter electrode 30 and a first receiver electrode 40 are mounted on the base portion 22, with the transmitter electrode 30 being positioned between the receiver electrode 40 and the back support portion 24. A second receiver electrode 50 iB mounted on the back support portion 24.
The transmitter electrode 30 generates an electric field r in response to an alternating current (A.C.) signal received ~rom a signal source (generator) 80 (preferably 50 to l00 k~z). For explanatory purposes, the electric field r is represented in Fig. l with a semi-circular dashed line located over the seat 20--the CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 electric field r is more accurately described as a substantially semi-spherical area located over the transmitter electrode 30.
Electric field "current paths" (indicated as lines r40 and r50 in Fig. 1) extend from transmitter electrode 30 to the receiver electrodes 40 and 50, respectively. In the following description, the current path designations identify the receiver electrode at which a current is measured in response to the electric ~ield signal generated by a transmitter electrode. For example, in Fig. l, path r40 represents the current at electrode 40 produced in response to an electric ~ield transmitted from electrode 3U.
The magnitudes of the receiver currents at the receiver electrodes 40 and 50 are inversely proportional to their distance from the transmitter electrode 30, and are proportional to the area of the electrodes (antennae), the transmission frequency and the signal power. Further, the extent to which a passenger is located in the current paths r40 and r50 ef~ects the magnitudes of the receiver currents. For example, if an adult passenger is sitting on the transmitter electrode 30 and receiver electrode 40, and leaning against the receiver electrode 50, then the receiver currents r40 and r50 are typically at or near a maximum value. However, when a passenger is positioned away from the transmitter electrode 30 but within the current path (such as a child in a child safety seat), the passenger tends to have less of an effect on the electric field such that the receiver current is less than the maximum value, but greater than an "empty seat" value.
In accordance with the present invention, the receiver currents at the receivers 40 and 50 are measured and compared with stored information to distinguish between a normally-seated passenger = = ~
CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 (typically indicated by maximum current values) and a child riding in child safety seat (typically indicated by intermediate current values). Further, by comparing intermediate current values ~or current paths r40 and r50 using predetermined criteria, the present invention provides a method ~or distinguishing between a child in a front-~acing safety seat and a child in a rear-~acing infant seat. For example, when a child is riding in a rear-~acing infant seat, the child's body is typically parallel to the base portion 22 of the seat 20 (see Fig. 1). This arrangement produces a relatively higher current on path r40 in comparison to the current on path r50. Conversely, a child in a forward facing child seat produces a relatively higher current on path r50 in comparison to the current on path r40. By comparing the measured currents, it is possible to determine the type o~ child sa~ety seat being used.
When a rear-facing in~ant seat is detected, the passenger restraint system de-activates adjacent airbags.
Fig. 2 shows a first electrode, Figs. 3(A) and 3(B) disclose a second electrode which may be used in the passenger sensing system according to the present invention. The electrodes disclosed in these ~igures generally include a conductive layer which is separated from a ground plane separated by an insulating layer.
The conductive layer is used both as a transmission antenna and as a reception antenna. The stability of the electric ~ield generated by these electrodes depends on the ground plane and the transmission frequency. With the disclosed electrodes, the transmission frequency should be in the range of 50 to approximately 100 kHz, with a preferred transmission frequency of approximately 100 kHz. The ground plane stabilizes, but weakens, the transmitted/received signal. When a high transmission frequency is used, a CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 low ground plane impedance is necessary. Therefore, an insulating layer provided between the conductive layer and the ground plane to reduce the weakening effect of the ground plane.
Fig. 2 shows a ~irst type of electrode 30, 40, 50 which includes a conductive fabric 231 mounted on a knitted fabric 233, which in turn is mounted on a foam layer 235. When mounted on a vehicle seat, a metal frame or spring arrangement of the vehicle seat serves as the ground plane. In one embodiment, the conductive fabric 231 of each electrode has a length o~ 300 mm and width of 170 mm. A suitable material for use as the conductive fabric 231 is produced by Seiren Co., Ltd.
of Osaka, Japan. The knitted fabric 233 is formed from polyester. The conductive material 231 is attached to the knitted fabric 233 by gluing or sewing.
Figs. 3(A) and 3~B) respectively show plan and side section views of a second type of electrode 30, 40, 50. The receiver electrode includes an upper metal layer 341 (preferably copper foil) formed over an upper insulating layer 342. The upper insulating layer 342 is formed over a lower insulating layer 343 (preferably a sheet of .254 mm thick FR4) and a lower metal layer 344 (preferably .254 mm thick copper). The upper metal layer 341 and upper insulating layer 342 are approximately 300 mm by 50 mm wide, and the lower insulating layer 343 and lower metal layer 344 are approximately 350 mm long and 110 mm wide. The lower metal layer 344 forms the ground plane. The upper insulating layer 342 is preferably 10 mm thick silicon rubber, but can also be another dielectric material such as plexiglass, Teflon or polyamide.
Fig. 4 shows a basic flow diagram illustrating a method for determining the presence o~ a passenger in accordance with the ~irst embodiment of the present invention.

-CA 02246~l0 l998-08-l7 Wo97/30864 PCT~S97/02797 In initial step 410, transmitter electrode 30 transmits the electric field r. During transmission, A.C. signals from the signal source 80 ~see Fig. 1) are applied to the conductive layer 231/341. This causes the conductive layer 231/341 to generate an electric field which is substantially located above the conductive layer 231/341.
In step 4~0, a magnitude o~ the electric field along path r40 causes current to ~low in the conductive layer 231/341 of the receiver electrode 40. These currents are transmitted through a lead ~and preferably via an amplifier, not shown) to the controller 60.
Likewise, in step 430, a magnitude of the electric field along path r40 causes a current in the receiver electrode 50. This current is transmitted ~rom receiver electrode 50 to controller 60.
In step 440, the currents associated with paths r40 and r50 are compared with preset minimum values.
I~ the measured currents ~rom paths r40 and r50 are below the minimum values (indicating an unoccupied seat), then the controller 60 generates a "de-activate~
command signal (step 460). If the measured strengths are greater than the minimum values, then, in step 450, the currents from paths r40 and r50 are compared. If the comparison indicates a rear-~acing in~ant seat (for example, when the measured current on path r50 is less than the measured current on path r40), then the controller 60 generates a "de-activate" command signal (step 460). Conversely, i~ the measured current on path r50 is greater than or equal to the current on path r40, the controller 60 generates an "activate"
command signal.
Re~erring again to Fig. 1, in accordance with the present invention, the "activate" and "de-activate"
command signals generated by the controller 60 are transmitted to a passenger restraint controller 70. ~n CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 response to the "activatel' command signal, the passenger restraint controller 70 activates an airbag 75 in response to a collision signal received, for example, from a collision detector 72. In response to 5 the "de-activate" command signal, the passenger restraint controller 70 ignores collision signals, thereby preventing activation of the airbag 75 in response to a collision signal.
The present invention is not limited to the speci~ic structure described above in the first embodiment. For example, although the transmitter electrode 30 is shown in Fig. 1 as mounted on the seat portion 22, the transmitter electrode 30 may be mounted on the back support portion 24 of the seat 20.
15 Additional variations of the present invention are discussed in the following description.
Fig. 5 shows an exploded perspective view of a passenger sensing arrangement in accordance with a second embodiment of the present invention. The 20 passenger sensing arrangement includes a lower electrode assembly 510 and an upper electrode assembly 520 which are mounted onto a foam seat cushion 530 and a foam back cushion 540, respectively. The foam seat -cushion 530 and foam back cushion 540 are in turn 25 supported by a seat frame 550.
The lower electrode assembly 510 includes a first electrode 511 and a second electrode 512 formed on a knitted ~abric (non-conductive) sheet 513. The first electrode 511 and second electrode 512 are attached via 30 individual conductors provided in a wire harness 514 to a connector 515. The conductors may be attached to the electrodes, for example, using a conductive adhesive or other structure. The first electrode also includes a first half-sized (~ifth) electrode 516 and a second 35 hal~-sized (sixth) electrode 517 which are attached via individual conductors provided in a wire harness 518 to CA 02246~10 lssx-08-17 W097/3086~ PCT~S97102797 a connector 519.
The upper electrode assembly 520 includes a third electrode 521 and a fourth electrode 522 formed on a knitted fabric (non-conductive) sheet 523. The third electrode 521 and fourth electrode 522 are attached via individual conductors provided in a wire harness 524 to a connector 525.
The foam cushions 530 and 540 are provided with trenches for recessing the edges of the sheets 513 and 523 and wire harnesses 514, 518 and 524 to provide a comfortable seat for a passenger. Specifically, a peripheral trench 533 is formed in a seat portion 531 which receives an outer edge of the sheet 513 when the lower electrode assembly 510 is mounted thereon.
15 Similarly, a first wire harness trench 534 and a second wire harness trench 535 are formed in the seat portion 531 and a front portion 532 of the ~oam seat cushion 530, respectively, for recessing the wire harnesses 514 and 518. The foam back cushion 540 similarly includes 20 a peripheral trench 541 which receives an outer edge of the sheet 523 and a wire harness trench 542 for receiving the wire harness 524 when the upper electrode assembly 520 is mounted thereon.
The seat frame 550 includes a base portion 551 and a back portion 552. The base portion 551 includes a plurality of springs 553 ~or supporting ~oam seat cushion 530, and the back portion 552 includes a matrix of springs 554 which support the back cushion 540.
Finally, a controller 555 is fixedly mounted onto the 30 seat ~rame 550 adjacent a front end of the base portion 551. The controller 555 includes a first socket 556 ~or receiving the connectors 515, 519 and 525, and a second socket 557 through which signals are transmitted via a separate conductor (not shown) to, for example, a 35 passenger restraint controller mounted on a vehicle body. A ground line 558 of the controller 555 is CA 02246~10 1998-08-17 W097/30864 PCT~S97/02797 connected to the springs 554, but can also be connected directly to the seat frame 550 (as indicated by the dashed line).
Fig. 6 shows a simplified diagram indicating connections between the controller 555 and the electrodes in accordance with the second embodiment.
The controller 555 includes an A.C. signal generator 610, a switching circuit 620 and a microprocessor or central processing unit (CPU) 630. The switching circuit 620 is connected between the electrodes 511, 512, 521 and 522 and the signal generator 610. The switching circuit 620 selectively connects one of the electrodes 511, 512, 521 and 522 to the signal generator 610 in response to a control signal received ~rom CPU 630. In addition, the switching circuit 620 is connected between the electrodes 511, 512, 516, 517, 521 and 522 and the CPU 630 such that the switching circuit 620 connects one or more o~ these electrodes to the inputs o~ the processor 620 ln response to control signals received from processor 620.
In accordance with the embodiment shown in Figs. 5 and 6, a selected one o~ the electrodes 511, 512, 521 and 522 is connected to the signal generator 610 such that to the selected electrode transmits an electric ~ield, and one or more o~ the remaining electrodes serve as receivers for detecting the electric ~ield strength along selected paths. For example, when electrode 512 is connected by switching circuit 620 to the signal generator 610, electrodes 511, 521 and 522 are connected through switching circuit 620 to the inputs of CPU 630. Similarly, when electrode 521 is connected by switching circuit 620 to the signal generator 610, electrodes 511, 512 and 522 are connected through switching circuit 620 to the inputs o~ CPU 630. By alternating the transmitting electrode in this manner, a total o~ N(N-1) (where N is the CA 02246~10 l99X-08-17 WOg7/30864 PCT~S97/02797 number of electrodes) current path measurements are available ~or sensing passengers. For example, as shown in Fig. 6, the ~our electrodes 511, 512, 521 and 522 provide twelve current path measurements (indicated as double-arrow lines extendin~ between these electrodes). However, it is also possible to obtain measurements ~rom the transmitting electrode, thereby increasing the total number o~ current measurements to N2 (that is, in the example o~ Fig. 6, a total of sixteen current measurements) which may be used to sense the presence/position o~ passengers. There~ore, the passenger sensing system of the embodiment shown in Figs. 5 and 6 provides an advantage over the ~irst embodiment (shown in Fig. 1) in that additional currents (electric ~ield paths) may be measured, thereby increasing the resolution o~ the passenger sensing process and improving the system's ability to avoid undesirable activation o~ an airbag when a rear-~acing in~ant seat is present. Further, the electrodes 516 and 517 may be used to detect an out-o~-position adult passenger, thereby preventing injury caused when an air bag deploys when the adult passenger is too close to the dashboard o~ the vehicle. Utilization of the sensors 516 and 517 is discussed below.
Fig. 7 is a simpli~ied circuit diagram o~ the controller 555 shown in Fig. 6. As discussed above, the controller 555 includes the signal generator 610, the switching circuit 620 and the CPU 630.
In addition, the controller 555 includes a RESET
circuit 703 which is connected between Vcc (5 volts) and the CPU 630. The RESET circuit resets the CPU in response to variations in Vcc or in response to a "runaway" condition of the CPU 630. Speci~ically, the RESET circuit 703 monitors Vcc and transmits a reset signal to the CPU 630 when an unusual reduction in Vcc is detected, thereby preventing a "runaway" condition CA 02246~10 1998-08-17 W097/30864 PCT~S97/02797 of the CPU 630. In addition, the RESET circuit 703 protects against the "runaway" condition at power up caused by instability in Vcc and system clock oscillations. Finally, the RESET circuit 703 acts as a watchdog timer circuit by monitoring a watchdog pulse signal generated by the CPU 630 which indicates normal operation. When the watchdog pulse signal breaks off, the RESET circuit 703 assumes a runaway condition, and transmits a reset signal to the CPU 630.
The A.C. transmission signal (approximately 100 kHz) generated by signal generator 610 optionally passes through a current sensing circuit (SENSE) 705 be~ore being applied to a selected elec~rode. The purpose of the current sensing circuit 705 is to detect the amount o~ current applied to the selected electrode, which varies in response to the presence or absence of a passenger. By detecting the current of the transmission signal, it is possible to detect the sitting position of a passenger in detail without increasing the number of electrodes utilized by the passenger sensing system. A detailed circuit diagram illustrating an example of the current sensing circuit 705 is shown in Fig. 8(A).
Transmission and reception signals are transmitted to and ~rom the electrodes through a first switch array 710 which is controlled by the CPU 630. The electrodes 511, 512, 521 and 522 (see Fig. 6) are connected to terminals Tx/Rxl, Tx/Rx2, Tx/Rx3 and Tx/Rx4 which are connected to one side o~ the first switch array 710.
The transmission signal from current sensing circuit 705 is selectively applied to one of the electrodes 511, 512, 521 and 522 through a first set of switches of the first switch array 710 in response to a transmission control signal from the CPU 630. The electrodes 516 and 517 are connected to terminals Rx5 and Rx6, respectively. The reception signal (current) CA 02246~l0 l998-08-l7 W097/30864 PCT~Sg7/02797 generated in a selected one of the electrodes current sensing circuit 705 iS selectively transmitted through a second set o~ switches of the first switch array in response to a reception control signal generated by the 5 CPU 630.
An amplifier block 720 is connected to the first switch array 710 and converts the reception signal currents generated in the electrodes 511, 512, 521, 522, 516 and 517 into voltage signals V (where V equals the current times 10,000), and then ampli~ies the voltage V using a gain of 6.
The amplified voltage signals are output from amplifier block 720 to a demodulator (DEMOD) circuit 730 and to a phase detection circuit 740.
The demodulator circuit 730 receives the signal from the ampli~ier block 720 and from the current sensing circuit 705. The demodulator circuit 730 changes the level (amplitude value) of the received signal (which is an A.C. signal) to a direct current 20 (D.C.) voltage signal which can be read by the CPU 630.
A detailed circuit diagram illustrating an example of the demodulator circuit 730 iS shown in Fig. 8(B).
The phase detection circuit 740 receives the reception signal from the amplifier block 720 and the 25 transmission signal ~rom the current sensing circuit 705. The phase detection circuit 740 detects the difference of the reception signal relative to the transmission signal. This enables detection of the sitting position of a passenger in detail without 30 increasing the number of electrodes utilized by the passenger sensing system. A detailed circuit diagram illustrating an example of the phase detection circuit 740 iS shown in Fig. 8(C). Transmission and reception signals are converted into square wave signals. The 35 rising edge of the transmission signal triggers the clock input o~ D-type flip flop ICl and generating a CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 high output signal. The rising edge of the reception signal causes flip-flop IC2 to generate a one-shot low signal which is applied to the RESET terminal of ICl through an inverter. This causes IC1 to generate a low output. As a result, IC1 generates an output signal representing a phase difference between the transmission and reception signals which is applied to the CPU 630 through an integrator.
The D.C. signal output from the demodulator circuit 730 is transmitted through a fine amplifier 750 and a coarse amplifier 760. A gain of the fine amplifier 750 is controlled by the CPU 630 through an offset exchange circuit 755. This allows detection of minute variations in the reception signal which provide more detailed information regarding a passenger's position on the vehicle seat. A detailed circuit diagram illustrating an example of the fine amplifier 750 and offset exchange circuit 755 is shown in Fig.
8(D).
Finally, output signals from the phase detection circuit 740, fine amplifier 750 and coarse amplifier 760 are selectively transmitted to the CPU 630 through a second switch array 770, which is controlled by the - CPU 630.
The CPU 630 communicates with the passenger restraint system (not shown) on lines 632 in response to the signals applied through the second switch array 770. The CPU 630 may also generate LED control signals on lines 631 which may be used to indicate operating conditions, and communicate with an EEPROM 635 on bus 633. A preferred controller 630 is a ~PD78052CG
microprocessor manufactured by NEC Corporation of Tokyo, Japan. Of course, the functions of controller 630 may be implemented by another digital processor.
Further, the functions of switching circuit 620 may be implemented in an application specific integrated CA 02246~10 1998-08-17 Wo 97/3086~ PCT/US97102797 circuit (ASIC).
As mentioned above, the controller 630 transmits control signals to the switching circuit 620, thereby connecting one of the electrodes to the signal generator 610, and connecting the reception signals on one or more of the rern~1n;ng electrodes to the inputs of the controller 630. The reception signals are converted into eight bit digital signals whose value is determined by the magnitude of the current (in proportion to a selected maximum current value). These eight bit digital signals are then subjected to mathematical operations and compared with stored data to determine whether the seat upon which electrodes are mounted is empty (unoccupied), occupied by a normally-seated passenger, occupied by a child in a rear-facing inEant seat, or occupied by a child in a front-:Eacing child safety seat or a booster seat. The stored data used to determine the occupancy o:E the seat is stored in EEPROM 635, which also stores instruction codes used by the controller 630. Once this determination is complete, the controller 630 generates passenger restraint command signals which are transmitted to a passenger restraint system (not shown) over lines 632.
Fig. 9 shows a flow diagram indicating a flow diagram illustrating a preferred method of controlling a passenger restraint controller in accordance with the second embodiment of the present invention. In the following description, the current path designations identify the transmitting electrode and receiving at which a current is measured. For example, path rl2 represents the current at electrode 512 produced in response to an electric field transmitted :Erom electrode 511 ~refer to Fig. 5 for electrode placement).
First, in step 901, the system is initialized by, for example, sending a reset command from the RESET

CA 02246~10 1998-08-17 W097/30864 PCT~S97102797 circuit 703 to the CPU 630 (see Fig. 7).
Next, in step 910, the CPU 630 controls the first switch array 710 to form selected pairs of the electrodes 511, 512, 521 and 522, each selected pair being formed by a transmitting electrode connected to the signal generator 610 and a receiving electrode from which currents are measured. Specifically, a current path rl2 is measured by transmitting from electrode 511 and measuring the resulting current in electrode 512.
Similarly, current paths rl3 (transmit from electrode 511, receive at electrode 521), r21 (transmit from electrode 512, receive at electrode 511), r24 (transmit from electrode 512, receive at electrode 522), and r31 (transmit from electrode 521, receive at electrode 511) are measured. Each of these measurements is compared with a first set of threshold values TP1 and TP2 which are experimentally determined and stored in EEPROM 635 or calculated using a first or second degree function.
A person is ~'detected" (yes branch of step 910) when the following logical expression is satisfied:
(rl2>TP1) AND ~rl3~TP1) AND (r21~TP1) AND (r24~TP2) AND
(r31~TP1). If a person is "detected", then control passes to optional step 915 which may include, for example, indicating detection using an ~ED or other indicator. Control is then passed to step 960 in which a "deploy" signal is transmitted from the CPU 630 to the passenger restraint system, thereby arming an airbag for deployment in the event of a collision. If the above logical expression is not satisfied, control is passed to step 920.
In step 920, the CPU 630 controls the first switch array 710 to form a second set of selected pairs of the electrodes 511, 512, 521 and 522. Specifically, measurements are taken on current paths r43 (transmit from electrode 522, receive at electrode 521), r42 (transmit from electrode 522, receive at electrode CA 02246~l0 Is9X-08-l7 W097/30864 PCT~S97/02797 512), r41 (transmit ~rom electrode 522, receive at electrode 511), r24 (transmit ~rom electrode 512, receive at electrode 522), r21 (transmit ~rom electrode 512, receive at electrode 511), rl4 (transmit ~rom electrode 511, receive at electrode 522), and rl2 (transmit ~rom electrode 511, receive at electrode 512). Each o~ these measurements is compared with a second set o~ threshold values TF1, TF2, TF3, TF4, TF5, TF6, TF7 and TF8 which are experimentally determined and stored in EEPROM 635 or calculated using a ~irst or second degree ~unction. A ~ront-~acing child seat (FFCS) is "detected" (yes branch o~ step 920) when the following logical expression is satis~ied: ~r43~TF1) AND (r42~TF2) AND (r41~TF3) AND ((r41-r42~TF4) OR (r41-r43>TF5) OR (r42-r43~TF6)) AND (r24-r21~TF7) AND (rl4-rl2~TF8) If a ~ront-~acing child seat is "detected", then control passes to optional step 925 which may include, ~or example, indicating detection using an ~ED
or other indicator. ~ontrol is then passed to step 960 in which a "deploy" signal is transmitted ~rom the CPU
630 to the passenger restraint system, thereby arming an airbag for deployment in the event of a collision.
I~ the above logical expression is not satis~ied, control is passed to step 930.
In step 930, the CPU 630 controls the ~irst switch array 710 to ~orm a third set o~ selected pairs of the electrodes 511, 512, 521 and 522. Speci~ically, measurements are taken on current paths ri3 ~transmit ~rom electrode 511, receive at electrode 521), r31 (transmit ~rom electrode 521, receive at electrode 511), rl2 (transmit ~rom electrode 511, receive at electrode 512), r21 (transmit ~rom electrode 512, receive at electrode 511), r34 (transmit ~rom electrode 521, receive at electrode 522), and r43 (transmit ~rom electrode 522, receive at electrode 521). Each o~
these measurements is compared with a third set o~

CA 02246~l0 l998-08-l7 W097/30864 PCT~S97/02797 threshold values TB1, TB2 and TB3 which are experimentally determined and stored in EEPROM 635 or calculated using a first or second degree function. A
booster seat is "detected" (yes branch of step 930) when the following logical expression is satisfied:
(rl3>TB1) AND (r31>TB1) AND (rl2>TB2) AND (r21>TB2) AND
(r34>TB3) AND (r43>TB3). If a booster child seat is "detected", then control passes to optional step 935 which may include, for example, indicating detection using an LED or other indicator. Control is then passed to step 960 in which a "deploy" signal is transmitted from the CPU 630 to the passenger restraint system, thereby arming an airbag for deployment in the event of a collision. If the above logical expression i~ not satisfied, control is passed to step 940.
In step 940, the CPU 630 controls the first switch array 710 to form a fourth set of selected pairs of the electrodes 511, 512, 521 and 522. Specifically, measurements are taken on current paths rl2 (transmit from electrode 511, receive at electrode 512), rl3 (transmit from electrode 511, receive at electrode 521), rl4 (transmit from electrode 511, receive at electrode 522), r31 (transmit from electrode 521, receive at electrode 511), and r43 (transmit from electrode 522, receive at electrode 521). Each of these measurements is compared with a fourth set of threshold values TR1, TR2, TR3, TR4 and TR5 which are experimentally determined and stored in EEPROM 635 or calculated using a first or second degree ~unction. A
rear-~acing infant seat (RFIS) is "detected" (yes branch of step 940) when the following logical expression is satisfied: (rl2>TR1) AND (rl3~TR2) AND
(rl4>TR3) AND (rl3-rl4~TR4) AND (r31-r43~TR5). I~ a rear-facing infant seat is "detected", then control passes to optional step 945 which may include, for example, indicating detection of the rear-facing infant CA 02246~l0 l998-0X-l7 W097/3086~ PCT~S97/02797 seat using an LED or other indicator. Control is then passed to step 970 in which a "non-deploy" signal is transmitted ~rom the CPU 630 to the passenger restraint system, thereby disarming an airbag adjacent the vehicle seat in the event of a collision. If the above logical expression is not satisfied, control is passed to step 950.
In step 950, the CPU 630 controls the first switch array 710 to form a fifth set of selected pairs o~ the electrodes 511, 512, 521 and 522. Speci~ically, measurements are taken on current paths rl2 (transmit from electrode 511, receive at electrode 512), rl3 (transmit from electrode 511, receive at electrode 521), rl4 (transmit ~rom electrode 511, receive at electrode 522), r24 (transmit from electrode 512, receive at electrode 522), and r34 (transmit ~rom electrode 521, receive at electrode 522). Each of these measurements iB compared with a ~ifth set of threshold values TEl, TE2, TE3, TE4, TE5 and TE6 which are calculated using a first degree function (e.g., y=ax~b) or a second degree ~unction (e.g., y=ax2+bx+c), and a sixth set of threshold values TEll, TE22, TE33, TE44, TE55 and TE66 which are stored in EEPROM 635. An empty seat is "detected" (yes branch of step 950) when the ~ollowing logical expression is satisfied:
(ABS(rl2-TEl)cTEll) OR (ABS(rl2-TE2)cTE22) AND
(ABS(rl3-TE3)~TE33) AND (ABS(rl4-TE4)cTE44) AND
(ABS(r24-TE5)cTE55) AND (ABS(r34-TE6)<TE66), where "ABS(r(i,j)-TE(k))" means the absolute value o~ r(i,]) minus TE(k). If an empty seat is "detected", then control passes to optional step 953 which may include, for example, indicating detection o~ the empty seat using an LED or other indicator. Control is then passed to step 970 in which a "non-deploy" signal is transmitted from the CPU 630 to the passenger restraint system, thereby disarming an airbag ad~acent the CA 02246~10 1998-08-17 W097/30864 PCT~S97/02797 vehicle seat in the event of a collision. If the above logical expression is not satisfied, control is passed to optional step 957 which may include, for example, indicating detection that something other than an empty - 5 seat has been detected.
After the CPU 630 sends a "deploy" command (step 960) or a l~non-deploy" command (step 970), control is passed again to step 910 and the process is repeated.
In this way, movement and or shifting of passengers is detected, thereby preventing undesirable deployment of an airbag, for example, when a passenger is leaning forward.
The above-described process only utilizes electrodes 511, 512, 521 and 522 to detect the presence of adult passengers, front-facing child seats, booster seats, rear-facing infant seats, empty seats or "something". Further resolution may be obtained using more sophisticated combinations of electrode pairs in order to detect the precise location of a passenger, thereby disa~ling the passenger restraint system when the passenger is in a position in which deployment may cause injury. For example, if a passenger is seated at an extreme forward position o~ the vehicle seat, deployment of an airbag may injure the passenger.
There~ore, step 957 o~ the above process may be modi~ied to verify the passenger's position by taking measurements on current paths rl5 (transmit ~rom electrode 511, receive at electrode 516) and rl6 (transmit from electrode 511, receive at electrode 517) (refer to Fig. 5). This information can be used to verify the passenger's extreme-forward position, and may be used, for example, to transmit an instruction telling the passenger to move back into a proper position on the seat so that the passenger restraining system will deploy.
Although the present invention has been described CA 02246~10 1998-OX-17 W097/30864 PCT~S97/0~797 in considerable detail with re~erence to certain pre~erred embodiments thereof, other embodiments are possible. For example, as shown in Figs. lO(A) through lO~D), one or more electrodes 518 and 519 may be positioned, on the dashboard (Figs. lO(A) and lO(B)), the dashboard and headrest (Fig. lO(C), or the dashboard and floor (Fig. lO(D) to provide additional resolution ~or detecting out-o~-position passengers who may be injured by activation o~ an airbag. There~ore, the spirit and scope o~ the appended claims should not be limited to the description of the pre~erred embodiments contained herein.

Claims (7)

WHAT IS CLAIMED IS:

1. ~A vehicle passenger sensing system comprising:
a signal generator for generating an alternating current signal;
a first electrode mounted on the vehicle and connected to the signal generator, the first electrode generating an electric field in response to said alternating current signal;
a second electrode mounted on the vehicle and spaced from the first electrode such that a first current is generated in the second electrode in response to the electric field and an object in the vehicle; and a controller for measuring the first current, and for generating a restraint system command signal in response to a magnitude of the first current.

2. ~The passenger sensing system according to Claim 1, wherein the seat includes a metal support member forming a ground plane; and wherein each of the first and second electrodes comprises a conductive layer formed on an insulating layer, the insulating layer being disposed between the conductive layer and the metal support member such that the conductive layer is electrically insulated from the metal support member.

3. ~The passenger sensing system according to Claim 1, wherein each of the first and second electrodes comprises first and second metal layers separated by an insulation layer, the first metal layer being selectively connected to one of the controller and the signal generator, and the second metal layer being connected to ground.

4. ~The passenger sensing system according to Claim 1, further comprising:
a third electrode mounted on the seat and spaced from the first and second electrodes, wherein a second current is generated in the third electrode in response to the electric field;
wherein the processor is connected to the third electrode and generates the command signal in response to a comparison between the first current and the second current.

5. ~The passenger sensing system according to Claim 4, wherein the second electrode is mounted on a base portion of the seat, and the third electrode is mounted on a back support portion of the seat.

6. ~The passenger sensing system according to Claim 4, wherein the controller further comprises a switching circuit for selectively connecting one of the first electrode and the second electrode to the signal generator, and for selectively connecting one of the first electrode and the second electrode to an input terminal of the processor.

7. ~A method for sensing a passenger in a vehicle seat, the method comprising the steps of:
transmitting an electric field from a first location on the vehicle seat by applying an alternating current to a first electrode located at the first location;
measuring a first current induced by the electric field in a second electrode located at a second location on the vehicle seat; and