United States Patent [19J
`Blackburn et al.
`
`[54] OCCUPANT SENSING APPARATUS
`[75]
`Inventors: Brian K. Blackburn, Rochester;
`Joseph F. Mazur, Washington; Scott
`B. Gentry, Shelby Township,
`Macomb County, all of Mich.
`[73] Assignee: TRW Vehicle Safety Systems Inc.,
`Lyndhurst, Ohio
`[21] Appl. No.: 682,908
`[22] Filed:
`Apr. 9, 1991
`Int. a.s .............................................. B60R 21/32
`[51]
`[52] u.s. Cl ..................................... 280/732; 280/735;
`180/273
`[58] Field of Search ............... 280/728, 730, 731, 732,
`280/735; 180/273
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,172,684 3/1965 Isaac ............................... 280/735 X
`3,340,523 9/1967 Whitman .
`3,672,699 6/1972 DeWindt .
`3,746,844 7/1973 Azurn eta!. .
`3,992,028 11/1976 Abe et a!. .
`4,700,180 10/1987 Vance .
`4,796,013 1/1989 Yasuda eta!. .
`4,804,859 2/1989 Swart .
`4,810,005 3/1989 Foh1 .
`4,833,457 5/1989 Graebe, Jr. .
`4,876.419 10/1989 Lodini .
`4,876,420 10/1989 Lodini .
`5,071,160 12/1991 White .................................. 280/735
`5,071,160 12/1991 White eta!. .
`5,074,583 12/1991 Fujita .................................. 280/735
`5,079,949 1/1992 Tarnori .
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US005232243A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,232,243
`Aug. 3, 1993
`
`FOREIGN PATENT DOCUMENTS
`0357225 3/1990 European Pat. Off ..
`3839134C1 2/1990 Fed. Rep. of Germany .
`
`OTHER PUBLICATIONS
`Interlink Electronics "Force and Position Sensing Re(cid:173)
`sistors: An Emerging Technology".
`Tekscan Corporate Capabillities.
`Kynar Piezo Film Technical Manual.
`Primary Examiner-Richard A. Schacher
`Attorney, Agent, or Firm-Tarolli, Sundheim & Covell
`ABSTRACT
`[57]
`An occupant sensing apparatus for use in an occupant
`restraint system comprises a fllm having an electrical
`characteristic with changeable states and a contact
`member. When an occupant sits in a vehicle seat, the
`contact member pushes on the film and changes the
`state of the electrical characteristic. An electric circuit
`is connected to the film for providing a first signal when
`the film indicates an occupant is not present, a second
`signal when an occupant is present, and a third signal if
`the film and the circuit become disconnected. The occu(cid:173)
`pant restraint system is enabled when either an occu(cid:173)
`pant is present or an electrical fault condition occurs. In
`accordance with another embodiment of the invention,
`an array of sensors located in the seat determines the
`occupant's position and weight and controls deploy(cid:173)
`ment of the occupant restraint system in response to the
`determined position and weight.
`
`9 Claims, 14 Drawing Sheets
`
`20"'
`
`IPR2016-00291 - Ex. 1012
`Toyota Motor Corp., Petitioner
`
`1
`
`

`
`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 1 of 14
`
`5,232,243
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`Aug. 3, 1993
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`
`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 3 of 14
`
`5,232,243
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`U.S. Patent
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`Aug. 3, 1993
`
`Sheet 5 of 14
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`U.S. Patent
`
`Aug. 3, 1993
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`Sheet 6 of 14
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`

`
`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 10 of 14
`
`5,232,243
`
`266
`
`BACK SENSOR ,
`
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`
`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 11 of 14
`
`5,232,243
`
`0:::
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`F~j5~~~~--~~--~----~
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`
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`
`FRONT
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`
`PERFECT
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`
`BACK
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`
`12
`
`

`
`. U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 12 of 14
`
`5,232,243
`
`402
`
`N
`
`DISABLE
`AIR BAG
`
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`
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`
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`
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`
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`
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`
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`
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`
`Fig.20
`
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`

`
`1
`
`5,232,243
`
`OCCUPANT SENSING APPARATUS
`
`TECHNICAL FIELD
`The present invention is directed to a vehicle occu(cid:173)
`pant restraint system and is particularly directed to an
`occupant sensing apparatus in an occupant restraint
`system.
`
`2
`ence of an occupant effects physical contact between
`the contact member and the film. The electrical charac(cid:173)
`teristic of the film has a first state when there is no
`contact between the film and contact member and a
`5 second state when there is contact between the film and
`the contact member. Electric circuit means are con(cid:173)
`nected to the film for providing a first signal when the
`film has the first state indicative of an occupant not
`being present, a second signal when the film has the
`10 second state indicative of an occupant being present,
`and a third signal when the film does not have the first
`or second state, thereby indicating that an electrical
`fault condition exits. The sensor further includes means
`for enabling the occupant restraint system when either
`the second signal or the third signal is provided by the
`electric circuit means.
`In accordance with a preferred embodiment, the film
`is a piezoelectric film. The electric circuit means pro(cid:173)
`vides an unstable feedback loop around the piezoelec(cid:173)
`tric film so that the film provides an electric output
`signal that oscillates. Contact between the contact
`member and the film effects a change in the output
`signal from the film. The electric circuit means provides
`the first, second, and third signals in response to the
`output signal from the film. Contact between the
`contact member and the film attenuates the oscillations.
`The oscillations are preferably within a predetermined
`frequency oand and the electric circuit means includes a
`band pass filter designed to respond to the predeter(cid:173)
`mined frequency band. The predetermined frequency is
`2 kilohertz. Preferably, a plurality of such piezoelectric
`films are mounted in a checker-board pattern with a
`plurality of individual force sensing resistor films so as
`to monitor weight and position of the occupant.
`
`BACKGROUND OF THE INVENTION
`Occupant restraint systems for use in vehicles are
`well known in the art. One such restraint system in-.
`eludes a crash sensor, an inflatable airbag, and an actua(cid:173)
`tion circuit that controls deployment of the airbag in
`response to an output from the crash sensor. The crash 15
`sensor can be an accelerometer that provides an electri(cid:173)
`cal signal having a value functionally related to the
`vehicle's deceleration. The actuation circuit includes a
`squib operatively connected to a source of inert gas.
`During a crash condition of a vehicle, the vehicle's 20
`accelerometer provides a signal indicative of such crash
`condition. The actuation circuit thereby applies a cur(cid:173)
`rent through the squib which causes the squib to ignite.
`When the squib ignites, the source of inert gas dis(cid:173)
`charges gas into the airbag, which results in inflation of 25
`the airbag.
`Certain vehicles have both a driver side airbag and a
`passenger side airbag ("dual airbags"). If such a vehicle
`is occupied only by the driver and is involved a crash,
`deployment of the passenger side airbag is unnecessary. 30
`Unnecessary deployment of the passenger side airbag
`can increase the cost of repairing the vehicle. Since a
`large percentage of vehicles on the highway are occu(cid:173)
`pied by only the driver, it is desirable to (i) be able to
`detect if a passenger is present in the vehicle and (ii) 35
`deploy the passenger side airbag during a crash only if
`the passenger is, in fact, present.
`The prior art has recognized the problem of unneces(cid:173)
`sary deployment of a passenger side airbag during a
`crash when no passenger is present. Prior art occupant 40
`restraint systems having dual airbags have provided
`various forms of occupant sensors and have enabled the
`passenger side airbag only when an occupant is present
`in the passenger seat.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`Further features of the present invention will become
`apparent to those skilled in the art to which the present
`invention relates from reading the following specifica(cid:173)
`tion with reference to the accompanying drawings, in
`which:
`FIG. 1 is a schematic of an occupant restraint system
`made in accordance with one embodiment of the pres-
`45 ent invention;
`FIG. 2 is top plan view of a film sensor made in ac(cid:173)
`cordance with the present invention with certain parts
`removed for clarity;
`FIG. 3 is a sectional view along line 3-3 of FIG. 2;
`FIG. 4 is an exploded view of the film sensor shown
`in FIG. 2;
`FIG. 5 is a circuit schematic of an occupant restraint
`system made in accordance with one embodiment of the
`present invention;
`FIG. 6 is a circuit schematic showing the film drive
`circuit shown in FJG. 5;
`FIG. 7 is a circuit schematic showing the band pass
`filter circuit shown in FIG. 6;
`FIG. 8 is a graphical representation of the output of
`the drive circuit shown in FIG. 6;
`FIG. 9 is a schematic of an occupant restraint system
`made in accordance with another embodiment of the
`present invention;
`FIG. 10 is a top plan view of the occupant position
`and weight sensor shown in FIG. 9;
`FIG. 11 is a side sectional view of the occupant posi(cid:173)
`tion and weight sensor taken along line 11-11 of FIG.
`10;
`
`SUMMARY OF THE INVENTION
`The present invention is directed to a new and useful
`occupant sensing apparatus. An occupant sensing appa(cid:173)
`ratus, accordance with the present invention, permits
`detection of (i) whether a vehicle seat is occupied, (ii) 50
`whether an object in the seat is animate or inanimate,
`(iii) the weight of an occupant, (iv) the position of an
`occupant in the seat, and (v) whc~cher there is an electri(cid:173)
`cal fault condition such as an electric open or short
`circuit in the occupant sensor. An occupant restraint 55
`system, having the occupant sensing apparatus of the
`present invention, is enabled if either an occupant is
`present or an electrical fault condition exists in the oc(cid:173)
`cupant sensor. The passenger side airbag is positioned,
`i.e., aimed, in response to the detected position of the 60
`occupant. The deployment of the airbag is controlled in
`response to the weight and position of the occupant.
`In accordance with one embodiment of the present
`invention, an occupant sensing apparatus for an occu(cid:173)
`pant restraint system comprises (a) a film having an 65
`electric characteristic with changeable states, and (b) a
`contact member. Means ar.e provided for mounting the
`film and the contact member in a vehicle so that pres-
`
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`3
`4
`bottom plate 92 is rigidly mounted to a substantially
`FIG. 12 is a circuit schematic of a portion of the
`occupant restraint system of FIG. 9;
`inflexible bottom portion of the seat 42.
`The bottom plate 92 and the top plate 94 include
`FIG. 13 is a circuit schematic of a portion of the
`interior surfaces 96, 98, respectively, which are spaced
`occupant restraint system of FIG. 9;
`FIG. 14 is a graph of resistance of the weight sensor 5 apart. An annular sealing member or gasket 100 is
`shown in FIG. 9;
`mounted between the support plate 92 and the upper
`FIG. 15 is a graph of the change of resistance of the
`plate 94. The sealing member 100 includes a central
`weight sensor as a function of the weight of the occu-
`opening 106. The plates 92, 94 are secured together by
`pant;
`means of a plurality of spaced apart screws 102. Specifi-
`FIG. 16 is a graph of the percentage of gas dis- 10 cally, the top plate 94 has holes 103 and the bottom plate
`92 has threaded holes 104. The gasket 100 has corre-
`charged from the airbag shown in FIG. 9 as a function
`of distance between the occupant and the dashboard or
`sponding holes 105. The screws 102 extend through the
`as a function of the weight of the occupant;
`holes 103 and lOS and are screwed into the holes 104.
`FIG. 17 is a graph depicting the angular position of
`The bottom plate 92, the upper plate 94, and the sealing
`the airbag shown in FIG. 9 as a function the occupant 15 member 100 form a chamber 107 in which a piezoelec-
`tric ftlm sensor 110 is mounted.
`leftward and rightward position in the vehicle seat;
`FIG. 18 is a graph depicting the angular position of
`The bottom plate 92 includes a recessed portion 112.
`the airbag shown in FIG. 9 as a functions the occupant
`The piezoelectric film sensor 110 spans the recess 112
`forward or rearward position in the vehicle seat;
`and is secured along opposed edges at locations 114, 116
`FIG. 19 is a graphical representation of the output of 20 by appropriate means, such as adhesive. The bottom
`plate 92 further includes a second recessed portion 120.
`the humanistic sensor shown in FIG. 9;
`FIGS. 20-22 are flow charts depicting the control
`A film drive circuit 122 is secured to a substrate 124
`process followed by the controller shown in FIG. 9;
`which is mounted in the recess 120 by appropriate
`and
`means. The drive circuit 122 is operatively connected to
`FIG. 23 is a top plan view of a position and weight 25 the piezoelectric film sensor 110 through terminals 126.
`sensor in accordance with another embodiment of the
`Electrical wires 128 connect the drive circuit 122 to the
`controller 36.
`present invention.
`The piezoelectric film sensor 110 is a multi-layer
`DESCRIPTION OF PREFERRED EMBODIMENT
`structure including a first piezoelectric layer 130 and a
`Referring to FIG. 1, an occupant restraint system 20 30 second piezoelectric layer 136 separated by an insulat-
`ing layer 138. The two piezoelectric layers and the
`includes an airbag assembly 22 mounted in an opening
`of a dashboard 24 of a vehicle. The airbag assembly 22
`insulating layer are secured together in an integral as-
`includes a airbag 26 folded within the interior of an
`sembly by appropriate means. Each layer 130, 136 has
`airbag housing 28. A cover 30 covers the airbag and is
`an associated electrode silk-screened on each side of the
`adapted to open easily upon inflation of the airbag 26. 35 layer. The electrodes of each piezoelectric layer 130,
`A source 32 of inert gas is mounted at the back of the
`136, is connected to an associated terminal126. Specifi-
`housing 28 and is operatively connected to the airbag
`cally, the top electrode for layer 130 is connected to
`26. Gas discharged from the source 32, which may be
`terminal 126'. The bottom electrode for layer 130 is
`generated by combustion of pyrotechnic material or
`connected to terminal126". The top electrode for layer
`simply released from a pressurized container, fills the 40 136 is connected to termina1126". The bottom electrode
`for layer 136 is connected to terminal 126'".
`airbag 26 to its inflated condition 26'. Once inflated, as
`occurs during a vehicle crash, the airbag 26 cushions an
`When energized by the film drive circuit 122, par-
`occupant located in a vehicle seat 34.
`tions of the piezoelectric layers, under their associated
`An electronic controller 36, such as a microcom-
`electrodes, flex. The drive circuit 122 is adapted so that
`puter, is operatively connected to a vehicle crash sensor 45 the piezoelectric film sensor 110 oscillates. The film
`38. The crash sensor 38 can be any of several known
`oscillation physically occurs over the recessed portion
`types. For example, the crash sensor 38 may be a me-
`112 of the bottom plate 92. This area of the film sensor
`chanica) inertia switch, such as a rolamite sensor, or an
`110 is referred to as the bridge.
`electrical accelerometer. If a normally open inertia
`The upper plate 94 includes an contact arm 140 that
`switch closes, this is an indication that a crash is occur- 50 extends normal to the plate 94 and the surface 98
`ring. Likewise, if a signal from an electrical accelerome-
`toward the film sensor 110. The upper plate 94 is made
`ter reaches a predetermined level or a predetermined
`from a flexible material. When an occupant sits on the
`level for a predetermined time, this is an indication that
`seat cushion 42, his weight is transferred through the
`a crash is occurring. Once the controller 36 determines
`material that makes up the seat cushion to the upper
`that a vehicle crash is occurring for which deployment 55 plate 94. The force applied to the upper plate 94 is
`of the airbag is necessary to protect the vehicle occu-
`functionally related to the occupar•'s weight. Since the
`pants, the controller 36 ignites squib 40 which, in turn,
`bottom plate 92 is rigidly secured relative to the vehicle
`actuates the gas source 32.
`floor, the upper plate 94 flexes in response to the occu-
`The occupant seat 34 with which the occupant re-
`pant's weight transferred through the seat material. As
`straint system 20 is used is preferably the passenger seat 60 the upper plate 94 flexes, the contact arm 140 moves
`in the vehicle. The seat 34 includes an occupant sensor
`toward the layers 130, 136 of fllm sensor 110.
`60 located in the bottom cushion 42 of the seat 34. The
`When no occupant is sitting on the seat cushion and
`occupant sensor 60 is used to control whether deploy-
`the film layers are energized by the film drive circuit
`ment of the airbag is to be prevented.
`122, the film oscillates. When the fllm oscillates, the
`FIGS. 2-4 show one embodiment of the present in- 65 output of the drive circuit oscillates with a certain peak-
`vention, including an occupant presence sensor 60. The
`to-peak voltage value. If an occupant who has a weight
`sensor 60 includes a housing 90 which incorporates a
`greater than a predetermined weight then sits on the
`bottom support plate 92 and a top cover plate 94. The
`seat cushion, the upper plate 94 flexes an amount suffi-
`
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`5,232,243
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`15
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`5
`6
`placed upon the substrate 124 so as to be close to the
`cient to insure that the contact arm 140 contacts the film
`film 110.
`110. If the arm 140 contacts the film 110, the oscillations
`The output 148 is connected to the input of the con-
`of the film 110 are attenuated. The resultant output from
`troller 36. The controller 36 includes a bandpass filter
`the film is a DC voltage value equal to approximately
`one-half the peak-to-peak voltage when no occupant is 5 150, of standard design well known in the art, con-
`nected to the output 148 of the film drive circuit 122.
`sitting on the seat cushion. By monitoring the output of
`the film 110, the controller is provided with an indica-
`The bandpass filter 150 is designed, in accordance with
`tion as to whether an occupant is located on the seat
`a preferred embodiment of the present invention, to
`cushion 60.
`pass frequencies of 2 kilohertz. The bandpass fllter 150
`FIGS. 5, 6 and 7, show a processing circuit, made in w· may be external to the sensor 60 or can be placed upon
`accordance with one embodiment of the present inven-
`the substrate 124 along with the drive circuit. As shown
`tion. In this embodiment, the occupant sensor is the
`sensor 60. The sensor 60 is used to determine whether
`in FIG. 8, the output 152 of the bandpass filter is an
`an occupant is sitting on the seat cushion 42. The con-
`oscillating voltage between zero volts DC (battery
`ground) and a voltage value V + (the positive battery
`troller 36 monitors the output signal from the sensor 60.
`The controller 36 enables actuation of the airbag when
`terminal) at a frequency of 2 kilohertz. The time period
`either the signal from the sensor 60 indicates that an
`P between pulses is equal to 0.5 msec., which is one
`occupant is sitting in the seat or the absence of a signal
`divided by the frequency of 2 kilohertz. The line 154
`from the sensor 60 indicates that an electrical fault con-
`shown in FIG. 8 represents force or stress against the
`dition exists such as an open or short circuit between the 20 top plate 94 of the sensor 60 as a result of an occupant
`sitting on the seat cushion 62. At time t1. the occupant's
`sensor 60 and the controller 36.
`weight deflects the top plate 94 a sufficient amount so
`FIG. 6 shows the drive circuit 122 in detail. The top
`electrode of piezoelectric layer 130
`is connected
`that the contact arm 140 contacts the film 110. When
`through terminal 126' to the inverting input of an op
`contact occurs between the arm 140 and the film 110,
`amp 144, connected as a differentiator, through a resis- 25 the output of the bandpass filter changes to a DC volt-
`age equal to approximately one-half V +, as shown by
`tor. The differentiator with op amp 144 has a feedback
`the line 168. The voltage goes to one-half V + because
`resistor 145. The output of the op amp 144 is connected
`to the inverting input of an op amp 142, connected as an
`the common ground of circuits 122, 150 is at one-half
`V +. Specifically, V + is the positive terminal of the
`amplifier, through an input resistor 147. The amplifier
`142 includes a feedback resistor 143. The output of the
`vehicle battery and chassis ground is the negative termi-
`amplifier is connected to the bottom electrode ofpiezo- 30 nal of the vehicle battery. Common ground for the
`electric layer 136 through terminal 126"'. The bottom
`drive circuit 122 and the band pass filter 150 is at one-
`electrode of piezoelectric layer 130 and the top elec-
`half the value of the battery voltage as is shown in FIG.
`trode of piezoelectric layer 136 are connected to a
`6. Therefore, when the oscillations of the piezoelectric
`source of electrical energy - V cc through terminal126".
`layers stop, the output of the band pass filter and the
`The noninverting inputs for the amplifiers 142, 144 are 35 drive circuit go to their common ground value, which is
`one-half v +, i.e., 6 VDC.
`connected to a common ground having a voltage value
`between + V cc and - V cc established through a resistor
`A counter 158, also part of the controller SO, is con-
`divider network 149 with equal value resistors. If the
`nected to the output 152 of the bandpass filter 150 and
`vehicle battery is used as the source of electrical energy,
`counts the output pulses from the bandpass filter. The
`- V cc is ground of the battery, and + V cc is the positive 40 output of the counter 158 is connected to a first input
`12 VDC of the battery. Therefore, the common ground
`159 of a digital comparator 160. The second input 161 of
`of the circuit 122 is at +6 VDC.
`the digital comparator 160 is connected to a hard wired
`The layers 130 and 136 have a predetermined amount
`count in digital form designated by 162 in FIG. 5.
`of capacitance across their associated terminals. The
`layers 130, 136 have a capacitance of 1.5 nanofarads. 45
`The counter is wired to be periodically reset by a
`reset circuit 163. The reset circuit 163 includes a clock
`The amplifier 142 with resistors 143, 147 has an ex-
`164 connected to a reset input of the counter 158
`tremely large gain. Because the amplifier 142 has a
`greater than 0 db gain at 180 degree phase shift, the
`through a delay circuit. The delay circuit includes two
`output 148 of the amplifier 142 will oscillate at a fre-
`inverters 165, 166 and a delay capacitor 167 connected
`so to the output of the inverter 166. The output of the
`quency equal to:
`clock 164 is further connected to the strobe input of the
`digital comparator 160.
`Tne clock outputs a periodic pulse. For the purpose
`of explanation only, the pulse occurs once per second.
`Also, assume the count from the hard wired count is
`2,000. The clock outputs a HIGH signal for the strobe
`and the reset of the counter 158. The delay circuit in(cid:173)
`sures that the count from the counter is strobed into the
`digital comparator prior to the reset of the counter 158.
`If the counter 158 is reset once a second, the counter
`output at the end of that one second, i.e., just prior to
`reset, will be equal to a count of 2,000.
`The output of the digital comparator 160 has three
`outputs including an output 170 which changes states
`based on the input of the counter 158. If no occupant is
`sitting on the seat cushion 42, the output of the counter
`158 will be a count value of 2,000. When this occurs, the
`output 170 is a digital HIGH. A digital HIGH at the
`
`where R is the value of resistor 145 and C is the series 55
`connected value of the capacitances of the layers 130,
`136. In the example given, the series capacitance is 0. 75
`nanofarads. In a preferred embodiment of the present
`invention, an output voltage is provided at a terminal
`148 having a frequency of approximately 2 kilohertz. It 60
`should be noted that the continuous oscillations at the
`output of the film drive circuit 122 are dependent upon
`the freedom of the layers in the film 110 to move.
`Because the capacitance values of the piezoelectric
`layers are used as component values in the drive circuit 65
`122, the circuit must be placed in close proximity to the
`piezoelectric film 110 to avoid line capacitance that
`would occur in long lead lines. The drive circuit 122 is
`
`I
`freq. = TriRC
`
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`8
`184 turns the FET 186 ON when the output of the OR
`output 170 thereby indicates no occupant is on the seat
`gate 183 is a digital HIGH. Turning the FET ON ignites
`cushion. If an occupant sits on the seat cushion 42 so
`that the arm 140 contacts the film 110, the output of the
`the squib 54 which, in turn, actuates the gas source 32
`and inflates the airbag 26. It should be understood that
`band pass filter 150 assumes a DC value, and the
`counter 158 counts zero pulses. The output 170 of the 5 the controller 36 enables deployment of the airbag 26
`digital comparator 160 then goes to a digital LOW.
`upon the detection of both a vehicle crash condition
`If an electrical fault such as an open or short circuit
`(with crash speed less than the predetermined value)
`occurs in the sensor 60, the output 148 .will go to a DC
`and either the presence of an occupant on the seat cush-
`voltage value equal to either zero volts DC (the battery
`ion 42 or the occurrence of an electrical open circuit
`ground) or V + (the positive battery terminal) because 10 between the film sensor 60 and the controller. Also, if
`input noise will result in saturation of one of the op
`the speed of the crash is greater than the predetermined
`amps 142, 144. In either case, the counter will count
`value, the airbag is deployed by the control algorithm
`180 and OR gate 183 independent of whether an occu-
`zero pulses when an electrical fault occurs between the
`sensor 60 and the controller 50. Therefore, if an electri-
`pant is present on the seat.
`cal fault occurs between the sensor 60 and the control- 15 An error detect circuit 188, which is part of the con-
`troller 36, is connected to the output 148 of the film
`ler 36, the output 170 goes to a digital LOW.
`It should be appreciated that the output 170 of the
`drive circuit 122. An output 190 from the digital com-
`digital comparator 160 will be a digital HIGH only
`parator is connected to an enable input of the error
`when (i) an occupant is not sitting on the cushion 42, (ii)
`detect circuit 188. Whenever an occupant is sitting on
`the sensor film is properly operating, i.e., the sensor film 20 the seat cushion 42 or an electrical fault occurs between
`is oscillating at 2 kilohertz, and (iii) all electrical wires
`the sensor 60 and the controller 36, the output 190 of the
`are connected from the sensor film up to the digital
`digital comparator 160 is a digital HIGH. In other
`comparator 160. Otherwise, the output 170 will be at a
`words, when output 170 is LOW, output 190 is HIGH.
`When the output 190 is HIGH, the error detect circuit
`digital LOW.
`The output 170 of the digital comparator 160 is con- 25 188 is enabled. When