||||l||||||||l|||||l|l||l|l||||lllllllllll|||l|||||l||||l|ll|ll|l||l|||||ll
`
`U8005232243A
`
`United States Patent
`Blackburn et a1.
`[45] Date of Patent: Aug. 3, 1993
`
`[19]
`
`[I 1] Patent Number:
`
`5,232,243
`
`
`
`[54] OCCUPANT SENSING APPARATUS
`
`[75}
`
`Inventors: Brian K. Blackburn, Rochester;
`Joseph F. Mazur, Washington; Scott
`B. Gentry. Shelby Township,
`Macomb County, all of Mich.
`
`173]
`
`Assignee:
`
`TRW Vehicle Safety Systems Inc,
`Lyndhurst. Ohio
`
`12 1]
`
`[22]
`
`[51}
`[52]
`
`{58]
`
`[56]
`
`App}. No.:
`Filed:
`
`682.903
`
`Apr. 9, 1991
`
`Int. (31.5 .............................................. 360R 21/32
`US. Cl. .................................... 280/732; 230/735;
`130/273
`Field of Search ............... 280/723, 730, 731, 732,
`280/735; 180/273
`
`References Cited
`
`.
`
`.
`
`U.S. PATENT DOCUMENTS
`3/1965
`Isaac ............................... 280/735 X
`3, 172,684
`9/1961
`Whitman .
`3.340.523
`6/1912
`DeWindl
`.
`3.6'1I 2.699
`Azum et a].
`1/1913
`3,746.8“
`11/1916
`Abe et a1.
`.
`3.992,028
`10/198?
`Vance .
`4,700,! 80
`1/1989
`Yasuda et a1.
`4396,11 l 3
`2/1939
`Swan .
`4,804,859
`3/1989
`Fohl
`.
`4. 81 0,005
`5/1939
`4.333.451f
`Graebe. Jr.
`10/1939
`Lodini
`.
`4, 87 6.4 19
`Lodini
`.
`10/1989
`4,816,420
`12/1991
`White .................................. 280/135
`5,0? l. l 60
`12/1991
`White et a].
`.
`5,011,160
`12/1991
`5.014.583
`Fujita .................................. 280/?35
`1/1992
`Tamori
`_
`5.079.949
`
`.
`
`FOREIGN PATENT DOCUMENTS
`
`0357225
`3339134C1
`
`.
`3/1990 European Fat. 011‘.
`2/1990 Fed. Rep. of Germany .
`OTHER PUBLICATIONS
`
`Interlink Electronics “Force and Position Sensing Re-
`sistors: An Emerging Technology".
`Tekscan Corporate Capabillities.
`Kynar Piezo Film Technical Manual.
`
`Primary Examiner—Richard A. Schacher
`Attorney, Agent, or Ft‘rm—Tarolli, Sondheim & Covell
`
`[5?]
`
`answer
`
`An occupant sensing apparatus for use in an occupant
`restraint system comprises a film 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-
`pant restraint system is enabled when either an occu-
`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-
`ment of the occupant restraint system in response to the
`determined position and weight.
`
`9 Claims, 14 Drawing Sheets
`
`
`
`HN-1007
`
`1
`
`HN-1007
`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 1 of 14
`
`5,232,243
`
` 'zo\
`
`CRASHSENSORCONTROLLER
`
`38
`
`2
`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 2 of 14
`
`5,232,243
`
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`US. Patent
`
`Aug. 3, 1993
`
`Sheet 3 of 14
`
`5,232,243
`
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`US. Patent
`
`Aug. 3, 1993
`
`Sheet 5 of 14
`
`5,232,243
`
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`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 6 of 14
`
`5,232,243
`
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`US. Patent
`
`Aug.3,1993
`
`Sheet 7 of 14
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`US. Patent
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`US. Patent
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`Aug. 3, 1993
`
`Sheet 9 of 14
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`5,232,243
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`US. Patent
`
`Aug. 3, 1993
`
`Sheet 10 of 14
`
`5,232,243
`
`255
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`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 11 of 14
`
`5,232,243
`
`3 YEAR
`
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`5% 30x 50x
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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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`
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`
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`
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`OF SEAT
`
`PERFECT
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`
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`
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`
`12
`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 12 of 14
`
`5,232,243
`
`SCAN
`SENSOR
`
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`
`
`
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`
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`
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`
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`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 13 of 14
`
`5,232,243
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`9US
`
`Patent
`
`Aug. 3
`
`9
`
`1993
`
`Sheet 14 of 14
`
`5,232,243
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`5,232,243
`
`1
`
`OCCUPANT SENSING APPARATUS
`
`TECHNICAL FIELD
`
`The present invention is directed to a vehicle occu-
`pant restraint system and is particularly directed to an
`Occupant sensing apparatus in an Occupant restraint
`system.
`
`BACKGROUND OF THE INVENTION
`
`2
`ence of an occupant effects physical contact between
`the contact member and the film. The electrical charac-
`teristic of the film has a first state when there is no
`contact between the film and contact member and a
`second state when there is contact between the film and
`the contact member. Electric circuit means are con-
`
`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
`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-
`vides an unstable feedback loop around the piezoelec-
`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 band and the electric circuit means includes a
`band pass filter designed to respond to the predeter-
`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.
`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-
`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-
`ent invention;
`FIG. 2 is top plan view of a film sensor made in ac-
`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 FIG. 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 or the occupant position
`and weight sensor shown in FIG. 9;
`FIG. 11 is a side sectional view of the occupant posi-
`tion and weight sensor talten along line 11~—ll of FIG.
`10;
`
`5
`
`10
`
`15
`
`20
`
`30
`
`35
`
`45
`
`Occupant restraint systems for use in vehicles are
`well known in the art. One such restraint system in-
`cludes a crash sensor, an inflatable airbag, and an actua-
`tion circuit that controls deployment of the airbag in
`response to an output from the crash sensor. The crash
`sensor can be an accelerometer that provides an electri-
`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
`accelerometer provides a signal indicative of such crash
`condition. The actuation circuit thereby applies a cur-
`rent through the squib which causes the squib to ignite.
`When the squib ignites,
`the source of inert gas dis-
`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.
`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-
`pied by only the driver, it is desirable to (i) be able to
`detect if a passenger is present in the vehicle and (ii)
`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-
`sary deployment of a passenger side airbag during a
`crash when no passenger is present. Prior art occupant
`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.
`
`SUMMARY OF THE INVENTION
`
`The present invention is directed to a new and useful
`occupant sensing apparatus. An occupant sensing appa-
`ratus. accordance with the present invention, permits
`detection of (i) whether a vehicle seat is occupied, (ii)
`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) whether there is an electri-
`cal fault condition such as an electric open or short
`circuit in the occupant sensor. An occupant restraint
`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~
`cupant sensor. The passenger side airbag is positioned,
`i.e., aimed, in response to the detected position of the
`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—
`pant restraint system comprises (a) a film having an
`electric characteristic with changeable states, and (b) a
`contact member. Means are provided for mounting the
`film and the contact member in a vehicle so that pres-
`
`50
`
`55
`
`60
`
`65
`
`16
`
`16
`
`

`

`5,232,243
`
`3
`FIG. 12 is a circuit schematic of a portion of the
`occupant restraint system of FIG. 9;
`FIG. 13 is a circuit schematic of a portion of the
`occupant restraint system of FIG. 9;
`FIG. 14 is a graph of resistance of the weight sensor
`shown in FIG. 9;
`FIG. 15 is a graph of the change of resistance of the
`weight sensor as a function of the weight of the oecu—
`Pant;
`FIG. 16 is a graph of the percentage of gas dis-
`charged from the airbag shown in FIG. 9 as a function
`of distance between the occupant and the dashboard or
`as a function of the weight of the occupant;
`FIG. 17 is a graph depicting the angular position of
`the airbag shown in FIG. 9 as a function the occupant
`leftward and rightward position in the vehicle seat;
`FIG. 18 is a graph depicting the angular position of
`the airbag shown in FIG. 9 as it functions the occupant
`forward or rearward position in the vehicle seat;
`FIG. 19 is a graphical representation of the output of
`the humanistic sensor shown in FIG. 9;
`FIGS. 20~22 are flow charts depicting the control
`process followed by the controller shown in FIG. 9;
`and
`FIG. 23 is a top plan view of a position and weight
`sensor in accordance with another embodiment of the
`present invention.
`
`DESCRIPTION OF PREFERRED EMBODIMENT
`
`Referring to FIG. 1, an occupant restraint system 20
`includes an airbag assembly 22 mounted in an opening
`of a dashboard 24 of a vehicle. The airbag assembly 22
`includes a airbag 26 folded within the interior of an
`airbag housing 28. A cover 30 covers the airbag and is
`adapted to open easily upon inflation of the airbag 26.
`A source 32 of inert gas is mounted at the back of the
`housing 28 and is operatively connected to the airbag
`26. Gas discharged from the source 32, which may be
`generated by combustion of pyrotechnic material or
`simply released from a pressurized container. fills the
`airbag 26 to its inflated condition 26'. Once inflated. as
`occurs during a vehicle crash, the airbag 26 cushions an
`occupant located in a vehicle seat 34.
`An electronic controller 36, such as a microcom-
`puter, is operatively connected to a vehicle crash sensor
`38. The crash sensor 38 can be any of several known
`types. For example. the crash sensor 38 may be a me—
`chanical inertia switch, such as a rolamite sensor, or an
`electrical accelerometer. If a normally open inertia
`SWitch closes, this is an indication that a crash is occur-
`ring. Likewise, if a signal from an electrical accelerome-
`ter reaches a predetermined level or a predetermined
`level for a predetermined time. this is an indication that
`a crash is occurring. Once the controller 36 determines
`that a. vehicle crash is occurring for which deployment
`of the airbag is necessary to protect the vehicle occu-
`pants, the controller 36 ignites squib 40 which, in turn,
`actuates the gas source 32.
`The occupant seat 34 with which the occupant re-
`straint system 20 is used is preferably the passenger seat
`in the vehicle. The seat 34 includes an occupant sensor
`60 located in the bottom cushion 42 of the seat 34. The
`occupant sensor 60 is used to control whether deploy-
`merit of the airbag is to be prevented.
`FIGS. 2—4 show one embodiment of the present in-
`vention, including an occupant presence sensor 60. The
`sensor 60 includes a housing 90 which incorporates a
`bottom support plate 92 and a top cover plate 94. The
`
`4
`bottom plate 92 is rigidly mounted to a substantially
`inflexible bottom portion of the seat 42.
`The bottom plate 92 and the top plate 94 include
`interior surfaces 96, 98, respectively, which are spaced
`apart. An annular sealing member or gasket 100 is
`mounted between the support plate 92 and the upper
`plate 94. The sealing member 100 includes a central
`opening 106. The plates 92. 94 are secured together by
`means ofa plurality of spaced apart screws 102. Specifi-
`cally, the top plate 94 has holes 103 and the bottom plate
`92 has threaded holes 104. The gasket 100 has corre-
`sponding holes 105. The screws 162 extend through the
`holes 103 and 105 and are screwed into the holes 104.
`The bottom plate 92, the upper plate 94, and the sealing
`member 100 form a chamber 10'lr in which a piezoelec-
`tric film sensor 110 is mounted.
`The bottom plate 92 includes a recessed portion 112.
`The piezoelectric film sensor 110 spans the recess 112
`and is secured along opposed edges at locations 114, 116
`by appropriate means, such as adhesive. The bottom
`plate 92 further includes a second recessed portion 120.
`A film drive circuit 122 is secured to a substrate 124
`which is mounted in the recess 120 by appropriate
`means. The drive circuit 122 is operatively connected to
`the piezoelectric film sensor 110 through terminals 126.
`Electrical wires 128 connect the drive circuit 122 to the
`controller 36.
`
`The piezoelectric film sensor 110 is a multi-layer
`structure including a first piezoelectric layer 130 and a
`second piezoelectric layer 136 separated by an insulat-
`ing layer 138. The two piezoelectric layers and the
`insulating layer are secured together in an integral as-
`sembly by appropriate means. Each layer 130. 136 has
`an associated electrode silk-screened on each side of the
`layer. The electrodes of each piezoelectric layer 130,
`136, is connected to an associated terminal 126. Specifi-
`cally. the top electrode for layer 130 is connected to
`terminal 126‘. The bottom electrode for layer 13!) is
`connected to terminal 126”. The top electrode for layer
`136 is connected to terminal 126". The bottom electrode
`for layer 136 is counected to terminal 126'".
`When energized by the film drive circuit 122, por-
`tions of the piezoelectric layers, under their associated
`electrodes, flex. The drive circuit 122 is adapted so that
`the piezoelectric film sensor 110 oscillates. The film
`oscillation physically occurs over the recessed portion
`112 of the bottom plate 92. This area of the film sensor
`110 is referred to as the bridge.
`The upper plate 94 includes an contact arm 14-0 that
`extends normal
`to the plate 94 and the surface 98
`toward the film sensor 110. The upper plate 94 is made
`from a flexible material. -When an occuth sits on the
`seat cushion 42, his weight is transferred through the
`material that makes up tire seat cushion to the upper
`plate 94. The force applied to the upper plate 94 is
`functionally related to the occupar"s weight. Since the
`bottom plate 92 is rigidly secured relative to the vehicle
`floor, the upper plate 94 flexes in response to the occu-
`pant‘s weight transferred through the seat material. As
`the upper plate 94 flexes, the contact arm 140 moves
`toward the layers 130, 136 of film sensor 110.
`When no occupant is sitting on the seat cushion and
`the film layers are energized by the film drive circuit
`122, the film oscillates. When the film oscillates, the
`output of the drive circuit oscillates with a certain peak-
`to-peait voltage value. If an occupant who has a weight
`greater than a predetermined weight then sits on the
`seat cushion, the upper plate 94 flexes an amount suffr-
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`6
`placed upon the substrate 124 so as to be close to the
`film 110.
`
`5
`cient to insure that the contact arm 140 contacts the film
`110. If the arm 140 contacts the film 110, the oscillations
`of the film 110 are attenuated. The resultant output from
`the film is a DC voltage value equal to approximately
`one-half the peak-to-peak voltage when no occupant is
`sitting on the seat cushion. By monitoring the output of
`the film 110. the controller is provided with an indica-
`tion as to whether an occupant is located on the seat
`cushion 60.
`
`FIGS. 5, 6 and 7, show a processing circuit, made in
`accordance with one embodiment of the present inven-
`tion. In this embodiment, the Occupant sensor is the
`sensor 60. The sensor 60 is used to determine whether
`an occupant is sitting on the seat cushion 4-2. The con-
`troller 36 monitors the output signal from the sensor 60.
`The controller 36 enables actuation of the airbag when
`either the signal from the sensor 60 indicates that an
`occupant is sitting in the seat or the absence of a signal
`from the sensor 60 indicates that an electrical fault con-
`dition exists such as an open or short circuit between the
`sensor 60 and the controller 36.
`
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`The output 148 is connected to the input of the con-
`troller 36. The controller 36 includes a bandpass filter
`150. of standard design well known in the art, con-
`nected to the output 143 of the film drive circuit 122.
`The bandpass filter 150 is designed, in accordance with
`a preferred embodiment of the present invention,
`to
`pass frequencies of 2 kilohertz. The bandpass filter 150
`may be external to the sensor 60 or can be placed upon
`the substrate 124 along with the drive circuit. As shown
`in FIG. 8, the output 152 of the bandpass filter is an
`oscillating voltage between zero volts DC (battery
`ground) and a voltage value V+ (the pesitive battery
`terminal) at a frequency of 2 kilohertz. The time period
`P between pulses is equal to 0.5 msec., which is one
`divided by the frequency of 2 kilohertz. The line 154
`shown in FIG. 8 represents force or stress against the
`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
`weight deflects the top plate 94 a sufficient amount so
`that the contact am 140 contacts the film 110. When
`contact occurs between the arm 140 and the film 110,
`the output of the bandpass filter changes to a DC volt—
`age equal to approximately one-half V+, as shown by
`the line 168. The voltage goes to one-half V+ because
`the common ground of circuits 122, 150 is at one-half
`V+. Specifically, V+ is the positive terminal of the
`vehicle battery and chassis ground is the negative termi-
`nal of the vehicle battery. Common ground for the
`drive circuit 122 and the band pass filter 150 is at one-
`half the value of the battery voltage as is shown in FIG.
`6. Therefore. when the oscillations of the piezoelectric
`layers stop, the output of the band pass filter and the
`drive circuit go to their common ground value, which is
`one-half V+, i.e., 6 VDC.
`A counter 158, also part of the controller 50, is con-
`nected to the output 152 of the bandpass filter 150 and
`counts the output pulses from the bandpass filter. The
`output of the counter 158 is connected to a first input
`159 of a digital comparator 160. The second input 161 of
`the digital comparator 160 is connected to a hard wired
`count in digital form designated by 162 in FIG. 5.
`The counter is wired to be periodically reset by a
`reset circuit 163. The reset circuit 163 includes a clock
`164 connected to a reset
`input of the counter 158
`through a delay circuit. The delay circuit includes two
`inverters 165, 166 and a delay capacitor 167 connected
`to the output of the inverter 166. The output of the
`clock 164 is further connected to the strobe input of the
`digital comparator 160.
`Tue 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-
`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
`
`FIG. 6 shows the drive circuit 122 in detail. The tap
`electrode of piezoelectric layer 130 is connected
`through terminal 126’ to the inverting input of an op
`amp 144, connected as a differentiator. through a resis-
`tor. The differentiator with op amp 144 has a feedback
`resistor 145. The output of the op amp 144 is connected
`to the inverting input of an op amp 142, connected as an
`amplifier, through an input resistor 14?. The amplifier
`142 includes a feedback resistor 143. The output of the
`amplifier is connected to the bottom electrode of piezo-
`electric layer 136 through terminal 126'“. The bottom
`electrode of piezoelectric layer 130 and the top elec-
`trode of piezoelectric layer 136 are connected to a
`source of electrical energy -—Vccthrough terminal 126“.
`The noninverting inputs for the amplifiers 142, 14-4 are
`connected to a common ground having a voltage value
`between + V“ and —Vm established through a resistor
`diVider network 149 with equal value resistors. If the
`vehicle battery is used as the source of electrical energy,
`— V“ is ground of the battery, and +11“- is the positive
`12 VDC of the battery. Therefore, the common ground
`of the circuit 122 is at +6 VDC.
`The layers 130 and 136 have a predetermined amount
`of capacitance across their associated terminals. The
`layers 130, 136 have a capacitance of 1.5 nanofarads.
`The amplifier 142 with resistors 143. 14'lr has an ex-
`tremely large gain. Because the amplifier 142 has a
`greater than 0 db gain at 180 degree phase shift. the
`output 148 of the amplifier 142 will oscillate at a fre-
`quency equal to:
`
`he
`
`a, .l—‘i' mac
`
`where R is the value of resistor 14-5 and C is the series
`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
`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
`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
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`7
`output 170 thereby indicates no occupant is on the seat
`cushion. If an occupant sits on the seat cushion 42 so
`that the arm 140 contacts the film 110, the output of the
`band pass filter 150 assumes a DC value, and the
`counter 158 counts zero pulses. The output 170 of the
`digital comparator 160 then goes to a digital LOW.
`If an electrical fault such as an open or short circuit
`occurs in the sensor 60, the output 148_will go to a DC
`voltage value equal to either zero volts DC (the battery
`ground) or V + (the positive battery terminal) because
`input noise will result in saturation of one of the op
`amps 142, 144. In either case, the counter will count
`zero pulses when an electrical fault occurs between the
`sensor 60 and the controller 50. Therefore, if an electri-
`cal fault occurs between the sensor 60 and the control-
`ler 36, the output 170 goes to a digital LOW.
`It should be appreciated that the output 170 of the
`digital comparator 160 will be a digital HIGH only
`when (i) an occupant is not sitting on the cushion 42, (ii)
`the sensor film is properly operating, i.e., the sensor film
`is oscillating at 2 kilohertz, and (iii) all electrical wires
`are connected from the sensor film up to the digital
`comparator 160. Otherwise, the output 170 will be at a
`digital LOW.
`The output 170 of the digital comparator 160 is con-
`nected to the input of an inverter 172. The output of the
`inverter 172 is connected to a first input of an AND gate
`174. The second input of the AND gate 175 is com
`nected to the output of a read-only memory 180 in the
`controller 36. The read-only memory 180 is connected
`to the output of the crash sensor accelerometer 38. A
`control algorithm is programmed in the read-only mem-
`ory 180 to process the output of the accelerometer 38.
`The control algorithm can take any of several forms.
`For example, if the accelerometer is of the type that
`provides a linear electric signal
`indicative of sensed
`deceleration (crash condition), the control algorithm
`may integrate the accelerometer signal. Once the inte-
`gral reaches a predetermined value indicative of a crash
`condition. the algorithm provides a digital HIGH to the
`AND gate 174. Thus, the controllerI as a result of the
`algorithm 180, provides a digital HIGH signal when the
`signal from the accelerometer indicates that the vehicle
`is in a crash condition.
`
`The output of the AND gate 174 is a digital HIGH
`when a crash condition occurs and either (i) an occu~
`pant is sitting on the seat cushion 42 or (ii) an electrical
`fault exists between the sensor 60 and the controller 36
`so that, in either case, the output of the inverter 172 is a
`digital HIGH. In all other situations, the output of the
`AND gate 174 is a digital LOW.
`The output of the AND gate 174 is connected to an
`input of an OR gate 183. The read-only memory 180 is
`connected to the other input of the OR gate 183 on line
`185. The read-only memory also includes a control
`algorithm that determines the speed of the crash. Pref—
`erably, this is accomplished by monitoring the sIOpe of
`an integration curve. If the speed of the crash exceeds a
`predetermined value, the control algorithm provides a
`digital HIGH on line 185. If the speed of the crash is
`greater than a predetermined value or the output of
`AND gate 174 is a digital HIGH, the output of OR gate
`183 is switched to a digital HIGH. The output of OR
`gate 183 is connected to a drive circuit 184. The output
`of the drive circuit 184 is connected to an electric
`switch 186, such as a field-effect transistor (“FIST").
`The FET 186 is connected in series with the squib 4-0
`across a source of electrical energy. The drive circuit
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`184 turns the FET 186 ON when the output of the OR
`gate 183 is a digital HIGH. Turning the PET 0N ignites
`the squib 54 which, in turn, actuates the gas source 32
`and inflates the airbag 26. It should be understood that
`the controller 36 enables deployment of the airbag 26
`upon the detection of both a vehicle crash condition
`(with crash speed less than the predetermined value)
`and either the presence of an occupant on the seat cush-
`ion 42 or the occurrence of an electrical open circuit
`between the film sensor 60 and the controller. Also, if
`the speed of the crash is greater than the predetermined
`value, the airbag is deployed by the control algorithm
`180 and OR gate 183 independent of whether an occu»
`pant is present on the seat.
`An error detect circuit 188, which is part of the con-
`troller 36, is connected to the output 148 of the film
`drive circuit 122. An output 190 from the digital corn-
`parator is connected to an enable input of the error
`detect circuit 188. Whenever an occupant is sitting on
`the seat cushion 42 or an electrical fault occurs between
`the sensor 60 and the controller 36, the output 190 of the
`digital comparator 160 is a digital HIGH.
`In other
`words. when output 170 is LOW, output 190 is HIGH.
`When the output 190 is HIGH, the error detect circuit
`188 is enabled. When enabled, the error detect circuit
`monitors the output voltage at the output 148 from the
`film drive circuit 122. If the voltage present on the
`output 148 is outside of a predetermined window cen-
`tered at one-half V+, an error indicator 192 located in
`the dash board 24 is energized to wam the vehicle oper-
`ator. The voltage on output 148 being outside of the
`window at one-half V+ indicates that an electrical fault
`has occurred. If the output 14-8 is within the window at
`one-halfV + , plus or minus a predetermined value. such
`condition indicates that an occupant is sitting on the seat
`cushion 42.
`
`FIG. 9 shows an occupant restraint system 220 made
`in accordance with another embod