U.S. patent application number 12/643478 was filed with the patent office on 2010-12-02 for occupant detection system.
This patent application is currently assigned to TK HOLDINGS INC.. Invention is credited to James G. Stanley, Robert A. Stopper, JR..
Application Number | 20100301880 12/643478 |
Document ID | / |
Family ID | 46327331 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100301880 |
Kind Code |
A1 |
Stanley; James G. ; et
al. |
December 2, 2010 |
OCCUPANT DETECTION SYSTEM
Abstract
An occupant detection system comprises a weight sensor and an
electric field sensor, each operatively coupled to a seat. The
electric field sensor generates an electric field from at least one
electrode in the seat bottom of the seat, provides for generating a
response to an influence of the occupant thereupon, and is adapted
to provide for discriminating from the response a seated infant or
child seating condition from another seating condition. If a
measure of weight from the weight sensor is less than a threshold,
or if a seated child seating condition is detected by the electric
field sensor, then a signal processor provides for disabling an
associated restraint actuator. The electric field sensor may
comprise a plurality of electrodes over separate first and second
regions of differing proximity to a seated infant or child, or at
least one electrode in cooperation with a shield or void over at
least one of the regions.
Inventors: |
Stanley; James G.; (Novi,
MI) ; Stopper, JR.; Robert A.; (Plymouth,
MI) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
TK HOLDINGS INC.
|
Family ID: |
46327331 |
Appl. No.: |
12/643478 |
Filed: |
December 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11708007 |
Feb 20, 2007 |
7635984 |
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12643478 |
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10987318 |
Nov 15, 2004 |
7180306 |
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11708007 |
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10153378 |
May 21, 2002 |
6825765 |
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10987318 |
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09614086 |
Jul 11, 2000 |
6392542 |
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10153378 |
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09474600 |
Dec 29, 1999 |
6520535 |
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09614086 |
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09474673 |
Dec 29, 1999 |
6283504 |
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09474600 |
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60143761 |
Jul 12, 1999 |
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60144161 |
Jul 15, 1999 |
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60207536 |
May 26, 2000 |
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Current U.S.
Class: |
324/686 |
Current CPC
Class: |
B60R 21/01532 20141001;
B60R 21/01516 20141001 |
Class at
Publication: |
324/686 |
International
Class: |
G01R 27/26 20060101
G01R027/26 |
Claims
1. An electric field sensor, comprising: a. at least one first
electrode; b. at least one second electrode; and c. at least one
third electrode, wherein said at least one second electrode is
located between said at least one first electrode and said at least
one third electrode, said at least one first electrode is located
proximate to a region to be sensed by said electric field sensor,
and said at least one second electrode is substantially the same
size as said at least one first electrode.
2. An electric field sensor as recited in claim 1, wherein said at
least one third electrode is electrically connected to a circuit
ground.
3. An electric field sensor as recited in claim 1, further
comprising a sensing circuit, wherein said sensing circuit is
operatively coupled to said at least one first electrode and to
said at least one said second electrode.
4. An electric field sensor as recited in claim 3, wherein said
sensing circuit applies a first applied signal to said at least one
first electrode and applies a second applied signal to said at
least one second electrode.
5. An electric field sensor as recited in claim 4, wherein said
second applied signal is equal so said first applied signal.
6. An electric field sensor as recited in claim 3, wherein said
sensing circuit is operatively coupled to at least one said third
electrode and said sensing circuit applies a third applied signal
to said third electrode.
7. An electric field sensor as recited in claim 6, wherein said
third applied signal is a circuit ground potential.
8. An electric field sensor, comprising: a. at least one electrode;
b. a first reference capacitor; c. a second reference capacitor;
and d. a sensing circuit comprising a plurality of states, wherein
in a first state said sensing circuit is operatively coupled to at
least one said electrode so as to provide for generating a first
signal responsive to the capacitance of said at least one said
electrode, in a second state said sensing circuit is operatively
coupled to said first reference capacitor so as to provide for
generating a second signal responsive to the capacitance of said
first reference capacitor, in a third state said sensing circuit is
operatively coupled to said second reference capacitor so as to
provide for generating a third signal responsive to the capacitance
of said second reference capacitor, and said sensing circuit is
adapted to provide for generating a measure responsive to the
capacitance of said at least one said electrode responsive to said
first signal, said second signal and said third signal.
9. An electric field sensor as recited in claim 8, wherein in said
third state, said sensing circuit is also operatively coupled to
said first reference capacitor so that said third signal is
responsive to the capacitance of a combination of said first and
second reference capacitors.
10. An electric field sensor as recited in claim 8, further
comprising at least one switch operatively coupling said at least
one electrode, said first reference capacitor and said second
reference capacitor to said sensing circuit, wherein said first
state, said second state, and said third state of said sensing
circuit correspond to corresponding states of said at least one
switch.
11. An electric field sensor as recited in claim 8, further
comprising at least multiplexer or demultiplexer operatively
coupling said at least one electrode, said first reference
capacitor and said second reference capacitor to said sensing
circuit, wherein said first state, said second state, and said
third state of said sensing circuit correspond to corresponding
states of said at least at least one multiplexer or
demultiplexer.
12. An electric field sensor as recited in claim 8, wherein said
plurality of states are repetitively sequentially cycled.
13. A method of generating a response from an electric field
sensor, comprising: a. operatively coupling an oscillatory voltage
signal across a voltage divider, wherein said voltage divider
comprises a first impedance operatively coupled to a switched
impedance at a node; b. switching said switched impedance in
accordance with a plurality of states, wherein in a first state
said switched impedance comprises at least one electrode of an
electric field sensor, in a second state said switched impedance
comprises a first reference capacitor, and in a third state said
switched impedance comprises a second reference capacitor; c.
generating a plurality of corresponding response signals from said
node corresponding to each of said first state, said second state
and said third state; and d. generating the response from the
electric field sensor responsive to said plurality of corresponding
response signals.
14. A method of generating a response from an electric field sensor
as recited in claim 13, wherein in said third state, said switched
impedance comprises a combination of said first and second
reference capacitors.
15. A method of generating a response from an electric field sensor
as recited in claim 13, wherein said plurality of states are
repetitively sequentially cycled.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The instant application is a divisional of U.S. application
Ser. No. 10/153,378, which is a continuation-in-part of U.S.
application Ser. No. 09/614,086 ("Application '086") filed on Jul.
11, 2000, now U.S. Pat. No. 6,392,542, which claims the benefit of
U.S. Provisional Application No. 60/143,761 filed on Jul. 13, 1999;
U.S. Provisional Application Ser. No. 60/144,161 filed on Jul. 15,
1999; and U.S. Provisional Application Ser. No. 60/207,536 filed on
May 26, 2000. Application '086 is a continuation-in-part of U.S.
application Ser. No. 09/474,600 filed on Dec. 29, 1999, now U.S.
Pat. No. 6,520,535; and a continuation-in-part of U.S. application
Ser. No. 09/474,673, filed on Dec. 29, 1999, now U.S. Pat. No.
6,283,504.
[0002] Application Ser. No. 10/153,378 is also a
continuation-in-part of U.S. application Ser. No. 09/474,470 filed
on Dec. 29, 1999, now U.S. Pat. No. 6,577,023, which claims the
benefit of U.S. Provisional Application Ser. No. 60/114,269 filed
on Dec. 30, 1998; U.S. Provisional Application No. 60/133,630 filed
on May 11, 1999; U.S. Provisional Application Ser. No. 60/133,632
filed on May 11, 1999; and U.S. Provisional Application Ser. No.
60/143,761 filed on Jul. 12, 1999.
[0003] Application Ser. No. 10/153,378 is also a
continuation-in-part of U.S. application Ser. No. 09/474,469 filed
on Dec. 29, 1999, now U.S. Pat. No. 6,563,231, which claims the
benefit of U.S. Provisional Application Ser. No. 60/114,269 filed
on Dec. 30, 1998; U.S. Provisional Application Ser. No. 60/133,630
filed on May 11, 1999; U.S. Provisional Application Ser. No.
60/133,632 filed on May 11, 1999; and U.S. Provisional Application
Ser. No. 60/143,761 filed on Jul. 12, 1999.
[0004] The instant application is also related to U.S. application
Ser. No. 09/520,866 filed on Mar. 6, 2000, now U.S. Pat. No.
6,348,862.
[0005] The above-identified patents and patent applications are
incorporated herein by reference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] In the accompanying drawings:
[0007] FIG. 1 illustrates an occupant detection system
incorporating a first embodiment of a seat weight sensor;
[0008] FIG. 2 illustrates an occupant detection system
incorporating a second embodiment of a seat weight sensor;
[0009] FIG. 3 illustrates a child in a typical rear facing infant
seat placed on a vehicle seat;
[0010] FIGS. 4a and 4b illustrate several electrode embodiments in
accordance with an electric field sensor;
[0011] FIGS. 5a and 5b illustrate other electrode embodiments in
accordance with an electric field sensor;
[0012] FIG. 6a illustrates a first embodiment of a circuit for
switching a calibration capacitor;
[0013] FIG. 6b illustrates a second embodiment of a circuit for
switching a calibration capacitor;
[0014] FIG. 7 illustrates a generalized sensing circuit for
measuring a capacitance;
[0015] FIG. 8 illustrates an embodiment of a circuit for sensing
capacitance and for controlling a restraint actuator responsive to
capacitance measurements and responsive to a measure of seat
weight;
[0016] FIG. 9 illustrates the operation of various switch elements
of the sensing circuit of FIG. 8;
[0017] FIG. 10 illustrates an alternate FET switch embodiment;
[0018] FIG. 11 illustrates another embodiment of a circuit for
sensing capacitance and for controlling a restraint actuator
responsive to capacitance measurements and responsive to a measure
of seat weight;
[0019] FIG. 12 illustrates a method of detecting an occupant and
controlling a restraint actuator responsive thereto;
[0020] FIG. 13 illustrates a first method of detecting a child seat
on a vehicle seat;
[0021] FIG. 14 illustrates an electric field sensor comprising a
plurality of electrodes;
[0022] FIG. 15 illustrates a second method of detecting a child
seat on a vehicle seat;
[0023] FIG. 16 illustrates a side-view of an embodiment of an
electric field sensor incorporating a driven shield;
[0024] FIG. 17a illustrates an embodiment of a capacitive sensing
pad comprising a front driven shield;
[0025] FIG. 17b illustrates another embodiment of a capacitive
sensing pad comprising a front driven shield;
[0026] FIG. 18a illustrates a cross-section of the embodiment
illustrated in FIG. 17a;
[0027] FIG. 18b illustrates a cross-section of the embodiment
illustrated in FIG. 17b;
[0028] FIG. 19 illustrates a front driven shield in a shielding
mode in accordance with a second aspect of the instant
invention;
[0029] FIG. 20 illustrates a front driven shield in a sensing mode
in accordance with a first embodiment of a second aspect of the
instant invention;
[0030] FIG. 21 illustrates a front driven shield in a sensing mode
in accordance with a second embodiment of a second aspect of the
instant invention;
[0031] FIG. 22 illustrates a third method of detecting a child seat
on a vehicle seat;
[0032] FIG. 23 illustrates a fourth method of detecting a child
seat on a vehicle seat;
[0033] FIGS. 24a and 24b illustrates the capacitance of the
occupant relative to an electric field sensor and relative to a
circuit ground;
[0034] FIG. 25 illustrates an embodiment of a second aspect of a
capacitive sensing pad;
[0035] FIG. 26 illustrates another embodiment of a second aspect of
a capacitive sensing pad;
[0036] FIG. 27 illustrates the performance of the instant invention
incorporating a capacitive sensing pad in accordance with the
embodiment illustrated in FIG. 26;
[0037] FIG. 28 illustrates yet another embodiment of a second
aspect of a capacitive sensing pad; and
[0038] FIG. 29 illustrates a third aspect of a capacitive sensing
pad.
DESCRIPTION OF EMBODIMENT(S)
[0039] Referring to FIG. 1, an occupant detection system 10
comprises a seat weight sensor 12 and an electric field sensor 14,
each operatively connected to a controller 16, for detecting an
occupant 18 in a vehicle 20. The seat weight sensor 12 is adapted
to generate a measure of weight upon the a vehicle seat 22, e.g.
upon the associated seat bottom 24. The electric field sensor 14
comprises at least one electrode 26 located, for example, in the
seat bottom 24 under the seat cover 28 and close to the top of a
foam cushion 30, and adapted to enable a type of occupant 18 or
object that may be upon the seat bottom 24 of the vehicle seat 22
to be distinguished.
[0040] The seat weight sensor 12 is responsive to a force upon onto
the vehicle seat 22. The seat weight sensor 12, for example, may
comprise one or more load cells 32 operatively coupled to at least
one load path between the seat bottom 24 and the vehicle 20, e.g.
between the seat frame 34 and the floor pan 36 of the vehicle 20,
e.g. at the corners 38 of the seat frame 34, so as to measure the
weight of the entire vehicle seat 22 and objects or occupants 18
placed thereon. For example, the one or more load cells 32 could
use a strain gage, a magnetostrictive sensing element, a force
sensitive resistive element, or another type of sensing element to
measure the associated load. For example, the seat weight sensor 12
may be constructed in accordance with the teachings of U.S. Pat.
No. 5,905,210, 6,069,325 or 6,323,444, each of which is
incorporated herein by reference.
[0041] The seat weight sensor 12 may alternately comprise at least
one weight sensing element, e.g. a force sensitive resistive
element, a membrane switch element, a pressure sensitive resistive
contact, a pressure pattern sensor, a strain gage, a bend sensor,
or a hydrostatic weight sensing element, operatively coupled to one
or more seating surfaces in the seat base or seat back, e.g. in
accordance with the teachings of U.S. Pat. No. 5,918,696,
5,927,427, 5,957,491, 5,979,585, 5,984,349, 5,986,221, 6,021,863,
6,045,155, 6,076,853, 6,109,117 or 6,056,079, each of which is
incorporated herein by reference. For example, referring to FIG. 2,
the seat weight sensor 12 may comprise a hydrostatic weight sensing
element--e.g. a fluid containing bladder 40, underneath the seat
cover 28 of the seat bottom 24 and supported by the seat frame
34--wherein a pressure sensor 42 operatively connected to the
bladder 40 measures the pressure of the fluid contained therein so
as to provide a measure of occupant weight. The pressure sensor 42
is operatively connected to the controller 16 so as to provide a
pressure signal thereto, which determines a measure of weight
therefrom. A seat weight sensor 12 within the cushion 30 of the
vehicle seat 22, e.g. in the seat bottom 24 only, would typically
not be as accurate as a seat weight sensor 12 that measures the
weight of the entire vehicle seat 22, but would still provide
information about the weight of an occupant on the vehicle seat 22
sufficient for the occupant detection system 10 to control a
restraint actuator 44, e.g. an air bag inflator module 44',
responsive thereto. The particular type of seat weight sensor 12 is
not considered to be limiting. The seat weight sensor 12 may, for
example, be integrated with either the seat frame 34 or the seat
bottom 24.
[0042] As used herein, the term "electric field sensor" refers to a
sensor that generates a signal responsive to the influence of that
being sensed, upon an electric field. Generally, an electric field
sensor comprises at least one electrode to which is applied at
least one applied signal; and at least one electrode--which could
be the same electrode or electrodes to which the applied signal is
applied--at which a received signal (or response) is measured. The
applied signal generates an electric field from the at least one
electrode to a ground in the environment of the at least one
electrode, or to another at least one electrode. The applied and
received signals can be associated with the same electrode or
electrodes, or with different electrodes. The particular electric
field associated with a given electrode or set of electrodes is
dependent upon the nature and geometry of the electrode or set of
electrodes and upon the nature of the surroundings thereto, for
example, the dielectric properties of the surroundings. For a fixed
electrode geometry, the received signal or signals of an electric
field sensor are responsive to the applied signal or signals and to
the nature of the environment influencing the resulting electric
field, for example to the presence and location of an object having
a permittivity or conductivity different from that of its
surroundings.
[0043] One form of electric field sensor is a capacitive sensor,
wherein the capacitance of one or more electrodes is measured--from
the relationship between received and applied signals--for a given
electrode configuration. The technical paper "Field mice:
Extracting hand geometry from electric field measurements" by J. R.
Smith, published in IBM Systems Journal, Vol. 35, Nos. 3 & 4,
1996, pp. 587-608, incorporated herein by reference, describes the
concept of electric field sensing as used for making non-contact
three-dimensional position measurements, and more particularly for
sensing the position of a human hand for purposes of providing
three dimensional positional inputs to a computer. What has
commonly been referred to as capacitive sensing actually comprises
the distinct mechanisms of what the author refers to as "loading
mode", "shunt mode", and "transmit mode" which correspond to
various possible electric current pathways. In the "shunt mode", a
voltage oscillating at low frequency is applied to a transmit
electrode, and the displacement current induced at a receive
electrode is measured with a current amplifier, whereby the
displacement current may be modified by the body being sensed. In
the "loading mode", the object to be sensed modifies the
capacitance of a transmit electrode relative to ground. In the
"transmit mode", the transmit electrode is put in contact with the
user's body, which then becomes a transmitter relative to a
receiver, either by direct electrical connection or via capacitive
coupling.
[0044] Accordingly, the electric field sensor 14 is either what is
commonly known as a capacitive sensor, or more generally an
electric field sensor operating in any of the above described
modes. The electric field sensor 14 comprises at least one
electrode 26 operatively coupled to at least one applied signal 46
so as to generate an electric field proximate to the at least one
electrode 26, responsive to the applied signal 46. The applied
signal 46, for example, comprises either an oscillating or pulsed
signal. At least one electrode 26 is operatively coupled to a
sensing circuit 48 that outputs at least one response signal 50
responsive to the electric field at the corresponding electrode 26,
wherein the response signal 50 is responsive to at least one
electric-field-influencing property--for example, dielectric
constant, conductivity, size, mass or distance--of an object
proximate to the electric field sensor 14. For example, for the
electric field sensor 14 as a capacitance sensor, the sensing
circuit 48 measures the capacitance of at least one electrode 26
with respect to either another electrode 26 or with respect to a
surrounding ground, for example, a seat frame 34 of the vehicle
seat 22, connected to circuit ground 52. The at least one applied
signal 46 is, for example, generated by the sensing circuit 48 that
also outputs the at least one response signal 50. The sensing
circuit 48 and associated at least one applied signal 46 may be
adapted to be responsive to the influence of a water soaked vehicle
seat 22, on measurements from the electric field sensor 14.
[0045] The electric field sensor 14 generates an electric field
from the applied signal 46 applied to at least one electrode 26 and
senses objects proximate to the associated at least one electrode
26, for example in the seat bottom 24 of a vehicle seat 22, from
the influence of the electric field on the response signal 50. The
at least one electrode 26 of the electric field sensor 14, the
applied signal 46 applied thereto, and the sensitivity of the
sensing circuit 48 are all adapted so that the electric field
sensor 14 is, for example, substantially non-responsive to objects
that are more than 50 mm above the seat bottom 24, but is
substantially responsive to occupants that are normally seated
directly on the vehicle seat 22.
[0046] The at least one electrode 26 of the electric field sensor
14 is adapted so as to provide for distinguishing seating
conditions for which a restraint actuator 44, for example an air
bag inflator module 44', should be deployed from seating conditions
for which the restraint actuator 44 should not be deployed, so as
to avoid causing more injury to an occupant 18 than the occupant 18
would otherwise incur without the deployment of the restraint
actuator 44. For example, the electrode 26 is adapted so that a
capacitance of the at least one electrode 26 with respect to a
circuit ground 52 is substantially greater for a seating condition
for which the restraint actuator 44 should be deployed, for example
an occupant 18 seated in substantially normal seating position on
the vehicle seat 22 or a large body immediately above the seat
bottom 24; than for a seating condition for which the restraint
actuator 44 should not be deployed, for example an empty vehicle
seat 22, an infant, child, or booster seat on the vehicle seat 22
with or without an infant or child seated therein, or an occupant
18 on the vehicle seat 22 in a position that is substantially
different from a normal seating position. The at least one
electrode 26 is, for example, located under the seat cover 28 and
substantially the same size as a region to be sensed on the vehicle
seat 22, extending from near the back of the seat bottom 24 to near
the front of the seat bottom 24. As described hereinbelow, sections
of the at least one electrode 26 are removed or selectively
shielded so as to selectively reduce the sensitivity thereof
proximate to regions where an infant or child, in an infant, child,
or booster seat, is closest to the vehicle seat 22, so as to
provide for distinguishing between a child seated in a child seat
and an occupant 18 that is seated directly on the vehicle seat 22.
Responsive to a child in a child seat on the vehicle seat 22, the
increase in capacitance of the electrode 26 of the electric field
sensor 14 in the seat bottom 24, relative to that of an empty
vehicle seat 22, is relatively small.
[0047] Stated in another way, the electric field sensor 14 has a
relatively short range and principally senses an occupant 18 when a
large surface of the occupant is relatively close to the sensor.
Occupants normally seated directly on the seat cover 28 typically
have a large surface of their body relatively close to the
electrode 26. When infants or children are in child seats, most of
their body is elevated several inches off the seat bottom surface,
resulting in a relatively small influence upon the electric field
sensor 14. The electric field sensor 14 in the seat bottom 24
distinguishes between a large body immediately above the seat cover
28--for example a normally seated, forward facing occupant in the
seat--and an infant or child seat--including rear facing, front
facing and booster seats--located on a vehicle seat 22. When the
vehicle seat 22 contains a child seat (including a rear facing
infant seats, a forward facing child seat and a booster seats), or
when the vehicle seat 22 is empty, no forward facing occupant is
detected near to the seat bottom and, as a result, the electric
field sensor 14 causes the restraint actuator 44 to be
disabled.
[0048] An electrode 26 of the electric field sensor 14 may be
constructed in a variety of ways, and the method of construction is
not considered limiting. For example, an electrode 26 may be
constructed using rigid circuit board or a flexible circuit using
known printed circuit board techniques such as etching or
deposition of conductive materials applied to a dielectric
substrate. Alternately, an electrode 26 may comprise a discrete
conductor, such as a conductive film, sheet or mesh that is
distinct from or an integral part of the vehicle seat 22 or
components thereof. The assembly of one or more electrodes 26
together with the associated substrate is sometimes referred to as
a sensing pad or a capacitive sensing pad 54.
[0049] In an exemplary embodiment, the electric field sensor 14
comprises a capacitive sensing pad 54 connected to an electronics
module 56 containing the sensing circuit 48 necessary to measure
the capacitance of the capacitive sensing pad 54 relative to the
circuit ground 52, or another measurement, responsive to the
influence of an electric-field-influencing medium upon the electric
field sensor 14. In operation, an occupant 18 seated on the seat
bottom 24 of vehicle seat 22 sufficiently increases the capacitance
of the electric field sensor 14 so as to indicate the presence of
the occupant. The capacitive sensing pad 54 is adapted so as to
provide a different response to large objects, such as normally
seated adults, on the seat bottom 24--for which an air bag
restraint system would be beneficial in a crash,--than to objects
such as rear facing infant seats, child seats, and booster seats on
the vehicle seat--for which an air bag restraint system would not
be beneficial in a crash.
[0050] The seat weight sensor 12, electric field sensor 14 and a
crash sensor 58 are operatively coupled to the controller 16, which
operates in accordance with known analog, digital, or
microprocessor circuitry and software, and in accordance with one
or more processes described hereinbelow, to control the actuation
of the restraint actuator 44 responsive to signals from the seat
weight sensor 12 and the electric field sensor 14 indicative of a
seat occupancy scenario; and responsive to a signal from the crash
sensor 58, indicative of a crash. For the example of a restraint
actuator 44 comprising an air bag inflator module 44', responsive
to a crash detected by the crash sensor 58, if the occupant
detection system 10 has enabled actuation of the restraint actuator
44, then the controller 16 generates a signal 60 which is
operatively coupled to one or more initiators 62 of one or more gas
generators 64 mounted in an air bag inflator module 44', thereby
controlling the activation of the air bag inflator module 44' so as
to inflate the air bag 66 as necessary to protect the occupant 18
from injury which might otherwise be caused by the crash. The
electrical power necessary to carry out these operations is
provided by a source of power 68, e.g. the vehicle battery. In
another embodiment, the occupant detection system 10 may make the
deployment enable/disable decision for the restraint actuator 44,
and communicate this decision to the controller 16 for controlling
the actuation of the restraint actuator 44. In yet another
embodiment, the occupant detection system 10 may incorporate the
crash sensor 58 and the elements of the controller 16 in a single
module that controls the actuation of the restraint actuator 44 as
described hereinabove.
[0051] Referring to FIG. 3, the occupant detection system 10 can be
used to distinguish infants or children in rear facing infant
seats, child seats or booster seats, from adults, on the basis that
the child 300 therein does not have a large surface of its body
very near to the seat bottom 24 and the at least one electrode 26
contained therein. For example, for the electric field sensor 14
providing a signal responsive to the capacitance of at least one
electrode 26 thereof, a normally seated occupant provides a
substantially larger increase in capacitance relative to an empty
seat, than does a child seat 302, e.g. a rear facing infant seat
304. The occupant detection system 10 can discriminate a rear
facing infant seat 304 (RFIS), or generally a child seat 302, from
an adult occupant 18 because the child 300 in a rear facing infant
seat 304 does not have a large surface of its body very near to the
seat bottom 24 and the at least one electrode 26 contained therein.
The seating contour 306 inside the rear facing infant seat 304 is
such that the buttocks of the child 300 are closest to the seat
bottom 24 of the vehicle seat 22. Usually there is a significant
gap 308, up to several inches, between the child 300 and the seat
bottom 24 of the vehicle seat 22. Since child seats are typically
made of plastic, the seats themselves are not sensed directly by
the electric field sensor 14. Even for a rear facing infant seat
304 for which the gap 308 between the child 300 and the seat bottom
24 of the vehicle seat 22 is relatively small, the inside seating
contour 306 still creates a significant gap between the at least
one electrode 26 and all parts of the child 300 except the
buttocks. Since only a small portion of the surface of the child
300 is near to the at least one electrode 26, the capacitance
measured by the electric field sensor 14 is relatively low, and
more particularly, less than the threshold capacitance, C.sub.norm
for detecting a normally seated occupant 18.
[0052] Referring to FIGS. 4a and 4b, the sensitivity to a rear
facing infant seat 304 of an elementary capacitive sensing pad
54.1, shown in FIG. 4a, comprising a continuous conductive sheet
electrode 26, can be reduced by the modification shown in FIG. 4b,
particularly for a rear facing infant seat 304 that exhibits a
relatively small gap 308 between the capacitive sensing pad 54.1
and the child 300. Referring to FIG. 4b, the portion of the child
seat 302 where the gap 308 is small, when the child seat 302 is
properly installed, is usually within a zone between 9 and 12
inches from the seat back and across the entire seat bottom 24. The
capacitive sensing pad 54.2 is adapted to make this zone less
sensitive than the remaining portion of the capacitive sensing pad
54.1 by removing at least one region 400 of the at least one
electrode 26 within the area of greatest sensitivity. Accordingly,
this increases the differentiation between a worst case signal for
a rear facing infant seat 304 and the signal for a normally seated
adult. Whereas, for example, rectangular slots are illustrated in
FIG. 4b, one of ordinary skill in the art will recognize that the
modification to the capacitive sensing pad 54.2 within the zone can
be accomplished with a variety of geometries so as provide for a
similar effect on the sensitivity pattern of the capacitive sensing
pad 54.2. For example FIGS. 5a and 5b illustrates at least one
region 400 within which the conductor is removed from the at least
one electrode 26 so as to reduce the sensitivity thereof to an
object proximate to the respective at least one region 400. In FIG.
5a, the capacitive sensing pad 54.3 comprises two regions 400
within which the conductor is removed, and in FIG. 5b, the
capacitive sensing pad 54.4 comprises one region 400 within which
the conductor is removed.
[0053] The temperature range that is possible in an automotive
environment can potentially adversely affect the sensing circuit 48
associated with the electric field sensor 14, causing a drift in
the "perceived" sensor reading. One way to combat this drift is to
use a reference capacitor that can be switched into the measurement
circuit in place of the sensing electrode. Because the reference
capacitor can be selected such that its value is relatively stable
over temperature, drift can be identified and this information can
be used to alter a decision threshold. An alternative scheme is to
always measure the difference between a reference capacitor and the
sensor capacitance. A second "calibration" capacitor can then be
switched in to take the place of the sensor to identify the
measurement system gain. Using a reference capacitor and a
calibration capacitor allows the system to continuously compensate
for variations in the measurement circuit. Rather than attempting
to measure the temperature and then make a correction, the
reference and calibration capacitor are used to measure the current
offset and gain of the measurement circuitry so that measurements
are always consistent. Switching between the reference capacitor,
the calibration capacitor, or a sensor can be done using a
combination of FET's or an analog demultiplexer such as a CD4051
from Texas Instruments.
[0054] Referring to FIGS. 6a and 6b, the sensing circuit 48 is
provided with a switchable calibration capacitor C.sub.cal that
enables an associated gain factor to be measured over time during
the operation of the electric field sensor 14, so as to provide for
drift compensation.
[0055] Accurately switching in and out a relatively small (e.g. 1
picofarad or less) calibration capacitance can be difficult. One
side of the calibration capacitor C.sub.cal is operatively
connected to the at least one electrode 26 and to the inverting
input of an amplifier 600 (U.sub.1). As illustrated in FIG. 6a, the
other side of the calibration capacitor C.sub.cal is switched to
ground by a first switch S.sub.1, so that when first switch S.sub.1
is closed, the capacitance of calibration capacitor C.sub.cal is
added to that of the electrode 26. However, one problem with this
arrangement of FIG. 6a with only a first switch S.sub.1 is that
when the first switch S.sub.1 is opened, the capacitance of the
first switch S.sub.1 is typically larger than the capacitance
C.sub.cal of the calibration capacitor C.sub.cal, thereby defeating
the purpose of the calibration capacitor C.sub.cal. For example, a
typical FET may have an OFF capacitance of 40 picofarads, so if the
capacitance C.sub.cal is 1 picofarad, then the series combination
is 0.98 picofarad, which means that effectively the calibration
capacitor C.sub.cal is never switched out of the circuit.
[0056] This problem is overcome by the arrangement of FIG. 6b,
wherein the other side of the calibration capacitor C.sub.cal is
switched to the non-inverting input of the amplifier 600 (U.sub.1)
by a second switch S.sub.2 When the first switch S.sub.1 is closed
and the second switch S.sub.2 is open, one side of the calibration
capacitor C.sub.cal is pulled to ground, thereby switching the
calibration capacitor C.sub.cal into the circuit. When the first
switch S.sub.1 is opened and the second switch S.sub.2 is closed,
both sides of the calibration capacitor C.sub.cal are driven by the
same signal, preventing any current from flowing through the
calibration capacitor C.sub.cal, thereby effectively switching the
calibration capacitor C.sub.cal out of the circuit.
[0057] Referring to FIG. 7, one technique for measuring a
capacitance C.sub.X is to measure the voltage from a capacitive
voltage divider 702 comprising a known capacitance C.sub.1 in
series
V X = V s ( C 1 C 1 + C X ) ##EQU00001##
with the capacitance C.sub.X to be measured, wherein an oscillating
voltage source V.sub.S is applied across the capacitive voltage
divider 702 and a voltage V.sub.X responsive to the capacitance
C.sub.X is measured at the junction 704 of the capacitive voltage
divider 702 between the known capacitance C.sub.1 and the
capacitance C.sub.X to be measured. For both the known capacitance
C.sub.1 and the capacitance C.sub.X to be measured represented as
pure capacitances for purposes of illustration, the voltage V.sub.X
is given by:
[0058] Accordingly, if both C.sub.1 and V.sub.S are known, then
C.sub.X can be determined from V.sub.X. However, as described
above, V.sub.S, C.sub.1 or the associated circuitry may subject to
drift over time
CS 1 = CR 1 + CR 2 ( 1 - VR 1 VS 1 1 - VR 1 VR 12 )
##EQU00002##
or as a result of environmental conditions, or subject to
system-to-system variation. The affect of this drift or variation
is compensated by repetitively switching the capacitance C.sub.X to
be measured from the unknown capacitance of an electric field
sensor to the known capacitance of one or more temperature stable
reference capacitors, wherein the repetitive switching process is
cycled sufficiently quickly so that that drift or variation over
the measurement cycle is negligible. For example, as illustrated in
FIG. 7, one or more various capacitances are switched into the
capacitive voltage divider 702 as capacitance C.sub.X by a
switching element 706. For example, as a first step, the switching
element 706 connects the at least one electrode 26 of the electric
field sensor 14 having a capacitance CS.sub.1 to the junction 704
of the capacitive voltage divider 702 as capacitance C.sub.X and a
corresponding voltage VS.sub.1 is measured as V.sub.X. Then as a
second step, the switching element 706 connects a first reference
capacitor CR.sub.1 to the junction 704 of the capacitive voltage
divider 702 as capacitance C.sub.X and a corresponding voltage
VR.sub.1 is measured as V. Then as a third step, the switching
element 706 adds a second reference capacitor CR.sub.2 to the
junction 704 of the capacitive voltage divider 702 so that the
capacitance Cx is given by the sum (CR.sub.1+CR.sub.2), and a
corresponding voltage VR.sub.12 is measured as V.sub.X. The period
of time between the first and third steps is sufficiently short for
there to be negligible drift in the measurement of V.sub.X over
that period of time. The three voltage measurements can then be
used to provide a measure of the capacitance CS.sub.1 of the at
least one electrode 26 of the electric field sensor 14--independent
of V.sub.S or C.sub.1--as follows:
[0059] The capacitance of at least one second electrode 26.2 of the
electric field sensor 14 containing first 26.1 and second 26.2
electrodes, is measured by repeating the above three step process,
except for switching the at least one second electrode 26.2 instead
of the at least one first electrode 26.1 during the first step.
[0060] Accordingly the electric field sensor 14 comprises at least
one electrode 26 operatively coupled to an applied signal Vs
thorough a capacitive voltage divider 702 so as to generate an
electric field proximate to the at least one electrode 26
responsive to a voltage V.sub.X on the at least one electrode 26.
The applied signal V.sub.S, for example, comprises an oscillating
signal. The at least one electrode 26 is operatively coupled to a
receiver 708 which outputs a response signal 710 responsive to the
electric field at the corresponding at least one electrode 26,
wherein the response signal 710 is responsive to at least one
electric-field-influencing property--for example dielectric
constant, conductivity, size, mass or distance--of an object
proximate to the electric field sensor 14. For example, for the
electric field sensor 14 as a capacitance sensor, the receiver 708
provides a measure of the capacitance of at least one electrode 26
with respect to a surrounding ground. The applied signal V.sub.S
is, for example, generated by an oscillator 712 incorporated in a
sensing circuit 714 that also incorporates the receiver 708.
[0061] The sensor measurements can be made by a single sensing
circuit 714 that incorporates a switching element 706 to
operatively couple either the at least one electrode 26, the at
least one first electrode 26.1, or the at least one second
electrode 26.2 to a common oscillator 712 and receiver 708 for
generating the respective measures of capacitance CS.sub.1,
CS.sub.2.
[0062] The capacitance of the at least one electrode 26, the at
least one first electrode 26.1, or the at least one second
electrode 26.2 relative to ground is relatively small, for example
less than about 300 picofarads. The temperature range that is
possible in an automotive environment can significantly affect the
components of the sensing circuit 714, causing drift that could be
erroneously interpreted as a measurement that could cause the
restraint actuator 44 to be erroneously enabled by the controller
16. The effects of this drift can be mitigated by incorporating a
temperature stable reference capacitor in the sensing circuit 714
that is switched in place of either the at least one first
electrode 26.1 or the at least one second electrode 26.2 so as to
provide a means for making comparative capacitive measurements.
[0063] Referring to FIG. 8, illustrating an exemplary sensing
circuit 714, an oscillator 802 generates an oscillating signal, for
example a sinusoidal signal, that is filtered by a first bandpass
filter 804 so as to create a first oscillating signal 806. The
first oscillating signal 806 is applied to a capacitive voltage
divider 808 comprising capacitor C.sub.1, resistors R.sub.1 and
R.sub.2, and one or more capacitive elements to be measured,
selected from at least one electrode 26, at least one first
electrode 26.1, at least one second electrode 26.2, a first
reference capacitor C.sub.R1, and a second reference capacitor
C.sub.R2, wherein the capacitive elements to be measured are
included or excluded responsive to the states of respective FET
switches Q.sub.1a, Q.sub.1b, Q.sub.2a, Q.sub.2b, Q.sub.3a,
Q.sub.3b, Q.sub.4a and Q.sub.4b. Capacitor C.sub.1, resistors
R.sub.1 and R.sub.2, and the FET switches Q.sub.1a, Q.sub.2a,
Q.sub.3a and Q.sub.4a--that when active switch in the respective
capacitive elements to be measured,--are all connected to one
another at a first node 810, which is connected to the input 812 of
a voltage follower U.sub.1. The output 814 of the voltage follower
U.sub.1 is connected to FET switches Q.sub.1b, Q.sub.2b, Q.sub.3b
and Q.sub.4b that when active, switch out the respective capacitive
elements so as to not be measured. The activation of the FET switch
elements of FET switch pairs Q.sub.1a and Q.sub.1b, Q.sub.2a and
Q.sub.2b, Q.sub.3a and Q.sub.3b and Q.sub.4a and Q.sub.4b are
respectively mutually exclusive. For example if FET switch Q.sub.1a
is activated or closed, then FET switch Q.sub.1b is deactivated or
open. A capacitive element being measured adds to the capacitance
at the first node, thereby affecting the strength of the signal at
the input 812 to the voltage follower U.sub.1. A capacitive element
not being measured is disconnected from the first node by its
respective first FET switch element, and connected to the output
814 of the voltage follower U.sub.1 by its respective second FET
switch element, wherein, in accordance with the characteristics of
the associated operational amplifier of the voltage follower
U.sub.1, the output 814 of the voltage follower U.sub.1 follows the
signal of the first node without that respective capacitive element
connected, and voltage follower U.sub.1 provides a current through
the associated capacitive element through the second respective FET
switch element. Moreover, when the respective second FET switch
element is activated, the source and drain of the respective first
FET switch element are separately coupled to the respective
operational amplifier inputs, so that to each is applied the same
potential, thereby eliminating the affect of the capacitance of the
respective first FET switch on the capacitance measurement.
[0064] The output 814 of the voltage follower U.sub.1 is then
coupled to a second bandpass filter 816 of the same pass band as
the first bandpass filter 804, the output of which is detected by a
detector 818 comprising diode D.sub.1, resistor R.sub.3 and
capacitor C.sub.2, and filtered by a first low pass filter 820. The
output 822 of the first low pass filter 820 has a DC component
corresponding to the capacitance at the first node 810. This DC
component is filtered by a blocking capacitor C.sub.3, and the
resulting signal is filtered by a second low pass filter 824 to
provide the amplitude 826 of the oscillating signal at the first
node 810, which is related to the total capacitance at that
location. The blocking capacitor C.sub.3 is adapted so as to
provide for a transitory measurement of the amplitude 826.
[0065] In operation, a microprocessor U.sub.2 controls the
activation of FET switches Q.sub.1a, Q.sub.1b, Q.sub.2a, Q.sub.2b,
Q.sub.3a, Q.sub.3b, Q.sub.4a and Q.sub.4b, for example in
accordance with the control logic illustrated in FIG. 9. With the
first reference capacitor C.sub.R1 switched in by microprocessor
U.sub.2, i.e. with Q.sub.2a activated and Q.sub.2b deactivated, the
controller measures a first amplitude. Then with the second
reference capacitor C.sub.R2 also switched in by microprocessor
U.sub.2, a second amplitude is measured corresponding to an
incremental increase of capacitance at the first node by the
capacitance of capacitor C.sub.R2. Then a sensitivity factor is
computed in Volts/picofarad given the known values of capacitance
of capacitors C.sub.R1 and C.sub.R2 as described hereinabove with
reference to FIG. 7. Then, the microprocessor U.sub.2 switches out
the first C.sub.R1 and second reference capacitor C.sub.R2,
switches in the capacitive sensing pad 102, measures a third
amplitude, and calculates the capacitance of either the at least
one electrode 26 or the at least one second electrode
26.2--depending upon which is being measured--using the calculated
sensitivity factor.
[0066] A control circuit 828 uses the measures of capacitance from
the electric field sensor 14 and the measure of weight W from the
seat weight sensor 12--in accordance with the steps described
hereinbelow--to control whether or not the restraint actuator 44 is
enabled responsive to a crash detected by a crash sensor 58.
Whereas FIG. 8 illustrates the microprocessor U.sub.2 and control
circuit 828 as separate elements, alternate arrangements are
possible. For example, both may be combined in one controller, or
the microprocessor may be adapted to sense the amplitude
measurements, calculate the capacitance of the first 12 and second
14 electric field sensors, and then output these capacitance values
to the control circuit 828.
[0067] The at least one electrode 26 and the at least one second
electrode 26.2 may be each modeled as a first capacitance C.sub.S1
in parallel with a series combination of a second capacitance
C.sub.S2 and a resistance R.sub.S, wherein the resistance R.sub.S
is inversely related to the wetness of the seat. The capacitance of
the capacitive sensor is dominated by C.sub.S1 for a dry seat, but
becomes affected by C.sub.S2 and R.sub.S as the wetness of the seat
increases.
[0068] The values of capacitance for capacitors C.sub.1, C.sub.R1,
and C.sub.R2 may be adapted to maximize the dynamic range of the
capacitance measurement over the range of expected capacitances of
the first 12 and second 14 electric field sensors.
[0069] Referring to FIG. 10, each FET switch Q.sub.1a, Q.sub.1b,
Q.sub.2a, Q.sub.2b, Q.sub.3a, Q.sub.3b, Q.sub.4a or Q.sub.4b may be
replaced by a pair of FET switches Q.sup.1 and Q.sup.2. Designating
the terminals of the original FET switch Q as G, S and D for the
gate, source and drain respectively, these terminals are mapped to
the terminals of the pair of FET switches Q.sup.1 and Q.sup.2 as
follows: 1) the respective gates G.sup.1 and G.sup.2 are connected
together and are mapped to G; 2) the sources S.sup.1 and S.sup.2
are connected together; 3) the drain D.sup.1 of FET switch Q.sup.1
is mapped to D; and 4) the drain D.sup.2 of FET switch Q.sup.2 is
mapped to S. This arrangement is beneficial for three-pin FET
switches for which the source is connected to the body, thereby
effectively creating a diode junction between the source and drain,
as is illustrated in FIG. 10. With the sources S.sup.1, S.sup.2
interconnected, these effective diode junctions are placed
back-to-back in series with opposing polarities, so as to prevent
the passage of a signal without being under control of the
respective gates G.sup.1, G.sup.2. Furthermore, the drain-source
capacitance of the pair of FET switches Q.sup.1 and Q.sup.2 is half
that of one FET switch Q.sup.1, because the respective capacitances
are connected in series.
[0070] FIG. 11 illustrates several other embodiments for various
aspects of the sensing circuit 714.
[0071] For example, the elements to be sensed at the first node 810
may be coupled via an analog demultiplexer 1102, such as a CD4051
from Texas Instruments, wherein under control of the microprocessor
U.sub.2, the elements to be sensed are coupled, one element at a
time, to the first node 810 by the analog demultiplexer 1102. For
example, first C.sub.R1a and second C.sub.R2a reference capacitors
and a capacitive sensor are each operatively connected to distinct
analog inputs of the analog demultiplexer 1102, and are operatively
connected--mutually exclusively--to the first node 810 by the
analog demultiplexer 1102. Accordingly, with this arrangement, the
calibration process differs from that illustrated in FIGS. 10a-b
for which two reference capacitors can be simultaneously
operatively connected to the first node 810. A plurality of analog
demultiplexers 1102 may be used if more analog channels are
required, in which case a separate set of reference capacitors, for
example C.sub.R1b and C.sub.R2b, may be used with each separate
analog demultiplexer 1102 to compensate for variations amongst the
various analog demultiplexers 1102.
[0072] As another example of another embodiment, an inductor L1 may
be placed between the sensing node 810 and the elements to be
sensed in order to reduce the effects of electromagnetic
interference.
[0073] As yet another example of another embodiment, a D/A
converter 1104 under control of the microprocessor U.sub.2 may be
used to cancel offsets in the associated amplitude signal, wherein
the output from the D/A converter 1104 is operatively connected to
an inverting amplifier 1106, and is subtracted from the filtered
detected amplitude signal 1108. By canceling the offset in the
amplitude signal, the associated circuit gain can be increased so
as to increase the dynamic range of the amplitude signal.
[0074] As yet another example of another embodiment, a super diode
detection circuit 1110 may be used for detecting the signal
amplitude.
[0075] Referring to FIG. 12, in accordance with a method 1200 of
detecting an occupant 18 and controlling a restraint actuator 44
responsive thereto, in step (1202), a measure of seat weight W is
either provided by or generated responsive to a signal provided by
the seat weight sensor 12. Then, in step (1204), if the measure of
seat weight W is less than a corresponding weight threshold
W.sup.Threshold, then, in step (1206), the restraint actuator 44 is
disabled. For example, the weight threshold W.sup.Threshold is
adapted to correspond to an upper bound of the weight of a small
occupant (e.g. about 60 pounds or 27 Kilograms) that would be
susceptible to injury from the deployment of the restraint actuator
44. Otherwise, from step (1204), if, in step (1300)--a method 1300
of detecting a child seat 302 on a vehicle seat 22,--a child seat
302 is detected on the vehicle seat 22 by the electric field sensor
14 in the seat bottom 24, then in step (1206), the restraint
actuator 44 is disabled. Otherwise, the restraint actuator 44 is
enabled. Accordingly, the restraint actuator 44 is disabled for
either an empty vehicle seat 22, or for an occupant 18 on the
vehicle seat 22 that is potentially at risk of injury from the
deployment of the restraint actuator 44, e.g. a sufficiently small
child, or a child in a child seat 302, e.g. a rear facing infant
seat 304. Otherwise, in step (1208), the restraint actuator 44 is
enabled, e.g. for a normally seated adult occupant 18 on the
vehicle seat 22.
[0076] Referring to FIG. 13, in accordance with a first method
1300.1 of detecting a child seat 302 on a vehicle seat 22, in step
(1302), the sensing circuit 48 generates a measure of the
capacitance C of the at least one electrode 26 of the electric
field sensor 14 in the seat bottom 24. The electrode 26 is adapted,
e.g. as illustrated in FIGS. 3a, 3b, 4a or 4b, so that the
capacitance thereof for a child seat 302 in the vehicle seat 22 is
substantially less (i.e. by a detectable difference) than the
capacitance of the electrode 26 for an occupant 18 seated on the
vehicle seat 22. Then, in step (1304), if the measure of the
capacitance C is less than a discrimination threshold
C.sup.Threshold, then in step (1306) a result is provided
indicating that a child seat 302 has been detected. For example,
for one particular electrode 26, the discrimination threshold
C.sup.Threshold was about 10 picofarads. Otherwise, from step
(1304), in step (1308), a result is provided indicating that a
child seat 302 has not been detected.
[0077] The measurements of the seat weight sensor 12 and electric
field sensor 14 as used in the above-described methods (1200, 1300)
are, in one set of embodiments, actually differential measurements
with respect to corresponding stored values of measurements for of
an empty vehicle seat 22. For example, for a seat weight sensor 12
that measures the weight of the entire vehicle seat 22, the stored
weight of the empty vehicle seat 22 is subtracted from the measured
seat weight so as to provide the weight of the object on the
vehicle seat 22, which is then used in the method 1200 of detecting
an occupant 18 and controlling a restraint actuator 44 responsive
thereto. Similarly, the stored capacitance measurement of the
electric field sensor 14 for an empty seat is subtracted from the
capacitance measurement of the electric field sensor 14, and this
difference is used in the method 1300 of detecting a child seat 302
on a vehicle seat 22.
[0078] Whereas a seat weight sensor 12 alone might otherwise have
difficulty distinguishing between the 60 lb. child on a 10 pound
booster seat (child seat 302) from a small adult occupant 18, the
electric field sensor 14 can distinguish between a child seat 302
and an adult occupant 18. Also, if the lap belt were cinched tight
on a rear facing infant seat 304, the force on the seat may be very
high, but the electric field sensor 14 can identify that there is
no adult occupant 18 seated directly on the seat bottom 24. A child
300 is seated directly on the seat bottom 24 can be detected by the
seat weight sensor 12.
[0079] Accordingly, the occupant detection system 10 provides for
enabling actuation of the restraint actuator 44, responsive to a
crash detected by the crash sensor 58, if the seat weight sensor 12
detects an occupant 18 (or object) of sufficient weight is on the
vehicle seat 22, and if the electric field sensor 14 indicates that
a child seat 302 is not on the vehicle seat 22. Otherwise, the
restraint actuator 44 is disabled so as to not be actuated
responsive to a crash detected by the crash sensor 58. A child seat
302 is typically secured to the vehicle seat 22 with a cinched seat
belt than can cause a substantial force on the vehicle seat 22, of
a magnitude that might otherwise be interpreted as an adult
occupant 18. In this case, the seat weight sensor 12 and the
electric field sensor 14 cooperate, wherein the electric field
sensor 14 detects the presence of the child seat 320 responsive to
an associated relatively low measure of capacitance so as to
prevent the restraint actuator 44 from otherwise being enabled. The
components of the seat weight sensor 12 and the electric field
sensor 14 can all be incorporated in the vehicle seat 22 so as to
provide for testing of the occupant detection system 10 in the
vehicle seat 22 prior to assembly in the vehicle 20. Furthermore,
electronics associated with the seat weight sensor 12, electric
field sensor 14 and controller 16 can be incorporated in a common
electronics module, or incorporated in separate electronics
modules.
[0080] Referring to FIG. 14, the capacitive sensing pad 54.5
mountable within the seat bottom 24 is adapted to detect a child
seat 302 thereon by incorporating a plurality of electrodes 26,
i.e. first 26.1 and second 26.2 electrodes, wherein the first
electrode 26.1 is located and shaped so as to principally sense a
region where the gap 308 between the child 300 and the capacitive
sensing pad 54.5 could be small, and the second electrode 26.2
senses the remaining portion of the seat bottom 24. Each of the
first 26.1 and second 26.2 electrodes is either operatively
connected to separate sensing circuits 48, or to separate
multiplexed channels of a common sensing circuit 48, so that the
one or more sensing circuits 48 provide separate first C.sub.1 and
second C.sub.2 measures of capacitance of the respective first 26.1
and second 26.2 electrodes. If the total signal, i.e. the sum of
C.sub.1 and C.sub.2, is relatively low and is dominated by the
signal from the first measure of capacitance C.sub.1, then the
corresponding object on the vehicle seat 22 is likely a child seat
302, e.g. a rear facing infant seat 304.
[0081] More particularly, referring to FIG. 15, in accordance with
a second method 1300.2 of detecting a child seat 302 on a vehicle
seat 22, in step (1502) the sensing circuit 48 generates a first
measure of capacitance C.sub.1 of the first 26.1 electrode, and in
step (1504) the sensing circuit 48 generates a second measure of
capacitance C.sub.2 of the second electrode 26.2 of the electric
field sensor 14 in the seat bottom 24. Then, in step (1506), if the
total measure of capacitance (C.sub.1+C.sub.2) is not less than a
discrimination threshold C.sup.Threshold2,--e.g. indicative of an
occupant 18 likely seated directly on the vehicle seat 22--then in
step (1508) a result is provided indicating that a child seat 302
has not been detected. Otherwise, in step (1510), if ratio of the
first measure of capacitance C.sub.1 of the first electrode
26.1--located so as to most proximate to the gap 308 of a child
seat 302 when the child seat 302 is on the vehicle seat 22--to the
total measure of capacitance (C.sub.1+C.sub.2), is greater than a
threshold, then in step (1512) a result is provided indicating that
a child seat 302 has been detected. Otherwise, from step (1510), in
step (1508), a result is provided indicating that a child seat 302
has not been detected.
[0082] The electric field sensor 14 may be adapted to reduce the
affect that liquids proximate to an electrode 26 can have on the
capacitance thereof with respect to a circuit ground 52, or with
respect to another electrode. For example, liquids spilled on and
absorbed by the foam cushion 30 can increase the capacitance of an
electrode 26 with respect to the circuit ground 52. Referring to
FIG. 16, the electric field sensor 14 can be adapted to reduce the
effect of a wetting of the foam cushion 30 by incorporating a third
electrode 1600, known as a driven shield 1600', and/or a fourth
electrode 1602, known as a ground plane 1602', under the at least
one first electrode 26.1, known as a sense electrode 26', wherein
the first 26.1, third 1600 and fourth 1602 electrodes are insulated
from one another, for example by at least one dielectric substrate.
For example, the first 26, third 1600 and fourth 1602 electrodes
may be integrated so as to form a single capacitive sensing pad
1604'. The driven shield 1600' is a second conductor under the
conductor of the sense electrode 26' that is driven at the same
potential as the sense electrode 26', resulting in a cancellation
of the electric field between the sense electrode 26' and the
driven shield 1600'. The driven shield 1600' substantially
eliminates the sensing capability of the capacitive sensing pad
704' on the side of the sense electrode 26' where the driven shield
1600' is located. A ground plane 1602' may be placed under the
driven shield 1600' so that the circuit driving the driven shield
1600' drives a consistent load.
[0083] Accordingly, as so adapted, the electric field sensor 14
further comprises at least one third electrode 1600 and at least
one fourth electrode 1602, wherein the at least one third electrode
1600 is located between the at least one first electrode 26.1 and
the at least one fourth electrode 1602, and the at least one third
electrode 1600 is operatively coupled to a second applied signal
1606. For example, the at least one third electrode 1600 is
substantially the same size as the at least one first electrode
26.1; the second applied signal 1606 is substantially the same as
the applied signal 46; the at least one fourth electrode 1602 is
located between the at least one first electrode 26.1 and a foam
cushion 30 of the vehicle seat 22; the at least one fourth
electrode 1602 is substantially the same size as the at least one
first electrode 26.1; and the at least one fourth electrode 1602 is
operatively connected to a circuit ground 52, or to a third applied
signal 1608, wherein the third applied signal 1608 is a circuit
ground 52 potential.
[0084] The driven shield 1600' and/or ground plane 1602' are, for
example, near to or slightly larger than the sense electrode 26',
and are provided to minimize the effects of liquid in the foam
cushion 30 below the driven shield 1600' and/or the ground plane
1602' on the capacitance of the sense electrode 26', rather than to
extend the range and sensitivity of the electric field sensor. The
driven shield 1600' and the sense electrode 26' essentially covers
the entire area to be sensed on the vehicle seat 22. Alternately, a
plurality of first electrodes 26.1 can be distributed sparsely
across the vehicle seat 22, thereby covering a smaller area than
the entire area to be sensed on the vehicle seat 22. Each electrode
26 can be embodied in a variety of sizes and shapes, and for a
plurality of first electrodes 26.1, the arrangement thereof can be
embodied in a variety of patterns.
[0085] Referring to FIGS. 17a and 18a, a capacitive sensing pad
54.6 comprising a sense electrode (S) 26 may be adapted to provide
similar functionality as the capacitive sensing pad 54.5
illustrated in FIG. 14 by incorporating a front driven shield (FDS)
1702 located and shaped similar to the first electrode 26.1 of the
capacitive sensing pad 54.5 illustrated in FIG. 14. The front
driven shield (FDS) 1702 is located on the side of the sense
electrode (S) 26 that is to be sensed thereby. The capacitive
sensing pad 54.6 further comprises a rear driven shield (RDS) 1704
that functions similar to the driven shield 1600' illustrated in
FIG. 16. A signal generator 1706 provides an oscillatory signal
1708 that is coupled directly to the rear driven shield (RDS) 1704
and indirectly through the sensing circuit 48 to the sense
electrode (S) 26. The oscillatory signal 1708 from the signal
generator 1706 is also coupled through a switch 1710 to the front
driven shield (FDS) 1702. When the switch 1710 is closed, the
charge on the front driven shield (FDS) 1702 is substantially the
same as on the corresponding region of the sense electrode (S) 26,
thereby substantially shielding that region of the sense electrode
(S) 26 from external influence. When the switch 1710 is open, the
front driven shield (FDS) 1702 is electrically floating, thereby
enabling an external electrostatic influence of the corresponding
region of the sense electrode (S) 26. The front driven shield (FDS)
1702 is insulated from the sense electrode (S) 26 by a first
insulator 1712, and the sense electrode (S) 26 is insulated from
the rear driven shield (RDS) 1704 by a second insulator 1714.
[0086] Referring to FIGS. 19 and 20, in accordance with another
embodiment, in a shielding mode, the front driven shield (FDS) 1702
is switched by a first switch 1716 (S.sub.1) to a buffered version
of the oscillatory signal 1708 so as to electrostatically shield
the sense electrode (S) 26. In a sensing mode, the first switch
1716 (S.sub.1) is opened, thereby disconnecting the front driven
shield (FDS) 1702 from the oscillatory signal 1708, and the front
driven shield (FDS) 1702 either is operatively connected to the
sense electrode (S) 26 by closing a second switch 1718 (S.sub.2)
therebetween, as illustrated in FIG. 20; or is electrically
floating, as illustrated in FIG. 21 and described hereinabove.
[0087] Referring to FIG. 22, the capacitive sensing pad 54.6 is
operated in accordance with a third method 1300.3 of detecting a
child seat 302 on a vehicle seat 22, wherein in step (2202), the
front driven shield (FDS) 1702 is activated so as to shield the
sense electrode (S) 26, and in step (2204) the sensing circuit 48
generates a second measure of capacitance C.sub.2 of the sense
electrode (S) 26. Then, in step (2206) the front driven shield
(FDS) 1702 is deactivated, and in step (2208) the sensing circuit
48 generates third measure of capacitance C.sub.3 of the sense
electrode (S) 26. Then, in step (2210), if the total measure of
capacitance C.sub.3 is not less than a discrimination threshold
C.sup.Threshold2,--e.g. indicative of an occupant 18 likely seated
directly on the vehicle seat 22--then in step (2212) a result is
provided indicating that a child seat 302 has not been detected.
Otherwise, in step (2214), if a ratio of a measure corresponding to
the first measure of capacitance C.sub.1=C.sub.3-C.sub.2 of the
sense electrode (S) 26 to the total measure of capacitance C.sub.3,
is greater than a threshold, then in step (2216) a result is
provided indicating that a child seat 302 has been detected.
Otherwise, from step (2214), in step (2212), a result is provided
indicating that a child seat 302 has not been detected.
[0088] Referring to FIGS. 17b and 18b, a capacitive sensing pad
54.7 comprising a sense electrode (S) 26 may be adapted to provide
similar functionality as the capacitive sensing pad 54.5
illustrated in FIG. 14 by incorporating a front driven shield (FDS)
1702' located and shaped similar to the second electrode 26.2 of
the capacitive sensing pad 54.5 illustrated in FIG. 14. The front
driven shield (FDS) 1702' is located on the side of the sense
electrode (S) 26 that is to be sensed thereby. The capacitive
sensing pad 54.7 further comprises a rear driven shield (RDS) 1704
that functions similar to the driven shield 1600', 1704 illustrated
in FIGS. 16 and 17a respectively. A signal generator 1706 provides
an oscillatory signal 1708 that is coupled directly to the rear
driven shield (RDS) 1704 and indirectly through the sensing circuit
48 to the sense electrode (S) 26. The oscillatory signal 1708 from
the signal generator 1706 is also coupled through a switch 1710 to
the front driven shield (FDS) 1702'. When the switch 1710 is
closed, the charge on the front driven shield (FDS) 1702' is
substantially the same as on the corresponding region of the sense
electrode (S) 26, thereby substantially shielding that region of
the sense electrode (S) 26 from external influence. When the switch
1710 is open, the front driven shield (FDS) 1702' is electrically
floating, thereby enabling an external electrostatic influence of
the corresponding region of the sense electrode (S) 26.
Alternately, the front driven shield (FDS) 1702' may be switched as
illustrated in FIGS. 19 and 20. The front driven shield (FDS) 1702'
is insulated from the sense electrode (S) 26 by a first insulator
1712, and the sense electrode (S) 26 is insulated from the rear
driven shield (RDS) 1704 by a second insulator 1714.
[0089] Referring to FIG. 23, the capacitive sensing pad 54.7 is
operated in accordance with a fourth method 1300.4 of detecting a
child seat 302 on a vehicle seat 22, wherein in step (2302) the
front driven shield (FDS) 1702' is activated so as to shield the
sense electrode (S) 26, and in step (2304) the sensing circuit 48
generates a first measure of capacitance C.sub.1 of the sense
electrode (S) 26. Then, in step (2306) the front driven shield
(FDS) 1702' is deactivated, and in step (2308) the sensing circuit
48 generates third measure of capacitance C.sub.3 of the sense
electrode (S) 26. Then, in step (2310), if the total measure of
capacitance C.sub.3 is not less than a discrimination threshold
C.sup.Threshold2,--e.g. indicative of an occupant 18 likely seated
directly on the vehicle seat 22--then, in step (2312), a result is
provided indicating that a child seat 302 has not been detected.
Otherwise, in step (2314), if ratio of the first measure of
capacitance C.sub.1 of the sense electrode (S) 26 to the total
measure of capacitance C.sub.3, is greater than a threshold, then,
in step (2316), a result is provided indicating that a child seat
302 has been detected. Otherwise, from step (2314), in step (2312),
a result is provided indicating that a child seat 302 has not been
detected.
[0090] Referring to FIGS. 24a and 24b, one potential source of
inconsistent capacitance measurements is inconsistent coupling to
circuit ground 52 by the occupant 18. The electric field sensor 14
is sensitive to this coupling because the magnitude of the
capacitance being sensed is relatively low. The electric field
sensor 14 measures the capacitance from the capacitive sensing pad
54 to circuit ground 52. Because the occupant 18 is very close to
the capacitive sensing pad 54 and the occupant 18 may be fairly
small, C.sub.so, the capacitance between the capacitive sensing pad
54 and the occupant 18, may be large compared to C.sub.og, the
capacitance between the occupant 18 and circuit ground 52. In this
case, the measurement of the capacitance from the capacitive
sensing pad 54 to circuit ground 52 will be dominated by C.sub.og
and the occupant 18 seated directly on the seat may be mistaken as
a child seat 302.
[0091] Referring to FIG. 25, the capacitive sensing pad 54.8 can be
adapted in accordance with the instant invention to provide
consistently high C.sub.og values. A group of relatively small
sense electrodes 2500 are distributed across the sensing area, with
relatively small ground planes 2502 distributed therebetween. An
occupant 18 seated directly on the vehicle seat 22 is seated close
to both the sense electrodes 2500 and the ground planes 2502.
Accordingly, C.sub.og will be consistently high such that the total
capacitance from the capacitive sensing pad 54.8 to the circuit
ground 52 will depend largely on C.sub.so. The ground planes 2502
should be placed far enough away from the sense electrodes 2500 so
that the corresponding range of capacitances of the capacitive
sensing pad 54.8 is not overly reduced so that the electric field
sensor 14.1 becomes impractical. This may require that the driven
shield 2504 extend beyond the sense electrode 2502. The driven
shield 2504 isolates the sense electrodes 2500 from the ground
planes 2502. One of ordinary skill in the art will recognize that
many variations of the capacitive sensing pad 54.8 are possible,
and that the arrangement of FIG. 25 is illustrative and not
limiting. The sense electrodes 2500, driven shield 2504 and ground
planes 2502 may be located either on a common plane, or on separate
planes in overlapping relationship with one another. The sense
electrodes 2500 are operatively coupled to the sensing circuit 48,
which measures the capacitance thereof with respect to the circuit
ground 52.
[0092] Referring to FIG. 26, another embodiment of a capacitive
sensing pad 54.9 with reduced sensitivity to a child seat 302
incorporates a sense electrode 2600 that comprises conductive
strips 2602 spaced apart in a lattice 2604. The capacitive sensing
pad 54.9 further comprises a ground plane 2606 that is located in
the region of the electric field sensor 14 where, when mounted in
the seat bottom 24, the gap 308 could be small between the seat
bottom 24 and a child in a rear facing infant seat 304, so as to
reduce the capacitance sensed when a rear facing infant seat 304 is
located on the vehicle seat 22. Accordingly, the ground plane 2600
substantially reduces the affect of any object immediately above
the area of the ground plane 2600, and precludes the need for a
driven shield, as described hereinabove.
[0093] Referring to FIG. 27, illustrating results from tests of an
electric field sensor 14 similar to that of FIG. 26--wherein the
data was taken with human subjects seated either directly on the
seat bottom 24 or in a child seat 302--there is a clear margin
between any of the child seat 302 cases and the occupants 18
weighing over 100 lbs. While a seat weight sensor 12 may have
difficulties distinguishing between the 60 pound child 300 on a 10
pound booster seat from a small adult occupant 18, the electric
field sensor 14 will identify that there is no adult occupant 18
seated directly on the seat bottom 24, and the system would
suppress the air bag inflator module 44'. Also, if the lap belt
were very tight on a rear facing infant seat 304, the force on the
vehicle seat 22 may be very high, but the electric field sensor 14
would identify that there is no adult occupant 18 seated directly
on the seat bottom 24 and, again, the air bag inflator module 44'
would be suppressed. If a child 300 is seated directly on the seat
bottom 24, a seat weight sensor 12 generally provides a reliable
measurement that can be used to control the air bag inflator module
44' deployment decision.
[0094] Referring to FIG. 28, in another embodiment of a capacitive
sensing pad 54.10 with reduced sensitivity to a child seat 302, the
sensor electrode 2800 comprises conductive strips 2802 spaced apart
in a lattice 2804 that is terminated at a plurality of first
terminals 2806, providing for improved redundancy and reliability.
The first terminals 2806 are operatively coupled to the sensing
circuit 48, which measures the capacitance at the first terminals
2806 with respect to circuit ground 52. A ground plane 2808 is
terminated at a second terminal 2810, which is either operatively
coupled to the sensing circuit 48, or directly coupled to circuit
ground 52.
[0095] The ground planes 2606, 2808 in FIGS. 26 and 28 can be
switched "in or out" to gain extra information. For example, the
ground planes 2606, 2808 could be left electrically floating,
resulting in a relatively small affect on the measurement, or could
be switched to circuit ground 52 to increase occupant-ground
capacitance C.sub.og. Additional information about the seat
occupancy scenario can be obtained by switching between these two
states. The relatively small area of the sense electrodes 2600,
2800 in FIGS. 26 and 28 also reduces the sensor-occupant
capacitance C.sub.so sufficiently so as to be significantly less
than the occupant-ground capacitance C.sub.og so that the
capacitance of the sense electrodes 2600, 2800 to circuit ground 52
is dominated by the sensor-occupant capacitance C.sub.so.
[0096] Referring to FIG. 29, in another embodiment of a capacitive
sensing pad 54.11 with reduced sensitivity to a child seat 302, the
electric field sensor 14 may be adapted with a receive electrode
2900 for sensing a signal transmitted from a sense electrode 2902
when an occupant 18 is seated proximate to both the receive
electrode 2900 and the sense electrode 2902. When the vehicle seat
22 is relatively wet, the foam cushion 30 may become saturated
causing the electric field sensor 14 in the seat bottom 24 to
identify an increase in signal large enough to represent an
occupant directly on the seat. The receive electrode 2900 in the
seat bottom 24 can be used to verify the occupant situation even
when the seat is saturated with water. The receive electrode 2900
is preferably in the same plane as the sense electrode 2902, and
the two electrodes 2900, 2902 are separated by a ground plane 2904
"gap". The ground plane 2904 also provides for reduced sensitivity
proximate to locations on the vehicle seat 22 that would be closest
to a child 300 in a child seat 302 thereon. The receive electrode
2900 senses the changes in the electric potential thereat caused by
changes in potential induced on the sense electrode 2902 through
capacitive coupling between the receive 2900 and sense 2902
electrodes. The amplitude of the signal from the receive electrode
2900 increase dramatically when there is a conductor coupling the
receive 2900 and sense 2902 electrodes, as is the case when a human
body part is well coupled to both electrodes 2900, 2902.
[0097] The relative amplitude of the signal from the receive
electrode 2900 is also dependent upon the signal frequency if the
vehicle seat 22/seat bottom 24/foam cushion 30 becomes wet. For
example, a signal having a relatively high frequency, e.g. above
about 1 Megahertz the signal is not conducted through the wet seat
materials as well as a signal with a relatively low frequency (or
long pulse length). A human body conducts is a relatively good
conductor of the relatively high frequency signal. Accordingly, at
frequencies above about 1 Megahertz, there can be a substantial
difference between a signal received by the receive electrode 2900
in an empty wet vehicle seat 22 and a signal received by the
receive electrode 2900 when an occupant 18 is seated directly on
the vehicle seat 22 (regardless of whether the vehicle seat 22 is
wet or dry).
[0098] Accordingly, the capacitive sensing pad 54.11 illustrated in
FIG. 29 provides for two sensing modes as follows: 1) sensing a
measure responsive to the capacitance of the sense electrode 2902,
and 2) sensing a signal from the receive electrode 2900 that is
coupled thereto from the sense electrode 2902 by an occupant 18.
Although the second sensing mode can be preferable with respect to
the first sensing mode when the vehicle seat 22 is wet, the first
sensing mode is beneficial when the vehicle seat 22 is dry because
of a relatively lower susceptibility to errors resulting from
various complicating child seat 302 cases. For example, a
relatively small piece of metal under the child seat 302 could
cause the second sensing mode to misidentify the situation as an
adult occupant 18 seated directly on the vehicle seat 22. Small,
ungrounded conductors generally do not substantially influence the
first sensing mode. Accordingly, both sensing modes used in
combination provide for improved robustness of the electric field
sensor 14. The effectiveness of a combination of the two sensing
modes is improved when a wet vehicle seat 22 is properly identified
and/or compensated, which can be done using frequency or phase
characteristics of the associated signals when the vehicle seat 22
is wet, as is disclosed in U.S. Pat. No. 6,392,543, which is
incorporated herein by reference. If the vehicle seat 22 is
sufficiently wet to significantly influence the measurements, then
the decision as to whether to deploy the restraint actuator 44 is
based on the results of the second sensing mode. It is possible to
further interdigitize the sense electrode 2902 and the receive
electrode 2900 so as to ensure that an occupant 18 seated directly
on the vehicle seat 22 will be coupled to both electrodes 2900,
2902 for most seating positions.
[0099] The seat weight sensor 12 and the electric field sensor 14
may be adapted to further cooperate with one another. For example,
for a seat weight sensor 12 comprising a pressure sensing system
that makes an assessment of the pressure pattern on the vehicle
seat 22, the electric field sensor 14 can be used as an additional
source of information to improve system robustness, e.g. so as to
properly accommodate otherwise complicating situations such as when
a towel is placed under a child seat 302. Furthermore, the electric
field sensor 14 in the seat bottom 24 can be integrated with a seat
weight sensor 12 comprising either a force sensing resistor or a
bend sensors because both sensor technologies could be incorporated
in the same sensing mat, possibly sharing one or more common
conductive elements thereof.
[0100] Generally, the sense electrode 2200, 2600, 2800, 2902 of the
capacitive sensing pad 54.8, 54.9, 54.10, 54.11 is distributed
sparsely across the vehicle seat 22, thereby covering a smaller
area than the entire area to be sensed on the vehicle seat 22. The
capacitive sensing pad 54.8, 54.9, 54.10, 54.11, and the elements
thereof, can be embodied in a variety of shapes.
[0101] While specific embodiments have been described in detail,
those with ordinary skill in the art will appreciate that various
modifications and alternatives to those details could be developed
in light of the overall teachings of the disclosure. Accordingly,
the particular arrangements disclosed are meant to be illustrative
only and not limiting as to the scope of the invention, which is to
be given the full breadth of the appended claims and any and all
equivalents thereof.
* * * * *