U.S. patent application number 12/633019 was filed with the patent office on 2011-06-09 for system and method of occupant detection with a resonant frequency.
This patent application is currently assigned to DELPHI TECHNOLOGIES, INC.. Invention is credited to Dennis P. Griffin, Mark C. Hansen.
Application Number | 20110133755 12/633019 |
Document ID | / |
Family ID | 44081393 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110133755 |
Kind Code |
A1 |
Griffin; Dennis P. ; et
al. |
June 9, 2011 |
System and Method of Occupant Detection with a Resonant
Frequency
Abstract
An occupant detection system that includes an electrode, an
electrical network, and a controller. The electrode is arranged to
be proximate to an expected location of an occupant for sensing an
occupant presence proximate the location. The electrode is
configured to provide an electrode impedance indicative of the
occupant presence. The electrical network is coupled to the
electrode to form a resonant circuit and is configured to provide a
network impedance. The resonant circuit is configured to exhibit a
resonant frequency dependent on the network impedance and the
electrode impedance. The controller is coupled to the resonant
circuit and is configured to determine the resonant frequency and
detect an occupant based on the resonant frequency.
Inventors: |
Griffin; Dennis P.;
(Noblesville, IN) ; Hansen; Mark C.; (Kokomo,
IN) |
Assignee: |
DELPHI TECHNOLOGIES, INC.
Troy
MI
|
Family ID: |
44081393 |
Appl. No.: |
12/633019 |
Filed: |
December 8, 2009 |
Current U.S.
Class: |
324/633 |
Current CPC
Class: |
B60N 2/002 20130101 |
Class at
Publication: |
324/633 |
International
Class: |
G01R 27/04 20060101
G01R027/04 |
Claims
1. An occupant detection system comprising: an electrode arranged
proximate to an expected location of an occupant for sensing an
occupant presence proximate thereto, said electrode configured to
provide an electrode impedance indicative of the sensed occupant
presence; an electrical network coupled to the electrode to form a
resonant circuit, said electrical network configured to provide a
network impedance, said resonant circuit configured to exhibit a
resonant frequency dependent on the network impedance and the
electrode impedance; and a controller coupled to the resonant
circuit, said controller configured to determine the resonant
frequency and thereby detect an occupant based on the resonant
frequency.
2. The occupant detection system in accordance with claim 1,
wherein the electrode is adjacent a seating surface of a vehicle
seat to sense the occupant seated in the vehicle seat.
3. The occupant detection system in accordance with claim 1,
wherein said electrode impedance has an electrode capacitive part
indicative of the occupant and an electrode resistive part
indicative of an environmental condition, and wherein the
controller detects an occupant based at least in part on the
electrode capacitance part and the electrode resistive part.
4. The occupant detection system in accordance with claim 3,
wherein the environmental condition is humidity.
5. The occupant detection system in accordance with claim 3,
wherein said network impedance has a network inductive part that
cooperates with the electrode capacitive part to influence the
resonant frequency.
6. The occupant detection system in accordance with claim 5, said
controller comprising a signal generator coupled to the resonant
circuit, said controller configured to output an excitation signal
having an excitation frequency.
7. The occupant detection system in accordance with claim 6, said
controller comprising a voltage detector arranged to measure a
network signal magnitude in response to the excitation signal.
8. The occupant detection system in accordance with claim 7, said
controller configured to vary the excitation frequency and measure
the network signal magnitude to determine the resonant frequency
and the environmental condition.
9. The occupant detection system in accordance with claim 8, said
controller configured to determine a wet seat fault condition based
on the network signal magnitude being less than a threshold.
10. The occupant detection system in accordance with claim 1, said
system comprising an air bag module receiving an activation signal
from the controller.
11. The occupant detection system in accordance with claim 1, said
electrical network impedance having a network capacitance part, and
said controller configured to change the network capacitance part
to reduce signal interference.
12. A method for detecting a vehicle occupant comprising the steps
of: arranging an electrode to provide an electrode impedance
indicative of an occupant presence sensed proximate thereto;
coupling the electrode to an electrical network to form a resonant
circuit that exhibits a resonant frequency indicative of the
electrode impedance; applying an excitation signal from a
controller to the resonant circuit, said excitation signal having
an excitation frequency; generating a network signal in response to
the excitation signal, said network signal having a network signal
magnitude dependent on the electrode impedance; measuring the
network signal magnitude; varying the excitation frequency;
determining the resonant frequency based on the network signal
magnitude; and detecting the presence of a sensed vehicle occupant
based on the resonant frequency.
13. The method in accordance with claim 12, wherein the step of
determining the resonant frequency is based on determining an
excitation frequency that causes a peak network signal
magnitude.
14. The method in accordance with claim 12, further comprising the
step of determining an environmental condition based on the network
signal magnitude.
15. The method in accordance with claim 14, wherein the
environmental condition is humidity.
16. The method in accordance with claim 15, wherein the step of
determining an environmental condition includes indicating a wet
seat fault condition if the network signal magnitude is below a
threshold.
17. The method in accordance with claim 12, wherein the step of
varying the excitation frequency includes selecting the excitation
frequency from a list.
18. The method in accordance with claim 12, further comprising the
step of determining the activation status of an air bag module
based on detecting the vehicle occupant.
19. The method in accordance with claim 12, said electrical network
impedance having a network capacitance part, said controller
configured to change the network capacitance part, and said method
further comprising changing the network capacitance part to reduce
signal interference.
20. A method for operating a vehicle occupant detection system
comprising an electrode for sensing an occupant presence, an
electrical network coupled to the electrode to form a resonant
circuit, and a controller configured to output an excitation signal
and measure a network signal magnitude, said method comprising:
selecting an excitation frequency range and an excitation frequency
step parameter for detecting an occupant presence; outputting a
plurality of excitation signals based on the excitation frequency
range and the excitation frequency step parameter; measuring a
plurality of network signal magnitudes arising from the plurality
of excitation signals; determining a resonant frequency based on a
maximum network signal magnitude; detecting an occupant presence
based on the resonant frequency; and adjusting the excitation
frequency range and the excitation frequency step parameter for
performing another occupant detection at the adjusted values.
Description
TECHNICAL FIELD OF INVENTION
[0001] The invention generally relates to vehicle passenger
occupant detection, and more particularly relates to a system and
method for detecting an occupant on a vehicle seat that includes an
electrode coupled to an electrical network configured to have a
resonant frequency that is dependent on presence of the
occupant.
BACKGROUND OF INVENTION
[0002] It is known to selectively enable or disable a vehicle air
bag or other occupant protection device based on the presence of an
occupant in a seat. It has been proposed to place electrically
conductive material in a vehicle seat to serve as an electrode for
detecting the presence of an occupant in the seat. For example,
U.S. Patent Application Publication No. 2009/0267622 A1, which is
hereby incorporated herein by reference, describes an occupant
detector for a vehicle seat assembly that includes an occupant
sensing circuit that measures the impedance of an electric field
generated by applying an electric signal to the electrode in the
seat. The presence of an occupant affects the electric field
impedance about the electrode that is measured by the occupant
sensing circuit. However, environmental conditions such as humidity
or moisture may interfere with the accuracy of measuring the
electric field impedance. Furthermore, such measurements may become
unreliable or unusable if liquid is present on or in the seat such
as due to a wet bathing suit or due a window being left open during
a rain shower. What is needed is a system that can determine the
presence of an occupant in a seat having an electrode that is not
adversely or unacceptably sensitive to varying humidity levels and
can sense when a seat is wet.
SUMMARY OF THE INVENTION
[0003] In accordance with one aspect of this invention, an occupant
detection system includes an electrode, an electrical network, and
a controller. The electrode is arranged to be proximate to an
expected location of an occupant for sensing an occupant presence
proximate the location. The electrode is configured to provide an
electrode impedance indicative of the sensed occupant presence. The
electrical network is coupled to the electrode to form a resonant
circuit and is configured to provide a network impedance. The
resonant circuit is configured to exhibit a resonant frequency
dependent on the network impedance and the electrode impedance. The
controller is coupled to the resonant circuit and is configured to
determine the resonant frequency. By determining the resonant
frequency, an occupant can be detected based on the resonant
frequency.
[0004] In another aspect, a method for detecting a vehicle occupant
that arranges an electrode to provide an electrode impedance
indicative of an occupant presence sensed proximate thereto. The
method also couples the electrode to an electrical network to form
a resonant circuit that exhibits a resonant frequency indicative of
the electrode impedance and applying an excitation signal having an
excitation frequency from a controller to the resonant circuit. The
resonant circuit generates a network signal in response to the
excitation signal. The network signal has a network signal
magnitude dependent on the electrode impedance. The method measures
the network signal magnitude, varies the excitation frequency, and
determines the resonant frequency based on the network signal
magnitude. By determining the resonant frequency, the presence of a
sensed vehicle occupant is detected based on the resonant
frequency.
[0005] In another aspect, a method for operating a vehicle occupant
detection system is described. The system includes an electrode for
sensing an occupant presence, an electrical network coupled to the
electrode to form a resonant circuit, and a controller configured
output an excitation signal and measure a network signal magnitude.
The method selects an excitation frequency range and an excitation
frequency step parameter for detecting an occupant presence. A
plurality of excitation signals are output based on the excitation
frequency range and the excitation frequency step parameter. In
response, a plurality of network signal magnitudes that arise from
the plurality of excitation signals are measured. A resonant
frequency is determined based on a maximum network signal
magnitude. An occupant presence is detected based on the resonant
frequency. The excitation frequency range and the excitation
frequency step parameter are adjusted for performing another
occupant detection.
[0006] Further features and advantages of the invention will appear
more clearly on a reading of the following detail description of
the preferred embodiment of the invention, which is given by way of
non-limiting example only and with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0007] The present invention will now be described, by way of
example with reference to the accompanying drawings, in which:
[0008] FIG. 1 block diagram of an occupant detection system,
according to one embodiment;
[0009] FIG. 2 is a perspective view of a seat assembly
incorporating the occupant detection system shown in FIG. 1;
[0010] FIG. 3 is a block/circuit diagram of the occupant detection
system shown in FIG. 1;
[0011] FIG. 4 is a graph of sensed voltage signals detected by the
controller in FIG. 3; and
[0012] FIG. 5 is a flowchart of a method of detecting an occupant
used by the system in FIG. 1.
DETAILED DESCRIPTION OF INVENTION
[0013] In accordance with an embodiment of an occupant detector,
FIG. 1 illustrates an occupant detection system 10 for detecting
the presence of an occupant 12. Determining an occupant presence in
a vehicle seat is useful for controlling various vehicle features
such as enabling or disabling an air bag module 14 in the vehicle
according to one embodiment. An air bag module 14 deploys an air
bag 16 to restrain the occupant 12 in the event of a vehicle
collision, as indicated by an arrow 18. It is advantageous to
disable the air bag module 14 if the vehicle seat is empty so the
air bag 16 is not unnecessarily deployed. Determining an occupant
presence may include determining the relative size of the occupant
12 so that an air bag deployment force may be adjusted
correspondingly. For example, if the occupant 12 is determined to
be a small adult or a child, it may be advantageous to deploy the
air bag 16 with less force than is used for a larger adult. As will
be explained in more detail below, the occupant detection system 10
includes an electrode 20 that receives a network signal 22 from an
electrical network 24 and generates an electric field 26 in
response to the network signal 22. The network signal 22 arises
from an excitation signal 28 output by a controller 30 to determine
an occupant presence for determining an air bag activation signal
13 to activate the air bag module 14. The air bag module 14
receives the activation signal 13 from the controller 30 to arm the
air bag module so that a signal from a collision detection system
(not shown) can deploy the air bag 16. It should be appreciated
that the occupant detection system 10 may be used for other vehicle
functions such as activating a seat belt warning if the seat belt
is not properly deployed.
[0014] FIG. 2 shows an exemplary seat assembly 32 suitable for use
by the occupant detection system 10 for sensing an occupant
presence to detect the occupant 12 (not shown in FIG. 2) proximate
to the seat assembly 32. The seat assembly 32 is illustrated in a
vehicle passenger compartment according to one embodiment, but
could be used in any kind of vehicle, such as an airplane. The seat
assembly 32 has a seat cushion 34 for providing a seating surface
36 to support the occupant 12. Seat cushion 34 is suitably made of
foam having characteristics suitable for seating use. Adjacent the
seating surface 36 is a mat 38 shown with the electrode 20 in the
form of a wire attached to the mat 38. The electrode 20 can be made
of any electrically conductive material suitable for use adjacent
the seating surface 36. Exemplary materials for forming the
electrode 20 include metal wire, conductive fiber, metal foil, and
metal ribbon. The cushion 34 is covered with covering 40 to protect
the cushion 34 and the electrode 20, and to make the appearance of
seat assembly 30 attractive. The electrode 20 is arranged to be
located near or proximate to the seating surface 36. Such an
arrangement improves occupant detection sensitivity and accuracy
for detecting an occupant near seating surface 36 by maximizing the
electrical field 26 coupling to the occupant 12. The electrode 20
is electrically coupled to a connector 42 so electrode 20 can be
readily connected to the occupant detection system 10.
[0015] FIG. 3 shows an exemplary circuit diagram 44 for
illustrating the operation of the occupant detection system 10. The
circuit diagram 44 includes an electrode/occupant model 46 for
illustrating the influences on an electrode impedance provided by
the electrode 20. Variation in the electrode impedance is caused by
the occupant 12 and other environmental factors. FIG. 3 shows a
capacitor CO having a capacitance value dependent on the presence
of an occupant. In general, capacitors may be characterized as two
spaced apart plates having material with a dielectric constant
occupying the space between the plates. The dielectric constant of
the material influences the capacitance value of the capacitor. In
the model 46, the electrode 20 corresponds to the plate of
capacitor CO connected to network signal 22. The other plate of
capacitor CO corresponds to the frame and body of the vehicle
surrounding the occupant 12 and is shown connected to a reference
ground 48. It follows that the dielectric constant of the material
in the region between the capacitor plates is influenced at least
in part by the presence or absence of the occupant 12. The presence
of a large adult versus a small child, or the absence of an
occupant effectively varies the model of the dielectric material
between the plates and thereby varies the capacitance value of
capacitor CO. As such, the electrode 20 has an electrode impedance
that is indicative of occupant presence and occupant size and/or
weight.
[0016] The model 46 also shows a resistor RH in parallel with
capacitor CO that models a resistive path for direct current that
is commonly associated with dielectric leakage of a capacitor. The
value of resistor RH is dependent on the material used to make
cushion 34 and seat cover 40, and on other environmental conditions
such as relative humidity, temperature, or changes due to wear and
breakdown of the materials used to form the seat assembly 32.
Increasing humidity decreases the value of resistor RH. A wet seat
due to a spilled drink, a wet bathing suit, or the seat being rain
soaked because a window was left open during a rain shower may also
reduce the value of resistor RH.
[0017] The electrode impedance of model 46 can be expressed as a
complex value including real parts and imaginary parts. For complex
values expressed in Cartesian coordinates, the real parts are based
on resistor values, and the imaginary parts are based on either
inductor values or capacitor values and the frequency being applied
to the inductor or capacitor. The electrode impedance has an
imaginary electrode capacitive part corresponding to the
capacitance value of capacitor CO that is indicative of the
occupant. A typical capacitance value for an empty seat assembly 32
in an automobile is about 50 pF to about 100 pF. When an adult
occupies the seat assembly 32, the capacitance value typically
increases about 30 pF to about 80 pF. The electrode impedance also
has a real electrode resistive part corresponding to the resistance
value of resistor RH that is indicative of an environmental
condition. A typical resistance value for a dry seat assembly 32 is
greater than 1.0 M.OMEGA. (1 million Ohms). If the humidity level
is high, the resistance value may be below 1.0M.OMEGA.. If the seat
is wet due to a spilled drink for example, the resistance value may
be below about 0.1M.OMEGA. according to one embodiment. The model
46 may optionally include a series combination of a dielectric
storage resistance RS and a dielectric storage capacitance CS to
provide a model to compensate for effects due to dielectric
storage. The model 46 may also include other parasitic elements
(not shown) such an inductor and or resistor corresponding to the
electrical characterizes of interconnecting devices such as
connector 43. The electrode impedances for an empty seat and
various sized seat occupants at various humidity levels are
determined empirically for a given seat/vehicle/electrode
configuration.
[0018] FIG. 3 shows an electrical network 24 coupled to the
electrode/occupant model 46 electrode to form a resonant circuit
having a resonant frequency. Since the electrode impedance is
capacitive and provides a capacitance part, a resonant circuit may
be formed if the electrical network has a network impedance that
provides an inductive part. The inductive part cooperates with the
capacitive part to influence the resonant frequency of the
combination of electrode 20 and electrical network 24. The
electrical network 24 provides the network inductive part by
including an inductor LN. The resonant circuit is characterized as
having a resonant frequency, and the resonant frequency is
dependent on the network impedance. The electrode impedance and the
network impedance combine to have a resonant circuit impedance ZR.
A typical inductor has measurable series resistance. As such, a
model of an inductor may optionally include a resistor RN in series
with the inductor LN.
[0019] The electrical network 24 is illustrated as being formed of
passive components. Alternately, the electrical network 24 may be
an impedance synthesizer configured to provide an electronic load
on the network signal 22 that mimics passive components such as
inductor LN. Such an impedance synthesizer may vary the apparent
impedance of electrical network 24 in response to a control signal
from a controller 30. By using an impedance synthesizer, the
resonant frequency of the resonant frequency for a given capacitive
value of CO can be adjusted to a desired value.
[0020] The electrical network 24 also shows a capacitor CN that,
when connected, varies the resonant frequency. One end of capacitor
CN is shown connected to the controller 30. If the connection to
the controller 30 is an open circuit, then CN will not affect the
resonant frequency. If a connection to the reference ground 48 is
provided, then capacitor CN and capacitor CO together determine the
capacitive part of the resonant circuit and the resonant circuit
impedance ZR. Being able to change the resonant frequency is
advantageous to prevent the occupant detection system from
radiating an electric field at certain frequencies, or to change
the resonant frequency in response to detecting radio frequency
type interference from some external source. Radio frequency
interference may be detected by monitoring the network signal 22
when the excitation signal 28 is not activated, or by determining
that an anomalous reading was observed when the resonant frequency
is being determined.
[0021] FIG. 3 shows the controller 30 coupled to the resonant
circuit formed by the electrical network 24 and the
electrode/occupant model 46 expressing the electrical impedance of
the electrode 20. The controller 30 is configured to determine the
resonant frequency of the resonant circuit, and thereby detect the
presence of an occupant 12 based on the resonant frequency of the
resonant circuit. The controller 30 may suitably include a signal
generator 52 to output the excitation signal 28 at an excitation
signal frequency. The excitation signal may be a sinusoidal voltage
according to one embodiment. A voltage divider network is formed by
the arrangement of a module impedance ZM and the resonant circuit
impedance ZR. In response to the excitation signal 28, the network
signal 22 is measured by a voltage detector 54. Voltage detector 54
suitably measures a network signal magnitude. Processor 50 is
configured to vary the excitation signal using signal generator 52
to determine the resonant frequency by determining the excitation
signal frequency that corresponds to a peak or maximum network
signal magnitude. Processor 50 may be a commercially available
microprocessor, or may be a commercially available digital signal
processor that includes the signal generator 52 and voltage
detector 54 blocks. the controller 30 may employ other control
circuitry according to other embodiments/
[0022] Module impedance ZM is preferably provided by a capacitor
CM. A suitable value for capacitor CM is 100 pF. If capacitor CM is
too large or too small, the voltage divider ratio of impedances ZM
and ZR will be such that the sensitivity of the network signal
magnitude near the resonant frequency will be reduced. Capacitors
around 100 pF having electrical characteristics that are stable
over time and temperature are readily available and economical.
[0023] FIG. 4 shows a graph 400 indicating frequency response
curves for various seat occupancy and environmental conditions.
Each curve shows the magnitude values of a network signal 22 from
an exemplary occupant detection system 10 for a seat occupancy and
environmental condition. The magnitude values are based on a
measured voltage, but may alternately be based on a binary number
from processor 50. The controller 30 is configured to vary the
excitation frequency, measure the network signal magnitude, and
determine the resonant frequency and the environmental condition
based on the peak or maximum network signal magnitude. Curve 401 is
an exemplary frequency response curve when seat assembly 32 is
empty or unoccupied and the humidity level is low, such as less
than 30% relative humidity for example. Curve 402 is an exemplary
frequency response curve when seat assembly 32 is empty or
unoccupied and the humidity level is high, such as greater than 90%
relative humidity for example. At low humidity, resistor RH is
relatively high, so the Q or quality factor of the resonant circuit
at low humidity conditions is higher than the Q of the resonant
circuit during high humidity conditions. Thus, the magnitude of the
network signal at the resonant frequency is an indicator of the
environmental condition of relative humidity. Curve 401 and curve
402 show a resonant frequency of about 82 kHz It is noted that the
change in humidity levels has little effect on the resonant
frequency.
[0024] Curve 403 is an exemplary frequency response curve when seat
assembly 32 is occupied by a person whose size is characterized as
being in the 95.sup.th percentile of adults, and the humidity level
is low. Being in the 95.sup.th percentile means that 95% of all
adults are smaller in size than the person in question, where size
is based on the volume, surface area, or weight of the person.
Curve 404 is an exemplary frequency response curve when seat
assembly 32 is occupied by a person whose size is characterized as
being in the 95.sup.th percentile of adults, and the humidity level
is high. Curve 403 and curve 404 show a resonant frequency of about
72 kHz. Comparing change in resonant frequency of curves 401 and
402 to the resonant frequency of curves 403 and 404 shows that the
presence of an occupant in the seat assembly 32 is indicated by the
resonant frequency and is relatively independent of humidity.
[0025] As the peak magnitude of the network signal 22 decreases
with increasing humidity, the resonant frequency becomes less
pronounced and the frequency response curves flatten. A wet seat
may cause the frequency response curve to flatten to such a degree
that it is difficult to determine the resonant frequency. As such,
it is advantageous if the controller 30 is also configured to
determine a wet seat fault condition based on the network signal
magnitude being less than a threshold, such as 0.5V for example.
According to one example, the threshold is determined empirically
for a given seat/vehicle/electrode configuration.
[0026] FIG. 5 is a flowchart 500 showing an embodiment of a method
of operating the occupant detection system 10 to detect the
presence of an occupant. The occupant detection system has an
electrode that has an electrode impedance indicative of an
occupant, and an electrical network coupled the electrode to form a
resonant circuit having a resonant frequency indicative of the
electrode impedance. At step 510, controller 30 applies an
excitation signal having an excitation frequency to the resonant
circuit. The excitation signal may be a sinusoidal signal,
according to one embodiment, since such a signal simplifies
determining a resonant frequency. At step 520 the resonant circuit
generates a network signal in response to the excitation signal.
The network signal has a network signal magnitude dependent on the
resonant frequency of the resonant circuit. At step 530, controller
30 measures the network signal magnitude. The network signal
magnitude is preferably based on a root-mean-square (RMS) value of
the network signal. Alternately, the network signal magnitude may
be based on a simple average value, or a peak-to-peak value. At
step 540 controller 30 varies the excitation frequency to generate
another network signal so another network signal magnitude can be
measured. The excitation frequency may be varied by selecting
frequencies from a list or may be determined using an algorithm.
The excitation frequency may also be selected based on previous
excitation frequencies and/or a previously determined resonant
frequency. Once a series of network signal magnitudes are measured,
the controller 30 determines the resonant frequency based on the
network signal magnitude for each frequency. At step 560 controller
30 determines an occupant based on the resonant frequency.
[0027] Another embodiment of a method of operating the occupant
detection system 10 may include determining the activation status
of an air bag module based on determining the occupant. Controller
30 outputs an activation signal 13 to the air bag module 14 for
controlling the activation status of the air bag module 14. If the
occupant detection system 10 determines that the seat assembly 32
is empty, then the air bag module may be deactivated to prevent
unnecessary deployment of the air bag 16. If an occupant of
sufficient size is detected in the seat assembly 32, then the air
bag module may be activated so that if a collision is detected the
air bag 16 may be deployed to protect the occupant 12.
[0028] Another embodiment of a method of operating the occupant
detection system 10 may include changing the electrical network
capacitance part to reduce signal interference. If an
electromagnetic signal is present around the electrode 20, the
electromagnetic signal may interfere with the controller 30
determining the resonant frequency. The presence of the
electromagnetic signal may be determined by configuring the
controller 30 to receive signals from the electrode 20 when no
excitation signal is being output. By changing the electrical
network capacitance part, for example by adding capacitor CN to the
resonant circuit, the resonant frequency can be shifted to avoid
interference from the electromagnetic signal.
[0029] Another embodiment of a method of operating the occupant
detection system 10 may include the controller 30 initially
selecting an excitation frequency range and an excitation frequency
step parameter for detecting an occupant presence. The excitation
frequency range needs to be large enough to generate excitation
signals at frequencies above and below the resonant frequency. The
excitation frequency step parameter may change the excitation
frequency a greater amount if the excitation frequency is not near
an expected resonant frequency, and make smaller changes near the
expected resonant frequency. Making small changes near the expected
resonant frequency may be useful when humidity is high or the seat
assembly 32 is wet which causes the resonant frequency to be less
pronounced. The selections of range and step parameter, or step
size, may be based on either predetermined values or values saved
from a previous time of operation. The controller outputs a
plurality of excitation signals based on the excitation frequency
range and the excitation frequency step parameter. The controller
30 measures a network signal magnitude arising from each excitation
signal at each of the plurality of frequencies. The controller 30
determines a resonant frequency by determining which of the
plurality of excitation signal frequencies results in a maximum
network signal magnitude. An occupant presence can then be
determined based on the resonant frequency. Determining an occupant
presence may include determining the size or classification of the
occupant. The size or classification may be used to indicate an
appropriate deployment force to the air bag module 14. The process
of varying the excitation frequency to determine a resonant
frequency may be repeated on a periodic basis, once every 10
seconds for example. By repeatedly determining a resonant
frequency, the occupant detection system 10 may increase a
confidence factor that an occupant has been accurately classified
and detect if the occupant 12 shifts to a position that may not be
optimum to protect the occupant if the air bag 16 was deployed.
Also, the excitation frequency range and the excitation frequency
step parameter may be adjusted more optimally the for performing
another subsequent occupant detection based on one or more prior
resonant frequency determinations, the occupants classification
and/or a determination of an environmental condition such as
humidity or that the seat is wet.
[0030] While this invention has been described in terms of the
preferred embodiments thereof, it is not intended to be so limited,
but rather only to the extent set forth in the claims that
follow.
* * * * *