Abstract:
An occupant detection system and method are provided for detecting an occupant seated in a vehicle seat. An electrode is arranged in a seat proximate to an expected location of an occupant for sensing an occupant proximate thereto. The electrode may be integrated with a seat heater. Control circuitry controls the seat heater. A signal generator is coupled to the electrode and configured output to the electrode a plurality of signals at a plurality of frequencies. Occupant detection circuitry detects voltages responsive to the plurality of signals at the plurality of frequencies and detects a state of occupancy based on the detected voltages. An LC circuit coupled to the electrode and the control circuitry suppresses capacitance generated by the control circuitry.

Description:
TECHNICAL FIELD 
     The present invention generally relates to occupant sensing systems, and more particularly relates to a system and method for detecting an occupant on a seat that includes an electrode configured to have a resonant frequency that is dependent on presence of an occupant. 
     BACKGROUND OF THE INVENTION 
     Automotive vehicles are commonly equipped with air bags and other devices that are selectively enabled or disabled based upon a determination of the presence of an occupant in a vehicle 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. Additionally, many vehicle seats are equipped with a seat heater which generally includes an electrically conductive mat for receiving the electrical current which, in turn, generates thermal energy to heat the seat. Moreover, the seat heater has circuit elements in the control circuitry which, such as transistors, may interfere with the accuracy of measuring the electric field impedance. What is needed is a system and method that can determine the presence of an occupant in a vehicle seat having an electrode that is not adversely affected by the seat heater and its control circuitry. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an occupant detection system for a seat is provided. The system includes an electrode arranged in a seat proximate to an expected location of an occupant for sensing an occupant proximate thereto. The system also includes control circuitry for controlling an electrical device proximate to the seat. The system has a signal generator coupled to the electrode and configured output to the electrode a plurality of signals at a plurality of frequencies. The system also includes occupant detection circuitry for detecting voltages responsive to the plurality of signals at the plurality of frequencies and detecting a state of occupancy based on the detected voltages. The system further includes an LC circuit coupled to the conductive element and the control circuitry for suppressing capacitance generated by the control circuitry. 
     According to another aspect of the present invention, a method for detecting presence of an occupant in a seat having an electrode is provided. The method includes the steps of applying a plurality of signals at a plurality of frequencies to an electrode to generate an electric field proximate to the vehicle seat, and detecting voltage responses to the plurality of signals. The method also includes the step of suppressing capacitance generated by control circuitry with the use of an LC circuit coupled to the electrode and the control circuitry. The method further includes the step of detecting a state of occupancy of the seat based on the detected voltages. 
     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a seat assembly incorporating an occupant detection system using the seat heater mat, according to one embodiment; 
         FIG. 2  is a block diagram of the occupant detection system; 
         FIG. 3  is a block/circuit diagram of the occupant detection system, according to one embodiment; 
         FIG. 4  is a flow diagram illustrating a routine for determining occupant classification by processing signals with the seat heater; and 
         FIG. 5  is a graph illustrating simulated amplitude/frequency behavior for two different capacitive loads under dry and wet environmental conditions, according to one example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an exemplary automotive vehicle seat assembly  10  is generally shown having a top side seating surface suitable for supporting an occupant (not shown). The seat assembly  10  is adapted to be installed in a vehicle passenger compartment such as a car seat, according to one embodiment, but could be used in any kind of vehicle, such as an airplane according to another embodiment. The seat assembly  10  has an electrode  12  installed in the seat assembly  10 . According to one embodiment, the electrode  12  employs an electrically conductive heater element in the form of a mat which serves as a seat heater and also serves as the electrode for sensing occupancy of the seat. The electrically conductive element  12  effectively serves both as a seat heater to heat the seat when energized and as an electrode or antenna to detect occupancy of the seat. The electrically conductive element  12  may be formed of suitable materials that allow for electrical conductivity for electrode sensing and electrical heating which includes metal wire, conductive fiber, metal foil, metal ribbon and conductive ink. The vehicle seat assembly  10  also includes an occupant detection system  20  which utilizes the electrically conductive element  12  as an electrode for sensing occupancy of the seat assembly  10 . 
     The occupant detection system  20  is illustrated in  FIG. 2 . The combination seat heater/electrode element or mat  12  is shown coupled to control circuitry shown as an electronic control unit (ECU)  16  for controlling an electrical device. In one embodiment, the electrical device is the seat heater and the control circuitry controls the seat heater to turn the seat heater on and off. This may be achieved by applying electrical current to the electrically conductive element  12 . The seat heater ECU  16  may include a plurality of field effect transistors (FETs) to turn the heater on and off as is generally known in the art. The field effect transistors employed in seat heater control circuitry are generally known to produce capacitance values which can interfere with capacitive sensing arrangements. 
     The occupant detection system  20  also includes passenger occupant detection system (PODS) control circuitry shown as an electronic control unit (ECU)  22  which controls the occupant detection applied via the combined seat heater/electrode element  12 . The passenger occupant detection system (PODS) ECU  22  is shown including a signal generator  24 , a voltage detection circuit  26 , and processing circuitry  28 . The signal generator  24  is configured to output a plurality of alternating current (AC) signals at different frequencies. This may include generating a first sine wave signal at a first frequency during a first time period, a second signal at a second sine frequency during a second time period, etc. A total of n AC signals at n frequencies may be generated, where n is a whole integer. The plurality of n signals may be output simultaneously or sequentially by the signal generator  24 . 
     The signal generator  24  is in communication with the seat heater/electrode element  12  which is configured to generate an electric field in response to the signals from the signal generator  24 . The electric field is projected to a location at which an object (occupant) is to be detected, such as the seating area of the seat assembly  10 . The impedance of a load  14  affects the voltage response received by the voltage detection circuitry  26 . The voltage detection circuitry  26  measures a voltage for each of the n frequencies at the n time periods. The measured voltages may depend upon the impedance of the load  14  which may include impedance caused by an occupant and environmental conditions such as humidity, moisture and temperature. 
     The PODS ECU  22  also includes processing circuitry  28  in communication with the voltage detection circuitry  26 . The signal processing circuitry  28  may include a processor, such as a microprocessor or other digital circuitry. The processor  28  is shown including a routine  100  which may be executed by the processor  28 . It should be appreciated that the processing circuitry  28  may include a plurality of noise filters (not shown) and may convert the measured voltages into digital voltage amplitudes. The voltage amplitudes are compared to determine if a change in voltage has occurred amongst the plurality of frequencies. A change or difference in voltages is indicative of the presence of an environmental condition that will affect the impedance of the load  14 . 
     The occupant detection system  20  may be used to enable, disable or change the response of a vehicle air bag system or other vehicle systems. In some applications, deployment of an air bag may be enabled when a person or object of a specific size or shape is seated in the vehicle. The size of a person may be proportional to the person&#39;s impedance and will affect the voltage sensed by the electrode element  12 . Additionally, environmental conditions may affect the loading on the system, such as humidity, moisture in the vehicle, temperature, and other environmental conditions. To actively control deployment of a system, the system  20  may compensate for detected environmental conditions. 
     The occupant detection system  20  includes an inductor and capacitor (LC) circuit  30  coupled to the PODS control circuitry  22  and the seat heater/electrode mat  12  to prevent or reduce adverse effects caused by control circuitry  16 . The occupant detection system  20  measures the frequency response and calculates the resistance, inductance and capacitance (RLC), and suppresses the capacitance values produced by ECU control circuitry  16 , including the field effect transistors inside the seat heater ECU control circuitry  16  by utilizing the LC circuit  30 . The RLC parameters are calculated using a best fit curve matching technique. According to one embodiment, the best fit curve matching technique  10  is a Levenburg-Marquardt algorithm. The use and processing of the LC circuit  30  advantageously allows for suppression of the interfering capacitance values such that the occupant detection may be performed without requiring that the seat heater  12  be turned on and off for heating purposes. 
     Referring to  FIG. 3 , the occupant detection system  20  is further illustrated showing the LC circuit  30  and the seat heater circuit generally coupled to the electrically conductive electrode element  12  and the seat heater control circuitry  16 . The LC circuit  30  includes a common mode choke L 1  to suppress the effects of external capacitance influences. The common mode choke L 1  is shown as an inductor having inductive L_choke. The LC circuit  30  also includes one or more capacitors coupled to ground. In the embodiment shown, first and second capacitors C 2  and C 3  are shown coupled to ground. The capacitors C 2  and C 3  maintain a stable ground coupling at the exterior of the choke L 1 . In one embodiment, the common mode choke L 1  may have an inductance L_choke of approximately 1 millihertz with a 5 amp maximum current, and the capacitors C 2  and C 3  may each have a capacitance value of approximately 3.3 microfarads. 
     The load  14  is generally illustrated having an occupant capacitance C 4  also represented by capacitance C_seat which generally is the capacitance of the occupant caused by the occupant impedance. Additionally, the load  14  has resistance values that include the heater resistance R 2  and resistance of the foam seat R 1  also shown as R_foam. The occupant capacitance C 4  is measured inside of the choke L 1 . It should be appreciated that the load  14  may vary based upon environmental conditions and occupant detection. An occupant generally affects the capacitance term, whereas certain environmental conditions such as humidity affect the resistance term, and other environmental conditions such as temperature affect the inductance term of the load  14 . By knowing the frequency response formula, three unknown values of the seat heater resistance to ground (R), changes to the choke inductance (L) due to temperature and part-to-part variation, and the capacitive load (C) of the seat are calculated using a best fit curve fitting technique, such as the Levenburg-Marquardt parameter estimation algorithm, according to one embodiment. The Levenburg-Marquardt parameter estimation algorithm may be represented by the following transfer function of the LCR circuit: 
               V   RS     =           V   ES     ·   2     ⁢   π   ⁢           ⁢     f   ·     C   0                 (       2   ⁢   π   ⁢           ⁢   f   ⁢           ⁢     C   0     ⁢   2   ⁢   π   ⁢           ⁢     fC   X       +       2   ⁢   π   ⁢           ⁢     fC   1         1   -         (     2   ⁢   π   ⁢           ⁢   f     )     2     ⁢     L   X     ⁢     C   1             )     2     +       (     1     R   X       )     2                 
where V RS  is the amplitude of the response signal, V ES  is the amplitude of the exciting signal, f is the frequency of the exciting signal, C O  and C 1  are constant parameters, and C X′ , R X′  and L X  are variable parameters.
 
     The occupant detection system  20  measures the voltage signal at n different frequencies over a desired frequency range and calculates the variable parameters C X′ , R X′  and L X  based on the curve fitting technique using the Levenburg-Marquardt algorithm. The curve fitting technique may be performed on the fly to achieve the best fit curve such that the capacitance value C 4 =C_seat indicative of occupancy can be solved for. For example, the occupant detection system  20  may measure voltages at each of frequencies from 100 kilohertz to 500 kilohertz at 50 kilohertz separations and at a sampling rate of about 100 milliseconds, according to one example. The range of frequencies may be from 10 kilohertz to 1 megahertz, according to one embodiment. The occupant capacitance C 4  is then compared to a threshold capacitance value to determine whether or not the seat has an occupant and whether the occupant has a minimal size. The capacitance threshold may be adjusted based upon environmental conditions, such as humidity or whether the seat is wet, according to one embodiment. This may be achieved by empirical testing. 
     Referring to  FIG. 4 , the routine  100  is illustrated beginning at step  102  and proceeding to step  104  to measure the voltage response of n frequencies from 50 to 500 kilohertz. This may include voltage measurements taken at frequencies of 50 to 500 kilohertz with 50 kilohertz separation according to one embodiment. Next, routine  100  calculates the best parameter fit for the foam resistance R_foam, the seat capacitance C_seat and the choke inductance L_choke, based on the n measurement values and a theoretical transfer function in step  106 . Next, at step  108 , routine  100  subtracts the offset capacitance k_C_empty_seat from the seat capacitance C_seat. Routine  100  then looks up the humidity indicator based on the fitted value for resistance of the foam R_foam in step  110 . Finally, at step  112 , routine  100  derives the occupant classification based on the seat capacitance C_seat and humidity classification. This is achieved by comparing the capacitance to a threshold capacitance value and adjusting the threshold based on the humidity. 
     Referring to  FIG. 5 , a simulation of the amplitude/frequency behavior for curves at two different capacitive loads, namely empty and occupied, is illustrated. In this example, the inductance is constant, and the resistance may vary depending upon the environmental conditions, such as humidity of the seat specifically whether the seat is dry or wet. The capacitance will vary depending on whether the seat is occupied or empty as indicated by 100 picofarads (pF) versus 200 picofarads (pF). As seen, an occupied seat be detected in curve  200  by a sensed voltage peak  202  at a certain frequency, and an empty seat condition can be detected by curve  220  by a different frequency response peak voltage  222  when the seat is dry. If the seat is wet, the voltage will generally be less due to the increased resistance and hence the lower curve  210  shows an occupied seat at peak voltage  212  and curve  230  shows an empty seat at peak voltage  232 . 
     Accordingly, the occupant detection system advantageously detects an occupant of a seat, such as a vehicle seat, in a manner that minimizes capacitive interference, particularly interference caused by control circuitry. The occupant detection system advantageously integrates the electrode with the seat heater and compensates for interference caused by control circuitry associated with the seat heater. Integration of the electrode and the seat heater allows for a reduced cost vehicle seat, as duplicative components are eliminated. Additionally, it should be appreciated that other control circuitry for controlling other devices may be employed, such as a seat cooler, and that the LC circuit may compensate for interference caused by other such devices. 
     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.