Patent Application
20110190987
The invention generally relates to vehicle passenger occupant detection, and more particularly relates to a system and method for determining an occupant near an electrode in response to an excitation signal.
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 an electrode impedance. The presence of an occupant affects the electrode impedance, predominately the capacitive part of the electrode impedance. Humidity and liquid moisture also affects the electrode impedance, predominately the resistive part of the electrode impedance
The electrode impedance may be measured by providing a reference impedance device such as a capacitor to form an alternating current voltage divider. Since the value of the reference impedance device is known, the value of the electrode impedance can be determined by applying a sinusoidal excitation signal at various frequencies to the voltage divider and comparing the electrode signal magnitude to the excitation signal magnitude. The electrode signal magnitude is generally limited to avoid violating certain radiated emissions standards. Typically the excitation signal magnitude is fixed at a value that will avoid excessive electrode signal magnitude for any of the excitation signal frequencies. However, as the capacitive part of the electrode impedance increases due to the presence of an occupant and the resistive part of the electrode impedance decreases due to increasing humidity or the presence of liquid moisture, the electrode signal magnitude decreases, thereby decreasing the accuracy of determining the electrode signal magnitude.
In accordance with one aspect of this invention, an occupant detection system is provided. The occupant detection system includes an electrode, a reference impedance device and a controller. The electrode is arranged proximate to an expected location of an occupant for sensing an occupant presence proximate thereto. The electrode is configured to provide an electrode impedance indicative of the occupant presence. The reference impedance device has a first terminal and a second terminal. The first terminal is coupled to the electrode to form a voltage divider network. The controller is coupled to the second terminal and is configured to output an excitation signal on the second terminal to generate an electrode signal on the electrode. The excitation signal has an excitation signal frequency and an excitation signal magnitude. The electrode signal has an electrode signal magnitude. The controller is configured to determine said controller further configured to determine an occupant presence based on the excitation signal frequency, the excitation signal magnitude, and the electrode signal magnitude. The controller is further configured to adjust the excitation signal magnitude based on the electrode signal magnitude.
In another aspect of the present invention, a controller in an occupant detection system is provided. The occupant detection system has an electrode arranged proximate to an expected location of an occupant for sensing an occupant presence proximate thereto. The electrode is configured to provide an electrode impedance indicative of the occupant presence. The controller includes a reference impedance device, a signal generator, a voltage detector, and a processor. The reference impedance device has a first terminal and a second terminal. The first terminal is coupled to the electrode to form a voltage divider network. The signal generator is configured to output an excitation signal on the second terminal to generate an electrode signal on the electrode. The excitation signal has an excitation signal frequency and an excitation signal magnitude. The electrode signal has an electrode signal magnitude. The voltage detector is configured to determine the electrode signal magnitude. The processor is configured to determine the electrode impedance based on the excitation signal frequency, the excitation signal magnitude, and the electrode signal magnitude. The processor is also configured to determine an occupant presence based on the electrode impedance. The processor is further configured to adjust the excitation signal magnitude based on the electrode signal magnitude.
In yet another aspect of the present invention, a method for detecting a vehicle occupant is provided. An electrode is arranged to provide an electrode impedance indicative of an occupant presence proximate thereto. The electrode is coupled to a reference impedance device to form a voltage divider network. An excitation signal is output to the voltage divider network. The excitation signal has an excitation signal frequency and an excitation signal magnitude. An electrode signal is generated in response to the excitation signal. The electrode signal has an electrode signal magnitude. An occupant presence is determined based on the excitation signal frequency, the excitation signal magnitude, and the electrode signal magnitude. The excitation signal magnitude is adjusted based on the electrode signal magnitude.
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.
The present invention will now be described, by way of example with reference to the accompanying drawings, in which:
In accordance with an embodiment of an occupant detector,
The electrode 20 radiates an electric field 26 in response to the electrode signal 22 and thereby provides an electrode impedance. The value of the electrode impedance in this embodiment is dependent, at least in part, on the coupling of the electric field 26 from the electrode 20 to the vehicle and is affected by the presence or absence of an occupant 12 residing in the seat assembly 32. The electrode 20 may be arranged to be located adjacent 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 coupling of electrical field 26 to the occupant 12. As such, the electrode impedance is indicative of the occupant presence. The electrode 20 may be coupled to the controller 24 by a connector 42 so electrode 20 can be readily connected to the controller 24.
It has been observed that a capacitor portion of the electrode impedance corresponding to a capacitance value of capacitor CO when the seat is empty is lower than the capacitance value of capacitor CO when the seat is occupied. The presence of a large adult versus a small child, or the absence of an occupant may vary the dielectric constant of the dielectric material between the plates and thereby varies the capacitance value of capacitor CO. 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 capacitive term typically increases about 30 pF to about 80 pF. As such, the electrode 20 has an electrode impedance that is indicative of occupant presence and occupant size.
The model 46 also illustrates a resistor RH in parallel with capacitor CO that suggests a resistive path for direct current that corresponds with dielectric leakage of a capacitor. The value of resistor RH may be dependent on the materials 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. It has been observed that the resistance value of resistor RH decreases as the humidity in and around the seat assembly 32 increases, or if liquid moisture is present in or on the seat assembly 32. A typical resistance value of resistor RH for a dry seat assembly 32 corresponding to a resistive portion of the electrode impedance is greater than 1.0MΩ (1 million Ohms). If the humidity level is high, the resistor RH may be below 1.0 MΩ. If the seat is wet due to a spilled drink for example, the resistor RH may be below about 0.1 MΩ.
In this embodiment of controller 24, a voltage detector 54 is coupled to the first terminal of reference impedance device ZR and electrode model 46 and may be configured to determine an electrode signal magnitude and send a magnitude signal 60 to processor 50. The processor 50 in this embodiment is configured to determine the electrode impedance based on the excitation signal frequency, the excitation signal magnitude, and the electrode signal magnitude. The processor 50 may be further configured to determine an occupant presence based on the electrode impedance. The signal generator 52 and the voltage detector 54 are shown as being separate from the processor 50. However, it should be understood that other control circuitry or devices that incorporate the functions of the processor 50, the signal generator 52 and the voltage detector 54 into a single device or alternative devices may be employed
As the capacitance value of capacitor CO increases due to the presence of an occupant, or the resistance value of resistor RH decreases due to the presence of humidity or liquid moisture, the electrode signal magnitude decreases if the excitation signal magnitude is fixed. As the magnitude of electrode signal 22 decreases, it may become difficult for the voltage detector 54 to determine electrode signal magnitude with sufficient accuracy to determine the presence of an occupant 12. The processor 50 may also be configured to adjust the excitation signal magnitude based on the electrode signal magnitude. Alternately, the adjustment of electrode signal magnitude may be by way of an arrangement of operational amplifiers and passive components configured to monitor the electrode signal magnitude and adjust the excitation signal magnitude accordingly.
Being able to control the electrode signal magnitude is advantageous in that the voltage detector 54 receives an electrode signal 22 having adequate magnitude for an accurate determination. However, the electrode signal magnitude is limited to avoid creating excessive radiated emissions. By way of an example, it has been observed that excessive radiated emissions are not generated if the electrode signal magnitude is less than 0.070 Volts root-mean-squared (RMS). If the electrode signal magnitude is greater than 0.070 Volts RMS, then the signal has sufficient magnitude to be readily measured by commercially available equivalents of voltage detector 54.
In one embodiment, impedance ZM is provided by a capacitor CM. A suitable value for CM is 100 pF, according to one example. If capacitor CM is too large or too small, the voltage divider ratio of the electrode impedance ZM and the electrode impedance will be such that a suitable electrode signal magnitude can not be generated. Capacitors around 100 pF having electrical characteristics that are stable over time and temperature are readily available and economical.
Excitation signal frequencies in the range of 1.0 kHz to 1000 kHz may be employed, according to one embodiment. At the lower end of the range of frequencies a decreasing value of resistor RH may lead to low excitation signal magnitudes. At the higher end of the range of frequencies an increasing value of capacitor CO may also lead to low excitation signal magnitudes. As such, it is advantageous for the excitation signal magnitude to be adjusted independently of the excitation signal frequency. The processor 50 may also be configured to adjust the excitation signal magnitude such that the electrode signal magnitude is constant for any excitation signal frequency. In one embodiment, the excitation signal may suitably be a sinusoidal waveform. Determining the electrode impedance is simplified when a sinusoidal waveform is used, particularly when excitation signals at multiple frequencies are used to separately determine capacitance and resistance portions of the electrode impedance corresponding to capacitor CO and resistor RH in the electrode and occupant model 46. If the model 46 is more complicated than having only capacitor CO and resistor RH, such as including dielectric storage resistor RS and dielectric storage capacitor CS as illustrated in
In another embodiment, a method may include adjusting the excitation signal magnitude such that the electrode signal magnitude is independent of the excitation signal frequency. Such an adjustment may be achieved such that the electrode signal magnitude is constant for any excitation signal frequency.
In another embodiment, a method may include the excitation signal being different sinusoidal waveforms. In this embodiment, the step of outputting an excitation signal may include outputting an excitation signal at a plurality of excitation frequencies. Such a method may include the step of determining the electrode impedance based on a plurality of electrode signal magnitudes at the plurality excitation frequencies. In another embodiment, a method may include the step of determining the activation status of an air bag module based on detecting the vehicle occupant.
Accordingly, an occupant detection system, a controller for an occupant detection system and a method of detecting an occupant is provided. The presence or absence of the occupant varies the dielectric properties of an area proximate to an electrode generating an electric field, and thereby influences the electrical impedance of the electrode. By determining the electrode impedance the presence of an occupant may be determined. The electric field is generated in response to an electrode signal arising from an excitation signal. The magnitude of the electrode signal is controlled by varying the magnitude of the excitation signal. By controlling the electrode signal magnitude, the electrode signal magnitude can be optimized to be large enough to be accurately determined using commonly available electronic devices, but not so large as to cause excessive radiated emissions that could interfere with the operation of other electrical devices. To determine the electrode impedance, the excitation signal may be output at more than one frequency, so the excitation signal magnitude may be adjusted for each frequency such that the electrode signal magnitude is optimized for each frequency. The system and method advantageously provide for enhanced signal-to-noise (s/n) ratio and improved measurement resolution that in turn improves the ability to differentiate between these loads and correctly classify the occupant. This is in contrast to other techniques that could be used to improve the magnitude of the signal at the input to the detector such as a gain stage that would amplify system noise as well as the desired signal.
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.
