CAPACITIVE VEHICLE SEAT OCCUPANCY DETECTION SYSTEM WITH DETECTION OF VEHICLE SEAT HEATER MEMBER INTERRUPTION

Abstract

A capacitive detection system for detecting occupancy of a vehicle seat includes a signal generating unit and a signal evaluation unit. The vehicle seat has a seat heater member configured for receiving pulse-width modulated electrical heater power of a pulse-width modulation frequency and time-varying output signals of the signal generating unit. The signal evaluation unit is configured to monitor a voltage difference between the electrical connection ends of the seat heater member, and is further configured to generate an output signal that is indicative of the seat heater member to be defective, on the basis of a fulfillment of at least one predetermined condition concerning the monitored voltage difference and a phase of the pulse-width modulated electrical heater power.

Claims

1. A combined seat heater and capacitive seat occupancy detection system, wherein the seat heater comprises at least one seat heater member having electrical connection ends for receiving pulse-width modulated electrical heater power of a pulse-width modulation frequency, and the capacitive seat occupancy detection system includes a signal generating unit provided for generating a time-varying output signal, wherein the signal generating unit is configured to be electrically connectable to at least one of said electrical connection ends of the at least one seat heater member for providing the time-varying output signal at least to the at least one seat heater member, a signal evaluation unit that is provided for sensing an electrical quantity that is indicative of a capacitance of the at least one seat heater member, and that is provided for generating an output signal that is representative of the sensed capacitance, wherein the signal evaluation unit is configured to monitor a voltage difference between the electrical connection ends of the at least one seat heater member, and is further configured to generate an output signal that is indicative of the at least one seat heater member being defective, when the voltage difference meets at least one predetermined condition.

2. The combined seat heater and capacitive seat occupancy detection system as claimed in claim 1, wherein the seat heater comprises a seat heater control unit that is configured to alternately provide pulse-width modulated (PWM) electrical heater power to the at least one seat heater member during a PWM time interval and perform a diagnostic operation during a diagnostic time interval, said pulse-width modulated (PWM) electrical heater power being provided by periodically connecting and disconnecting one of said first and second connection end to, respective from, a respective one of a power source and ground, and wherein the signal evaluation unit is configured to perform, during a first predetermined time interval of a length corresponding to the length of a PWM time interval, the steps of a) sampling the monitored voltage difference periodically (n samples) and comparing the sampled voltage difference with a first threshold value (V.sub.thresh), b) increase a counter value each time that the sampled voltage difference exceeds the first threshold value, c) after the n samples, comparing the counter value with a predetermined second threshold value (C.sub.thresh), and d) assessing that the seat heater member is not defective if said counter value is lower than said second threshold value (C.sub.thresh).

3. The combined seat heater and capacitive seat occupancy detection system as claimed in claim 2, wherein said signal evaluation unit is configured to perform, if in step c) said counter value is higher than or equal to said second threshold value, the steps of monitoring, during a second predetermined time interval of a length corresponding to the length of a PWM time interval, said second predetermined time interval being adjacent to said first predetermined time interval, a voltage at the respective connection end that is disconnected from the respective one of ground or power source during a phase of the pulse-width modulated electrical heater power in which a momentary electrical heater power is zero, and assessing that said seat heater member is not defective if a variation of the voltage at the respective connection end exceeds a specific threshold value during said second predetermined time interval or generating said output signal that is indicative of the at least one seat heater member to be defective if variation of the voltage at the switched connection end does not exceed said specific threshold value.

4. The combined seat heater and capacitive seat occupancy detection system as claimed in claim 2, wherein said first threshold value (V.sub.thresh) is lower than 1 V (0V?V.sub.thresh?1V).

5. The combined seat heater and capacitive seat occupancy detection system as claimed in claim 2, wherein said second threshold value (C.sub.thresh) is higher than 90% of the number of samples n (0.9.Math.n?C.sub.thresh?n) and preferably higher than 95% of the number of samples n (0.95.Math.n?C.sub.thresh?n).

6. The combined seat heater and capacitive seat occupancy detection system as claimed in claim 1, wherein the signal generating unit further includes a first current source and a second current source for providing a diagnostic output signal, and wherein the first current source is electrically connectable to one electrical connection end of the at least one seat heater member and the second current source is electrically connectable to the other electrical connection end of the at least one seat heater member.

7. A vehicle seat with a capacitive vehicle seat occupancy detection system as claimed in claim 1, the vehicle seat comprising a seat base configured for taking up a seat base cushion, the seat base and the seat base cushion being provided for supporting a bottom of a seat occupant, a backrest configured for taking up a backrest cushion provided for supporting a lumbar and back region of the seat occupant, a seat heater control unit that is configured to provide pulse-width modulated electrical heater power of a pulse-width modulation frequency to at least one seat heater member, and at least one seat heater member being at least partially disposed at at least one out of the seat base cushion and the backrest cushion, and having electrical connection ends that are connected to the seat heater control unit for receiving the pulse-width modulated electrical heater power, wherein the at least one seat heater member is electrically connected to the signal generating unit of the capacitive vehicle seat occupancy detection system.

8. A method of operating a capacitive vehicle seat occupancy detection system for use with a seat heater member of a vehicle seat, wherein a desired heating of the vehicle seat is designed to be carried out by providing pulse-width modulated electrical heater power of a pulse-width modulation frequency from a seat heater control unit that is connected to electrical connection ends of the at least one seat heater member, and wherein a time-varying output signal is being provided at least to the electrical connection ends of the at least one seat heater member for sensing an electrical quantity that is indicative of a capacitance of the at least one seat heater member, the capacitance being usable as a basis for at least one out of detecting and classifying a seat occupancy by comparison to at least a first predetermined value for the sensed capacitance, the method comprising steps of monitoring a voltage difference between the electrical connection ends of the at least one seat heater member and generating an output signal that is indicative of the at least one seat heater member being defective, when the voltage difference meets at least one predetermined condition.

9. The method according to claim 8, comprising the steps of determining a voltage difference between the electrical connection ends of the at least one seat heater member, checking if the determined voltage difference exceeds a predetermined third threshold value that is larger than zero, and generating an output signal that is indicative of the at least one seat heater member to be defective if the determined voltage difference exceeds the predetermined third threshold value.

10. The method according to claim 8, wherein the seat heater comprises a seat heater control unit that is configured to alternately provide pulse-width modulated (PWM) electrical heater power to the at least one seat heater member during a PWM time interval (PWM cycle) and perform a diagnostic operation during a diagnostic time interval, said pulse-width modulated (PWM) electrical heater power being provided by periodically connecting and disconnecting one of said first and second connection end to, respective from, a respective one of a power source and ground, said method further comprising, during a first predetermined time interval of a length corresponding to the length of a PWM time interval, the steps of a) sampling the monitored voltage difference periodically (n samples) and comparing the sampled voltage difference with a first threshold value (V.sub.thresh), b) increase a counter value each time that the sampled voltage difference exceeds the first threshold value, c) after the n samples, comparing the counter value with a predetermined second threshold value (C.sub.thresh), and d) assessing that the seat heater member is not defective if said counter value is lower than said second threshold value (C.sub.thresh).

11. The method according to claim 10, further comprising, if in step c) said counter value is higher than or equal to said second threshold value, the steps of monitoring, during a second predetermined time interval of a length corresponding to the length of a PWM time interval, said second predetermined time interval being adjacent to said first predetermined time interval, a voltage at the respective connection end that is disconnected from the respective one of ground or power source during a phase of the pulse-width modulated electrical heater power in which a momentary electrical heater power is zero, and assessing that said seat heater member is not defective if a variation of the voltage at the respective connection end exceeds a specific threshold value during said second predetermined time interval or generating said output signal that is indicative of the at least one seat heater member to be defective if variation of the voltage at the switched connection end does not exceed said specific threshold value.

12. The method as claimed in claim 8, wherein the step of checking if the determined voltage difference exceeds a predetermined threshold value includes low-pass filtering of a voltage at one of the electrical connection ends of the at least one seat heater member.

13. A non-transitory digital memory unit comprising a software module having program code that is stored in the digital memory unit and that is executable by a processor unit of the capacitive vehicle seat occupancy detection system or a separate control unit to carry out the method of claim 8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0066] FIG. 1 schematically illustrates a configuration of an embodiment of a capacitive vehicle seat occupancy detection system in accordance with the invention as being partially installed in a vehicle seat of a passenger car;

[0067] FIG. 2 illustrates a view onto details of a layout of the embodiment of a capacitive vehicle seat occupancy detection system pursuant to FIG. 1; and

[0068] FIG. 3 is a flow chart of an embodiment of the method in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0069] FIG. 1 schematically illustrates a configuration of an embodiment of a capacitive vehicle seat occupancy detection system 10 in accordance with the invention as being partially installed in a vehicle seat 26 of a passenger car.

[0070] The vehicle seat includes a seat base 26, i.e. a seat frame or seat pan, configured for taking up a seat base cushion, the seat base and the seat base cushion being provided for supporting a bottom of a seat occupant. The vehicle seat further comprises a backrest configured for taking up a backrest cushion provided for supporting a lumbar and back region of the seat occupant. Seats of this kind are well known in the art and shall therefore neither shown in a figure nor be described in further detail herein.

[0071] The vehicle seat may comprise a plurality of seat heater members that are formed e.g. by pieces of electric wires made from a copper-nickel alloy with low temperature coefficient, such as Constantan?, and having an outer electrical insulation layer. Alternatively the seat heater members may comprise printed traces of a resistive material on a carrier layer, for instance traces of a PTC material (positive temperature coefficient). The seat heater members 36 are arranged in a meandering manner at various locations of the vehicle seat, as is known in the art. One seat heater member 36 may e.g. be attached to a seat base cushion front member, and two more seat heater members 36 may be attached to a two-part seat base cushion center member, close to the A surface of the seat base cushion. The meandering patterns formed by the seat heater members cover a major portion of the surface of the seat base cushion center member. The seat heater members comprise electrical connection ends 38, 40 for receiving pulse-width modulated electrical heater power of a pulse-width modulation frequency.

[0072] The vehicle further comprises a seat heater control unit 28 that is connected to a battery 44 of the vehicle (FIG. 2). The seat heater control unit 28 is configured to provide pulse-width modulated electric power of variable duty cycle to the seat heater members 36. To this end, the electrical connection ends 38, 40 of each one of the plurality of seat heater members are electrically connected to the seat heater control unit 28 by shielded cables 30 and via common mode chokes 32 for decoupling purposes (exemplarily shown for seat heater member 36 in FIG. 1). The pulse-width modulation is symbolized in FIG. 2 by a switch 34 meaning that in this specific embodiment a ground line which usually has a common voltage potential to a chassis of the vehicle, is switched (low side switch) to generate the pulse-width modulated electric power. In other embodiments, a line connected to the positive terminal of the battery 44 may be switched (high side switch) to generate the pulse-width modulated electric power. The seat heater control unit 28 includes a temperature sensor 42, e.g. a thermistor made of an NTC material (negative temperature coefficient), that is arranged at an appropriate position in the vehicle seat 26 and that is configured for providing signals indicative of a temperature of the vehicle seat 26 to the seat heater control unit 28 for control purposes.

[0073] Moreover, the capacitive vehicle seat occupancy detection system 10 includes a signal generating unit 12 and a signal evaluation unit 18, which are installed remotely from the vehicle seat 26.

[0074] The signal generating unit 12 is provided for generating a time varying output signal, e.g. a sinusoidal wave signal or square-wave signal, and for applying the output signal to seat heat member 36. The time-varying output signal may e.g. be designed as a sinusoidal wave signal with a frequency of 100 kHz but other signal forms and/or frequencies are also contemplated that appear to be suitable to the person skilled in the art. The electrical connection ends 38, 40 of the seat heater member 36 are connected to the signal generating unit 12 to enable applying the time-varying output signal.

[0075] The electrical connection ends 38, 40 of the seat heater member 36 are connected to the signal generating unit 12 which, in turn, is connected to the signal evaluation unit 18 (FIG. 1). The signal evaluation unit 18 comprises a micro-controller 20 (FIG. 2) and is provided for sensing an electrical quantity that is indicative of a capacitance of the seat heater member 36 that is connected to the signal generating unit 12.

[0076] The signal evaluation unit 18 is further provided for generating an output signal that is representative of the sensed capacitance and that is usable as a basis for detecting and classifying a seat occupancy by comparison to predetermined values for a sensed capacitance. In this specific embodiment, the signal evaluation unit 18 is configured to generate output signals representing seat occupancy classes child/unoccupied seat (class 1) and adult (class 2). In other embodiments, different seat occupancy classes may be chosen that appear to be suitable to the person skilled in the art.

[0077] Referring again to FIG. 1, the signal evaluation unit 18 is equipped with a communication interface 22 that is connected to an airbag control unit 46 of the vehicle, and is configured for transferring output signals to the airbag control unit 46 for the purpose of air bag activation control. For instance, if the transferred output signal represents the occupancy class adult, an airbag of the vehicle seat will be deployed.

[0078] In order to be able to detect a failure in the seat heater element used for capacitive sensing, the signal evaluation unit 18 is further configured to monitor a voltage difference between the electrical connection ends 38, 40 of the seat heater member 36 that is connected to the signal generating unit 12 by determining the voltage difference between measuring points P1, P2 (FIG. 2), and is also configured to generate an output signal that is indicative of the seat heater member 36 to be defective, on the basis of a fulfillment of at least one predetermined condition concerning the monitored voltage difference, as will be described in more detail in the following.

[0079] For the purpose of monitoring the voltage difference, the signal evaluation unit 18 includes e.g. a low-pass filter 24 connected to one of the electrical connection ends 40 of the seat heater member 36.

[0080] From the layout shown in FIG. 2 it is evident to those skilled in the art that during a phase of the pulse-width modulated electrical heater power in which the momentary electrical heater power is larger than zero, a voltage difference between measuring points P1, P2 is the same for a functional seat heater member 36 and a defective seat heater member 36. In particular, a distinction between a functional seat heater member 36 and a defective seat heater member 36 is not possible by determining the voltage difference between the measuring points P1, P2 in case that pulse-width modulated electrical heater power is provided at a duty cycle of 100%.

[0081] During a phase of the pulse-width modulated electrical heater power in which the momentary electrical heater power is zero and the electrical connections are established as shown in FIG. 2, a voltage difference between the measuring points P1, P2 is zero for a functional seat heater member 36, and is larger than zero for a defective seat heater member 36. A summary of expected voltage differences between the measuring points P1, P2 for various scenarios is given in table 1 below.

TABLE-US-00001 TABLE 1 momentary electrical heater momentary electrical heater power is larger than zero power is zero potential potential voltage potential potential voltage scenario at P1 at P2 difference at P1 at P2 difference seat heater 12 V 0 V 12 V 12 V 12 V 0 V member functional seat heater 12 V 0 V 12 V 12 V <12 V >0 V member defective counter increase ? YES NO

[0082] It will be noted that a third scenario occurs if the seat heater control unit 28 is not switched on, i.e. if the 12 V of the battery 44 are not applied to measurement point P1. In order to be able to detect a defective seat heater in this scenario, the signal generating unit 12 furthermore includes its own on-board diagnosis circuit comprising a first current source 14 and a second current source 16 for providing an output diagnosis signal (FIG. 2), e.g. a diagnosis voltage of about 3 V, which can be applied across the connection ends 38, 40 of the seat heater member 36. The first current source 14 is electrically connected to one electrical connection end 38 of the seat heater member 36 and the second current source 16 is electrically connected to the other electrical connection end 40 of the seat heater member 36. If the seat heater member 36 is intact, the diagnostic voltage is detected at both measuring points P1, P2. If however the potential at measuring point P2 is lower than the diagnostic voltage, a defect of the seat heater element may be reliably detected.

[0083] Hereinafter, an embodiment of a method of operating the capacitive vehicle seat occupancy detection system 10, in particular a method of operating the system 10 for detecting a defective seat heater member 36 of the vehicle seat 26 is described. A flow chart of the method is given in FIG. 3. In preparation of operating the capacitive vehicle seat occupancy detection system 10, it shall be understood that all involved units, devices and systems are in an operational state and configured as illustrated in FIGS. 1 and 2.

[0084] In order to be able to carry out the method, the signal evaluation unit comprises a software module. The method steps to be conducted are converted into a program code of the software module. The program code is implemented in a digital data memory unit of the signal evaluation unit 18 and is executable by a processor unit of the signal evaluation unit 18. The digital data memory unit and the processor unit are components of the micro-controller 20. Alternatively, the software module may as well reside in and may be executable by a control unit of the vehicle, for instance by the airbag control unit 46, and established data communication means 22 between the signal evaluation unit 18 and the airbag control unit 46 of the vehicle could be used for enabling mutual transfer of data.

[0085] In this specific embodiment of the method, a momentary phase of the pulse-width modulated electrical heater power is monitored by a simple counter member as follows. In other embodiments of the method, different means may be chosen that appear to be suitable to the person skilled in the art.

[0086] In a first step 48 of the method, a voltage difference between the electrical connection ends 38, 40 of the seat heater member 36 is monitored during a predetermined time interval of a length corresponding to the length of a PWM time interval one PWM cycle by taking a plurality of samples of the voltage difference between the electrical connection ends 38, 40 of the seat heater member 36 connected to the signal generating unit 12. The diagnosis takes for instance 64 samples during one PWM cycle of the seat heater control unit. After each sampling, it is checked in step 50, if the determined voltage difference exceeds a predetermined threshold value V.sub.thresh for the voltage difference that is larger than zero, for instance V.sub.thresh=1.0 V. If the condition is not fulfilled, i.e. if the voltage difference is lower than or equal to V.sub.thresh, the next sampling is started. If the condition is fulfilled, a counter value is increased by one in a next step 52.

[0087] After the required number of samplings is made, i.e. after 64 samples, the momentary counter value is compared to an appropriately selected predetermined counter threshold value in step 54. In this specific embodiment with a total of 64samples taken per PWM cycle, the counter threshold value may e.g. be set to 62. In other embodiments of the method, a different predetermined counter threshold value may be chosen that appears to be suitable to the person skilled in the art. If the momentary counter value is less than or equal to the predetermined counter threshold value, it is determined that during the PWM cycle the voltage difference was dropped below the threshold value (during a non heating period) and accordingly the seat heater member 36 is assessed to be fully functional and the next diagnostic cycle may be started.

[0088] If however the momentary counter value is larger than the predetermined counter threshold value, i.e. the voltage difference was exceeding the threshold value Wrest, during substantially the entire PWM cycle, then either

[0089] the duty cycle of the pulse-width modulated electrical heater power is 100%, or [0090] the seat heater member 36 is defective.

[0091] If the heater member is powered at a duty cycle of 100% during the first predetermined time interval, the second, adjacent predetermined time interval will necessarily contain a diagnostic time interval in which the seat heater control unit performs a diagnostic operation. During this diagnostic operation, the seat heater member is not powered and therefore the voltage difference between the connection ends will drop below the threshold value if the seat heater is intact.

[0092] In order to detect the occurrence of a diagnostic operation by the seat heater control unit, the signal evaluation unit (18) will monitor, during a second predetermined time interval, a voltage at measuring point P2, i.e. at the respective connection end that is disconnected from ground during a diagnostic operation of the seat heater control unit.

[0093] If no voltage shift is detected by the microcontroller 20 during the second predetermined time interval, i.e. if no NTC pause is detected, an output signal that is indicative of the seat heater member 36 to be defective is generated in step 56 by the signal evaluation unit 18.

[0094] If the voltage at P2 briefly raises to 12 V during the second predetermined time interval, due to switch 34 being opened during the diagnostic operation (NTC pause), the signal evaluation unit 18 can determine that an NTC pause has occurred and assess that the seat heater member 36 is fully functional. The counter is then reset in a next step 58, and the sampling of the voltage difference is resumed.

[0095] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

[0096] Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.