SENSOR DIAGNOSTICS FOR A CAPACITIVE SENSING SYSTEM

Abstract

A capacitive sensor device configured for being connected to at least one electric heating member serving as a capacitive antenna electrode. The capacitive sensor device includes a common mode choke connected between the heating current supply and the electric heating member, a capacitive sensing circuit configured to determine a complex electrical impedance between the electric heating member and a counter electrode, an electric measuring shunt for providing a voltage that is representative of an electric current flowing through the electric heating member, and a remotely controllable source of direct current electrically connected in parallel to the heating current supply. The remotely controllable source of direct current is configured to provide, upon receiving a remote control signal, an electric pulse having a predetermined amount of electric charge to the at least one electric heating member.

Claims

1. A capacitive sensor device that is configured to be connected to at least one electric heating member and to a heating current supply for providing electric power to the at least one electric heating member, wherein the at least one electric heating member is configured to serve as a capacitive antenna electrode, the capacitive sensor device comprising: a common mode choke having first and second inductively coupled windings, wherein the first winding is configured to be connected between a first output terminal of the heating current supply and a first terminal of the electric heating member, and wherein the second winding is configured to be connected between a second terminal of the electric heating member and a second output terminal of the heating current supply, a capacitive sensing circuit that is configured to inject a periodic measurement signal into the at least one electric heating member via a measurement node and to measure, in response to the injected measurement signal, an electrical quantity at the measurement node that is usable to determine a complex electrical impedance between the at least one electric heating member and a counter electrode, an electric measuring shunt that is electrically connected in series to the at least one electric heating member for providing a voltage that is representative of an electric current flowing through the at least one electric heating member, and a remotely controllable source of direct current that is electrically connectable in parallel to the heating current supply, wherein the remotely controllable source of direct current includes a capacitor that is configured to provide, upon receiving a remote control signal, an electric pulse having a predetermined amount of electric charge to the at least one electric heating member during a transition between two different states of charge of the capacitor.

2. The capacitive sensor device as claimed in claim 1, wherein the remotely controllable source of direct current further includes a current supply that is configured to provide the predetermined amount of electric charge uniformly during a predetermined pulse duration.

3. The capacitive sensor device as claimed in claim 1, wherein one of the inductively coupled windings of the common mode choke serves as the electric measuring shunt.

4. A capacitive measurement system including: a capacitive sensor device as claimed in claim 1, and a microcontroller that is configured for remotely controlling the remotely controllable source of direct current.

5. The capacitive measurement system as claimed in claim 4, wherein the microcontroller includes at least one analog-to-digital converter having an input port that is electrically connected to the electric measuring shunt for determining the voltage across the electric measuring shunt.

6. A seat occupancy detection system for detecting an occupancy of a seat, in particular a vehicle seat, the seat occupancy detection system comprising: a capacitive measurement system as claimed in claim 4, at least one electric heating member that is arranged at a cushion or a backrest forming part of the seat and that is employable as the capacitive antenna electrode, and a heating current supply for providing electric power to the at least one electric heating member.

7. A method of operating the capacitive measurement system as claimed in claim 4 with regard to a functional test of the at least one electric heating member, the method comprising steps of: (a) sending a remote control signal to the remotely controllable source of direct current to enable provision of an electric pulse having a predetermined amount of electric charge at least to the at least one electric heating member, (b) determining the voltage across the electric measuring shunt, (c) comparing an electric quantity that is derivable from the determined voltage with a predetermined threshold for the electric quantity, and (d) if the electric quantity is lower than the predetermined threshold for the electric quantity, generate a signal that is indicative of the at least one electric heating member to be defective.

8. The method as claimed in claim 7, comprising preceding steps of: (e) determining an output voltage of the heating current supply, and (f) carrying out the steps (a) through (d) upon a fulfilled condition that the determined output voltage is smaller than or equal to a predetermined lower threshold that is close to zero, or (g) carrying out the steps (a) through (d) using a second predetermined threshold for the electric quantity that is distinct from the predetermined threshold upon a fulfilled condition that the determined output voltage is larger than the predetermined lower threshold that is close to zero.

9. The method as claimed in claim 8, further comprising steps of: after executing step (e) and steps (a) through (c), redetermining an output voltage of the heating current supply, and executing step (d) upon a fulfilled condition that the redetermined output voltage is equal to the output voltage determined in step (e) within a predetermined margin of tolerance.

10. The method as claimed in claim 9, comprising preceding steps of: determining the voltage across the electric measuring shunt, carrying out the steps (a) and (b), calculating a difference between the voltage across the electric measuring shunt as determined in step (b) and the voltage across the electric measuring shunt as determined in the first step of this claim, and using the calculated difference as the electric quantity while executing steps (c) and (d).

11. The method as claimed in claim 7, wherein the steps are automatically and periodically carried out.

12. A non-transitory computer-readable medium for controlling an execution of the method as claimed in claim 7, wherein the method steps are stored on the computer-readable medium as a program code, wherein the computer-readable medium comprises a part of the capacitive measurement system or a separate control unit and the program code is executable by a processor unit of the capacitive measurement system or a separate control unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:

[0050] FIG. 1 is a layout of a conventional seat occupancy detection system,

[0051] FIG. 2 shows a layout of an embodiment of a seat occupancy detection system comprising a capacitive sensor device in accordance with the invention,

[0052] FIG. 3 is a flowchart of an embodiment of a method in accordance with the invention, and

[0053] FIG. 4 is a flowchart of another embodiment of a method in accordance with the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0054] FIG. 2 illustrates a layout of an embodiment of a seat occupancy detection system 20 comprising a capacitive measurement system 22 with a capacitive sensor device 50 in accordance with the invention.

[0055] The seat occupancy detection system 20 is configured to detect an occupancy of a seat, in particular a vehicle seat of a passenger car, and includes the capacitive measurement system 22, a first electric heating member 24 that is arranged at a backrest of the vehicle seat and a second electric heating member 26 that is arranged at a seat cushion forming part of the vehicle seat. The second electric heating member 26 is employed as a capacitive antenna electrode. Moreover, the seat occupancy detection system 20 comprises a heating current supply 32 designed as an electronic control unit for providing electric power to the electric heating members 24, 26. In an operational state, the heating current supply 32 is configured to switch the electric power that is provided between a first output terminal 34 and a second output terminal 36 periodically on and off to control an electric heating power that is supplied to the seat heating members 24, 26 according to a pulse-width modulation scheme. A typical switching frequency may, for example, be 25 Hz.

[0056] The capacitive sensor device 50 is electrically connected to the electric heating members 24, 26 and to the heating current supply 32. The second electric heating member 26 serves as a capacitive antenna electrode for the capacitive sensor device 50. The capacitive sensor device 50 includes a common mode choke 52 with first and second inductively coupled windings 54, 56. The first winding 54 is electrically connected between the first output terminal 34 of the heating current supply 32 and a first terminal 28 of the second electric heating member 26. The second winding 56 is electrically connected between a second terminal 30 of the second electric heating member 26 and the second output terminal 36 of the heating current supply 32. The first electric heating member 24 is electrically connected directly to the first output terminal 34 and the second output terminal 36 of the heating current supply 32.

[0057] The capacitive sensor device 50 further includes a capacitive sensing circuit 58 that is configured to inject a periodic alternating measurement signal into the second electric heating member 26 via a measurement node 60. The measurement node 60 is arranged at the electrical connection between the second winding 56 of the common mode choke 52 and the second terminal 30 of the second electric heating member 26. The capacitive sensing circuit 58 is configured to measure, in response to the injected measurement signal and using this a reference voltage, a complex electric voltage at the measurement node 60. From the measured complex electric voltage, the capacitive sensing circuit 58 is configured to determine a complex electrical impedance between the second electric heating member 26 and the vehicle chassis, which forms a counter electrode at ground potential. In this way, the second electric heating member 26 and the capacitive sensing circuit 58 are configured for operating in loading mode.

[0058] By the described arrangement, the common mode choke 52 AC-decouples the capacitive sensing circuit 58 from the first electric heating member 24 and the heating current supply 32.

[0059] Moreover, the capacitive sensor device 50 comprises an electric measuring shunt 62 that is electrically connected in series to the second electric heating member 26 for providing a voltage that is representative of an electric current flowing through the second electric heating member 26. The electric measuring shunt 62 is arranged between the second winding 56 of the common mode choke 52 and the second output terminal 36 of the heating current supply 32.

[0060] A remotely controllable source of direct current (DC) 64 forms part of the capacitive sensor device 50. The DC source 64 is electrically connected in parallel to the first output terminal 34 and the second output terminal 36 of the heating current supply 32. The remotely controllable DC source 64 includes a DC voltage source 66, a capacitor 68, a resistor 70 and two coupled, remotely controllable switches 72, 74 whose switching statuses are persistently opposite to each other. When the first switch 72 is closed (and the second switch 74 is open), the capacitor 68 is charged by the DC voltage source 66 via the resistor 70. When the first switch 72 is open (and the second switch 74 is closed), the capacitor 68 supplies a current pulse that flows through the first winding 54 of the common mode choke 52, the second electrical heating member 26, the second winding 56 of the common mode choke 52 and the electric measuring shunt 62. For the sake of completeness it is noted that the capacitor 68 also supplies a portion of the pulse current to the first electrical heating member 24, but this fact is not relevant for the further considerations. Thus, upon receiving a remote control signal for a selected duration of the current pulse, the remotely controllable DC source 64 is configured to provide the electric current pulse having a predetermined amount of electric charge to the electric heating members 24, 26. The amount of flowing electric charge is given by the product of the capacitance of the capacitor 68 and the voltage difference of the capacitor 68 at the start and at the end of the current pulse, i.e. during a transition between two different states of charge of the capacitor 68.

[0061] The capacitive measurement system 22 further comprises a microcontroller 38. The microcontroller 38 includes a processor unit 40, a digital data memory unit 42 to which the processor unit 40 has data access, and a microcontroller system clock that forms part of the processor unit 40. The digital data memory unit 42 comprises a non-transitory computer-readable medium. The microcontroller 38 is configured for remotely controlling the remotely controllable DC source 64. To this end, the microcontroller 38 is equipped with control outputs 44 formed as a plurality of pulse width modulation (PWM) units that are able to provide mutually independent PWM signals.

[0062] Moreover, the microcontroller 38 includes a plurality of analog-to-digital converters (ADCs) 46. Input lines of two ADCs 46 are electrically connected to ends of the electric measuring shunt 62 for determining the voltage across the electric measuring shunt 62 in a differential amplifier configuration. Other ADCs are also electrically connected (not shown) to the first output terminal 34 and the second output terminal 36 of the heating current supply 32 for determining an output voltage of the heating current supply 32.

[0063] A control link (indicated by a double arrow in FIG. 2) between the microcontroller 38 and the capacitive sensing circuit 58 enables data transfer and control.

[0064] In the following, an embodiment of a method of operating the seat occupancy detection system 20 pursuant to FIG. 2 with regard to a functional test of the second electric heating member 26 will be described. A flowchart of the method is provided in FIG. 3. In preparation of operating the seat occupancy detection system 20, it shall be understood that all involved units and devices are in an operational state and configured as illustrated in FIG. 2.

[0065] In order to be able to carry out the method automatically and periodically and in a controlled way, the microcontroller 38 comprises a software module 48 (FIG. 2). The method steps to be conducted are converted into a program code of the software module 48. The program code is implemented in the digital data memory unit 42 of the microcontroller 38 and is executable by the processor unit 40 of the microcontroller 38.

[0066] Referring now to FIG. 3, in a first step 76 of the method, an output voltage of the heating current supply 32 is determined by the ADCs 46 of the microcontroller 38. After that, a bifurcation in the flowchart occurs.

[0067] If the determined output voltage of the heating current supply 32 is smaller than or equal to a predetermined lower threshold that is close to zero, namely 1.0 V, i.e. if the heating members 24, 26 are not powered by the heating current supply 32, the microcontroller 38 sends, in the following step 78, a remote control signal to the remotely controllable DC source 64 to enable provision of an electric pulse having a predetermined amount of electric charge to the electric heating members 24, 26. All predetermined threshold and tolerance values mentioned herein reside in the digital data memory unit 42 of the microcontroller 38 and can readily be retrieved by the processor unit 40.

[0068] In a next step 80, the voltage across the electric measuring shunt 62 is determined by the microcontroller 38 via the ADCs 46 in a periodic manner such that a plurality of voltage measurements is carried out during the duration of the electric pulse, and a time integral of the determined voltage is calculated as an electric quantity derived from the determined voltage by the processor unit 40. Then, in the next step 82, the electric quantity is compared with a predetermined threshold for the electric quantity. As an optional step 84 of the method (optional steps are indicated in the flowchart of FIG. 3 by dashed lines), an output voltage of the heating current supply 32 is redetermined and then compared to the previous determined output voltage in another step 86. If the redetermined output voltage is equal to the previous determined output voltage within a predetermined margin of tolerance of 20%, the next step is carried out. If not, the method is restarted with the step 78 of sending a remote control signal to the remotely controllable DC source 64. If the derived electric quantity is lower than the predetermined threshold for the electric quantity, a fault signal that is indicative of the second electric heating member 26 to be defective is generated by the microcontroller 38 as the next step 88. If the derived electric quantity is equal to or larger than the predetermined threshold for the electric quantity, the method steps are started all over again.

[0069] If the output voltage of the heating current supply 32 determined in step 76 is larger than the predetermined lower threshold, i.e. if the heating members 24, 26 are powered by the heating current supply 32, there is already heating current flowing and the voltage drop across the electric measuring shunt 62 caused by the heating current may be used to check the integrity of the heater. It follows that there is no need to switch on the DC source 64 to feed the heater and the integrity test of the heating member 26 may be achieved in a conventional way by directly executing steps 80, 82 and 88.

[0070] In the embodiment of the method as shown in FIG. 4, the integrity check of the heater member 26 uses the DC source also in the case, in which the output voltage of the heating current supply 32 determined in step 76 is larger than the predetermined lower threshold, i.e. if the heating members 24, 26 are powered by the heating current supply 32. In this case, it is preferable to consider the superposition of the electric power provided by the heating current supply and the electric pulse of the remotely controllable source of direct current. In this embodiment, the microcontroller 38 replaces in another step 90 the predetermined threshold for the electric quantity by a second predetermined threshold that is distinct from the predetermined threshold and retrieved from the digital data memory unit 42. Then, the method steps are executed, starting with step 78 and using the second predetermined threshold for the electric quantity.

[0071] 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.

[0072] 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, which is meant to express a quantity of at least two. 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.