Capacitive sensor arrangement

Abstract

A capacitive sensor arrangement includes a sensing electrode having a capacitance (Cx) which depends on the presence of an object in a detection space; a measurement device connected to the sensing electrode and configured to detect the capacitance (Cx) of the sensing electrode; and a conducting structure, wherein the capacitance (Cx) of the sensing electrode depends on a potential of the conducting structure. In order to obtain a reliable capacitive measurement, the measurement device is connected to a power supply between a first potential (Vs) and a second potential (GND), the measurement device being connected to the second potential exclusively via the structure.

Claims

1. A capacitive sensor arrangement for detecting an object, comprising: a sensing electrode having a capacitance which depends on the presence of the object in a detection space; a measurement device connected to the sensing electrode and configured to detect the capacitance of the sensing electrode; and a conducting structure, wherein the capacitance of the sensing electrode depends on a potential of the structure; wherein the measurement device is configured to receive a supply of power between a first potential and a second potential, the measurement device being connected to the structure by a conductor, and to the second potential exclusively via the structure.

2. The capacitive sensor arrangement according to claim 1, wherein the conductor connecting the measurement device to the structure is a first connecting line.

3. The capacitive sensor arrangement according to claim 2, wherein the measurement device is operable when the first connecting line is at least indirectly connected to the second potential.

4. The capacitive sensor arrangement according to claim 2, wherein a plurality of conducting structures are connected in series between the second potential and the first connecting line.

5. The capacitive sensor arrangement according to claim 1, wherein the first potential is a supply voltage supplied by a power source to which the measurement device is connected via a power supply line.

6. The capacitive sensor arrangement according to claim 1, wherein the structure is connected to the second potential by a second connecting line.

7. The capacitive sensor arrangement according to claim 1, wherein at least a portion of the structure is connected in series between a first and a second connecting line.

8. The capacitive sensor arrangement according to claim 1, wherein the structure is a ground electrode.

9. The capacitive sensor arrangement according to claim 1, wherein the measurement device is connected to a communication line for communication with another device.

10. The capacitive sensor arrangement according to claim 9, wherein operability of the measurement device is detectable by another device through the communication line.

11. The capacitive sensor arrangement according to claim 1, wherein the measurement device is configured to output a signal indicative of the presence of the object.

12. The capacitive sensor arrangement according to claim 1, wherein the structure is part of a vehicle seat or a steering wheel.

13. The capacitive sensor arrangement according to claim 12, wherein the measurement device is configured to detect the presence of an occupant on the vehicle seat.

14. The capacitive sensor arrangement according to claim 12, wherein the measurement device is configured to detect whether at least one hand is in contact with the steering wheel.

15. The capacitive sensor arrangement according to claim 1, wherein the second potential is ground potential (GND).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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 drawings, wherein:

(2) FIG. 1 is a schematic view showing a first embodiment of an inventive capacitive sensor arrangement; and

(3) FIG. 2 is a schematic view showing a second embodiment of an inventive capacitive sensor arrangement.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

(4) FIG. 1 by way of example illustrates a first embodiment of a capacitive sensor arrangement 1 according to the invention. The sensor arrangement 1 comprises a sensing electrode 2 which could be arranged in a vehicle seat as an occupancy sensor. The sensing electrode 2 is associated with a capacitance C.sub.X relative to ground. The capacitance C.sub.X is unknown and varies with the presence of an object 20 entering a detection space 12 of the sensing electrode 2. The object 20 could be the body of a person. The part of the capacitance C.sub.X associated with the object 20 may be described as C.sub.O. However, the total capacitance of the sensing electrode 2 also comprises a contribution C.sub.S which results from the presence of a conducting structure 3 in the vicinity of the sensing electrode 2. The conducting structure 3 may be a metal structure like a seat frame in the vehicle or a metal core of the steering wheel. It could also be a ground electrode, i.e. an electrode that needs to be kept at ground potential. This capacitance C.sub.S and therefore the total capacitance C.sub.X largely depends on whether the structure 3 is connected to ground or not.

(5) The sensing electrode 2 is connected to a measurement device 4 which is configured to detect the total capacitance C.sub.X of the sensing electrode 2. For this purpose, the measurement device 4 may apply a sinusoidal voltage to the sensing electrode 2 and detect the flowing current. It is well-known that the capacitance is directly related to the current, the voltage and the frequency of the alternating voltage. In order to operate, the measurement device 4 needs to be supplied externally with power. A power supply line 5 is connected to the measurement device and to a control device 30, which supplies a first potential, namely a supply voltage V.sub.S, e.g. from a vehicle battery, a generator or the like. The measurement device 4 is also connected to the control device 30 by a communication line 6, which may be part of one the same bus connection as the power supply line 5. The measurement device 4 may be configured to determine whether the object 20 is present or not and output a signal via the communication line 6 indicating the presence of the object 20. Also, the control device 30 may be able to detect whether or not the measurement device 4 is operable by the status of the communication line. E.g. if no communication from the measurement device 4 is received for a predetermined amount of time, this may be interpreted by the control device 30 as an inoperable state or off state of the measurement device 4.

(6) The measurement device 4 is connected in series between the power supply line 5 and a first connecting line 7. While the power supply line 5 supplies the supply voltage V.sub.S to the measurement device 4, the latter only becomes operable when it is connected to a second potential, namely ground potential GND, via the first connecting line 7. The first connecting line 7 is connected to the structure 3 at a first connection point 8. Therefore, operability of the measurement device 4 depends on whether the structure 3 is connected to ground potential GND or not.

(7) A second connecting line 10 is connected to the structure 3 at a second connection point 9. This second connecting line 10 is directly connected to ground potential GND. Therefore, as long as the first and second connecting lines 7, 10 are intact, the measurement device 4 is operable and the measurement results are reliable since the structure 3 is connected to ground potential GND. It should be noted that the structure 3 may further be connected to ground potential GND by a secondary connection 11, which could be due to a mechanical connection to ground. E.g. if the structure 3 is a vehicle seat frame, this seat frame is mounted directly or indirectly to the vehicle body, which could also imply an electrical connection. However, such an electrical connection may be unreliable compared to the second connecting line 10.

(8) If the first connecting line 7 is disconnected or both the second connecting line 10 and the secondary connection 11 are disconnected, the measurement device 4 becomes inoperable. In this case, it cannot communicate with the control device 30, which detects the inoperability of the measurement device 4 and may output a corresponding warning signal.

(9) In the embodiment shown, both connection points 8, 9 are located spaced-apart on the structure 3 so that at least a portion of the structure 3 is connected in series between the first connecting line 7 and the second connecting line 10. Therefore, the measurement device 4 is connected to ground potential GND exclusively via the structure 3 i.e. there is no connection of the measurement device 4 to ground potential GND except through the structure 3.

(10) FIG. 2 illustrates a second embodiment of a capacitive sensor arrangement 1 according to the invention. This embodiment is largely identical to the embodiment shown in FIG. 1 and insofar will not be explained again. However, in this embodiment, there are two conducting structures 3, 13 which may represent ground electrodes of the sensor arrangement, i.e. electrodes that are supposed to be kept at ground potential GND. As indicated in the figure, an electric field between the sensing electrode 2 and each of the structures (ground electrodes) 3, 13 gives rise to contributions C.sub.S1, C.sub.S2 to the total capacitance C.sub.X. These capacitances C.sub.S1, C.sub.S2—and therefore the total capacitance C.sub.X—depend on whether the structure 3, 13 is connected to ground potential GND or not.

(11) The first structure 3 is connected to the measurement device 4 by a first connecting line 7. The second structure 13 is connected to the first structure 3 by a second connecting line 10 and is connected to ground potential GND by a third connecting line 14. All in all, the measurement device 4 is connected for power supply between the supply voltage V.sub.S and the ground potential GND. If any of the first, second or third connection line 7, 10, 14 is disconnected, the measurement device 4 becomes inoperable. At the same time, disconnection of the second or third connection line 10, 14 means that at least one of the structures 3, 13 is no longer connected to ground potential GND. In such a case, the potential of at least one structure 3, 13 would be floating, which would make any measurement of the total capacitance C.sub.X unreliable. Since the structures 3, 13 are connected in series between the ground potential GND and the first connecting line 7, the grounded condition of both structures 3, 13 can be easily monitored by monitoring the operational state of the measurement device 4. It is understood that the embodiment shown in FIG. 2 could be extended to more than two structures 3, 13, all of which would be connected in series.