Capacitance-based sensor and occupant sensing system

1. A capacitance-based sensor that is arranged in a seat of a vehicle and senses presence of an occupant in the seat, the capacitance-based sensor comprising:

an occupant sensing electrode that is embedded in the seat;

an empty seat capacitance reducing electrode that is placed between the occupant sensing electrode and a seat frame of the seat, which is connected to a vehicle ground, wherein the occupant sensing electrode is connected to the vehicle ground, and the empty seat capacitance reducing electrode is connected to an intersecting point which connects between the occupant sensing electrode and the vehicle ground;

an electric current sensing device that measures an electric current and is inserted in one of a first location, which connects between the occupant sensing electrode and the intersecting point, and a second location, which connects between the vehicle ground and the intersecting point; and

a drive device that generates a drive voltage and is inserted in the other one of the first location and the second location, wherein the empty seat capacitance reducing electrode is connected to the intersecting point to reduce a capacitance that is measured with the electrical current sensing device in an empty state of the seat where the occupant is not present on the seat; wherein

in an occupant sensing operational mode of the capacitance-based sensor for sensing the occupant on the seat, the drive device applies a voltage between the occupant sensing electrode and the vehicle ground, and the electric current sensing device measures an electric current between the occupant sensing electrode and the vehicle ground;

in a malfunction sensing operational mode of the capacitance-based sensor for sensing a malfunction of the capacitance-based sensor, the drive device applies a voltage between the occupant sensing electrode and the empty seat capacitance reducing electrode, and the electric current sensing device measures an electric current between the occupant sensing electrode and the empty seat capacitance reducing electrode;

in the occupant sensing operational mode of the capacitance-based sensor, the drive device and the electric current sensing device are connected in series between the occupant sensing electrode and the vehicle ground in this order, and the empty seat capacitance reducing electrode has the same electric potential as that of the vehicle ground; and

in the malfunction sensing operational mode of the capacitance-based sensor, the drive device and the electric current sensing device are connected in series between the occupant sensing electrode and the empty seat capacitance reducing electrode in this order.

2. The capacitance-based sensor according to claim 1 , wherein:

the electric current sensing device is placed in the first location; and

the drive device is placed in the second location.

3. The capacitance-based sensor according to claim 2 , wherein the occupant sensing electrode has the same electric potential as that of the empty seat capacitance reducing electrode upon operation of the drive device.

4. The capacitance-based sensor according to claim 1 , wherein:

the electric current sensing device is placed in the second location; and

the drive device is placed in the first location.

5. The capacitance-based sensor according to claim 4 , wherein the empty seat capacitance reducing electrode has the same electric potential as that of the vehicle ground upon operation of the drive device.

6. The capacitance-based sensor according to claim 1 , wherein the electric current sensing device includes an operational amplifier, which keeps the same electric potential and electric insulation between corresponding two of the occupant sensing electrode, the empty seat capacitance reducing electrode and the vehicle ground, between which the electric current sensing device is connected.

7. The capacitance-based sensor according to claim 1 , wherein:

the occupant sensing electrode and the intersecting point are connected to each other through a first conductive line;

the vehicle ground and the intersecting point are connected to each other through a second conductive line;

at least a portion of the first conductive line and at least a portion of the second conductive line are formed as a shielded line that has a shield portion, which has the same electric potential as that of the empty seat capacitance reducing electrode upon operation of the drive device.

8. The capacitance-based sensor according to claim 7 , wherein the empty seat capacitance reducing electrode is electrically connected to the electric current sensing device and the drive device through the shield portion of the shielded line.

9. The capacitance-based sensor according to claim 1 , further comprising a dielectric layer, wherein the occupant sensing electrode is provided on a first side of the dielectric layer, and the empty seat capacitance reducing electrode is provided on a second side of the dielectric layer, which is opposite from the first side of the dielectric layer.

10. The capacitance-based sensor according to claim 9 , wherein a surface of the occupant sensing electrode and a surface of the empty seat capacitance reducing electrode are coated with a dielectric material.

11. The capacitance-based sensor according to claim 1 , wherein a surface area of the empty seat capacitance reducing electrode is smaller than a surface area of the occupant sensing electrode.

12. The capacitance-based sensor according to claim 1 , wherein a surface area of the empty seat capacitance reducing electrode is larger than a surface area of the occupant sensing electrode.

13. The capacitance-based sensor according to claim 1 , wherein in each of the occupant state sensing operational mode and the malfunction sensing operational mode of the capacitance-based sensor, the occupant sensing electrode has a high electric potential, and the empty seat capacitance reducing electrode and the vehicle ground has a low electric potential, which is lower than the high electric potential.

14. The capacitance-based sensor according to claim 13 , further comprising:

a first conductive line that has the drive device, which is interposed between a first end and a second end of the first conductive line, wherein the first end of the first conductive line is directly connected to the occupant sensing electrode;

a second conductive line that has the electric current sensing device, which is interposed between a first end and a second end of the second conductive line, wherein the first end of the second conductive line is directly connected to vehicle ground, and the second end of the second conductive line is connected to the second end of the first conductive line;

a third conductive line that has a first end, which is directly connected to the empty seat capacitance reducing electrode, and a second end, which is connected to the second end of the first conductive line and the second end of the second conductive line; and

a line selecting switch device that executes switching for the second conductive line and the third conductive line, wherein:

the line selecting switch device divides the second conductive line into:

a first part that includes the first end of the second conductive line, which is directly connected to the vehicle ground; and

a second part that includes the second end of the second conductive line and the electric current sensing device;

the line selecting switch device divides the third conductive line into:

a first part that includes the first end of the third conductive line, which is directly connected to the empty seat capacitance reducing electrode; and

a second part that includes the second end of the third conductive line, which is connected to the second end of the first conductive line and the second end of the second conductive line;

in the occupant state sensing operational mode of the capacitance-based sensor, the line selecting switch device electrically connects between the first part of the second conductive line and the second part of the second conductive line and also electrically connects between the first part of the third conductive line and the second part of the third conductive line; and

in the malfunction sensing operational mode of the capacitance-based sensor, the line selecting switch device electrically connects between the first part of the third conductive line and the second part of the second conductive line, so that the occupant sensing electrode is electrically connected to the empty seat capacitance reducing electrode through the drive device and the electric current sensing device.

15. The capacitance-based sensor according to claim 1 , wherein:

the occupant sensing electrode includes a group of electrode portions, each of which is electrically separated from the rest of the electrode portions of the group; and

the capacitance-based sensor further comprising an electrode portion selecting switch device, which electrically connects one of the electrode portions of the group to the drive device and the electric current sensing device.

16. The capacitance-based sensor according to claim 15 , wherein upon operation of the drive device, the empty seat capacitance reducing electrode has an electrical potential that is the same as that of the vehicle ground.

17. The capacitance-based sensor according to claim 15 , wherein the electrode portions of the group are arranged in parallel and are spaced from one another in one of a fore-and-aft direction of the vehicle and a transverse direction of the vehicle.

18. An occupant sensing system comprising:

the capacitance-based sensor according to claim 1 ; and

an occupant sensing ECU that determines an occupant state of the seat based on an output of the capacitance-based sensor.

19. The occupant sensing system according to claim 18 , wherein:

the occupant sensing ECU receives an output of a belt buckle switch device of the seat; and

when the occupant sensing ECU senses an unbuckled state of a seat belt of the seat based on the output of the belt buckle switch device, the occupant sensing ECU determines that the seat is empty and has no child restraint system installed therein.

20. A capacitance-based sensor that is arranged in a seat of a vehicle and senses presence of an occupant in the seat, the capacitance-based sensor comprising:

an occupant sensing electrode that is embedded in the seat;

an empty seat capacitance reducing electrode that is placed between the occupant sensing electrode and a seat frame of the seat, which is connected to a vehicle ground, wherein the occupant sensing electrode is connected to the vehicle ground, and the empty seat capacitance reducing electrode is connected to an intersecting point which connects between the occupant sensing electrode and the vehicle ground;

an electric current sensing device that measures an electric current and is inserted in one of a first location, which connects between the occupant sensing electrode and the intersecting point, and a second location, which connects between the vehicle ground and the intersecting point; and

a drive device that generates a drive voltage and is inserted in the other one of the first location and the second location, wherein the empty seat capacitance reducing electrode is connected to the intersecting point to reduce a capacitance that is measured with the electrical current sensing device in an empty state of the seat where the occupant is not present on the seat; wherein

in the occupant sensing operational mode of the capacitance-based sensor, the drive device and the electric current sensing device are connected in series between the occupant sensing electrode and the vehicle ground in this order, and the empty seat capacitance reducing electrode has the same electric potential as that of the vehicle ground; and

in the malfunction sensing operational mode of the capacitance-based sensor, the drive device and the electric current sensing device are connected in series between the occupant sensing electrode and the empty seat capacitance reducing electrode in this order;

in the occupant sensing operational mode of the capacitance-based sensor, the electric current sensing device and the drive device are connected in series between the occupant sensing electrode and the vehicle ground in this order, and the occupant sensing electrode has the same electric potential as that of the empty seat capacitance reducing electrode; and

in the malfunction sensing operational mode of the capacitance-based sensor, the electric current sensing device and the drive device are connected in series between the occupant sensing electrode and the empty seat capacitance reducing electrode in this order;

in the occupant sensing operational mode of the capacitance-based sensor, the vehicle ground has a relatively high electric potential, and the occupant sensing electrode and the empty seat capacitance reducing electrode have a relatively low electric potential;

in the malfunction sensing operational mode of the capacitance-based sensor, the occupant sensing electrode and the vehicle ground have a relatively low electric potential, and the empty seat capacitance reducing electrode has a relatively high electric potential; the capacitance-based sensor further comprising:

a first conductive line that has the electric current sensing device, which is interposed between a first end and a second end of the first conductive line, wherein the first end of the first conductive line is directly connected to the occupant sensing electrode;

a second conductive line that has the drive device, which is interposed between a first end and a second end of the second conductive line, wherein the first end of the second conductive line is directly connected to the vehicle ground, and the second end of the second conductive line is connected to the second end of the first conductive line;

a third conductive line that has a first end, which is directly connected to the empty seat capacitance reducing electrode, and a second end, which is connected to the second end of the first conductive line and the second end of the second conductive line; and

a line selecting switch device that executes switching for the second conductive line and the third conductive line, wherein:

the line selecting switch device divides the second conductive line into:

a first part that includes the first end of the second conductive line, which is directly connected to the vehicle ground; and

a second part that includes the second end of the second conductive line and the drive device;

the line selecting switch device divides the third conductive line into:

a first part that includes the first end of the third conductive line, which is directly connected to the empty seat capacitance reducing electrode; and

a second part that includes the second end of the third conductive line, which is connected to the second end of the first conductive line and the second end of the second conductive line;

in the occupant state sensing operational mode of the capacitance-based sensor, the line selecting switch device electrically connects between the first part of the second conductive line and the second part of the second conductive line and also electrically connects between the first part of the third conductive line and the second part of the third conductive line; and

in the malfunction sensing operational mode of the capacitance-based sensor, the line selecting switch device electrically connects between the first part of the third conductive line and the second part of the second conductive line, so that the occupant sensing electrode is electrically connected to the empty seat capacitance reducing electrode through the electric current sensing device and the drive device.

21. A capacitance-based sensor that is arranged in a seat of a vehicle and senses presence of an occupant in the seat, the capacitance-based sensor comprising:

an occupant sensing electrode that is embedded in the seat;

a capacitance reducing electrode that is disposed between the occupant sensing electrode and a frame of the seat, the frame and the occupant sensing electrode being connected to a vehicle ground;

an electric current sensing device that measures an electric current, the electric current sensing device being electrically connected in series between the occupant sensing electrode and the vehicle ground;

a drive device that generates a drive voltage, the drive device being electrically connected in series between the occupant sensing electrode and the vehicle ground; wherein

the capacitance reducing electrode is electrically connected at a point between the electric current sensing device and the drive device;

the electric current sensing device is electrically connected in one of a first location between the point of connection of the capacitance reducing electrode and the occupant sensing electrode and a second location between the point of connection of the capacitance reducing electrode and the vehicle ground;

the drive device is electrically in the other of the first and second locations; and

the capacitance reducing electrode reduces a capacitance that is measured by the electrical current sensing device when the occupant is not present in the seat.