Methods and apparatus for a capacitive sensor

1. A capacitive sensor capable of detecting a liquid, comprising:

a first electrode;

a single, continuous second electrode arranged planar with and surrounding the first electrode, wherein the first and second electrodes form a sensing capacitor; and

a single, continuous third electrode connected to a ground potential, wherein the third electrode:

is planar with the first and second electrodes;

adjacent to an outer edge of the second electrode; and

surrounds the first and second electrodes.

2. The capacitive sensor according to claim 1 , wherein the first electrode comprises a reception electrode.

3. The capacitive sensor according to claim 1 , wherein the second electrode comprises a drive electrode connected to a voltage source.

4. The capacitive sensor according to claim 1 , wherein the second electrode arranged is concentric with the first electrode.

5. The capacitive sensor according to claim 4 , wherein the third electrode is arranged concentric with the first and second electrodes.

6. The capacitive sensor according to claim 1 , wherein the first electrode comprises a set of continuous linear segments.

7. The capacitive sensor according to claim 6 , wherein the second electrode is arranged to form a comb-shape that interleaves the first electrode.

8. The capacitive sensor according to claim 1 , wherein the third electrode surrounds the outer edge of the second electrode.

9. The capacitive sensor according to claim 1 , wherein the third electrode is configured to couple the liquid, which is adjacent to a surface of the third electrode, to the ground potential.

10. A method for detecting rain using a capacitive sensor, comprising:

forming the capacitive sensor comprising:

a first electrode;

a single, continuous second electrode arranged planar with and surrounding the first electrode, wherein the first and second electrodes form a sensing capacitor; and

a single, continuous third electrode planar with and surrounding the first and second electrodes and connected to a common ground having a fixed ground potential;

electrically coupling the rain to the ground potential via the second electrode and the third electrode;

measuring a capacitance of the sensing capacitor;

converting the capacitance to a digital value;

comparing the digital value to a predetermined threshold value; and

generating a logic signal according to the comparison;

wherein:

the logic signal is a first value if the digital value is less than the threshold value and indicates an absence of rain; and

the logic signal is a second value if the digital value is greater than or equal to the threshold value and indicates a presence of rain.

11. The method according to claim 10 , wherein:

the second electrode arranged is concentric with the first electrode; and

the third electrode is arranged concentric with the first and second electrodes and directly adjacent to the second electrode.

12. The method according to claim 10 , wherein:

the second electrode is arranged to form a comb-shape;

the first electrode is nested within the second electrode; and

the third electrode is directly adjacent to and surrounds an outer perimeter of the second electrode.

13. The method according to claim 10 , wherein:

the first electrode comprises a reception electrode; and

the second electrode comprises a drive electrode connected to a voltage source.

14. The method according to claim 10 , wherein:

the first electrode is configured to operate as one of a reception electrode and a drive electrode; and

the second electrode is configured to operate as one of the reception electrode and the drive electrode.

15. A system for detecting rain on a surface, comprising:

a capacitive sensor affixed to the surface and comprising:

a first electrode;

a single, continuous second electrode arranged planar with and surrounding the first electrode, wherein the first and second electrodes form a sensing capacitor; and

a single, continuous third electrode connected to a common ground having a fixed ground potential, wherein the third electrode is planar with and surrounding the first and second electrodes;

a sensing circuit connected to the capacitive sensor and configured to:

measure a capacitance of the sensing capacitor;

convert the capacitance to a digital value;

compare the digital value to a threshold value;

generate a logic signal according to the comparison;

wherein:

the logic signal is a first value if the digital value is less than the threshold value and indicates an absence of rain; and

the logic signal is a second value if the digital value is greater than or equal to the threshold value and indicates a presence of rain; and

a control circuit connected to an output terminal of the sensing circuit and configured to:

receive the logic signal; and

generate a control signal according to the logic signal, wherein the control signal controls an operation to remove the rain from the surface.

16. The system according to claim 15 , wherein the third electrode is configured to couple the rain to the ground potential.

17. The system according to claim 15 , wherein:

the second electrode arranged is concentric with the first electrode and surrounds an outer perimeter of the first electrode; and

the third electrode is arranged concentric with the first and second electrodes and surrounds an outer perimeter of the second electrode.

18. The system according to claim 15 , wherein:

the second electrode comprises a comb-shape that interleaves the first electrode; and

the first electrode is nested within the second electrode.

19. The system according to claim 15 , wherein:

the first electrode comprises a reception electrode; and

the second electrode comprises a drive electrode connected to a voltage source.

20. The system according claim 15 , wherein:

the first electrode is configured to operate as one of a reception electrode and a drive electrode; and

the second electrode is configured to operate as one of the reception electrode and the drive electrode.