Capacitive sensing and method

1. An occupant sensing system for a seat, comprising:

a sensing electrode configured to generate an electric field when power is supplied to the electrode;

a shield electrode configured to form a layer parallel to the sensing electrode, the shield layer configured to reduce the electromagnetic influence of electric fields generated by objects located outside of the electric field; and

a circuit coupled to the sensing electrode and configured to sense a change in the electric field caused by an occupancy state of the seat, the circuit further configured to ground the shield layer for measuring a first impedance at the sensing electrode, to ground the sensing electrode for measuring a second impedance at the shield layer, to electrically couple the sensing electrode and shield electrode together for measuring a third impedance at the sensing electrode and shield electrode, and to determine the impedance from the sensing electrode to ground based on the first, second, and third impedances,

wherein the determined impedance from the sensing electrode to ground does not include an impedance from the sensing electrode to the shield layer.

2. The occupant sensing system of claim 1 , wherein the first impedance comprises an inverse sum of the impedance from the sensing electrode to ground and the impedance from the sensing electrode to the shield electrode.

3. The occupant sensing system of claim 1 , wherein the second impedance comprises an inverse sum of the impedance from the shield electrode to ground and the impedance from the shield electrode to the sensing electrode.

4. The occupant sensing system of claim 1 , wherein the third impedance comprises an inverse sum of the impedance from the sensing electrode to ground and the impedance from the shield electrode to ground.

5. The occupant sensing system of claim 1 , wherein the first impedance comprises an inverse sum of the impedance from the sensing electrode to ground and the impedance from the sensing electrode to the shield layer, the second impedance comprises an inverse sum of the impedance from the shield layer to ground and the impedance from the shield electrode to the sensing electrode, and the third impedance comprises an inverse sum of the impedance from the sensing electrode to ground and the impedance from the shield electrode to ground.

6. The occupant sensing system of claim 1 , wherein the circuit demodulates a current for each of the first, second, and third impedances and a current for at least one source impedance between the sensing electrode and a current source and the shield electrode and the current source, the circuit normalizing the demodulated current values and determining a current from the sensing electrode to ground based on the normalized current values, the circuit determining the impedance from the sensing electrode to ground based on the determined current from the sensing electrode to ground.

7. The occupant sensing system of claim 1 , wherein the circuit is further configured to ground the sensing electrode for measuring a source impedance at the sensing electrode, to ground the shield electrode for measuring a source impedance at the shield electrode, and to disconnect a current source from the sensing electrode for measuring first and second reference impedances, wherein the determined impedance from the sensing electrode to ground is also based on the measured source impedances and reference impedances.

8. The occupant sensing system of claim 1 , wherein the circuit is further configured to ground the sensing electrode and shield layer for measuring a source impedance, and to disconnect a current source from the sensing electrode for measuring first and second reference impedances, wherein the determined impedance from the sensing electrode to ground is also based on the measured source impedance and reference impedances.

9. The occupant sensing system of claim 1 , wherein the shield electrode comprises a driven shield, and wherein the circuit is further configured to electrically connect a known impedance between the driven shield and a load current amplifier to compensate for a driven shield signal.

10. The occupant sensing system of claim 9 , wherein electrically coupling the sensing electrode and shield electrode together for measuring the third impedance further comprises electrically coupling the known impedance in parallel with the sensing electrode and shield electrode.

11. A method for measuring a change in capacitance at a vehicle sensor based on an impedance from the sensor to ground, comprising the steps of:

generating an electric field at a capacitive sensing electrode, the electromagnetic influence of electric fields generated by objects located outside of the electric field being reduced by a shield layer parallel to the sensing electrode;

grounding the shield layer and measuring a first impedance at the sensing electrode;

grounding the sensing electrode and measuring a second impedance at the shield electrode;

electrically coupling the sensing electrode and shield electrode together and measuring a third impedance at the sensing electrode/shield electrode; and

determining the impedance from the sensing electrode to ground based on the first, second, and third impedances,

wherein the determined impedance from the sensing electrode to ground does not include an impedance from the sensing electrode to the shield layer.

12. The method of claim 11 , further comprising:

demodulating a current for each of the first, second, and third impedances;

demodulating a current for at least one source impedance between the sensing electrode and a current source and the shield electrode and the current source;

normalizing the demodulated current values;

determining a current from the sensing electrode to ground based on the normalized current values; and

determining the impedance from the sensing electrode to ground based on the determined current from the sensing electrode to ground.

13. The method of claim 11 , further comprising:

grounding the sensing electrode for measuring a source impedance at the sensing electrode;

grounding the shield electrode for measuring a source impedance at the shield electrode; and

disconnecting a current source from the sensing electrode and measuring first and second reference impedances,

wherein the determined impedance from the sensing electrode to ground is also based on the measured source impedances and reference impedances.

14. The method of claim 11 , further comprising:

grounding the sensing electrode and shield layer for measuring a source impedance;

disconnecting a current source from the sensing electrode for measuring first and second reference impedances,

wherein the determined impedance from the sensing electrode to ground is also based on the measured source impedance and reference impedances.

15. The method of claim 11 , further comprising:

electrically coupling a known impedance between the shield electrode and a load current amplifier, the shield layer comprising a driven shield; and

electrically coupling the known impedance in parallel with the sensing electrode and shield electrode when electrically coupling the sensing electrode and shield electrode together for measuring the third impedance.

16. A capacitive vehicle sensor, comprising:

a sensing electrode configured to generate an electric field;

a shield electrode configured to reduce the electromagnetic influence of electric fields generated by objects outside of the electric field;

a circuit coupled to the sensing electrode and shield electrode that allows a plurality of impedances to be coupled between the sensing electrode and the shield electrode; and

a control system that configures the impedances between the sensing electrode and the shield electrode, is configured to measure current to the sensing electrode in each configuration, and is configured to calculate a measure related to the impedance from the sensing electrode to ground.

17. The capacitive vehicle sensor of claim 16 , wherein the calculated impedance from the sensing electrode to ground is used to classify an occupant of a vehicle seat.

18. An occupant sensing system for a seat, comprising:

a sensing electrode configured to generate an electric field when a signal is supplied to the electrode;

a shield electrode configured to form a layer parallel to the sensing electrode;

a circuit coupled to the sensing electrode and configured to sense a change in the electric field caused by an occupancy state of the seat, the circuit further configured to allow a plurality of coupling impedances between the sensor and shield;

wherein the system calculates a measure related to the impedance from the sensing electrode to ground using the results of separate measurements taken while the coupling impedances are in different configurations.

19. The occupant sensing system of claim 18 , wherein the calculated impedance from the sensing electrode to ground is used to classify an occupant of a vehicle seat.

20. A method for measuring a change in capacitance at a vehicle sensor based on an impedance from the sensor to ground, comprising the steps of:

generating an electric field at a capacitive sensing electrode;

driving the shield layer with a signal substantially equal to the sensing signal and measuring the current sent to the sensing electrode;

driving the shield layer with a signal marginally different than the sensing signal and measuring the current sent to the sensing electrode;

driving the shield layer with a signal substantially equal to the sensing signal while electrically coupling the sensing electrode and shield electrode together and measuring the current sent to the sensing electrode;

driving the shield layer with a signal marginally different than the sensing signal while electrically coupling the sensing electrode and shield electrode together and measuring the current sent to the sensing electrode; and

determining the impedance from the sensing electrode to ground based on the four measurements,

wherein the determined impedance from the sensing electrode to ground does not include an impedance from the sensing electrode to the shield layer.

21. The method of claim 19 , wherein the determined impedance from the sensing electrode to ground is used to classify an occupant of a vehicle seat.