Electrostatic seat occupant detection system

An electrostatic seat occupant detection system of a vehicle having a vehicle body and a seat, the electrostatic seat occupant detection system comprising:

• an antenna unit that includes a main electrode and a sub-electrode disposed side by side;
• an electric power source for supplying AC voltage to the antenna unit;
• a switch unit for switching supply of the AC voltage to the antenna unit to form one of a first electric field between the vehicle body and the main electrode thereby providing a seat-occupant detecting mode and a second electric field between the main electrode and the sub-electrode thereby providing a wet condition detecting mode;
• and
• a seat-occupant detecting ECU, wherein the seat-occupant detecting ECU comprises:
• an admittance calculating section that calculates a first admittance of the first electric field and a second admittance of the second electric field;
• a conductance-susceptance separating section that separates conductance of the first and second admittances from the susceptance of the first and second admittances;
• a water content estimating section that estimates an amount of water content of the seat based on the conductance and the susceptance of the second admittance;
• and
• a judging section that judges whether a seat occupant is present or not based on the conductance and susceptance of the first admittance and the estimated amount of the water content of the seat, wherein the judging section provides a threshold plane that is a function of the conductance and the susceptance of the first admittance and judges that an adult is seated on the seat when the amount of water content estimated by the water content estimating section falls below the threshold plane.