Multiple frequency grain moisture sensor for combines

1. A grain moisture sensing system comprising:

an excitation signal source for producing an excitation signal;

a sensor cell having a driven plate for applying the excitation signal and a sense plate proximate to and substantially parallel with the driven plate for capacitive measurement across a spacing between the driven plate and the sense plate such that a sense current is produced at the sense plate;

the excitation signal source electrically connected to the driven plate of the sensor cell;

a first synchronous detector adapted for measuring components of the excitation signal, the synchronous detector electrically connected to the excitation source; and

a second synchronous detector adapted for measuring components of the sense current, the second synchronous detector operatively connected to the sense plate.

2. The grain moisture system of claim 1 wherein the first synchronous detector is adapted for alternatively measuring imaginary components of the excitation signal and real components of the excitation signal.

3. The grain moisture system of claim 1 wherein the second synchronous detector is adapted for alternatively measuring imaginary components of the sense current and real components of the sense current.

4. The grain moisture system of claim 1 wherein the first synchronous detector is a mixer and the second synchronous detector is a mixer.

5. The grain moisture sensor of claim 1 wherein the excitation signal source is a switch adapted for alternatively selecting one of a first frequency in-phase signal, a first frequency quadrature signal, a second frequency in-phase signal, and a second frequency quadrature signal.