Debris filter sensor

1. A system comprising:

a debris fence having a largest side formed from structure and gaps of the debris fence, the largest side of the debris fence defining a first plane;

a piezoelectric device positioned adjacent to the debris fence, the piezoelectric device having a largest surface oriented in a second plane perpendicular to the first plane such that air flowing through the debris fence perpendicular to the first plane passes in parallel along the largest surface; and

a resonant frequency detection circuit connected with the piezoelectric device;

wherein a resonant frequency of the piezoelectric device is responsive to debris adjacent to the debris fence, the debris comprising dust.

2. The system of claim 1 wherein the debris fence comprises a plurality of parallel slats.

3. The system of claim 1 wherein the piezoelectric device comprises a piezoelectric disk.

4. The system of claim 1 wherein the piezoelectric device comprises a piezoelectric element with first, second, and third electrodes, and wherein the resonant frequency detection circuit grounds the first electrode and feeds back signal from the second electrode to the third electrode.

5. The system of claim 1 wherein the resonant frequency detection circuit comprises a Hartley oscillator.

6. The system of claim 1 wherein the resonant frequency detection circuit comprises a frequency-to-voltage converter and an analog-to-digital converter.

7. The system of claim 1 further comprising a mounting plate, the debris fence and piezoelectric device connected with the mounting plate.

8. The system of claim 7 further comprising a filter of an electronics housing, wherein the mounting plate connects the debris fence over the filter.

9. The system of claim 1 further comprising a mounting plate, the debris fence and piezoelectric device connected with the mounting plate such that the first plane of the debris fence is parallel with a filter over which the mounting plate is connected.

10. The system of claim 9 wherein the mounting plate is connected against the filter.

11. The system of claim 1 further comprising a mounting plate, the debris fence and piezoelectric device connected with the mounting plate such that the first plane of the debris fence is parallel with a largest surface of the mounting plate.

12. A system comprising:

a debris fence distributed along a first plane;

a piezoelectric device positioned adjacent to the debris fence, the piezoelectric device having a largest surface oriented in a second plane perpendicular to the first plane such that air flowing through the debris fence perpendicular to the first plane passes in parallel along the largest surface;

a mounting plate, the debris fence and piezoelectric device connected with the mounting plate,

a filter of an electronics housing, wherein the mounting plate connects the debris fence over the filter; and

a resonant frequency detection circuit connected with the piezoelectric device;

wherein a resonant frequency of the piezoelectric device is responsive to debris adjacent to the debris fence, the debris comprising dust.

13. A system comprising:

a debris fence distributed along a first plane;

a filter of an electronics housing;

a piezoelectric device positioned adjacent to the debris fence, the piezoelectric device having a largest surface oriented in a second plane perpendicular to the first plane such that air flowing through the debris fence perpendicular to the first plane passes in parallel along the largest surface;

a mounting plate, the debris fence and piezoelectric device connected with the mounting plate such that the first plane of the debris fence is parallel with the filter over which the mounting plate is connected;

a resonant frequency detection circuit connected with the piezoelectric device;

wherein a resonant frequency of the piezoelectric device is responsive to debris adjacent to the debris fence, the debris comprising dust.