Compact, low-cost electric field mill
An electric field mill (EFM) incorporates a novel rotor and shaft grounding mechanism providing a low-impedance path to ground via a bearing packed with electrically conductive grease. A removable bearing washer allows for servicing. The EFM includes a data processing scheme built around a peak detection algorithm and moving much of the signal processing to digital processing. A GPS disciplined sampling clock and cellular connectivity allow for use and maintenance of arrays of widely scattered EFMs.
1. An electric field mill (EFM) for measuring an electric field comprising:
fixed sensor plates configured to sense the electric field via accumulated charge on the sensor plates;
a conductive rotor attached to a conductive rotating shaft, the rotor having spaced-apart blades configured to alternately block and unblock sets of the sensor plates from sensing the electric field;
conductive grounded housing;
an isolation plate for containing the sensor plates and electrically isolating the sensor plates from the shaft and rotor;
a detector for sensing the rotation of the shaft;
analog circuitry attached to the sensor plates and the detector, the analog circuitry configured to amplify the sensed accumulated charge to form an accumulated charge signal, and
signal processing circuitry attached to the analog circuitry configured to generate an electric field measurement based upon the amplified accumulated charge signal and the sensed rotation;
wherein the rotor and the shaft are grounded by a bearing attached to the housing and packed with electrically conductive grease configured to allow the shaft to rotate; and
wherein the signal processing circuitry is digital and is configured to
convert the amplified accumulated charge signal into a digital charge signal,
convert the sensed rotation into a digital rotation signal,
combine the digital charge signal and the digital rotation signal and compute signal polarity,
extract amplitude from the digital charge signal, and
generate the electric field measurement based on the extracted amplitude and the signal polarity.
2. The EFM of claim 1 further comprising a GPS receiver which provides time-tagging of the electric field measurement.
3. The EFM of claim 2 further comprising cellular communication equipment.
4. The EFM of claim 3 , further comprising equipment configured to provide remote commands to the digital circuitry.
5. The EFM of claim 4 wherein the commands include restart and motor speed commands.
6. Multiple spaced-apart EMS according to claim 3 forming an array and further comprising apparatus for collecting data from each EFM via the cellular communication equipment and synchronizing the data based on the GPS receiver time-tagging.
7. The EFM of claim 6 , further comprising equipment configured to provide remote commands to the digital circuitry.
8. The EFM of claim 7 wherein the commands include restart and motor speed commands.
9. The EFM of claim 1 further comprising an electronically commutated motor configured to rotate the shaft and the rotor.
10. The EFM of claim 9 wherein the digital circuitry further comprises a dedicated motor controller integrated circuit configured to control the motor in order to maintain shaft rotation speed.
11. The EFM of claim 1 wherein the digital circuitry is further configured to extract amplitude using a Hilbert Transform.
12. The EFM of claim 1 wherein the digital circuitry is further configured to extract amplitude using a spline-based envelope extraction.
13. The EFM of claim 1 wherein the digital circuitry is further configured to apply a calibration map and site correction to the electric field measurement.
14. The EFM of claim 1 wherein the bearing includes a removable washer configured to allow the grease to be replenished.
15. The EFM of claim 14 wherein the grease comprises conductive carbon grease.
16. The EFM of claim 1 further comprising temperature sensors configured to sense temperature at the analog circuitry and at the digital circuitry.