Design of a Low-Cost, High-Speed Electric Field Mill for Continuous Electric Field Monitoring

We present the design and performance characteristics of a new electric field mill (EFM) instrument designed and built at CU Boulder. Our EFM was developed for the RELAMPAGO field campaign in Argentina in Fall 2018, where we will deploy an array of eight of these field mills. The electric field is measured by two sets of sense plates that are alternatively exposed and shielded by a grounded rotating vane, similar to other field mills. The outputs from the two sets of sensors are buffered and differentially amplified. An innovation of our field mill is that rather than converting the AC signal into a DC electric field using analog electronics, we directly sample the AC signal as well as the phase of the rotating vane, as measured by an infrared photo-interrupt device. The translation from AC signal to DC electric field is then implemented in software. The vane rotates at 33 Hz and is actively controlled by a PWM signal; with two sets of three sense plates, the output AC signal is at 100 Hz, enabling measurement of electric fields at 10 ms resolution. The ADC, data acquisition, and motor control are implemented in a Teensy microcontroller running Arduino code, and the data are time-tagged using a GPS receiver. Data are sampled and recorded at 1 kHz on a micro-SD flash card, capable of storing 45 days of continuous data without interruption. The entire instrument uses less than 5 W of power, and is powered by a single 50 W solar panel and 150 Wh battery and charge controller. Planned updates to the design are to include real-time data decimation and a built-in modem to transfer data over a cellular network. In this paper, we present the instrument design as discussed above, calibration techniques and results, and preliminary data from the RELAMPAGO field campaign.