ELECTRIC FIELD METER -- E100
Principle of Operation

Case removed; view of interior circuit boards.	Field meter in use on an outdoor stand.	Rotating shutters on downward-facing end.

Also see the electrical, mechanical, material, and environmental specifications.

NOTE: The following description of the E-100 electric field meter was published as Appendix A in: Winn, W. P., Aircraft measurement of electric field: Self-calibration, J. Geophys. Res., 98, 7,351--7,365, 1993.

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The electric field meters on SPTVAR [Special Purpose Test Vehicle for Atmospheric Research, New Mexico Tech's instrumented research airplane] are also known as field mills. Chalmers [1967] describes early instruments of this sort as well as other types of instruments that measure electric field. In a field mill, a conducting, grounded, rotating shutter alternately shields and exposes electrodes from the electric field to be measured. The charge induced on the electrodes, which is periodic, passes through an amplifier. After amplification, the signal is demodulated and filtered to produce a voltage that is proportional to the electric field.

Figure 10 shows the main mechanical components. The motor rotates both the shutter and the light chopper. The electric field induces charge on the parts of the electrodes that are not covered by the shutter. The signal from the chopped light beam is used to demodulate the periodic signal from the electrodes. Slip rings on the rotating motor shaft are wiped by brushes attached to the instrument frame to maintain the shutter at the potential of the skin of the airplane.

Figure 10. Diagram of the main mechanical components of a field mill.

Figure 11 shows in some detail how the signals from the electrodes are processed, and Figure 12 shows what the signals look like as they progress through the electronic circuits. There are four electrodes. Pair A is exposed when Pair B is covered, and vice versa. Thus the periodic signal from one pair is 180 degrees out of phase with the signal from the other pair. Each pair is amplified separately by a charge amplifier, in which the induced charge is converted to a voltage. A differential amplifier following the charge amplifiers adds the signals from the two pairs by compensating for the 180 degree phase shift. The demodulator is simply an amplifier whose gain is +/- 1 depending on the signal from the light chopper; the rectified signal produced by the demodulator is shown in Figure 12. Filtering and buffering completes the demodulation and produces a voltage that is proportional to the local electric field amplitude. Additional components at the inputs and outputs provide protection against transient voltages such as those produced by lightning flashes. Some properties of the field meters are listed in Table 3.

Figure 11. Circuit diagram for the insensitive channel of the field mill. All resistances are ohms; capacitances greater than 1 are picofarads; capacitances less than 1 are microfarads. All operational amplifiers are National Semiconductor type LF441 or LF444. A circuit for the sensitive channel (not shown) branches off at the output of the differential amplifier and has an extra stage of gain before demodulation. All the data used in the self-calibration were from the insensitive channel. Waveforms of voltages VA, VB, VC, V2, V3, and V4 are shown in the next figure.

Figure 12. voltage waveforms at various nodes in the circuit shown in Figure 11.

TABLE 3. Properties of the Electric Field Meters

Property	Parameter
Input voltage	21 to 30 V dc
Input current	0.4 A start-up, 0.1 A running
Motor angular velocity	50 rev/s
Mass	1.6 kg
Diameter	11.43 cm
Length	17.8 cm
Electrode and shutter material	Type 303 stainless steel
Insulator material	Fluorinated ethylene propylene (FEP Teflon)
Input range, insensitive channel	-1700 to +1700 kV/m at face of mill
Input range, sensitive channel	-26 to +26 kV/m at face of mill
Output voltage range	-10 to +10 V

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Read about how the E100 field meter is used in a lightning warning system; and how it is used to determine when to trigger lightning using small rockets.

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Last updated 01 February 2002 by kieft@nmt.edu.