Rotationally Vibrating Electric-Field Mill

The disadvantages of rotary couplings in conventional field mills could be avoided.

A proposed instrument for measuring a static electric field would be based partly on a conventional rotating- split-cylinder or rotating-split-sphere electric-field mill. However, the design of the proposed instrument would overcome the difficulty, encountered in conventional rotational field mills, of transferring measurement signals and power via either electrical or fiber-optic rotary couplings that must be aligned and installed in conjunction with rotary bearings. Instead of being made to rotate in one direction at a steady speed as in a conventional rotational field mill, a split-cylinder or split-sphere electrode assembly in the proposed instrument would be set into rotational vibration like that of a metronome. The rotational vibration, synchronized with appropriate rapid electronic switching of electrical connections between electric-current-measuring circuitry and the split-cylinder or split-sphere electrodes, would result in an electrical measurement effect equivalent to that of a conventional rotational field mill.

The figure depicts a version of the proposed instrument, the electrode assembly of which would include a hollow metal hemisphere split into four electrodes. Instead of a conventional rotary bearing, the instrument would include a flexural bearing that would be part of a metronome-like actuator. The measurement- signal and power connections between the electrode assembly and external instrumentation would be made via optical fibers that would flex with the flexural bearing.

The flexural bearing and actuator would be anchored to a stationary base, on which data-acquisition and power-supply electronic circuits would be mounted. In addition to the electrodes, the electrode assembly would contain electronic circuits for switching the electrical connections to the electrodes, measuring the electric currents that flow between connected electrodes as the assembly rotates in the ambient electric field, digitizing the current measurements, and transmitting the digitized measurement signals to the data-acquisition circuitry via one of the optical fibers. Power would be transmitted from a light-emitting diode on the stationary base, via another optical fiber, to photovoltaic circuitry in the electrode assembly.

Because the flexural bearing, its actuator, and the electrode assembly taken together would constitute a resonant mechanical system like a metronome, little power would be needed to maintain the large angular excursions needed to produce sufficiently large measurement signals. The precise nature of the actuator has not yet been determined; it seems likely that a magnetic drive could easily be implemented. The actuator could be equipped with a rotary position encoder, which could provide feedback for adjusting the excitation of the actuator to correct for small deviations of the rotational vibration from constant frequency and amplitude.

This work was done by Harold Kirkham of Caltech for NASA’s Jet Propulsion Laboratory.

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