A proposed brushless shaft-angle sensor for use in extreme cold would offer significant advantages over prior such sensors:

  • It would be capable of operating in extreme cold; and
  • Its electronic circuitry would be simpler than that of a permanent- magnet/multiple- Hall-probe shaft-angle sensor that would otherwise ordinarily be used to obtain comparable angular resolution.

The Series Capacitance between the stationary electrodes would vary as the shaft turned, causing the frequency of the tunnel-diode oscillator to vary.
As in the case described in the immediately preceding article, the design of this sensor would exploit the wide temperature range and other desirable attributes of differential transducers based on tunnel-diode oscillators as described in "Multiplexing Transducers Based on Tunnel-Diode Oscillators" (NPO-43079), NASA Tech Briefs, Vol. 30, No. 9 (September 2006), page 42.

The principle of operation of the proposed shaft-angle sensor requires that the shaft (or at least the portion of the shaft at the sensor location) be electrically insulating. The affected portion of the shaft would be coated with metal around half of its circumference. Two half-circular-cylinder electrodes having a radius slightly larger than that of the shaft would be mounted on the stator, concentric with the shaft, so that there would be a small radial gap between them and the outer surface of the shaft. Hence, there would be a capacitance between each stationary electrode and the metal coat on the shaft.

The stationary electrodes would be connected into a tunnel-diode oscillator circuit, so that the series combination of the two capacitances would be part of the capacitance that determines the oscillation frequency. As the shaft is rotated, the stationary-electrode/metal-coat overlap area would change, causing the series capacitance and the oscillation frequency to change. The frequency would be measured and used to infer the shaft angle from the known relationships among shaft angle, capacitance, and frequency.

It should be noted that a given frequency could signify either of two distinct shaft angles. If necessary, one could resolve the shaft-angle ambiguity by use of two sensors at different angular positions.

This work was done by Talso Chui of Caltech for NASA's Jet Propulsion Laboratory.


This Brief includes a Technical Support Package (TSP).
Shaft-Angle Sensor Based on Tunnel- Diode Oscillator

(reference NPO-43328) is currently available for download from the TSP library.

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