It is often desirable to sense the angular position of a rotating part. Numerous kinds of rotation sensors have been developed over the years; one type is a capacitive sensor, where a capacitance varies according to angular position. Rotary sensors known in the art are lacking in resolution and generally have an oversized form factor, making them impractical for applications requiring maximum signal production for a limited sensing area, in addition to having differential cancellation of offset bias and gain terms to reject noise caused by motion not in the direction of rotation of the sensor. What is needed is a rotary position sensor with high resolution and a reduced form factor that is able to operate for long periods of time, while having a reduced cost of fabrication.

The rotary sensor prototype includes two flat discs patterned with conductive material.

To address this need, a rotary position sensor is provided in which two flat discs are patterned with conductive material and laid over one another, creating capacitive coupling between the two discs at the conductor locations. One disc, referred to as the sensing disc, has four capacitive sensing pads that appear in at least eight locations (at least two locations each) in a regular pattern around the surface of the sensing disc. In general, each capacitive sensing pad has N-fold rotational symmetry, and the overall pattern formed by all capacitive sensing pads has 4N-fold rotational symmetry.

The other disc, referred to as the scale, has a pattern of rounded conductive pads on its surface that produces four sinusoidal waveforms in capacitance, each 90 degrees out of phase with the previous, with the capacitive sensing pads of the sensing disc as the two discs are rotated relative to one another to provide angular position information. This is achieved by making the overlapping area of the capacitive sensing pads change in a sinusoidal fashion with rotation. Thus, the conductive pattern on the scale, and the capacitive sensing pads on the sensing disc, form the two plates of the capacitors, and the capacitances of these capacitors provide angular position information.

This sensor is designed to improve the movement of rotary joints. It produces a highly robust, friction-free angular measurement using non-contact sensing technology. The sensing discs can be as thin as a few sheets of paper, and have a center cutout to allow the passing of a shaft or cabling. The sensor is alignment-insensitive — it is capable of rejecting motion noise in all directions not intended to be sensed. This allows for high-precision angular measurements with low-precision mounting. The sensor is also completely shielded to reject environmental electrical noise. Larger sensors or sensors with improved readout electronics will produce even higher resolution.

For more information, contact Evan Elder, Licensing Associate, at 650-725-9558.

Tech Briefs Magazine

This article first appeared in the October, 2018 issue of Tech Briefs Magazine.

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