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Dual Plane Capacitive Coupling Encoder

IP.com Disclosure Number: IPCOM000077825D
Original Publication Date: 1972-Sep-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Flaherty, RJ: AUTHOR [+3]

Abstract

Optical gratings and capacitive encoders usually have noise and alignment problems that are difficult to resolve. This capacitive encoder minimizes noise and alignment difficulties. The noise reduction is obtained by using dual-plane receptor plates. The use of offset receiver channels provides a positional, as well as directional encoder.

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Dual Plane Capacitive Coupling Encoder

Optical gratings and capacitive encoders usually have noise and alignment problems that are difficult to resolve. This capacitive encoder minimizes noise and alignment difficulties. The noise reduction is obtained by using dual-plane receptor plates. The use of offset receiver channels provides a positional, as well as directional encoder.

The basic hardware, excluding circuits and logic, consists of one triple-plane printed-circuit emitter board 1 and two single or dual-plane printed-circuit receptor boards 2 and 3, Fig. 1. The center plane 1a of the triple plane board is excited with an AC voltage from source 4. The outer planes 1b and 1c are grounded and have a grating pattern etched in them. For a ten-pitch encoder, the grid pattern is formed by rectangular etched apertures at fixed intervals. Receptor boards 2 and 3 have rectangular plates 2a and 3a, respectively, of the same size as the grid apertures. They are connected in parallel and arranged in a pitched pattern to match the grid pattern on the emitter board. The receptor boards are attached to a carrier, not shown, which moves with respect to the emitter board.

When the receptor plates 2a and 3a are directly over the grounded areas 1d and 1e of the emitter board 1, they have minimum capacitive coupling to the center plane and maximum capacitive coupling to ground. As the plates move from this position to a position over the apertures 5, the capacitive coupling to the center plane increases to a maximum value. The amplitude of the AC coupled voltage on the receptor plates has a maximum and minimum, as the capacitive coupling to the center plane 1a is varied. This amplitude modulated signal can be demodulated, so that a DC signal proportional to the relative position of the plates is obtained on a repetitive basis equal to the grid pitch.

If fringing is neglected, the capacitive coupling between two parallel plates is C = KA Epsilon(o) over S K is the relative dielectric constant, A is the common area, Epsilon(o) is the permittivity constant, S is the distance between the plates. The distance D between the top and bottom receptor plates is a constant, because they are mounted a fixed dist...