Position sensor for video disc

Abstract

A stylus position sensor for a video disc player is described. An electrode secured in fixed relation to a movable stylus with respect to the stylus supporting carriage is disposed between first and second electrodes to form first and second variable capacitors therewith. The capacitance of the variable capacitors change in accordance with the motion of the stylus related electrode between the first and second electrodes. First and second varactor diodes are respectively serially connected with the first and second variable capacitors and poled in a complementary manner with respect to each other. The net capacitance at said third electrode comprises the parallel combination of the two respective series combinations of variable capacitor and varactor diode. The capacitance of the variable diodes is modulated by an AC signal potential coupled from a low impedance source through respective coupling capacitances. As the center electrode is displaced from the center position with respect to the first and second electrodes, the net capacitance at the center electrode increases and decreases in phase with the modulating capacitance of the varactor diode connected to the electrode toward which the stylus moved. A capacitance sensitive pickup circuitry converts the net capacitance change to a signal potential for controlling a servo motor to adjust the carriage position.