Patent ID: 8339122

Claim:
A speed sensor for use in measuring the speed of rotation of a rotationally salient rotating member, the speed sensor comprising an electrode and a sensor circuit; the sensor circuit comprising: a voltage source for supplying a voltage to the electrode to generate an electric field in a dielectric medium between, the electrode and the rotating member; a current detector for detecting current flow between the voltage source and the electrode due to perturbation of the electric field by passage of at least one salient feature of the rotating member through the electric field as the rotating member rotates, the current detector outputting a first signal modulated at a frequency corresponding to the frequency of perturbation of the electric field; and an amplifier circuit comprising a signal amplifier for amplifying the first signal and outputting a second signal modulated at a frequency corresponding to the frequency of perturbation of the electric field, wherein the first signal is a modulated voltage, and wherein the current detector comprises a capacitor and a resistor connected in series across the voltage source, wherein the electrode is connected to a node between the voltage source and a first terminal of the capacitor, and the first output signal is taken from a node between a second terminal of the capacitor and the resistor.