Importance

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Abstract

A rotational device comprises a first portion, a rotatable body rotatable relative to the first portion and comprising at least one salient member, and a speed sensor arrangement for use in measuring a speed of rotation of the at least one salient member, the speed sensor arrangement comprises a signal electrode, at least a portion of which is located between the rotatable body and the first portion, there being an electric potential difference between the signal electrode and the first portion in use, the signal electrode being configured to output a first signal in use which is a function of the speed of rotation of the at least one salient member; a guard electrode, at least a portion of which is located between the signal electrode and the first portion, the guard electrode being separated from the signal electrode by at least a first electrically insulating portion, and the guard electrode being separated from the first portion by at least a second electrically insulating dielectric portion; and a buffer arrangement configured, in use, to provide a second electrical signal to the guard electrode; the second electrical signal being arranged to place the guard electrode at an electrical potential such that the potential difference between the signal electrode and the guard electrode is less than the potential difference between the signal electrode and the first portion.

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First Claim