Capacitive strain gage having fixed capacitor plates

Claim 1

A capacitive strain gage for measuring strain in a test specimen, comprising: a housing attachable to the test specimen at a first location, said housing including a passage extending therethrough between spaced-apart opposing surfaces, one of said opposing surfaces having first and second electrodes disposed thereon and the other of said opposing surfaces having a third electrode disposed thereon, said electrodes each having fixed positions on said housing, and said first and second electrodes each being positioned for capacitive coupling to said third electrode; and a ground plane attachable to the test specimen at a second location in movable relation to said housing, said ground plane including an arm that extends into the passage in said housing, said arm including an aperture for electric fields that is aligned with said electrodes in a quiescent position, said aperture being sized and shaped such that when strain in said test specimen causes relative movement of said housing and said arm, the capacitive coupling between said third electrode and at least one of said first and second electrodes is changed, wherein said first and second electrodes comprise parallel conductive strips perpendicular to the direction of relative movement of said housing and said ground plane, wherein said first and second electrodes have parallel outer edges perpendicular to the direction of relative movement and wherein the aperture in said ground plane includes parallel opposite edges that are respectively aligned with and parallel to the outer edges of said first and second electrodes.