Displacement tamper sensor and method

摘要

A displacement sensor for sensing displacement of a first surface relative to a second surface includes a coil mounted on the first surface, and encircling a region in plane; and a conductor extending lengthwise from the second surface through the region. The conductor has a volume that varies along its length. An AC source drives the coil to generate a magnetic field that pierces the region and induces eddy currents in the conductor. Eddy current losses resulting from the eddy currents vary in dependence on the volume of the conductor above and beneath the plane. A sensing circuit is coupled to the AC source to detect changes in eddy current losses, and thereby displacement. Detection of displacement may be used to signal tamper/intrusion. The sensor may further include a visual or audible output to signal such displacement.

主权项

1. A displacement sensor for sensing displacement of a first surface relative to a second surface, said displacement sensor comprising
a substantially flat coil mounted on said first surface, and encircling a region in a plane; a conductor extending lengthwise from said second surface through said region, said conductor having a volume above and beneath said substantially flat coil, and a cross-section that is non-uniform normal to said conductor's lengthwise extent; wherein said substantially flat coil forms part of a oscillator, and wherein said oscillator comprises:
a tuned resonator;an AC supply to drive said tuned resonator to a resonant frequency dictated by said substantially flat coil, and to drive said substantially flat coil to generate a magnetic field that pierces said region and induces eddy currents in said conductor, and whereby eddy current losses resulting from said eddy currents vary in dependence on the relative volume of said conductor above and beneath said substantially flat coil; and a sensing circuit coupled to said oscillator to detect changes in power consumption of said AC supply resulting from said eddy current losses by detecting changes in amplitude of oscillation of said oscillator, and thereby displacement of said relative volume of said conductor above and beneath said substantially flat coil.