Displacement tamper sensor and method

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.

2. The displacement sensor of claim 1 , wherein said conductor comprises a frustro-conical portion.

3. The displacement sensor of claim 1 , wherein said conductor comprises a bolt having frustro-conical head.

4. The displacement sensor of claim 1 , wherein said resonant frequency is between about 1 kHz and 1 MHz.

5. The displacement sensor of claim 1 , further comprising a gasket, and wherein said substantially flat coil is mounted to said gasket.

6. The displacement sensor of claim 5 , wherein said gasket is formed of at least one of rubber, silicone, plastic, Textolite, Teflon, paper, or other insulating material.

7. The displacement sensor of claim 6 , wherein said oscillator and said sensing circuit are mounted to said gasket.

8. The displacement sensor of claim 5 , wherein said gasket is mounted to a cabinet.

9. The displacement sensor of claim 1 , wherein said sensing circuit further comprises an audible or visible output for signaling displacement.

10. A method of sensing displacement of a first surface relative to a second surface, said method comprising

mounting a substantially flat coil on said first surface, encircling a region in a plane, said substantially flat coil forming part of an oscillator, wherein said oscillator comprises a tuned resonator;

providing a 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,

extending lengthwise from said second surface through said region;

driving said tuned resonator by an AC supply to a resonant frequency dictated by said substantially flat coil to drive said substantially flat coil to generate a time varying 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

detecting changes in power consumption of said AC supply resulting from said eddy current losses by detecting changes in amplitude of oscillation of said oscillator to detect displacement of said relative volume of said conductor above and beneath said substantially flat coil.

11. The method of claim 10 , further comprising signaling a tamper condition in response to sensing displacement of said first surface relative to said second surface.

12. The displacement sensor of claim 1 , wherein said oscillator is formed as a Colpitts oscillator.