The following are patents on eddy curent techniques, primarily methods used in non-destructive testing (NDT). NDT is a close relative of induction metal detection and many of the techniques found in metal detector designs originated in NDT devices.
Download	Patent #	Title	Description
698K	US3337796	Eddy Current Testing Device with Means for Sampling the Output Signal to Provide a Signal Proportional to the Instantaneous Value of Said Output Signal at a Particular Phase	Helluva title, the basis for most modern sampled discriminators.
2061K	US3478263	Wide Frequency Range Eddy Current Testing Instrument
1225K	US4006407	Non-destructive Testing Systems Having Automatic Balance and Sample and Hold Operational Modes
1107K	US4095180	Method and Apparatus for Testing Conductivity using Eddy Currents
1973K	US4188577	Pulse Eddy Current Testing Apparatus for Magnetic Materials, Particularly Tubes
1204K	US4191922	Electromagnetic Flaw Detection System and Method Incorporating Improved Automatic Coil Error Signal Compensation
785K	US4230987	Digital Eddy Current Apparatus for Generating Metallurgical Signatures and Monitoring Metallurgical Contents of an Electrically Conductive Material
1196K	US4303885	Digitally Controlled Multifrequency Eddy Current Test Apparatus and Method
810K	US4424486	Phase Rotation Circuit for an Eddy Current Tester
1155K	US5508610	Electrical Conductivity Tester and Methods Thereof for Accurately Measuring Time-Varying and Steady State Conductivity Using Phase Shift Detection
218K	US5952879	Device for the Simultaneous Demodulation of a Multifrequency Signal, Particularly for an Eddy Current Measurement