Eddy current surface flaw detection employing signal correlation

摘要

An eddy current flaw detection system for inspecting a workpiece for surface flaws is described. The system has a search unit for mounting a pair of eddy current search coils in close proximity to a workpiece at locations which are longitudinally spaced with respect to the workpiece. The system is disclosed in the environment of a workpiece support and transport mechanism which causes a workpiece to rotate and move axially with respect to the search unit. Coil energizing and detection circuitry is provided having a pair of eddy current channels. Each channel energizes a connected one of the search coils to induce eddy currents within a workpiece at slightly different energization frequencies. Each channel has an output for producing signals indicating the presence and severity of detected flaws. The coil energization at different frequencies results in each coil responding to self induced eddy currents rather than currents induced by the other of the two coils. Processing circuitry is provided having a pair of inputs each connected to a respective eddy current channel output. The processing circuitry algebraically multiplies concurrently received eddy current channel output signals to produce a resultant signal. The disclosed signal correlation circuitry will not process a signal from a single coil. Accordingly, the axial separation of the search coils establishes a minimum length of surface flaw which will be detected. The value of the resultant signal indicates the severity of the detected surface flaw.