David C. Clark

Introduction to the Meandering Winding Magnetometer (MWM) and the grid measurement approach

A new sensor called the Meandering Winding Magnetometer (MWM) and associated grid measurement algorithms is described. The MWM can be used to determine property profiles for ferrous and nonferrous components, to provide repeatable and reproducible measurements on curved surfaces or inspection of difficult to access locations, and to detect and characterize cracks. This paper describes the application of the MWM to detection and characterization of early stage fatigue damage in aluminum and stainless steel. Other potential applications include coating characterization, case depth measurement, crack detection, and embedded sensing. The MWM is a thin and conformable sensor that incorporates both eddy current type sensing and magnetic induction sensing methods to measure conducting and magnetic properties of nonferrous and ferrous metals. The grid measurement approach is a model-based technique used to measure two properties independently, at a single frequency. This grid method also provides a convenient framework for MWM system calibration and processing of multiple frequency data. For example, this permits measurement over a wide frequency range using a single MWM sensor geometry. This paper provides a general introduction to the MWM technology and specific capability demonstrations on ferrous and nonferrous alloys.

New quasi-static magnetic and electric field imaging arrays and algorithms for object detection, identification, and discrimination

Unlike radar-based imaging technologies that use electromagnetic waves, quasistatic imaging technologies operate at lower frequencies where electric and magnetic fields are decoupled. Magnetoquasistatic (MQS) devices, such as metal detectors, that impose magnetic fields satisfy the diffusion equation in conducting media and Laplaces equation in air or poorly conducting soils. Electroquasistatic (EQS) devices satisfy Laplaces equation. In Laplacian or diffusion decay, the amplitude of the magnetic and electric fields decay exponentially with distance from the drive windings or electrode. For quasistatic sensors, objects are detected and imaged through perturbations to the applied magnetic or electric fields that change the mutual transimpedances or transadmittances at the sensor terminals, rather than through time delays of reflected electromagnetic waves as in GPR.