Digital Eddy Current Probe for Tube Nondestructive Testing Using Binary Excitation

Eddy current probes are ubiquitous in nondestructive testing (NDT) of safety critical tubing such as heat exchanger tubes in power plants. The eddy current excitation is usually multiple sinusoidal harmonics. Because of limited space inside the tube, the probe consists only of transmitter and receiver coils. A remote eddy-current instrument with the task of exciting the probe and acquiring its signals is connected to the probe, sometimes via cables tens of meters long. Deployment of the probe circuitry next to the coils inside the tube would improve signal integrity, enable sensing of the environment inside the tube, and reconfiguration of the probe settings accordingly. A necessary step towards such a digital eddy current probe is to minimize the complexity and the component count of the transmitter circuitry. In this paper, we investigate a switched mode operation of the transmitter circuitry based on an H-bridge driven by a binary signal. The binary signal, with specified main excitation frequencies and spurious free range, is constructed using a crest factor minimization algorithm. We present a compact demonstrator of a digital eddy current probe that fits completely inside the tube. We show that it performs well in difficult, but common NDT task of detecting tube defects near a tube support plate.