A robot-arm-based mobile HTS-SQUID nondestructive inspection (NDI) system was developed to inspect advanced structures such as hydrogen fuel-cell tanks. In order to realize the stable operation of a HTS-SQUID gradiometer, exposed in Earth’s magnetic field and noise from the robot arm, without flux-trapping, flux-jumping and unlocking during motion, a new active magnetic shielding (AMS) technique using fluxgate and a compensation coil were introduced. The highly sensitive fluxgate, which can measure a magnetic field of up to several tens of μT, was mounted near a HTS-SQUID gradiometer, cooled by a cryocooler on the robot arm. This was done to measure the ambient noise and feed back its output to a compensation coil, which surrounded both the SQUID and fluxgate, to cancel the ambient noise around them. The AMS technique successfully enabled the HTS-SQUID gradiometer to be moved by the robot arm at up to 100 mm/s in an unshielded environment without flux-trapping, jumping and unlocking. The detection of hidden slots in multi-layer composite-metal structures imitating the fuel tank was demonstrated.
A nondestructive inspection method to detect wire breakage in power transmission lines using an HTS SQUID gradiometer was examined. Hard-aluminum transmission lines composed of 19 twisted Al wires, with or without wire breakage, were prepared as specimens. While applying AC voltage to the specimens to induce the equivalent current in each wire, distributions of magnetic field gradient above the transmission lines were scanned by the gradiometer. A periodic pattern in the gradient distribution due to wire breakage was observed in the results of the line with wire breakage, while such pattern was not observed in that of the line without wire breakage. The effect of wire breakage in the transmission line appearing in field gradient generated by the line with current was also investigated by computer simulation, and the result was in approximate agreement with the experimental results.