Abstract
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.
