The effect of sample movement in fault detection using eddy currents

Eddy-current methods of non-destructive testing applied to travelling metal objects on production lines show two advantages over other techniques. These are the comparative ease with which an instrument can be coupled to test objects and the high speed of response, making possible the fast scanning rates required with fast manufacturing process. Experience in recent years has shown that relative motion between test object and measuring head gives results which differ from those of a static system, and may severely limit permissible scanning rates. The paper draws attention to dynamically-induced currents, as causing the speed effect experienced in eddy-current testing. The theoretical arguments advanced have been used to evolve three methods of overcoming the speed effect.