Differential Sensor Measurement With Rotating Current Excitation for Evaluating Multilayer Structures

Rotating current probe using rotating electromagnetic fields and currents generated by orthogonal excitation coils is required for achieving similar sensitivity to defects in all radial directions around fastener. However, orthogonal excitation scheme only has a single null field point. When using array sensors for rapid inspection, sensors located away from the point measure a strong background field that must be eliminated. This paper presents a new approach using two giant magnetoresistive sensor arrays in a differential configuration to reduce the impact of the background field and increase signal-to-noise ratio. The operation of the differential probe is validated using a finite-element model. A prototype probe was designed, built, and tested. Experimental results obtained using the prototype probe to inspect a two-layer aluminum sample are presented.