Remote edge detection of subcutaneous stiffeners

Construction of narrow-gap double skin components requires, at some stage, blind welding from one side. During construction, due to thermal distortion, the hidden stiffeners (spacers or stringers) may move sufficiently far from their designated locations that assembly welds, made from one side, could miss the stringers completely. Thus, a real-time sensor capable of identifying and accurately locating spacer edges beneath the outer skin is required. Outer skin magnetic properties and plate/spacer separation seriously influence the capabilities of the best candidate detecting methods. Initial trials were undertaken using a non-destructive testing (NDT) eddy current method to detect beneath aluminium plate. Probe design and construction concentrated on air-cored absolute coils, on which finite element analysis (FEA) was used to predict electrical response. When the refined detector system was tested on magnetic material (mild steel), it was found to be preferable if a static magnetic field was applied to the plate component. Applied magnetism had a profound effect on coil field penetration and hence detectability (maximum plate thickness rose swiftly from 3mm, unmagnetized, to over 12mm, magnetized). As a sensor, the eddy current detector was shown to be capable of finding subcutaneous edges in both aluminium and mild steel fabrications. What has been developed is a sensor simply requiring computer-controlled movement to allow completely automatic hidden edge finding.