A New Design of MFL Sensors for Self-Driving NDT Robot to Avoid Getting Stuck in Curved Underground Pipelines

The magnetic flux leakage (MFL) type pipeline inspection gauge (PIG) is commonly used to detect defects on the underground pipelines as one of the nondestructive testing instruments. Although it is effective for the inter-city scale large size pipeline, it is hard to detect defects for the small size pipeline installed inside the city. So, it is necessary to adopt self-driving robot system to drive PIG module because the small gas pressure inside a small size pipeline is not enough to push PIG module. Even though it is enough to pass in straight line, when this module passes through bent pipe or welded pipe joint, the magnetic force between pipe and inspection module is increased sharply so that the PIG module could be stuck eventually. This paper proposed a new design structure of MFL system to decrease stuck forces on the pipe wall. All structural parameters for MFL sensors based on the magnetic field theory were designed to minimize the magnetic force and maximize the detection sensitivity of defect signals. The performance of the proposed system was verified by simulation results using 3-D finite-element method and experimental measured ones.