Robotic inspection of fastener holes with hybrid visual and ultrasonic motion control

Fasteners are widely used in mechanical structures, where stress concentrations around fastener holes can lead to crack initiation and fatigue failures. In the aerospace industry, routine fastener hole inspections are critical to ensure structural integrity. Ultrasonic testing is one of the main inspection approaches. Conventionally, it involves a single-element probe that must be manually placed at multiple locations and orientations so that the ultrasound beam insonifies the area around the hole from different angles. The received ultrasonic time-domain signals at each location are analyzed, which is time-consuming, operator-dependent, and prone to inconsistencies. 2D ultrasonic array probes enable 3D volumetric images of fastener hole defects to be obtained from a single probe position, offering the potential for more efficient automated inspection and data interpretation. To achieve this, the 2D array probe must be accurately located over the centre of the hole and the ultrasonic coupling with the component must be consistent over the entire probe contact surface. This paper presents an automated robotic system for ultrasonic fastener hole inspection, that is designed to address these issues. A 7 degree-of-freedom robot arm is used with a vision module, and a customized probe adapter integrates a 2D ultrasonic array and coupling block into the robot end effector. A novel hybrid probe manipulation method is proposed, which combines camera-based visual localization with real-time ultrasound signal feedback to ensure accurate probe alignment and consistent coupling. The whole inspection workflow is scheduled and a graphical user interface is developed to demonstrate this automatic inspection. Experimental validation demonstrates that the robotic system performs accurate, repeatable inspections, significantly enhancing efficiency and reliability compared to manual techniques. The proposed approach addresses key challenges in robotic ultrasonic inspection and offers a scalable solution for intelligent maintenance in aerospace and other high-reliability industries.