Development of an automated ultrasonic aircraft rivet inspection system

This paper describes an automated ultrasonic testing system which utilises a single-arm robot. The system developed in this paper is designed to inspect for cracks around rivets in the bottom wing surface of a fighter aircraft. The wing is made ofaluminium, has several thicknesses and has flush head and protruding rivets of three different diameters. The cracks may exist on the top and bottom surface and just under the countersunk head of the flush rivets. In the developed system, an Automated Guide Vehicle (AGV) carries a single-arm robot to an appropriate position under the wing. A vision system then locates the centre of a rivet by observing the circumference (or part of the circumference) of the rivet on the wing surface. The AGV moves the robot to seven appropriate locations to test the whole wing surface. A water-irrigated probe scanning head (which is attached to a miniature x-y manipulator) on the end of the robot arm is then guided to the rivet. In order to ensure the head is perpendicular to the wing and at the correct distance from the wing, three laser beams are used which strike areas just around the rivet. The scanning head consists ofan unfocused 8 mm-diameter ultrasonic immersion probe which produces a compression wave in a water chamber which refracts to a 45° wave in the aluminium wing. Half-skip and full-skip are used to test for the top and bottom cracks. On the same head there is another probe forming a 70° refracted angle beam to test for the countersunk rivet cracks. The miniature x-y manipulator guides the probe to the appropriate position for scanning. The robot arm then guides the probe around the rivet. Signal amplitude is used to assess the acceptability ofa crack. Prior to scanning a crack, the system is automatically, calibrated on a test-piece with known cracks. The calibration process is automatically repeated at intervals. The coupling is monitored by another probe which tests a signal reflected normally from the wing surface.