DEVELOPMENT OF AN AUTOMATED ULTRASONIC AIRCRAFT RIVET INSPECTION SYSTEM

This paper describes an automated ultrasonic testing system which util ises a single-arm robot. The system developed in this paper is designe d to inspect for cracks around rivets in the bottom tying surface of a fighter aircraft. The wing is made of aluminium, has several thicknes ses and has flush head and protruding rivets of three different diamet ers. The cracks may exist on the top and bottom surface and just under the countersunk head of the flush rivets. In the developed system, ar t Automated Guide Vehicle (AGV) carries a single-arm robot to an appro priate position under the wing. A vision system then locates the centr e of a rivet by observing the circumference (or part of the circumfere nce) of the rivet on the wing surface. The AGV moves the robot to seve n appropriate locations to test the whole wing surface. A water-irriga ted probe scanning head (which is attached to a miniature x-y manipula tor) 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 di stance from the wing, three laser beams are used which strike areas ju st around the rivet. The scanning head consists of an unfocused 8 mm-d iameter ultrasonic immersion probe which produces a compression wave i n a water chamber which refracts to a 45 degrees 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 degrees refracted angle beam to test for the countersunk rivet cracks. The min iature x-y manipulator guides the probe to the appropriate position fo r scanning. The robot arm then guides the probe around the rivet. Sign al amplitude is used to assess the acceptability of a crack. Prior to scanning a crack, the system is automatically calibrated on a test-pie ce with known cracks. The calibration process is automatically repeate d at intervals. The coupling is monitored by another probe which tests a signal reflected normally from the wing surface.