A method for rapid detection and evaluation of position errors of patterns of small holes on complex curved and freeform surfaces

This paper presents an evaluation method for the rapid and automatic detection of position errors of arrays of small holes on complex-curved and freeform surfaces that can satisfy the special demands of the aviation and automobile industries. The evaluation is based on the dual-sensor autofocusing method. The dual-sensor unit is the combination of a tactile probe and an optical vision sensor. The tactile probe detects the focal position for the optical vision sensor by probing the distance between the objective lens of the microscope and the location of each small hole. The optical vision sensor focuses to this position for capturing the image of the artifact under inspection. As a case study, a pattern of φ 0.5 mm small holes centripetally drilled with equal-angular distribution on the circumference of an elliptical cylinder shell is investigated. The autofocusing errors caused by the radius of the tactile probe and the position errors of the small holes are evaluated mathematically. Subsequently, a standalone dual-sensor autofocusing unit is built and integrated into a user-controllable 3D coordinate test rig. It is used to autofocus and capture the images of small holes. The centroid positions and deviations of the holes are automatically and rapidly detected from the captured images.