Vector extraction from digital images of steel bars produced by an inductive scanning system using a differential gradient method combined with a modified Hough transform

A method of extracting positional information from images of steel bars emb edded in concrete using a set of image pre-processing algorithms combined w ith a modified Hough transform is described. The images are formed using a portable inductive scanning system in which a sensing element detects the m agnetic field emanating from the bars. Eddy currents induced on the surface s of the bars by excitation coils generate these magnetic fields. The resul ts from an edge detection algorithm based on the two-dimensional separable smoothing and local maxima detection are applied to a two-stage Hough trans form which locates the major axes of the bars and hence their alignment and number. An effective method of minimising the redundant information in the Hough array is also described. The complete system incorporates fully inte grated control, acquisition and processing software, which makes it conveni ent for on-site use. (C) 2000 Elsevier Science Ltd. All rights reserved.