Visual detection of hexagonal headed bolts using method of frames and matching pursuit

Abstract

In this paper we focus on the problem of automatically detecting the absence of the fastening bolts that secure the rails to the sleepers. The proposed visual inspection system uses images acquired from a digital line scan camera installed under a train. The general performances of the system, in terms of speed and detection rate, are mainly influenced by the adopted features for representing images and by their number. In this paper we use overcomplete dictionaries of waveforms, called frames, which allow dense and sparse representations of images and analyze the performances of the system with respect to the sparsity of the representation. Sparse means a representation with only few no vanishing components. In particular we show that, in the case of Gabor dictionaries, dense representations provide the highest detection rate. Moreover, the number of no vanishing components of 1% of the total reduces of 10% the detection rate of the system, indicating that very sparse representations do not heavily influence the performances. We show the adopted techniques by using images acquired in real experimental conditions. © Springer-Verlag Berlin Heidelberg 2005.