Optoelectronic Differential Cloudy Triangulation Method for Measuring Geometry of Hot Moving Objects

Introduction

Machine vision-based optoelectronic methods are widely used in multiple fields of industrial applications. Development of new measuring industrial technologies is associated with the creation of measurement methods using the element base of modern optoelectronics and photonics (Meledin, 2011). Especially topical is an enhancement of methods for dynamic control of geometrical parameters in the industry (Degarmo, 2003). Presently, optical methods for measuring the geometry of complex shaped and fast-moving objects actively develop (Feng et al., 2011). High-precision measurement of moving products’ thickness is a very important and complicated technological problem. Laser triangulation methods are being developed to solve this problem (Li et al., 2014; Sicard, Sirohi, 2013; Semenov et al., 2010; Penney, Thomas, 1989; Batbakov et al., 2004; Plotnikov, 1995; Komissarov, 1990; Lee, 2001; Deponte, 2004; Gordon, Jacob, 1969; etc). They provide thickness measurement error of up to 10^-4 in the differential scheme under laboratory conditions (Raymond, 1984).

There are various high-precision methods of thickness measurement, based on a differential laser triangulation (Dvoynishnikov et al., 2015). Triangulation measuring instruments are placed on different sides of the measured object. Information on thickness is obtained based on the difference in indications of the measuring instruments.

The error of the mentioned methods considerably increases at measuring the thickness of hot moving objects, for example, hot rolling in metallurgy. The error growth is caused, mainly, by the influence of phase inhomogeneity in thermal gradient medium on the structure of light beam tracers in the measuring scheme. Optical measurements are performed under intense air heating by hot surface of the measured object. The high temperature of the surface and high speed of motion form intensive non-stationary vortex flows near the object surface. These flows mix up with the convective motion of air masses, caused by an intense heat and mass transfer near a heated surface of the object. Air convection and motion of phase inhomogeneous in the medium form temperature phase gradients and change refraction index in the local medium. In such conditions, optical signals of laser triangulation of the measuring instruments are subjected to considerable non-stationary distortions. These distortions lead to significant and hard-to-predict errors of measurements (Dvoynishnikov et al., 2015). Therefore to improve the accuracy of laser triangulation methods, it is necessary to study and consider peculiarities of laser radiation propagation in the conditions of intensive temperature and phase distortions of optical signals.

Propagation of electromagnetic waves in the air was widely discussed. Most of these works aim at studying the propagation of electromagnetic waves in a large scale from a few hundred meters to hundreds of kilometers (Zuev et al., 1983; Bol'basova et al., 2009; Kravcov et al., 1983; Gurvich et al., 2014). In particular, a large number of studies are devoted to refractive distortions of laser radiation in a turbulent atmosphere (Pollinger et al., 2012; Aksenov et al., 2012; Nosov et al., 2009; Kandidov et al., 1998). The mentioned works touch upon the related areas of knowledge. However, direct use of the results of these studies in the analysis of propagation of laser radiation near hot dynamic objects is complicated. This is due to the specificity of phase and structural parameters of turbulent vortices in the air near the moving hot surfaces. The statistical distribution of these vortices’ parameters is substantially different from the well-studied parameters of atmospheric vortices. This chapter presents a statistical analysis of propagation of laser radiation in the phase inhomogeneous media appropriated for laser measurements near hot dynamic objects.