A robust signal processing algorithm for linear displacement measuring optical transmission sensors

Abstract

Optical triangulation sensors fall into a general category of noncontact height or displacement measurement devices and are widely used for their simple structure, high resolution, and long operating range. However, there are several factors that must be taken into account in order to obtain high accuracy and reliability: Measurement errors from inclinations of an object surface, probe signal fluctuations generated by speckle effects, power variation of a light source, electronic noises, and so on. In this article, we propose a signal processing algorithm, named expanded average square difference function (EASDF), for a newly designed optical triangulation sensor which is composed of an incoherent source, a line scan array detector, a specially selected diffuse reflecting surface, and several optical components. The EASDF, which is a modified correlation function, can calculate displacement between the probe and the object surface effectively even if there are inclinations, power fluctuations, and noises. This optical triangulation sensor system with the EASDF shows an 8 mm linear operation range and 1 mu m resolution without averaging and the maximum measurement error is 2.4 mu m for +/- 10 degrees inclinations. (C) 2000 American Institute of Physics. [S0034-6748(00)01608-7].