New approaches to machine vision based displacement analysis

The paper describes the design and manufacturing of optical components; calibration and evaluation procedures; together with thermally stable housings. All these elements are required for the implementation of accurate optical displacement and orientation sensors. Printed hexagonal and rectangular dot-patterns on optical glass are investigated with respect to their spatial scattering properties of an incident laser beam, and as a means of implementing semi-transparent windows to image laser spots on two parallel targets. The positions of the laser spots are observed by two cameras. The mapping from pixel coordinates to real world coordinates is implemented via a bivariate tensor polynomial product, whereby the calibration coefficients are determined in a manner such that the distortion associated with the optical component, e.g., lens distortion is compensated. The covariance propagation is explicitly computed for the calibration process and used to optimize the selection of the polynomial degree. This reflects the trade-off minimizing between the systematic and stochastic errors. The new technique and methods are demonstrated in the design of and implementation of an active optical laser target for machine guidance control. This measurement instrument, rigidly fixed on a Tunnel Boring Machine (TBM), measures the orientation and position of the machine where an off-vehicle reference laser beam is projecting its laser spots on two parallel targets. Precise vision-based measurements of the laser spot positions are achieved, yielding a standard deviation of the displacement error of 0.05 [mm] and for yaw and pitch of 0.02 [degree].