A New Fast Differentiable Ray-Tracing Environment for Stable Deflectometric Measurements

Abstract

In Concentrated Solar Power Tower systems, sunlight is harnessed through the reflection and concentration of sun rays by numerous mirrors, known as heliostats, onto a receiver situated atop a solar tower. The efficiency of energy collection relies heavily on the accurate alignment of heliostats, which dynamically track the sun’s position throughout the day. Additionally, the surface geometry of heliostat mirrors plays a crucial role. As a result, assessing and measuring the mirror shapes during production and periodically post-installation in the heliostat field are imperative. While contemporary deflectometry systems exhibit remarkable precision in sub-millimeter range measurements of heliostat mirror surfaces, their initial calibration poses challenges. This research introduces enhancements to a differential ray-tracing environment to not only implement light source alternatives to the sun, such as projectors, but also making it applicable for quality control within the production process. The proposed approach builds upon state-of-the-art deflectometry methods, reconstructing mirror surfaces from reflected fringe patterns projected onto the mirror by a projector. It promises a simplified initial setup by incorporating system parameters into the optimization loop. In the development of the differentiable ray-tracing environment, solutions are devised to model custom-shaped heliostat mirrors, including focusing convex shapes. Through in silico stability analysis, the method’s capability to reconstruct mirror surfaces under the influence of inaccurate projector parameters in a compact manufacturing setup is explored. The analysis reveals stable surface reconstruction for small projector directional errors up to 10 mrad and positional errors up to 0.5 meters. Moreover, the method shows potential in handling larger errors, warranting further investigation. Furthermore, an initial attempt to reconstruct a heliostat mirror facet using real captured images is undertaken, and the adaptation of the new deflectometry method to a larger setup within the heliostat field is discussed.