Design considerations of an inductive sensor for segmented mirror telescopes

Abstract

The Segmented mirror technology has become natural choice for any optical telescope larger than 8 meter in size, where small mirror segments are aligned and positioned with respect to each other to an accuracy of few tens of nanometer. Primary mirror control system with the help of edge sensor and soft linear actuator maintains that alignment which changes due to gravity and wind loading. For any segmented mirror telescope edge-sensor plays very critical role. It should have very high spatial resolution (few nanometer), large range, multidimensional sensing, high temporal stability as well as immunity towards relative change in temperature and humidity. Though capacitive sensors are widely used for this purpose, however, their inherent sensitivity towards humidity and dust make them unsuitable for telescopes operating at humid low altitude regions. Whereas, inductance based sensors, working on the principal of mutual inductance variation between two planar inductor coils, produce promising results in such a situation. Looking at stringent requirements, design and development of a planar inductive sensor is a challenge. As a first step toward sensor development, we have explored the design aspects of it. The inductive coils are first simulated and analyzed using electromagnetic FEA software for different coil parameters. The design considerations include optimization of coil parameters such as geometry of coils, trace densities, number of turns, etc. and operational requirements such as number of degree of freedoms to be sensed, range of travel, spatial resolution, as well as required sensitivity. The simulation results are also verified through experimentation. In this first paper we report the design and analysis results obtained from FEA simulations.