Dirk Voigt and Arthur S. van de Nes, “Capacitive Sensor Calibration with Fabry-Pérot Interferometry”, talk, Macroscale 2014, Recent Developments in Traceable Dimensional Metrology, 28-30 October 2014, BEV – Bundesamt für Eich- und Vermessungswesen, Vienna, Austria.
Optical displacement interferometry has achieved an advanced level of maturity in commercially available instrumentation, many based on counting and interpolation. Periodic nonlinearities typically are a major limitation to the measurement uncertainty. In contrast, Fabry-Pérot interferometry (FPI) exploits high resolution resonant optical frequency measurements without suffering from these nonlinearities. FPI thus allows for sub-nanometer measurement accuracy and may provide a valuable tool for sensor calibration. Dedicated implementations of FPI showed remarkable performance such as, e.g. for large displacement stroke or in differential configuration. In this work, we consider FPI as a tool for highly accurate calibration of capacitive displacement sensors, aiming for sub-nanometer measurement uncertainty under practical ambient conditions. Our instrumentation is based on a Metrological FPI engineered with a very precise actuation mechanism for up to 300 mm displacement of a reference target electrode disk. The challenge for sub-nanometer calibration uncertainty is in the thermo-mechanical stability of the set-up and in the ambient climate stability to millikelvin-level over hour-scale periods that are required for full calibration runs. We report on progress toward a methodology for testing and calibration of capacitive displacement sensors, dealing with sensor mounting, alignment and the dedicated measurement procedures. Whereas we can establish system drift rates below 1 nm/hour, minute uncontrolled air pressure fluctuations will require special attention in the measurement methodology.
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