Robust white-light interferometric sensing system for fast displacement measurement

A robust white-light interferometric sensing system for fast displacement measurement is presented. In order to increase the speed of the sensing system in comparison with the standard realization based on the postprocessing of the captured interferometric signal, a real-time algorithm based on the modified centroid algorithm has been implemented. The modified centroid algorithm generates the error signal that is proportional to the interferometer optical path difference (OPD) during every scan of the used white-light source coherence zone. In order to keep zero OPD, an amplified version of the error signal has been brought to the input of the piezo bimorph actuator (PBA) that on the other side serves for the fast coherence zone scan. In this way the PBA tracks the position of the object whose displacement is to be measured. Therefore, the voltage signal at the PBA input is proportional to the measured displacement. The realized sensing system has an overall bandwidth of almost 10 Hz, where the sensor full scale range of 200 μm has been measured with a resolution of 3 nm.