Twin-scale Vernier Micro-pattern for Visual Measurement of 1-D in-plane Absolute Displacements with Increased Range-to-Resolution Ratio

This article presents a visual method for 1-D in-plane displacement measurement which combines a resolution of a few nanometers with an unambiguous excursion range of 168 µm. Furthermore, position retrieval is only based on elementary phase computations and, thus, might become compatible with high-rate processing by implementing the processing algorithm on high speed computing architectures, like a DSP or a FPGA device. The method is based on a twin scale Vernier micro-pattern fixed on the moving target of interest. The two periodic grids have slightly different periods in order to encode the period order within the phase difference observed between the two sub-patterns. As a result, an unambiguous range of 168 µm is obtained from grid periods of 8 µm and 8.4 µm. The resolution is evaluated to be of 11.7 nm despite remaining mechanical disturbances. Differential measurements demonstrated indeed a measurement accuracy better than 5 nm.