En-plane displacement measurement with sub-pixel resolution: Application to vibration characterization of a shear-force scanning probe

摘要

This paper reports on a sub-pixel resolution vision approach for the characterization of in-plane rigid-body vibration. It is based on digital processing of stroboscopic images of the moving part. The method involves a sample preparation step, in order to pattern a periodic microstructure on the vibrating device, for instance by focused ion beam milling. An image processing has then been developed to perform the optimum reconstruction of this a priori known object feature. In-plane displacement and rotation are deduced simultaneously with a high resolution (better than 0.01 pixel and 0.0005 rad. respectively). The measurement principle combines phase measurements - that provide the high resolution - with correlation - that unwraps the phase with the proper phase constants. The vibration modes of a tuning fork were fully characterized for the demonstration of the method capabilities. Then the tuning fork was loaded with a tungsten wire sharpened in a sub-micrometer tip for use in shear-force microscopy. The vibrations of the scanning probe were also characterized furnishing representative data on its actual vibration amplitude. The technique could however be applied to many kinds of micro-devices, for instance comb driven electrostatic actuators. For applications allowing the sample preparation, the proposed methodology is more convenient than common interference methods or image processing techniques for the characterization of the vibration modes, even for amplitudes in the nanometer range.