Vibration-resistant interference microscope with assistant focusing for on-machine measurement of surface topography

The interference microscope is a powerful tool for surface topography measurement, but its high sensitivity to vibration hinders its application to on-machine use. To measure surface roughness on a machine for the ultra-precision machining, a vibration-resistant interference microscope (VRIM) with an assistant focusing function is developed. The basic principle of VRIM is an error-compensated phase-shifting interferometry. An iterative algorithm is presented to calculate the surface phase with the phase shift amounts as unknown variables, where the phase shift amounts are calculated and compensated with least-squares method. A narrow bandwidth illumination is employed to alleviate coherence envelop influence, and a simplified intensity model is established to decouple the variables. Assisting the microscope to find fringe quickly, the focusing is realized by introducing an off-axis thin beam to generate two spots, of which their relative position relates to the defocus. The focusing method is directional and determinant, and has a large range up to 0.3 mm. In the vibration disturbances of 0.2 μm and 0.4 μm amplitudes over 0 Hz to 20 Hz frequency region, the roughness accuracy and repeatability of measuring an ultra-precision machined surface are both up to the sub-nanometer level. The developed instrument is applied to a single-point diamond turning machine and achieves a sub-nanometer accuracy and repeatability.