Study of a noncontact type micro-CMM with arch-bridge and nanopositioning stages

A high precision micro-CMM (coordinate measuring machine) is under development. The expected measuring range is 25×25×10 mm and the resolution is about 1 nm. For the enhancement of machine accuracy, some new design concepts are introduced. The arch-bridge is proven to have higher stiffness than the conventional rectangular bridge. The proposed co-planar stage can reduce the Abbé error in the vertical direction. The long travel of each axis is activated by the piezo-ceramic ultrasonic motor with its AC drive mode. The fine positioning is achieved by the same motor with its DC drive mode, which can perform nanometer steps proportional to the input voltages. A linear diffraction grating interferometer (LDGI) is developed as the position feedback sensor with the resolution to 1 nm after the waveform interpolation. The autofocusing probe is adopted from DVD pickup head and modified to achieve long-range and short-range detections. Using its focus error signals (FES), the probe can reach to 30 nm accuracy in the short range. In this paper, the design principles of the developed micro-CMM are described.