Digital holography in a machine tool: measuring large-scale objects with micron accuracy

Abstract

We present digital multi-wavelength holography measurements generated inside a machine tool. The digital holographic sensor HoloTop NX used in the application has a field of view of 12.5 mm × 12.5 mm, each field of view consists of 3008 px × 3008 px. Three separately stabilized diode lasers connected to a fiber switch serve as light sources. The unambiguous measurement range of the holographic sensor defined by the laser wavelengths is approximately 140 µm and the axial reproducibility is ~0.4 µm. The sensor is mounted to a standard 5-axis machine tool Hermle C 32U. An automatically created numerical control (NC) program enables the machine tool to meander the sensor over larger test objects, here 280 mm × 94 mm. For each field of view, a height map with 9 million data points is generated using digital holography. The images are acquired with 2 mm lateral overlap and are then stitched together to achieve a height map of the complete sample with more than 1.4 billion data points. For comparison the shape of the sample was measured with a state-of-the-art coordinate measurement machine (CMM). The datasets from the CMM and the holographic measurement are semi-automatically referenced to each other. The holographic height data are averaged over a rectangular area with radius 50 µm around each of the 360 measurement locations where the tactile CMM measurement data have been acquired. This allows us to evaluate deviations between the holographic and the CMM dataset. It turns out that the root mean square (RMS) error between the two data sets is smaller than 0.4 µm.