Non-incremental interferometric displacement measurement using a laser Doppler sensor with phase coding

The inspection of fast rotating objects with rough surfaces is an important task in the emerging field of process control. However, this is challenging since fast and non-contact inspection techniques with a measurement uncertainty in the nanometer range are often required. We present a novel optical sensor allowing non-incremental interferometric displacement measurement of moving solid state objects with rough surfaces. It features three wavelength coded interference fringe systems which are superposed slightly tilted. The displacement is determined by evaluating the phase shift between the resulting scattered light signals. Experimentally, a measurement uncertainty of 660 nm was obtained. This displacement uncertainty is independent of the lateral object velocity in principle. Due to this unique feature, the sensor can be utilized advantageously for precise displacement and vibration measurements of high speed objects as demonstrated by vibration measurements at a turbo pump shaft rotating with 48 000 rpm.