Shape measurement of precision components by use of a dynamically tracking interferometric line sensor

• Applicants: Dr.-Ing. Gerd Ehret; Professor Dr.-Ing. Peter Lehmann
• Subject Area: Measurement Systems
• Term: from 2011 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 209933375

This project deals with the accurate form measurement of precisioncomponents based on a line-shaped multi-distance sensor, which isdynamically tracked to the contour of a measuring object. During thefirst phase of the project the concept for the measurement ofapproximately rotationally symmetrical optical surfaces wasdeveloped and verified experimentally. The measuring object islocated on a precise rotary table. The interferometric line sensor usesa sinusoidal modulation of the optical path length in the reference armof the interferometer and a line scan camera to record theinterference signals. This results in approx. 800,000 height values persecond. During a single revolution of the rotary table, the sensordetects the geometry of a circular-ring-shaped segment of the surfaceof the measuring object. Repeating such measurements for differentradial positions and stitching of the individual overlapping circular ringsegments, yields the complete geometry of the specimen. During thesecond project phase, the measuring system was extended by a fiberbasedillumination, an absolute height measurement option by meansof white light interferometry for calibration and scanning, and themotorized inclination tracking of the sensor. In addition, stitchingalgorithms have been developed and successively improved. Thesestitching algorithms selectively utilize the redundant height data fromthe overlapping regions to calculate 3D topographies with highprecision. In the proposed third project phase, the line-based formmeasuring system is to be extended by a multi-line camera, whichallows laterally displaced, simultaneous measurement of heightvalues. This will result in new evaluation possibilities andmeasurement strategies which are to be investigated. For example,surface gradients in the scanning direction can be detected and takeninto account during sensor tracking, or evaluation methods based onshearing algorithms can be used. In addition, the wobble error of therotary table, which currently limits the achievable measurementuncertainty, will be recorded during the measuring process and itsinfluence will be subsequently eliminated from the topographymeasurement values. The project further intends to extend themeasuring volume of the system, so that specimens of up to 200 mmin diameter can be measured. In addition, an on-the-fly tracking of thesensor is to be implemented so that free-form surfaces can also bemeasured. At the end of the third project phase, it is expected thatconsiderably more reliable measurements with lower measurementuncertainty will be even possible if the rotary table, as the only axisthat is moved during a circular ring measurement, does not achievethe highest accuracy requirements. This is intended to provide optimalpreconditions for the possible transfer of the measuring system intoapplication, e.g. within the framework of a transfer project.

• DFG Programme: Research Grants