High-precision applications of multiwavelength holography typically requires stable laboratory-like environments, which is hard to achieve in industrial applications. The influence of schlieren is crucial, especially in large fieldof-view applications, where long working distances can result in optical paths up to one meter. Schlieren is a well-known effect in interferometry and can be seen in the reconstructed object wavefronts. Small perturbations in temperature change the refractive index of air resulting in local variations of the optical path. Proper encapsulation or vacuum techniques are typically employed to compensate for this. In this work, we investigate the impact of schlieren on multiwavelength holography and propose a compensation method. The sensor used is a Mach-Zehnder-based interferometer with a field-of-view of 17.9 mm × 13.4 mm and a camera with 9344 px×7000 px. A mirror was positioned at a distance of 1 m in front of the sensor. We performed holographic and interferometric measurements with and without an encapsulating pipe around the beam to investigate the influence of schlieren. The deviations of the phase shifts of the holographic data were laterally resolved using a modified version of the algorithm proposed by Cai et al. The fringe patterns of the interferometric data captured with different exposure times and frame rates were analyzed using a sinusoidal fit and discrete Fourier transformations (DFT) to show lateral frequency deviations. Both methods show that encapsulation leads to improved measurements. A potential compensation method is proposed.
Hybrid manufacturing processes, high level of automation, short product service life and decreasing vertical range of manufacture in production request for increasing flexibility and speed of quality control. With HoloCut we previously introduced the world’s first wireless digital-holographic sensor system prototype for fast and precise measurements inside a machine tool. With the experience gained so far, we now present an improved, even more compact sensor system, for the use on various multi-axis systems such as coordinate measuring machines (CMM), robots and machine tools and show first results with different handling systems. Besides improved mechanical stability, a size and weight reduction resulted from a new design approach: The arrangement of components around a central "core" made it possible to create a very compact design with a diameter of 125 mm, a height of ~180 mm and a weight of ~2 kg. The system features a 12.5 × 12.5 mm² measuring field with a lateral sampling of 4 μm. An NVIDIA Xavier embedded system enables pre-evaluations of the recorded measurement data in order to allow re-recording them, even before the complete data transmission (up to 160 MB with 2 Hz measuring rate) and evaluation. This is especially important for the use in vibration-prone environments such as multi-axis systems. Various handling systems such as a HERMLE C32U machine tool, an undamped LEITZ Reference HP 15.9.7 CMM and a UNIVERSAL ROBOT UR16e are examined with regard to vibrations. In future work, the behavior of the system under higher vibration amplitudes will be characterized.
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