The production metrology of today is still dominated by tactile probing systems. However some special metrological tasks cannot be fulfilled by this technique, one example is in the high precision manufacturing of surfaces and structures, which become ultra-miniaturized, complex and fragile. The inspection of small boreholes and cavities is also an example with very tight tolerances which demands non-contact miniaturized sensors. Particularly the measurement of the shape of spray holes in modern fuel injection nozzles for diesel engines fits this statement, as its shape represents the key factor for maximal motor efficiency, as well as minimal pollutant emissions. Any deviation from its design shape significantly affects spray breakup and can lead to unequal distribution of flow and pressure changes. These holes can have diameter of 150 microns, with a tendency to even smaller diameters in future systems.

Within this work the integration of a fiber optic sensor for distance measurements in measuring machines, specifically for borehole inspection, is described. The used device is a form-tester (Mahr GmbH, MMQ-400) with 3 degrees of freedom. The motion of the machine axis will be controlled with help of image processing operation which are based on pictures taken from the specimen’s top surface. For this mean a micro camera will be mounted on the form-tester. By applying in-house developed MATLAB codes, the exact position of the boreholes and that of the fiber optic probe is obtained, so that an automated positioning and measurement (e.g. round-out and roundness tests) could be performed. This process enhances both the precision due to an optimized sensor positioning and speed of the measurement rather than manual execution. Different positioning scenarios will be discussed and compared in this paper, to prove the capability of the proposed system as well as its adaptivity.

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