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2822 |
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Interferometrical System for High Precision Measurements of Flatness, Thickness and Parallelism of Mechanical Parts |
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Optics and Interferometry |
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| Content |
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An interferometrical system which allows for flatness, thickness and parallelism measurements of precision mechanical parts with tolerances of less than 2µm has been developed, built, tested and already used under shop floor conditions.
This optical system is based on a concept of conventional Michelson white light interferometry with a depth resolution of 1 nm. One of the basic advantages of white light interferometry is that the depth resolution is independent of the aperture of the objectives. The high depth resolution is necessary to perform measurements of form on mechanical parts which have tolerances of less than 2 µm.
The reference mirror and the object are illuminated by a plane wave. White-light interference displays a maximum contrast when the optical path length in the reference arm is equal to the optical path length in the object arm. In order to obtain the 3D-shape of the object, a change of the path length in the reference arm is necessary. This is called a depth scan. The depth scan is performed by scanning the reference mirror mounted on a piezo actuator. The maximum length of the depth scan is 500 µm. During the depth scan, an intensity variation occurs in each pixel of the CCD-camera. This variation is called correlogram. The maximum contrast of the correlogram defines the location of the equal path lengths. The data evaluation has to detect the location z0(x,y) of equal path lengths for each pixel (x,y). The interferometer has a unique architecture of optical elements which allow for fast measurements on both parallel surfaces without moving the part. The images of the top and bottom surface of the object are overlaid on the camera and separated by coherence multiplexing. Therefore, a high depth resolution and repeatability can be achieved. The interferometer unit contains a high aperture, long working distance telecentric objective, a high power LED and an optimized illumination optic. With this configuration, the measurement system will only allow for measurements on parts with a low reflectivity and only up to 30 mm in diameter without having to stitch the surfaces together.
The interferometer is designed to be used for very fast measurements of flatness, thickness and parallelism. The design of the interferometer allows for measuring all three features simultaneously or any feature independently. The measuring head is placed on a displacement table which allows for measurement of parts between 0 to 100 mm in length. This also means it is possible to measure the distance between two separate surfaces on the part. To ensure for a very precise measurement, a gage measurement is made before each part measurement. As a result, any errors from the interferometer can be compensated for in the final measurement results.
Typical parts for this system are mechanical components such as distance plates, bodies, gage blocks and optical plates. The stable mechanical construction allows for automation of the measurement process by integrating special fixturing and a robot for part loading and unloading.
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