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Moire-based position sensor

For all microrobot handling, a so-called "Global Positioning system" must give the position of the robots to the robot control. At the University of Karlsruhe a position sensing system called "MPS" is currently being developed. With a resolution of 1µm and a turn-around-time of 0.2s this system is very exact and fast. Working touchless, its measurement principle works two-staged: the first approximation step is using the photogrammetrical principle, this results based on interferometrical effect delivers the mentioned resolution of 1µm.

The freely movable robot is equipped with three marks working both as a photogrammetrical and as a Moire-based mark by having three or four coincidental cosinusodial shaped full circles printed upon it. A high-resolution CCD-camera is watching the whole scene permanently. Because of the Moire-effect, there are two grids needed to generate the Moire-fringes, which have to be analysed by the connected computer with the developed software. The first grid is the grid on the robot marks. The second grid, which is fixed to the working plate, is virtually created inside the computer (this grid is also circular and cosinusodial shaped). Merging the CCD-picture (after some image processing) with the virtual grid, the Moire-effect will appear. Using some more image processing, the intersection points of the Moire-Fringes are extracted. With a over-specified linear equation system, the centre point of the mark can be calculated then. Creating and solving the error function of this equation system, the resulting error is about 0.1µm within the resolution of 1µm.

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