| 摘要 |
The invention relates to a standard of length for checking the accuracy of measurement of coordinate measuring devices, in which there are provided two scannable balls which are held at a known separation (spacing, distance) from one another and have a precisely machined surface. The position of at least one of the two balls can be varied inside the measurement volume of the coordinate measuring device. In order to exclude temperature-induced changes in length such as occur with spherical end-measuring gauges (spherical gauge blocks), and in order to be able to use the standard of length universally and for comprehensive checking, the balls are held on a carrier such that the separation can be varied. The separation of the balls is monitored continuously interferometrically. For this purpose, the balls are hollow-bored at least partially along the course of the measurement beam of the interferometer arrangement and arranged to have the same axis relative to the measurement beam. The optically active parts of the interferometer arrangement - interferometer divider with active layer and triple reflector with a centrally symmetrical point - are arranged in the interior of the calibrated balls in such a way that their critical geometric location is at the centre of the ball. As a result, only the centre separation is determined irrespective of any possible swinging of the balls. So as to permit different positions of the balls inside the measurement volume and to avoid collisions with components of the coordinate measuring device, the carrier can itself be inherently capable of changing length as well as pivoting and inclining. <IMAGE>
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