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When the carriage strays from the straight and narrow

A new sensor detects deviations from straight-line motion


[es] Like a moving ship, that part of a machine which must move in a straight line - the carriage - may start to roll: unless its guide way is sufficiently exact, the carriage rolls about its longitudinal axis. To allow this so-called roll error to be measured, a novel sensor has been developed at the Physikalisch-Technische Bundesanstalt (PTB). It uses a laser beam, meaning that any contacting is avoided, and it is much more precise than the methods applied to date. At the Hannover Fair (March 20 to 25), the PTB will present the new sensor which can measure great errors, for example of planing machines, but above all very small deviations of highly precise products manufactured in the hightec sector.

Optical measuring methods are increasingly winning through. Their great advantage over mechanical methods: they can look at the process to be examined from the outside, so to speak; the object measured is not touched and, therefore, not changed.

The optoelectronic sensor developed at PTB makes use of the properties of polarized light: when a polarizer is placed into the light, only light of a certain vibration direction is transmitted. When the vibration direction is turned, less light, or no light at all, is transmitted. "In the case of our sensor, the laser light is first polarized", explains Dr. Michael Schulz, one of the design engineers. "It then falls upon a second polarizer which also travels on the carriage and is first adjusted so that no light should pass through it. If light passes nevertheless, we know that the instrument to be tested has produced a roll error." In such a case, the carriage, which should actually move without twisting, has turned about its longitudinal axis - and with it the polarizer carried quasi piggyback.

It is the great advantage of this method that the properties of polarized light do not change over wide distances. The sensor is, therefore, capable of carrying out measurements along arbitrarily long paths. Moreover, its field of application is extremely large: it extends from deviations which cannot be seen with the naked eye to orders of magnitude which are of significance in the hightec sector. "This is the most important field of application of the new sensor", says Schulz.

A new ultra-precise apparatus which he and his colleagues have developed and into which the new sensor has been integrated as an important component is also intended for such applications. This instrument measures the shape of certain surfaces used in optical systems, and this with extremely high accuracy - with a height resolution in the order of magnitude of the diameter of an atom.

However, quite normal machine tools, too, could be improved by the new measuring technique. "Our sensor will certainly not be of interest to craftsmen, for example joiners who use planing machines", says Schulz. "But no doubt for firms which manufacture such machines for joiners' workshops."

Additional information:
At the Hannover Fair (March 20 to 25, 2000)
PTB stand (Hall 18, stand B 020, telephone at Fair: +49 (0)511 89-438-04)

Dr. Michael Schulz, telephone: +49 (0)531 592-4211, e-mail:michael.schulz(at)ptb.de
Dipl.-Ing. Andreas Fricke, telephone: +49 (0)531 592-4213, e-mail:andreas.fricke(at)ptb.de
"Imaging Optics and Spectrometry" section