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High-precision length measurements in space

A new interferometric measuring system has been developed at PTB. The LaserTracer can carry out high-precision length measurements in space. With the patented laser beam guide a ten times higher accuracy was achieved in comparison to conventional laser trackers.

The laser beam of the LaserTracer automatically follows a reflector attached to the probe of a coordinate measuring machine (CMM) or to the tool head of a production machine.

The LaserTracer developed at PTB belongs to the category of laser trackers. These are laser-based 3D coordinate measuring machines that determine positions in a 3-dimensional space by measuring two solid angles and an interferometric length. By neglecting the angular information they can still be deployed, in principle, for intereferometric length measurements in a volume – however often with insufficient measurement accuracy, impaired by the mechanical quality of the axes of movement.

The new LaserTracer carries out interferometric length measurements exclusively whereas the increased accuracy is provided through a novel tracing system for the laser beam. A stationary steel precision sphere serves as spherical reference mirror, while the entire optical unit is guided around its centre. Because of the low shape deviation of the sphere (< 50 nm) all interferometric measurements relate to the centre of the sphere. This principle significantly reduces the measuring uncertainty: at a measuring length of 5 m the measurement uncertainty of a conventional laser tracker is roughly 13 µm, that of the LaserTracer, however, is merely 1.5 µm.

One application for the LaserTracer is to improve the accuracy of coordinate measuring machines. The uncertainty in profile deviation of a gear mea-surement can be reduced from 1.5 µm to 0.8 µm, for instance, by means of a special procedure. In result PTB can provide standards to industry with distinctly higher accuracy.

A second field of deployment is the recording and correction of systematic errors of coordinate measuring devices and production machines. This procedure, which was developed together with the National Physical Laboratory (NPL) in England, not only enables the considerable increase in accuracy but also a significant reduction of the measurement time.
A complete parameter set for correction can be recorded within four hours. In the mean time the LaserTracer and this procedure have become available on the market.

Contact at PTB:

Division 5.3
Phone: 0531-592-5300