PTB News 1/2005 (135 kB) PTB News 1/2005, English edition, Issue May 2005Now, a universal calibration device is available for the control of inspection equipment in production measurement technology. Special merit is the low measurement uncertainty, even under shop floor conditions and a continuous range of nominal values from 0 mm to 300 mm.
Inspection equipment monitoring is an indispensable part of quality control in production. Handheld measuring equipment for length measurements (micrometer gauges, callipers and dial gauges) are regularly tested by comparison with solid measures like slip gages and ring gages. Therefore multiple solid measures must be held which are associated with high costs. In addition, the testing measures often do not cover the measurement range for which the particular hand-held measuring device is used in sufficient manner.
A universal calibration device with a linear slide unit, on the other hand, offers a continuous application scale and can substitute various solid measures. Such a calibration device was developed in a new international project lead by PTB and funded by the EU. Heart piece of the calibration device is a diode laser interferometer. A measurement slide is integrated to accommodate the measuring device to be tested. The slide has a three-speed manual drive with 0.1 µm step size. It is positioned along a 300 mm guide way. The device is particularly robust and easy to transport.
A particular challenge was to construct a special laser interferometer. In general, the uncertainty of a practical interferometric length measurement is essentially determined by influences of the temperature of the measurement object, and the refractive index of the air. These influences were to be compensated. This was achieved by applying a diode laser tuneable over an extended wave length range. The wavelength of the laser was dynamically adjusted to the environmental conditions. This was achieved by stabilizing the laser frequency on to the transmission maximum of an open Fabry- Perot-interferometer. The hitherto required sensors to determine the environmental conditions are thus superfluous.
With one single calibration device a wide spectrum of hand-held measuring devices can be precisely calibrated with ease on a length scale up to 300 mm. Depending on the particular measurement device an uncertainty of at most 1 µm + 10–5 · L (L: length) has been achieved even under shop floor conditions. Presently, the measuring system is undergoing further development to become an interferometer for absolute measurements. This will facilitate practical applications even further.