Accurate roughness measurements by laser interferometer calibration, VFM-uncertainty calculations and noise reduction

摘要

Normally, roughness measurements are carried out with a rather high uncertainty ranging to a few percent up to 10%. This is a rather bad situation considering the accuracy of primary length standards (1:5·10~(11)) and common dimensional measurements (1:10~6). In this paper we show that with laser interferometer calibration techniques and extensive uncertainty evaluations this situation can be improved. As the probe of a roughness measuring instrument moves dynamically, a probe calibration should be carried out dynamical as well, in the same frequency range as in which the probe normally operates. For this, a simple yet very effective dynamic probe calibration device has been designed, where the traceability is achieved by a laser interferometric read-out with 30 kHz and 10 nm resolution. With this device a probe can be fully characterized for various amplitudes and frequencies. It is shown that for a Mitutoyo roughness measuring machine the deviations stay well within 1%. Uncertainty estimations for roughness measurements are not straightforward, therefore one takes for safety often quite high estimates for effects like probe size and measurement force. With the VFM ('Virtual Form Measurement') concept this problem is treated by simulating the measurement on the same surface as actually measured, while varying probe diameter, filtering, probe angle, noise etc. and estimating the influence of all these circumstances on the calculated parameter. In this way a task-specific uncertainty estimation is obtained with an outcome which is mostly lower than expected. This is shown with some examples. With improved noise reduction techniques the basic uncertainty can be reduced to the nm-level for the roughness tester used.