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Production sequence of Si-spheres and interferometrical determination of the sphere volume

The networked primary angle comparator of the PTB

22.12.2020

The Heidenhain IK320 electronics previously used for processing the scanning head signals on the PTB's primary angle comparator were based on a 32-bit VME bus interface. As a replacement, new EIB8 electronics with a 1 Gbit/s Ethernet interface are now being used. The EIB8 interpolates the phase of the incremental measuring signal within one signal period with a factor of 65536 corresponding to 0.000075" resolution. The IK320 works with an interpolation factor of 4096 corresponding to a resolution of 0.0012", which approximately corresponds to the achievable measurement uncertainty. The EIB8 com-bines the correction of non-linearities of several harmonics used by the IK320 by means of premeasurement with the Heidemann method, in which non-linearities in the signal period and the first harmonic are corrected dynamically with higher precision.


Figure 1: (top): Mean measurement deviations in the measurement range of ± 40 angular seconds of the autocollimator, (middle): Associated differences of the individual measurement deviation to the common mean value, (bottom): The associated standard deviations of the individual and the mean measurement deviation.

First measurements with the new electronic platform could prove that the previous measurement uncertainty for the calibration of autocollimators is already reached again. Figure 1 above on top shows the mean measurement deviation of a very good autocollimator for a calibration with 36 forward and backward measurements in 401 static measurement positions each (total measurement time = 162 hours) at a measurement step size of 0.1". Over the measuring range shown, the essentially long-period systematic measurement deviations are in the range of ± 0.0066". Converted to the circumference of a circle with a radius of 200 mm, as used in the measuring system of the primary angle comparator WMT 220, these are systematic measurement deviations in the range of ± 6.6 nm. In the middle of Figure 1, the differences of the individual calibrations from one forward and backward measurement to the mean calibration curve are shown. These individual differences are within ± 0.0083". The standard deviations of the individual calibrations from one forward and backward measurement each are shown below. All values lie between a maximum value of 0.0030" and a minimum value of 0.0013". By using the new possibilities of the data acquisition system regarding the interpolation of the periodic non-linearities and the increased data acquisition rate, which creates new possibilities in data analysis, there is a justified hope to further reduce the measurement uncertainty.

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