A high-precision angle encoder for a 10-m submillimeter antenna

We report measurements of angle error characteristics of new high-precision multi-speed resolvers developed for the 10-m submillimeter telescope of the Nobeyama Radio Observatory. We have found that the resolvers have an accuracy of 0.03 arcsec rms and 0.26 arcsec peak-to-peak, which is well below the error budget assigned to the angle encoder in the LMSA/ALMA project (0.15 to 0.2 arcsec). The error pattern in the raw readout had simple characteristics and was only composed of a few Fourier components that are associated with the number of winding poles and the core slot number of the stator. The 4-arcsec peak-to-peak error in the raw readout is reduced to less than 1/15 by a PROM correction. We have also found that shaft misalignment causes only small changes of the error pattern (0.08 arcsec error per 0.10 mm shaft misalignment), which enables us to install the resolver simply by the fitting. The temperature dependence of the error pattern also seems to be small enough for the operation temperature range specified in the project. (Author)