Wire position monitor(WPM)is designed to monitor contraction of the cold masses during the cooling-down operation in an accelerator driven system.Because of material difference,machining error,assembly error,etc.,each WPM has to be calibrated.The sensing voltage and wire position are of a nonlinear relationship,which is expressed by high order polynomial.Root mean square(RMS)of the polynomial fitting error were 3.8μm and 7.4μm at x and y directions,respectively.The alignment test was carried out on the beta cryostat.Optical instruments were used to verify the WPM measuring results.The differences between WPM measuring results and optical measurements were 0.044 and 0.05 mm in x and y direction,respectively.A significant asymmetric contraction was detected,and asymmetry of material was taken as the main reason through analysis.
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