Research on methods to suppress the error accumulation in the elevation direction for accelerator tunnel control network

In recent years, the 4th generation synchrotron radiation light sources have gradually emerged. However, the accumulated errors in Laser Tracker multi-station measurements make it challenging to meet the accuracy requirements for the control network measurement, especially in the direction of elevation. To address this issue, this paper proposes to utilize the Hydrostatic Leveling System (HLS) to provide high-precision external constraints for the control network measurement. Firstly, based on the measurement principle of capacitive HLS sensor, a system is designed and constructed which was used to optimize the measurement accuracy of the HLS for level difference between difference points and to obtain the external reference. Comparison of the HLS measured values with the nominal values measured by the CMM shows that the optimized HLS can achieve a level difference measurement accuracy of 5 μm. On this basis, some constraint methods are proposed, which have been verified by simulations and measurements. The results show that the error accumulation in the elevation direction is suppressed compared with the classical adjustment. For the 70 m linear tunnel control network of Hefei Light Source, the maximum error of the measurement is reduced by 90 μm (23% improvement in accuracy) and the standard deviation of the error is reduced by 26 μm (35% improvement in accuracy) after adding constraint.